![2018, secure in the knowledge that the sector will continue to progress across many
fronts even before the ink in this chapter has had time to dry.
1.2 In the beginning
So rather than ask ‘where did it all start’, it is more illuminating to ask ‘why did it
all start’?
Energy. Society needs energy to produce or process material into more valu-
able goods. This has always been the case and over history energy has been pro-
duced by the manual labours of humans, or their domesticated animals.
For most of human history, the most valuable commodity was food and nearly
all energy harvesting systems went into this. In most cases, some natural energy
flow was harvested, initially the wind and in due course the flow of rivers. All of
these techniques require technology to turn flowing fluids into useful work and
most relied upon grinding or processing plant material into food stuffs.
This gives the first example of a tidal site I have encountered: Eling Mill in
Hampshire. The mill is believed to have Roman foundations. It has been built
across the river at Eling where it enters Southampton Water and the sea. The gates
of the mill are left open as the tide rises so allowing the mill runs releasing pond to
be filled with the incoming tide. At high tide the gates are closed, so impounding
the tidal water in the mill pond. Once the tide has fallen outside the pond, the mill is
free to run by releasing the impounded sea through its mill wheels. It has been
doing this for nearly 2,000 years.
There must be other sites where this has been done but forgotten. Indeed, there
are multiple tidal mills shown on maps in Wales and Cornwall, but the dates of
them are often unclear. However, they never achieved the ubiquity that wind mills
achieved, so they have often been ‘the first of a kind’ when the idea was
re-discovered again and again during history. However, as an idea they were
eclipsed by the bigger and more powerful forces of fossil fuels. There are practically
no examples of new tidal schemes being built, commissioned and successfully run
for any period of time once the technology to burn fossils was developed. Where
there are examples then they tended to be commissioned at times of crisis when oil
supplies were looking fragile, or when the costs looked to be rising uncontrollably.
Another driver for the development of renewables seems to have been the
development of the means to generate and use electricity. This occurred around
1880 when shaft power could be turned into kilowatts and gave rise to a flurry of
schemes.
This leads to my earliest wave scheme: Herring Cove, Nova Scotia. The Par-
sons Ocean Power Company was incorporated in 1922 [1] and by 1925 had built a
machine which successfully generated power, until its later destruction in a storm.
This scheme relied upon capturing the movement of a tethered float. The move-
ment of the float caused machinery to rotate by way of a lever arm and gear
mechanism and generated sufficient electricity to light ‘several 60 watt bulbs’.
The Pacific coast also seemed to attract those who saw the opportunity and
there are several examples of around a dozen schemes such as the oscillating water
column at Parallel Point, San Francisco.
2 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-27-320.jpg)
![Schemes like Herring Cove aimed to generate electricity for the local area, but
the advent of the electricity supply grid generally marked the end of their useful-
ness, or certainly their competitiveness.
In the space of a few short decades, a new era of coal, steam and electric
power, together with the creation of an electricity grid, disconnected the need to co-
locate generation and the users of energy and heralded a long hiatus in the need for,
or an interest in, renewable power.
Industrial economies became used to plentiful and relatively cheap electricity
provided through increasingly large fossil fuel power plants streaming electrical
energy to people’s homes. Motor transport grew on the back of plentiful, cheap
oil. This meant that, with very few exceptions, there was little need to consider
renewables.
Yet in some places renewables found a role. In France, La Rance Tidal Power
Station was opened in 1966 as the world’s first tidal power station, feeding power
into the country’s grid. It was for decades the largest tidal power station in the world.
Whilst in the Highlands of Scotland – geographically remote and poorly con-
nected – massive hydro schemes were built throughout the middle of the twentieth
century to ‘power the glens’ and provide economic stimulus to what was until then
a socio-economic backwater. Likewise in the USA, where, by 1920, 40 per cent of
all electricity came from hydro [2]. Similarly, other one-off renewable projects
could be found around the world. But by and large, there was limited interest or
need to consider new forms of power until, like the industrial revolution, another
tectonic shift occurred.
1.3 Shocks to the system
The first OPEC oil price shock in 1973–74 [3], which saw the world oil price
quadrupled in less than a year (from $3 to $12 per barrel), was a major blow to the
world’s economy and meant – for the first time, that politicians, policymakers and
politicians would need to consider new forms of power. This shock sparked a new
interest in alternative sources of energy, with wave power leading the charge for
ocean technologies.
A further shock in 1979 saw the price double again (to nearly $40 a barrel),
whilst by the 1980s, a growing recognition of climate change demanded action on
the world’s stage.
Interest in onshore wind began to grow.
By 1987 the Montreal Protocol was agreed, restricting chemicals that damage
the ozone layer, then in 1989 the UK Prime Minister Margaret Thatcher – possessor
of a chemistry degree – warned in a speech to the UN that
We are seeing a vast increase in the amount of carbon dioxide reaching the
atmosphere... The result is that change in future is likely to be more fun-
damental and more widespread than anything we have known hitherto.
It has been this recognition – and the resulting global treaties on climate
change – that has spurred action around the world and has sparked the massive
growth in renewables we see today.
A review of progress on ocean energies 3](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-28-320.jpg)
![The industrial revolution, the growth of grids and the recognition of climate
change have all resulted in radical shifts in how we make and consume energy. At a
political level, nations states must decide – for political and social ends – how
energy is created and used, and the plain numbers in $ per kWh must somehow or
other stack up.
And these decisions are apt to change. In the last decade alone two further
shocks – the Fukushima nuclear disaster and the U.S. shale gas revolution – have
marked ground-breaking shifts in the ways in which different countries view
energy and energy security.
Mature economies including Japan and Germany have decided to move away
from nuclear energy, whilst China – now living with the very real health and cli-
mate risks caused by an over-reliance on coal – has become the world’s largest
developer of onshore wind.
Looking ahead it is clear we cannot know what future changes we might see.
Climate change continues to accelerate, the move towards electric vehicles can
only grow and we must all find new ways to lighten our load on our crowded
planet. Other ‘unknown unknowns’ – to paraphrase Donald Rumsfeld – inevitably
lurk around the bend.
Renewable energy is making an incredible contribution having gone from
practically zero in the 1940s to now being the dominant electricity source in several
countries such as Norway and Scotland. In 2016, cumulative grid-connected
onshore wind capacity reached 451 gigawatts (GW) and wind power accounted for
almost 4 per cent of global electricity generation [4].
Onshore wind capacity is now expected to reach almost 750 GW by 2022 –
and although ocean energy technologies lag significantly behind, once each sector
develops a commercially available product, we can with some confidence expect
similar long-term exponential growth.
1.4 The challenge of ocean energy
More than 70 per cent of our planet is covered in ocean, and whilst in many areas
our seas are already heavily exploited, for food, oil and gas – they still represent a
massive bounty of renewable energy waiting to be harvested.
Waves, tides, wind, salinity, temperature gradients and the sun all offer
tempting new sources of power – power which can often be generated offshore,
with little or no conflict with other sea users.
In some cases, these technologies are already in the market – indeed some have
been around for more than a century, and in others they are not quite there yet.
But what is clear is that – in an ever-changing world – we cannot afford to shut
out alternative new sources of energy, whatever they may be.
The challenge – as will become clear in this book – is not only to develop
new technologies that work, but also to develop new technologies that work com-
mercially. And to understand this, we need a brief exploration of technology
readiness.
4 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-29-320.jpg)
![1.5 Technology Readiness Levels
Trying to describe how ready a technology is for deployment has always been
difficult. Every inventor is enthusiastic about their project, often overly so, and this
tends to lead to wild claims. Fortunately, NASA came up with a Technology
Readiness Levels (TRL) scale to measure the readiness of any technology and this
has now been widely adopted as a means of comparison and categorisation.
Technological and commercial maturation occurs over several phases, from
concept design to commercial deployment at sea. Each technology project is
evaluated against the parameters for each technology level and is then assigned a
TRL rating based on the project’s progress. Moving from one phase to the next
requires increased deployment, leading to technological and hence economic
improvements.
There are nine technology readiness levels. TRL 1 is the lowest and TRL 9 is
the highest.
NASA defines TRLs as follows [5]:
When a technology is at TRL 1, scientific research is beginning, and those
results are being translated into future research and development.
TRL 2 occurs once the basic principles have been studied and practical
applications can be applied to those initial findings. TRL 2 technology is very
speculative, as there is little to no experimental proof of concept for the technology.
When active research and design begin, a technology is elevated to TRL 3.
Generally, both analytical and laboratory studies are required at this level to see if a
technology is viable and ready to proceed further through the development process.
Often during TRL 3, a proof-of-concept model is constructed.
Once the proof-of-concept technology is ready, the technology advances to
TRL 4. During TRL 4, multiple component pieces are tested with one another. TRL
5 is a continuation of TRL 4; however, a technology that is at 5 is identified as a
breadboard technology and must undergo more rigorous testing than technology
that is only at TRL 4. Simulations should be run in environments that are as close to
realistic as possible.
A TRL 6 technology has a fully functional prototype or representational model.
TRL 7 technology requires that the working model or prototype be demon-
strated in a space environment. TRL 8 technology has been tested and ‘flight
qualified’ and it’s ready for implementation into an already existing technology or
technology system. Once a technology has been ‘flight proven’ during a successful
mission, it can be called TRL 9.
To look specifically at ocean energy, the EU has described a number of stages
for the development of ocean energy technologies as they pass through these levels
(Figure 1.1).
A crucial question is how quickly any given technology will progress through
these stages. It is essential to understand that any timeline is highly dependent on
overcoming the barriers faced by ocean energy developers and the level of public
support offered in the short and medium terms by national and regional govern-
ments and international institutions such as the EU.
A review of progress on ocean energies 5](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-30-320.jpg)
![To put above in context, if we take the example of onshore wind, it took
40 years from the first experiments until early rollout (Figure 1.2) – and the speed
of rollout was highly dependent on the public policy framework.
One should also note that it was not the first inventors (Scotland and the USA)
which capitalised on the industrial rollout and economic benefit which ensued that
prize went to Denmark. They put in place the specific tariffs, investment vehicles
and consents processes that gave the fledgling technology somewhere to take flight.
The lesson here is the benefit went to the country which commercialised the
technology, not the one that invented it, and there are parallels here for ocean
energy policymakers [6].
Wave Energy Scotland, an organisation established to bring wave energy to the
point of commercialisation, has taken the TRL model and matched it to its own
four-stage progression. EMEC provides an overview of the technology develop-
ment pathway described here (Figure 1.3).
Applying this discipline to marine energy is useful. It provides a clear pathway
from the lab to the sea, and all of the steps that need to be taken in between.
It is not, however, the end of the story. As in the example of the onshore wind
industry, it is not just about inventing a technology that works, it is about finding a
path to make a technology work commercially and this can take many years. This is
a theme I will return to in the closing chapter.
In the last decade, a number of ocean energy technologies have made tre-
mendous strides towards TRL 9 and here we will look at them all in turn.
R&D
• Small-scale device
validated in lab
• Component testing
and validation
• Small-/medium-scale
pilots
Prototype
• Representative
single-scale devices
with full-scale
components
• Deployed in relevant
sea conditions
• Ability to evidence
energy generation
Demonstration
• Series or small array
of full-scale devices
• Deployed in relevant
sea conditions
• Ability to evidence
power generation to
Grid
• For OTEC and
salinity gradient: full
functionality down-
scaled power plant
Pre-Commercial
• Medium-scale array
of full-scale devices
experiencing
interactions
• Grid connected to a
hub or substation
(array)
• Deployed in relevant/
operational sea
conditions
• For OTEC and
salinity gradient:
scalable
Industrial Roll-Out
• Full-scale commercial
ocean energy power
plant or farms
• Deployed in operati-
onal real sea
conditions
• Mass production of
off-the-shelf
components and
devices
TRLs 1–4 TRLs 3–6 TRLs 5–7 TRLs 6–8 TRLs 7–9
Figure 1.1 From EU Ocean Energy Forum Strategic Roadmap
10% EU
power
demand
Average
onshore
turbine size
= 1 MW
Average
offshore
turbine size
= 2 MW
Offshore
farm 450-kW
turbines
(Denmark)
Renewable
obligation
in USA
3-bladed
200-kW
turbine
(Denmark)
12-kW
turbines
(Scotland
& Ohio)
Wind
2014
2002
2001
Prototype to industrial roll-out: 40+ years
1991
1978
1956
1887
Figure 1.2 Development of wind turbines, from early experiments to industrial
rollout, Ocean Energy Europe
6 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-31-320.jpg)
![1.6 Wave energy – TRL 7
Wave energy remains one of the world’s last great untapped sources of renewable
energy – with a total theoretical wave energy potential of 32 PWh/year, roughly
twice the global electricity supply [7]. But thus far, it has proved the most chal-
lenging to develop.
Today there are an estimated 218 active wave energy developers around the
globe [8], ranging from start-ups and small-to-medium enterprises, to utilities,
original equipment manufacturers and industrial engineering firms, each seeking to
develop one of at least nine different types of wave energy converter. Yet we still
have not seen a dominant technology emerge at full-scale TRL 8, with various
wave energy concepts in development.
A full explanation of the different types of wave energy converters can be
found on the EMEC website at the following link: www.emec.org.uk/marine-
energy/wave-devices/.
In Europe, it is estimated ocean energy could meet 10 per cent of EU electrical
demand by 2050 [9], whilst worldwide it is recognised that ‘blue growth’ has
the potential to regenerate peripheral areas affected by declining traditional indus-
tries [10]. This is particularly true for wave energy, where areas of high resource
are closely aligned with remote and economically fragile areas – places where new,
high value jobs have a disproportionate impact.
I can vouch personally for the massive impact the industry has had in Orkney,
where it is estimated that EMEC has spent over £16 million in Orkney with around
200 people currently employed in the marine renewables sector [11]. An economic
impact assessment commissioned by government agency Highlands and Islands
Enterprise estimates that EMEC has generated a gross value added to the wider
UK economy of £284.7 million, with 4,224 full-time equivalent (FTE) job years
so far [12].
Figure 1.3 EMEC pathway to commercialisation
A review of progress on ocean energies 7](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-32-320.jpg)
![In the modern era, I consider the wave energy story began in earnest in the
1970s in the UK at the Wave Power Department at the University of Edinburgh,
led by Prof Stephen Salter whose invention – known as the Edinburgh or Salter
Duck – excited considerable international attention and showed that the words
‘wave’ and ‘energy’ do actually fit together. However, his work was not done in
isolation as he gathered a team of researchers and there were others such as
hovercraft inventor Sir Christopher Cockrell also working on schemes in parallel.
By the late seventies however, the UK had struck oil in the North Sea and
public support for wave energy waned. It is regrettable that Salter’s work was
undermined by inaccurate reporting by UK government’s advisors of the time
which led to the programme’s cancellation. And although wave energy remained a
significant topic at an R&D level, it was not until the 2000s that we saw a sig-
nificant amount of progress – again led by a Scottish concern.
The rise of Pelamis in the early years of this century was a tremendous story of
endeavour, and one of the firsts in the world. It dovetailed seamlessly with the
establishment of EMEC in 2003 and enabled the Edinburgh pioneers to success-
fully pilot the world’s floating first grid-connected device.
For the next decade, Scotland and the UK gave tremendous support to the
sector until – by 2014, the technological and financial challenges in creating energy
from waves overwhelmed the leaders in the sector – with the high-profile collapses
of Pelamis and, shortly thereafter, Aquamarine Power.
What these financial collapses, or as I would prefer to term them lessons, have
shown us is that some technological challenges are too big or too complex for
single private firms to tackle alone, even with public support.
1.7 Tidal and current energy
1.7.1 Tidal range – TRL 9
Whilst wave energy continues to demonstrate its ability to generate electricity, tidal
range power is already well proven.
As I mentioned earlier, Eling Mill in Hampshire has been utilising tidal range
power for over 2,000 years, whilst the 240-MW la Rance Tidal Power Station in
France has been operating since 1966. In Canada, the 20-MW Annapolis Royal
Generating Station Opened in 1984, whilst the 254-MW Sihwa Lake Tidal Power
Station in South Korea has been operational since 2011.
Tidal range technology shares characteristics similar to hydropower, essen-
tially utilising the height difference of two bodies of water created by a dam or
barrier in order to produce electricity. Its main advantage is that is it very pre-
dictable. However, its capacity factor of 25 per cent is not as high as offshore wind
due to the nature of tidal cycles and turbine efficiency [13].
In recent years, tidal range has evolved with the concept of tidal lagoons: these
are artificial basins built in bays and estuaries and are considered less intrusive than
tidal barrages (which tend to span entire estuaries and alter the salinity and
8 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-33-320.jpg)
![ecology). In the UK, the 320-MW Swansea Bay Lagoon project in Wales has (at
the time of writing) failed to secure the government support it requires to proceed.
This was intended to be a pathfinder project, leading to five larger proposed
tidal lagoon projects in the United Kingdom: Cardiff (3,000 MW), Newport
(1,800 MW), Colwyn Bay, West Cumbria and Bridgwater Bay.
One recent review [14] estimates that 5,792 terawatt-hours (TWh, around a
third of the total global electricity supply) could be produced by tidal range power
plants. However, 90 per cent of the resource is distributed across just five countries,
with both the UK and France having a significant share of that resource.
In common with other forms of energy, it is not the technology per se that is an
obstacle to progress – tidal range clearly works. The issue appears to be its
uncompetitiveness versus unsustainable and incumbent fossil-based fuels, or
heavily subsidised nuclear generation. However, the technology does require a very
specific set of physical and geographical circumstances to make a scheme viable –
and the necessary revenue stream or public support to offset the high initial
capital costs.
1.7.2 Tidal stream – TRL 8
In contrast to wave energy, the tidal stream sector has made significant advances
towards commercialisation in recent years. Orkney technology firm Orbital Marine
Power has developed a floating turbine already providing significant power to
Orkney Mainland, whist in the Pentland Firth the MeyGen scheme, owned by
SIMEC Atlantis, has the world’s largest operational array, powered by Andritz
Hydro Hammerfest and Atlantis Resources turbines.
Tidal stream generators draw energy from water currents in much the same
way as wind turbines draw energy from air currents. However, the higher density of
water relative to air (water is 800 denser) means that tidal generators have smaller
blades than wind turbines and operate at lower velocities.
Given that power varies with the density of medium and the cube of velocity,
water speeds of nearly one-tenth the speed of wind provide the same power for the
same size of turbine system; however, this limits the application in practice to
places where tide speed is at least 2 knots (1 m/s) even close to neap tides.
In Europe, the majority of high resource tidal stream sites which meet this
criteria lie around the coasts of the UK, Ireland and France with some hotspots in
the Mediterranean. Worldwide other tidal hotspots include Nova Scotia, Chile, the
Philippines and Indonesia.
In the modern era, the development of tidal energy was effectively led by
Dr Peter Frankel when he installed his 15-kW machine in Loch Linnhe, Scotland,
in 1994–95 [15]. This two-bladed rotor project by IT Power had been sponsored by
Scottish Nuclear Electric and was the forerunner of the 300-kW SeaFlow project
later installed off Lynmouth in Devon in 2003 by Peter’s company Marine Current
Turbines (MCT).
That device was not grid connected but provided invaluable information on the
practicalities of a stand-alone generating structure at sea. It led directly to the iconic
A review of progress on ocean energies 9](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-34-320.jpg)
![twin rotor 1.2-MW SeaGen device at Strangford Loch in Northern Ireland installed
in 2008. This turbine operated successfully for several years and generated over
350 MWh into the grid until it was shut down in 2017.
In parallel with MCT’s work, a number of other companies also began
development on their own tidal turbines, with several notable examples.
In the Straits of Messina, Ponte de Archimede developed a 150-kW vertical
axis turbine (KOBOLD). This largely forgotten device was the first to generate into
the grid in Europe in 2001 [16]. The company was owned by ship owner Dr Elio
Matacena and used his experience of the Voith Schneider vertical propellers that
powered his car ferries to come up with the design.
In Hammerfest, Norway, a fully submerged horizontal axis turbine was
installed in 2003 [17]. This 300-kW machine generated into the local grid and led
to the development of the 1-MW horizontal axis turbine installed at EMEC in 2011.
Herbert Williams in Florida developed a rim generator in the 1990s to work
in the Gulf Stream [18] which eventually became Irish-owned OpenHydro. They
installed a test rig at EMEC in 2006 and gird-connected their first 6-m diameter
open-centred turbine in 2008. They narrowly beat MCT to the accolade of the first
to generate to the GB National Grid and continued to test at EMEC until the
company’s demise in 2018. OpenHydro had gone on to install a 10-m diameter
machine in the Bay of Fundy in Canada in 2009 before installing a 16-m diameter
machine at a nearby site and also two machines at Roche Blanchard in France [19].
In the middle of all this Tidal Generation Limited, a company made up in part
of ex-MCT employees, developed a buoyant sub-sea turbine. This initially 500-kW
machine led to a 1-MW machine through Rolls Royce’s support and eventual
ownership of the company before its sale to Alstom.
In more recent times, Scotland has seen three most notable successes.
In 2016, Nova Innovation deployed the world’s first fully operational, grid-
connected offshore tidal energy array, at Bluemull Sound in Shetland. The first
Nova M100 turbine was deployed at the site in March 2016, and the second was
deployed in August 2016, making this the first offshore tidal array in the world to
deliver electricity to the grid. A third turbine was added to the array in early 2017.
The first phase of the MeyGen project passed the 8-GWh generation milestone
in July 2018 [20] through its use of two Hammerfest turbines and an Atlantis
machine – both companies having undertaken testing work at EMEC in the pre-
vious years. This scheme is set to expand its site with the next phase being planned
at the time of writing.
And finally, the device of which Orkney is most proud is the locally envisaged,
designed, managed and installed SR1-2000 made by Scotrenewables (now known
as Orbital Marine Power), which repeatedly generated around 7 per cent of
Orkney’s electricity through much of 2018. This machine is the first pre-
commercial prototype of the company and follows years of development by the
local team. The first open water tests of a large-scale model were undertaken within
sight of EMEC’s offices and led to the 250-kW machine being developed and
connected in 2012 [21]. The success of that device led to the 2-MW machine that
effectively powered Orkney for a day a fortnight in 2018 [22].
10 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-35-320.jpg)
![RED uses the transport of (salt) ions through membranes. RED consists of a
stack of alternating cathode and anode exchanging permselective membranes. The
compartments between the membranes are alternately filled with seawater and
freshwater. The salinity gradient difference is the driving force in transporting ions
that results in an electric potential, which is then converted to electricity.
Two main applications exist: as stand-alone plants in estuaries where freshwater
rivers run into the sea and as hybrid energy generation processes recovering energy
from high-salinity waste streams, which could be, for example, brine from desali-
nation or salt mining. A possible third application is salinity gradient technologies
applied to land-based saltwater lakes or other types of saltwater reserves [23].
The total technical potential for salinity gradient power is estimated to be
around 647 GW globally; however, despite this high theoretical potential, chal-
lenges both in proving the technology and in finding a route to commercialisation
means current salinity gradient schemes are currently on hold.
Norwegian hydro utility Statkraft was an early champion of the sector and
developed its first PRO power plant at Tofte on Hurum, near Oslo in 2009. It used
2,000 m2
of flat sheet membranes and could theoretically produce 10 kW, although
the actual production was around 5 kW and powered an electric kettle for demon-
stration purposes.
Yet by 2013, Statkraft had decided to withdraw from the sector, suggesting that
the technology would not be sufficiently developed to become competitive within
other technologies.
Making his assessment of the technology, Statkraft department manager Stein
Erik Skilhagen said:
We have proven that the technology works and have achieved substantial
improvements through our efforts. Our main challenge has been to make
the technology efficient enough to achieve energy production costs on par
with competing technologies. With the current market conditions, we see
that we cannot achieve this in the foreseeable future. There are other
technologies which have developed enormously in recent years. These are
more competitive and relevant investments for us in the future [24].
In 2013, Dutch firm REDstack officially opened 50-kW RED pilot, situated on
the sea defence causeway, which separates relatively clean freshwater on one side
from relatively clean seawater present in the Wadden Sea/North Sea [25]. This pro-
ject is still operational, although has not published plans for any further development.
1.9 Offshore wind – TRL 8
Unique amongst the other forms of ocean energy explored in this book, offshore
wind is already a highly successful competitive product, fully proven at TRL 8 and
close to TRL 9.
Compared to other forms of power, offshore wind remains a relative newcomer –
but already this decade deployment of offshore wind has more than quintupled from
A review of progress on ocean energies 13](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-38-320.jpg)
![3.2 GW in 2010 to 18.7 GW in 2017 by which time it contributed some 56 TWh or
0.3 per cent of global electricity generation [26].
The key factor behind the rise of the offshore wind market is a concerted series
of public/private initiatives undertaken by countries bordering the North Sea in
Europe. More than 80 per cent of global offshore wind capacity is located in
Europe, of which the United Kingdom with installed capacity of 6.8 GW and
Germany with 5.4 GW are the two largest countries.
Beyond Europe, only the People’s Republic of China has large-scale offshore
wind capacity installed, at 2.7 GW, while smaller offshore wind facilities are
located in the United States, Korea and Japan.
Although it uses a fundamentally similar technology to onshore wind, offshore
wind enjoys some distinctive advantages: the main ones are that offshore installa-
tions are able to tap more consistent and higher winds speeds, and there are fewer
restrictions on ground area and height. As a result, project sizes and turbines are
typically larger and performance is higher.
Costs relative to other forms of renewables are higher. However, recent
energy auctions across Europe have seen offshore wind energy prices fall to below
£58 per MWh [27].
And despite the vast resource potential that could be tapped with fixed-bottom
configurations (i.e. turbines with foundations on the seabed), studies have found
that 80 per cent of estimated offshore wind resources in Europe are located in
waters at depths greater than 60 m, with the corresponding figure being 80 per cent
in Japan and 60 per cent in the United States [28].
This has sparked a new wave of innovation – to site offshore wind farms in
ever-deeper waters – with a growing appetite to surmount any technological chal-
lenges. Already there is a vast supply chain engaged in the development of huge
offshore wind parks, and the ever-present pressure to reduce cost and exploit new
resources will continue to spur new advances.
In Japan, they are successfully testing a number of floating turbine concepts
at scale at the Fukushima Floating Offshore Wind Farm [29], whilst in late
2017 Statoil switched on the world’s first commercial floating wind farm –
Hywind – off Peterhead in Scotland. This comprises five 6-MW floating turbines
anchored by means of three suction anchors in 95–120 m of water.
Industry voices expect we will see ever larger turbines – potentially 20 MW or
more by the middle years of the next decade – and it is clear offshore wind is here
to stay.
1.10 Marine solar
Marine solar on the other hand is a new concept. It relies upon the open and
unfettered expanse of water providing a flat and unshaded area to place floating
solar panels. This technology may find a better home on lakes and reservoirs where
the added benefit of reducing evaporation may strengthen water resources in
arid areas.
14 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-39-320.jpg)
![It is a technology to watch and will clearly owe much to the technical devel-
opments of terrestrial solar and its application is expected to centre in more equa-
torial regions.
1.11 Enabling technologies and actions
For the purposes of this opening chapter, I do not propose to go into detail in what
I have termed ‘enabling technologies and actions’ each of which merits their own
section within this book, including the following:
● offshore support structure design
● electrical power transmission and grid integration
● offshore energy storage
● multi-purpose platforms
● installation, operation and maintenance of offshore renewable.
What I do observe, however, is that all of these topics – and they are major
ones – involve what we do with ocean energy technologies at sea.
How do we build structures that can withstand the extraordinary forces they
will experience in winter storms?
How do we harness new technologies such as High-Voltage Direct Current
(HVDC) to bring power ashore? (Or can we, indeed store energy at sea?) Can we
reduce cost through multi-purpose platforms, and how can we install, maintain and
operate these new technologies in real ocean conditions?
For someone starting off on their journey of creating energy from the oceans
with a radical new idea, these enabling technologies and actions can appear far
away from the early days spent in the laboratory.
But as soon as a technology crosses the shoreline in earnest, and aims to go to
sea, then these are the areas which present some of the biggest challenges we face.
In order to operate at sea, each technology must be able to operate – and operate
reliably – whilst experiencing forces far beyond the average sea state.
Robustness and reliability are key, and they do not come cheap. So, whilst in
the ocean the first challenge is to prove ‘it works’ the next challenge is to show it
works reliably. Then we must show there is at least a route to making it work
commercially – given whatever support mechanisms may be in place.
This is a topic I will return to in the closing chapter – where themes such as
‘learning by doing’ and serial production will come to the fore.
Meanwhile, dip in, and enjoy exploring the fascinating journey to realise
renewable energy from our oceans.
References
[1] https://otcns.ca/news/2012-06-01/ocean-wave-energy-a-nova-scotia-first-,
accessed on 5 March 2019.
A review of progress on ocean energies 15](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-40-320.jpg)
![[2] ‘History of Hydropower’. U.S. Department of Energy, https://www.energy.
gov/eere/water/history-hydropower accessed on 5 March 2019.
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[4] https://www.iea.org/topics/renewables/wind/, accessed on 5 March 2019.
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accordion1.html, accessed on 5 March 2019.
[6] The Danish wind industry 1980–2010: Lessons for the British marine energy
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Marine_2016.pdf, accessed on 5 March 2019.
[8] EMEC. http://www.emec.org.uk/services/
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OceanEnergyForum_Roadmap_Online_Version_08Nov2016.pdf, accessed on
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[10] https://ec.europa.eu/maritimeaffairs/policy/ocean_energy_en, accessed on
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[11] http://www.hie.co.uk/growth-sectors/energy/marine-energy.html, accessed
on 5 March 2019.
[12] Atlantic Marine Energy Test Site (AMETS) Economic Impact Case Study
on EMEC in Orkney, December 2017.
[13] WEC, 2016. https://www.worldenergy.org/wp-content/uploads/2016/10/World-
Energy-Resources-Full-report-2016.10.03.pdf
[14] https://phys.org/news/2018-05-tidal-range-power-potential-electricity.html#jCp,
accessed on 5 March 2019.
[15] https://www.supergen-marine.org.uk/sites/supergen-marine.org.uk/files/
publications/taiwan_2009_mct.pdf, accessed on 5 March 2019.
[16] https://tethys.pnnl.gov/annex-iv-sites/enermar-project, accessed on 5 March
2019.
[17] https://www.powerengineeringint.com/articles/print/volume-19/issue-1/features/
orkney-tidal-power-a-hotbed-on-the-seabed.html, accessed on 5 March 2019.
[18] https://www.bloomberg.com/news/articles/2016-06-09/the-ex-con-inventor-
disrupting-underwater-energy, accessed on 5 March 2019.
[19] https://www.rpsgroup.com/images/UK/PDFs/MARITIME_TIDAL_161012.
aspx, accessed on 3 February 2019.
[20] https://simecatlantis.com/2018/07/11/meygen-phase-1a-operational-update/,
accessed on 5 March 2019.
[21] http://www.scotrenewables.com/technology-development/sr250, accessed on
5 March 2019.
[22] http://www.scotrenewables.com/technology-development/sr2000, accessed
on 5 March 2019.
[23] http://www.irena.org/documentdownloads/publications/salinity_energy_v4_
web.pdf, accessed on 5 March 2019.
[24] https://www.statkraft.com/media/news/News-archive/2013/Statkraft-halts-
osmotic-power-investments/, accessed on 5 March 2019.
16 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-41-320.jpg)
![[25] http://www.redstack.nl/en/home, accessed on 5 March 2019.
[26] https://www.iea.org/publications/freepublications/publication/WEO2017
Special_Report_OffshoreEnergyOutlook.pdf, accessed on 5 March 2019.
[27] https://www.gov.uk/government/publications/contracts-for-difference-cfd-
second-allocation-round-results, accessed on 5 March 2019.
[28] Carbon Trust, 2015. https://www.carbontrust.com/resources/reports/technology/
floating-offshore-wind-market-technology-review/
[29] https://www.4coffshore.com/windfarms/project-dates-for-fukushima-floating-
offshore-wind-farm-demonstration-project-(forward)-phase-2-jp13.html,
accessed on 5 March 2019.
A review of progress on ocean energies 17](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-42-320.jpg)
![Chapter 2
Wave energy
Gianmaria Sanninoa
, Adriana Carilloa
, Arne Voglerb
,
Giovanni Braccoc
, Giuliana Mattiazzoc
, Diego Vicinanzad
,
Pasquale Contestabilee
, Domenico P. Coirof
,
Giancarlo Troiseg
, Luca Castellinih
, and John V. Ringwoodi
Wave energy has an estimated global potential of 3.7 TW, almost double that of
current world electrical energy consumption, and has, to date, remained virtually
untapped [1]. Wave also has a relatively low correlation with, for example, wind
and solar [2], meaning that a balance of wave with other renewable technologies
can offer greater consistency of supply. Despite these attractive features, the drive
to make wave energy economic has made relatively poor progress, due to a com-
bination of the following factors: the ocean environment is hostile, wave energy is
reciprocating (rather than unidirectional, therefore requiring rectification) and
variable, and the technology is still in its relative infancy. These difficulties are
manifest in the current unattractively high levelized cost of energy (LCoE) for
wave energy, currently estimated at €225/MW h compared to the LCoE for off-
shore wind, for example, at €165/MW h [3]. This chapter examines the various
facets of wave energy, including resource quantification and wave measurement
(Section 2.1), the wide variety of onshore and offshore wave energy devices
(Section 2.2), the variety of power take-off (PTO) mechanisms which convert wave
power into other useful forms (Section 2.3) and concluding with some insight to
how wave energy devices are modelled and controlled (Section 2.4).
a
ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Develop-
ment, Rome, Italy
b
Department of Marine Energy Research, University of the Highlands and Islands – Lews Castle
College, Stornoway, Isle of Lewis, UK
c
Dipartimento di Ingegneria Meccanica e Aerospaziale, Politecnico di Torino, Torino, Italy
d
Inter-University National Consortium for Marine Sciences (CoNISMa), Italy
e
Department of Engineering, University of Campania Luigi Vanvitelli, Aversa (Caserta), Italy
f
Department of Industrial Engineering – Aerospace Section, University of Naples Federico II, Naples,
Italy
g
SeaPower Scarl – Research Consortium, Naples, Italy
h
UMBRAGROUP S.p.A., Foligno, Italy
i
Centre for Ocean Energy Research, Maynooth University, Ireland](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-44-320.jpg)
![While this chapter can only give a limited overview of the challenges and some
potential solutions to successfully harness wave energy, it belies the truly vast and
diverse array of prototype wave energy converter (WEC) devices [4]. This diversity
demonstrates the lack of technology convergence in WEC design and, combined
with the multitude of PTO modalities described in Section 2.3, the potential variety
of wave energy systems is truly broad. Nevertheless, from the overview and
examples presented in this chapter, it is hoped that the reader will gain a better
understanding of the challenges, and potential benefits, of adding wave energy to
the current mix of commercial renewable energy technologies.
2.1 The wave resource
2.1.1 Wave resource assessment1
Waves are generated from the transfer of energy from wind blowing over the
oceans and can travel thousands of kilometres from the generation area. During
their propagation, the wave characteristics change and waves become progressively
more regular and increase their wavelength, becoming the so-called swell waves.
Due to the superposition of wind and swell waves, the distribution of wave heights
does not coincide with that of winds. The highest latitudes both in the Northern and
in the Southern Hemisphere are characterized by the largest values of wave height.
Monthly mean values over 5 m are reached in both hemispheres during winter but,
while there is a strong seasonality in the Northern hemisphere, values are more
constant over the year in the Southern hemisphere [5]. Wave energy is proportional
to the square of height and to the mean period. Due to the difference in frequency
characteristics of wind waves and swell waves also the relative occurrence of the
two types of waves has been investigated at a global scale [6].
Wave energy atlases have been created both on global scale and on particular
regions (e.g. [1]). They are based on wave measurements obtained from buoys,
satellite data and output from model simulations. In the last decades, satellite alti-
meters have been launched and are providing significant wave height data covering
the entire ocean surface at constantly increasing resolution. This data is com-
plemented by a variety of wave models that are actually running over different
domains and at different resolutions. These models simulate numerically the
growth, propagation and decay of waves forced by surface winds. They produce
homogeneous data over long periods of time and provide a series of wave variables
permitting a detailed characterization of the waves. In situ wave measurements
from wave buoys represent the reference for the validation of models, recording
very high temporal resolution and good quality data. Even if a few centres exist
worldwide that collect and maintain buoys networks, observations are sparse and
the maintenance of instruments expensive. Time series from wave buoys describe
wave climate only locally and often present large data gaps, caused by temporary
failure or by routine maintenance operations.
1
Written by Gianmaria Sannino and Adriana Carillo.
20 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-45-320.jpg)
![In deep water, the available wave energy flux per meter of wave crest in Watt
can be expressed as
J ¼
rg2
64p
TeHs
2
(2.6)
where g is the gravity acceleration, r the sea water density assumed to be
r ¼ 1,025 kg/m3
.
Energy matrices are one of the most useful means to synthetize the wave
characteristics over a long time period in order to assess the productivity of a
specific wave converter. All the data collected are divided into bins of equal period
and height, and then the energy in each bin is computed using the previous formula.
The productivity of a device in a specific site can be determined multiplying
the distribution of occurrences of the different sea states as a function of Hs and Te
and the power matrix of the device.
2.1.1.2 Mediterranean wave assessment
Global wave energy atlases lack the spatial resolution required to correctly describe
the wave energy distribution in small and semi-enclosed basins as the Mediterranean
Sea. In such regions, wind waves dominate as the fetch is limited. Wave height and
period show substantial spatial variations due to wind variability and to the presence
of complex topography. In these regions, specific high-resolution wave models
represent the most important tool to assess wave-energy distribution.
Different wave energy climatologies have been realized for the Mediterranean
Sea in the last years. They are based on data collected from the Italian Wave Buoys
Network, operating since 1989 (e.g. [7]), or from model simulations (e.g. [8]).
The wave energy climatology for the entire Mediterranean Sea presented in [9]
covers the period 2001–10. The wave model used is a parallel version of WAM
(WAve Model) Cycle 4.5.3 at a horizontal resolution of 1/16
[10]. The model has
been forced with six-hourly wind fields obtained from ECMWF operational ana-
lysis at 1/4
spatial resolution. The main integral wave parameters, wave height
(Hs), mean wave period (Tm), significant wave period (Te) and mean direction (qm)
have been collected over the entire model domain, every 3 h.
An accurate validation of the wave parameters obtained from the model
simulation has been performed against available buoys data, from the Italian Wave
Buoy Network, managed by ISPRA; wave heights have also been compared against
satellite radar altimeters data. Both the comparisons have shown very good statis-
tical agreement. Maps of the available seasonal wave power flux per unit crest
averaged over the entire 10 years of the Mediterranean simulation are shown in
Figure 2.1.
In the Mediterranean basin, the winter season is the most energetic, followed
by autumn, while summer is characterized by very low values almost everywhere.
The most productive area is located in the Western Mediterranean, between the
Balearic Islands and the western coast of Sardinia where an average energy flux of
around 24 kW/m is reached in a large area, during winter.
22 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-47-320.jpg)
![Other areas characterized by high levels of wave energy in most of the seasons
are the north-western coast of Sicily and the Central Mediterranean. Values of
energy lower than 9 kW/m are observed in summer everywhere.
2.1.1.3 Wave assessment at Pantelleria
Results from the model covering the entire Mediterranean Sea have shown that the
Sicily Channel represents a promising area for wave-energy production. The area
around the island of Pantelleria (located in the Sicily Channel) has been selected to
perform a higher resolution study, which can be used in support for the installation
of a specific device [11].
Data from the Mediterranean simulation performed using WAM have been
used to laterally force a higher resolution version (1/120
corresponding to about
1 km) of the same model. The nested model grid is centred on the Island of Pan-
telleria and has the same discretization of the directional wave energy density
function of the coarse model, and the same surface forcing. The map of the energy
averaged over the 10-year model is shown in Figure 2.2. The main contribution to
wave energy comes from waves propagating from North-Northwest. The sheltering
effect of the island is evident in the decrease of energy in the Southern coast.
The distribution of wave energy among different sea states has been analysed
in detail at a near-shore site on the northern coast.
The scatter plot in Figure 2.3 represents the distribution of the yearly average
energy in terms of Te and Hs, evaluated over the 10-year period. The contribution
to the total energy given by individual sea states are lumped in 0.25 s wide intervals
of Te and 0.25 m wide intervals of Hs. Rectangles are coloured according to the
total energy computed over the 3-h sea states extracted from the model. Lines of
constant power are drawn on the scatter plot. The maximum amount of energy is
available for significant wave heights between 2 and 3 m and for mean periods
between 6 and 7 s.
46°N
44°N
42°N
40°N
38°N
36°N
34°N
32°N
30°N
0 2 4 6 8 10 12 14 16 18 20 22 24
Autumn
0° 10°E 20°E
Energy flux (kW/m)
30°E
Figure 2.1 (Continued )
24 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-49-320.jpg)
![than what was previously possible with staff gauges, a reduced ability to capture
shorter period waves with an increased deployment depth was also observed.
Significant advances in wave measurement technology were made in the 1960s
with the use of accelerometers mounted in ships or floating buoys. Through the
process of double integration, the accelerometer output can be converted to dis-
tance, thus providing continuous readings of vertical wave displacement. Limita-
tions to this system were found in the inertia and size of the carrier platform, which
prevented accurate readings to be taken for shorter wavelengths and periods as in
wind waves. This limitation was to some extent overcome by installing accel-
erometers in smaller size buoys with a reduced mass, and thus providing an
increased ability to follow the surface motion related to smaller short period waves.
Another constraint in the case of floating buoys was due to the mooring connection,
which limits the ability of a floating body to follow the sea surface freely. By
deploying buoys free floating, which provides accurate wave data in a non-
stationary way as the buoy moves around with the wind, wave and current drift, or
more often by using a flexible and damped mooring configuration, this constraint
was largely eliminated.
A shared shortcoming of the methods described in the above section was the
non-ability to capture wave directionality or multimodality for situations with
waves transiting the sensors from more than one direction. An initial solution to
resolving directional wave data was found by installing sensors in an array con-
figuration, often triangular, which allowed the tracing of individual waves through
the array, and subsequent calculation of direction and wave speed.
By fitting out a buoy with multiple accelerometers along three perpendicularly
aligned axes, two in the horizontal pane and the third one vertically aligned, a
breakthrough was achieved in the 1980s, as it was now possible for the first time to
gather wave directional data from a single instrument by combining horizontal and
vertical motions [12]. Around a similar time, successful results on measuring wave
spectra were also obtained from an acoustic sensor system utilizing the Doppler
effect [13], and this has now become a standard feature of many acoustic Doppler
current profilers (ADCPs) available on the market. Generally, the use of buoys and
acoustic profilers to measure waves is confined to providing time series or time-
averaged spectral data for single-spot locations. Additional modern wave-sensing
technologies and approaches include the use of radar and satellite imagery to also
allow continuous wave monitoring across wider spatial areas, and a more detailed
introduction into the different sensor types is given in Section 2.1.2.2.
2.1.2.1 Wave characterization for energy applications
To provide annual energy production estimates of WEC farms, a sound under-
standing of the wave resource is imperative. Such understanding and estimates are
an essential part of project design and development and are not only required to
match and refine WEC technology against a local wave resource, but also to
increase confidence in cash flow projections, thus supporting the investment case.
In addition to the requirement for detailed long-term wave resource assessment
during the project development stages, ongoing wave forecasting and monitoring is
Wave energy 27](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-52-320.jpg)
![also important throughout the construction and operational phases of WEC farms.
Offshore installation works such as lifting of heavy loads during anchor handling or
tasks that involve crew transfer from support vessels onto individual WECs, e.g. to
undertake electrical connections between WECs and offshore hubs, can only be
safely undertaken during moderate or calm sea states and thus careful weather
windowing is required. During operations, the constant tuning of energy converters
to the impacting sea states allows an increased energy production by operating the
WECs close to their resonance frequencies. A further improvement to the sea state
dependent tuning of energy converters is suggested through the implementation of
a fast adaptive control approach, based on an individual wave-by-wave input that
combines instantaneous measured wave data a short distance up-wave of a WEC
with a short-term wave forecasting algorithm [14,15]. Wave monitoring during the
operational phase at an energy site is further required to implement mitigation and
survival strategies during wave conditions exceeding the operational design win-
dow of the deployed technology.
The requirement to use measured wave data for the development of wave
resource assessments is defined in standard IEC 62600-101:215 published by the
International Electrotechnical Commission [16]. Although the primary standard
tools for wave resource assessments are numerical spectral wave models such as
SWAN or DHI Mike 21 SW, typically applied for a 10-year hindcasting period,
access to measured data for a number of locations within the model domain is
considered essential for the calibration and validation of model outputs. The rele-
vant IEC standard prescribes that ‘all numerical modelling shall be validated using
measured wave data. Whenever possible the numerical model output should be
validated using data from one or more locations close to where wave energy con-
verters might realistically be deployed’ [16, p. 16]. The same standard furthermore
recommends measured field data be used covering a minimum of a consecutive
year to avoid any seasonal bias in the model outputs.
In the context of wave resource assessments, the importance of using measured
wave data for model calibration is visualized in Figure 2.4 for an area off the Isle of
Lewis, north-west Scotland. Shown is the distribution of significant wave height of
a prospective WEC development site, based on a Mike21 spectral wave model,
together with the locations of two acoustic wave and current profilers (AWAC 1
and AWAC 2) deployed at a spacing of 600 m between sensors. A considerable
difference in wave heights between both sensor locations was found with an energy
hotspot at location AWAC 1. By calibrating against the sensor data, it was possible
to replicate the hotspot in the numerical model, thus increasing model confidence
and applicability for the wider area. This example also highlights the importance to
deploy multiple sensors at a development site during the resource assessment
process to capture a wider range of localized phenomena. To this end, it is helpful
to set up an initial numerical model to identify potential anomalies prior to the
deployment of sensors. Once initial results are obtained from such a model,
appropriate data acquisition locations can be selected to confirm and help improve
initial model outputs. In addition to the two AWACs shown in Figure 2.4, addi-
tional three-wave measurement buoys were deployed at the same time slightly
28 Renewable energy from the oceans](https://image.slidesharecdn.com/24385665-250510174618-70fc9448/85/Renewable-Energy-From-The-Oceans-From-Wave-Tidal-And-Gradient-Systems-To-Offshore-Wind-And-Solar-Domenico-P-Coiro-53-320.jpg)
![further offshore to help inform boundary conditions for the localized model, and
also as additional calibration points for a larger model domain.
Where the calibration of phase averaged wave models requires spectral wave
data, i.e. data related to sea state parameters derived from the wave spectrum and
generally with directional energy flux discretized across a range of frequencies, the
analysis of statistical parameters from wave time series data is relevant to some
other applications. Although standard wave models allow for extreme value ana-
lysis including assessment of a 50-year return periods to assess probabilities of
occurrence of severe sea states, model outputs are limited to the assessment of
wave geometry, such as differences between back and front wave steepness,
maximum individual wave heights or detection of rogue waves across the model
domain. If the interest is on assessment of individual wave-by-wave parameters,
this is best facilitated through directional time series data from in situ sensors
followed by statistical processing. However, the decomposition of spectral data
following theoretical models such as Jonswap [18] or [19] wave spectra offers an
alternative route for such analysis if no time series data is available.
An overview of different wave sensor types suitable for measurement of time
series or spectral data at single spot locations or across wider domains is given in
the following section.
2.1.2.2 Instrumentation for wave measurement
Where the earliest method to parameterize sea states relied on visual observations
by trained, often ship based, observers, a range of technological solutions is now
available to provide vastly improved detail and accuracy. Although it is widely
accepted that a visual observer is well able to estimate the significant wave height
(Hs), defined as the average of the highest third of waves during an observation
period, this method provides poor results with view to detection of wave period,
Sign.wave height (m)
AWAC 2
1 km
AWAC 1
Above 3.00
2.80 – 3.00
2.60 – 2.80
2.30 – 2.60
2.10 – 2.30
1.90 – 2.10
1.70 – 1.90
1.50 – 1.70
1.30 – 1.50
1.00 – 1.30
0.85 – 1.00
0.65 – 0.85
0.45 – 0.65
0.20 – 0.45
0.00 – 0.20
Below 0.00
Figure 2.4 DHI Mike 21 SW model at 200 m mesh size. Close up of the AWAC
location clearly confirms the energy hot spot 600 m SW of AWAC2
at AWAC1. An additional hotspot is visible 3.5 km to the NNE of
the AWACs (adapted from [17])
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- 6. IET ENERGY ENGINEERING 129 Renewable Energy from the Oceans
- 7. Other volumes in this series: Volume 1 Power Circuit Breaker Theory and Design C.H. Flurscheim (Editor) Volume 4 Industrial Microwave Heating A.C. Metaxas and R.J. Meredith Volume 7 Insulators for High Voltages J.S.T. Looms Volume 8 Variable Frequency AC Motor Drive Systems D. Finney Volume 10 SF6 Switchgear H.M. Ryan and G.R. Jones Volume 11 Conduction and Induction Heating E.J. Davies Volume 13 Statistical Techniques for High Voltage Engineering W. Hauschild and W. Mosch Volume 14 Uninterruptible Power Supplies J. Platts and J.D. St Aubyn (Editors) Volume 15 Digital Protection for Power Systems A.T. Johns and S.K. Salman Volume 16 Electricity Economics and Planning T.W. Berrie Volume 18 Vacuum Switchgear A. Greenwood Volume 19 Electrical Safety: A guide to causes and prevention of hazards J. Maxwell Adams Volume 21 Electricity Distribution Network Design, 2nd Edition E. Lakervi and E.J. Holmes Volume 22 Artificial Intelligence Techniques in Power Systems K. Warwick, A.O. Ekwue and R. Aggarwal (Editors) Volume 24 Power System Commissioning and Maintenance Practice K. Harker Volume 25 Engineers’ Handbook of Industrial Microwave Heating R.J. Meredith Volume 26 Small Electric Motors H. Moczala et al. Volume 27 AC–DC Power System Analysis J. Arrillaga and B.C. Smith Volume 29 High Voltage Direct Current Transmission, 2nd Edition J. Arrillaga Volume 30 Flexible AC Transmission Systems (FACTS) Y.-H. Song (Editor) Volume 31 Embedded Generation N. Jenkins et al. Volume 32 High Voltage Engineering and Testing, 2nd Edition H.M. Ryan (Editor) Volume 33 Overvoltage Protection of Low-Voltage Systems, Revised Edition P. Hasse Volume 36 Voltage Quality in Electrical Power Systems J. Schlabbach et al. Volume 37 Electrical Steels for Rotating Machines P. Beckley Volume 38 The Electric Car: Development and future of battery, hybrid and fuel-cell cars M. Westbrook Volume 39 Power Systems Electromagnetic Transients Simulation J. Arrillaga and N. Watson Volume 40 Advances in High Voltage Engineering M. Haddad and D. Warne Volume 41 Electrical Operation of Electrostatic Precipitators K. Parker Volume 43 Thermal Power Plant Simulation and Control D. Flynn Volume 44 Economic Evaluation of Projects in the Electricity Supply Industry H. Khatib Volume 45 Propulsion Systems for Hybrid Vehicles J. Miller Volume 46 Distribution Switchgear S. Stewart Volume 47 Protection of Electricity Distribution Networks, 2nd Edition J. Gers and E. Holmes Volume 48 Wood Pole Overhead Lines B. Wareing Volume 49 Electric Fuses, 3rd Edition A. Wright and G. Newbery Volume 50 Wind Power Integration: Connection and system operational aspects B. Fox et al. Volume 51 Short Circuit Currents J. Schlabbach Volume 52 Nuclear Power J. Wood Volume 53 Condition Assessment of High Voltage Insulation in Power System Equipment R.E. James and Q. Su Volume 55 Local Energy: Distributed generation of heat and power J. Wood Volume 56 Condition Monitoring of Rotating Electrical Machines P. Tavner, L. Ran, J. Penman and H. Sedding Volume 57 The Control Techniques Drives and Controls Handbook, 2nd Edition B. Drury Volume 58 Lightning Protection V. Cooray (Editor) Volume 59 Ultracapacitor Applications J.M. Miller
- 8. Volume 62 Lightning Electromagnetics V. Cooray Volume 63 Energy Storage for Power Systems, 2nd Edition A. Ter-Gazarian Volume 65 Protection of Electricity Distribution Networks, 3rd Edition J. Gers Volume 66 High Voltage Engineering Testing, 3rd Edition H. Ryan (Editor) Volume 67 Multicore Simulation of Power System Transients F.M. Uriate Volume 68 Distribution System Analysis and Automation J. Gers Volume 69 The Lightening Flash, 2nd Edition V. Cooray (Editor) Volume 70 Economic Evaluation of Projects in the Electricity Supply Industry, 3rd Edition H. Khatib Volume 72 Control Circuits in Power Electronics: Practical issues in design and implementation M. Castilla (Editor) Volume 73 Wide Area Monitoring, Protection and Control Systems: The enabler for smarter grids A. Vaccaro and A. Zobaa (Editors) Volume 74 Power Electronic Converters and Systems: Frontiers and applications A.M. Trzynadlowski (Editor) Volume 75 Power Distribution Automation B. Das (Editor) Volume 76 Power System Stability: Modelling, analysis and control B. Om P. Malik Volume 78 Numerical Analysis of Power System Transients and Dynamics A. Ametani (Editor) Volume 79 Vehicle-to-Grid: Linking electric vehicles to the smart grid J. Lu and J. Hossain (Editors) Volume 81 Cyber-Physical-Social Systems and Constructs in Electric Power Engineering S. Suryanarayanan, R. Roche and T.M. Hansen (Editors) Volume 82 Periodic Control of Power Electronic Converters F. Blaabjerg, K.Zhou, D. Wang and Y. Yang Volume 86 Advances in Power System Modelling, Control and Stability Analysis F. Milano (Editor) Volume 87 Cogeneration: Technologies, optimisation and implementation C.A. Frangopoulos (Editor) Volume 88 Smarter Energy: From smart metering to the smart grid H. Sun, N. Hatziargyriou, H.V. Poor, L. Carpanini and M.A. Sánchez Fornié (Editors) Volume 89 Hydrogen Production, Separation and Purification for Energy A. Basile, F. Dalena, J. Tong and T.N. Veziroğlu (Editors) Volume 90 Clean Energy Microgrids S. Obara and J. Morel (Editors) Volume 91 Fuzzy Logic Control in Energy Systems with Design Applications in MATLAB‡ /Simulink‡ İ.H. Altaş Volume 92 Power Quality in Future Electrical Power Systems A.F. Zobaa and S.H.E.A. Aleem (Editors) Volume 93 Cogeneration and District Energy Systems: Modelling, analysis and optimization M.A. Rosen and S. Koohi-Fayegh Volume 94 Introduction to the Smart Grid: Concepts, technologies and evolution S.K. Salman Volume 95 Communication, Control and Security Challenges for the Smart Grid S.M. Muyeen and S. Rahman (Editors) Volume 96 Industrial Power Systems with Distributed and Embedded Generation R Belu Volume 97 Synchronized Phasor Measurements for Smart Grids M.J.B. Reddy and D.K. Mohanta (Editors) Volume 98 Large Scale Grid Integration of Renewable Energy Sources A. Moreno- Munoz (Editor) Volume 100 Modeling and Dynamic Behaviour of Hydropower Plants N. Kishor and J. Fraile-Ardanuy (Editors) Volume 101 Methane and Hydrogen for Energy Storage R. Carriveau and D.S.-K. Ting Volume 104 Power Transformer Condition Monitoring and Diagnosis A. Abu-Siada (Editor) Volume 106 Surface Passivation of Industrial Crystalline Silicon Solar Cells J. John (Editor) Volume 107 Bifacial Photovoltaics: Technology, applications and economics J. Libal and R. Kopecek (Editors)
- 9. Volume 108 Fault Diagnosis of Induction Motors J. Faiz, V. Ghorbanian and G. Joksimović Volume 110 High Voltage Power Network Construction K. Harker Volume 111 Energy Storage at Different Voltage Levels: Technology, integration, and market aspects A.F. Zobaa, P.F. Ribeiro, S.H.A. Aleem and S.N. Afifi (Editors) Volume 112 Wireless Power Transfer: Theory, technology and application N. Shinohara Volume 115 DC Distribution Systems and Microgrids T. Dragičević, F. Blaabjerg and P. Wheeler Volume 117 Structural Control and Fault Detection of Wind Turbine Systems H.R. Karimi Volume 119 Thermal Power Plant Control and Instrumentation: The control of boilers and HRSGs, 2nd Edition D. Lindsley, J. Grist and D. Parker Volume 120 Fault Diagnosis for Robust Inverter Power Drives A. Ginart (Editor) Volume 123 Power Systems Electromagnetic Transients Simulation, 2nd Edition N. Watson and J. Arrillaga Volume 124 Power Market Transformation B. Murray Volume 126 Diagnosis and Fault Tolerance of Electrical Machines, Power Electronics and Drives A.J.M. Cardoso Volume 128 Characterization of Wide Bandgap Power Semiconductor Devices F. Wang, Z. Zhang and E.A. Jones Volume 130 Wind and Solar Based Energy Systems for Communities R. Carriveau and D.S.-K. Ting (Editors) Volume 131 Metaheuristic Optimization in Power Engineering J. Radosavljević Volume 132 Power Line Communication Systems for Smart Grids I.R.S. Casella and A. Anpalagan Volume 139 Variability, Scalability and Stability of Microgrids S.M. Muyeen, S.M. Islam and F. Blaabjerg (Editors) Volume 155 Energy Generation and Efficiency Technologies for Green Residential Buildings D. Ting and R. Carriveau (Editors) Volume 157 Electrical Steels, 2 Volumes A. Moses, K. Jenkins, Philip Anderson and H. Stanbury Volume 905 Power System Protection, 4 volumes
- 10. Renewable Energy from the Oceans From wave, tidal and gradient systems to offshore wind and solar Edited by Domenico Coiro and Tonio Sant The Institution of Engineering and Technology
- 11. Published by The Institution of Engineering and Technology, London, United Kingdom The Institution of Engineering and Technology is registered as a Charity in England & Wales (no. 211014) and Scotland (no. SC038698). † The Institution of Engineering and Technology 2019 First published 2019 This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may be reproduced, stored or transmitted, in any form or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publisher at the undermentioned address: The Institution of Engineering and Technology Michael Faraday House Six Hills Way, Stevenage Herts, SG1 2AY, United Kingdom www.theiet.org While the authors and publisher believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgement when making use of them. Neither the authors nor publisher assumes any liability to anyone for any loss or damage caused by any error or omission in the work, whether such an error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. The moral rights of the authors to be identified as authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. British Library Cataloguing in Publication Data A catalogue record for this product is available from the British Library ISBN 978-1-78561-766-9 (hardback) ISBN 978-1-78561-767-6 (PDF) Typeset in India by MPS Limited Printed in the UK by CPI Group (UK) Ltd, Croydon
- 12. Contents Preface xv List of contributing authors xix 1 A review of progress on ocean energies 1 Neil Kermode 1.1 A risky business 1 1.2 In the beginning 2 1.3 Shocks to the system 3 1.4 The challenge of ocean energy 4 1.5 Technology Readiness Levels 5 1.6 Wave energy – TRL 7 7 1.7 Tidal and current energy 8 1.7.1 Tidal range – TRL 9 8 1.7.2 Tidal stream – TRL 8 9 1.8 Thermal and salinity gradient systems 11 1.8.1 Ocean thermal energy conversion – TRL 8 11 1.8.2 Salinity gradient – TRL 4 12 1.9 Offshore wind – TRL 8 13 1.10 Marine solar 14 1.11 Enabling technologies and actions 15 References 15 2 Wave energy 19 Gianmaria Sannino, Adriana Carillo, Arne Vogler, Giovanni Bracco, Giuliana Mattiazzo, Diego Vicinanza, Pasquale Contestabile, Domenico P. Coiro, Giancarlo Troise, Luca Castellini, and John V. Ringwood 2.1 The wave resource 20 2.1.1 Wave resource assessment 20 2.1.2 Wave measurements 26 2.2 Wave energy devices 38 2.2.1 The ISWEC plant at Pantelleria 38 2.2.2 Harbour breakwaters for wave energy conversion 47 2.2.3 Overview of the development of a pivoting buoy system 53 2.3 PTO development 66 2.3.1 Introduction 67 2.3.2 Design and test methods 68
- 13. 2.3.3 Innovative PTOs 69 2.3.4 Magnetic gears 70 2.3.5 Dielectric elastomer PTO 71 2.3.6 Electromechanical ballscrew-based PTO 73 2.3.7 Conclusions 75 2.4 Modelling and control 75 2.4.1 WEC models 76 2.4.2 WEC control 81 References 84 3 Tidal and current energy 95 Brian Kirke and Domenico P. Coiro 3.1 Introduction 95 3.1.1 Characteristics of tidal current energy 95 3.1.2 Basic physics of tidal energy 100 3.2 The resource 101 3.3 Tidal rise and fall concepts 102 3.3.1 Tidal barrages 102 3.3.2 Tidal lagoons 102 3.3.3 Dynamic tidal power barriers 102 3.3.4 Turbines for tidal rise and fall schemes 103 3.4 Tidal bridges and fences 105 3.5 Tidal, ocean current and river HKTs 106 3.6 Differences between hydrokinetic and wind energy 107 3.6.1 Limited depth suitable for turbines 107 3.6.2 Tidal and ocean current flow velocities are lower than wind 109 3.6.3 Much higher fluid density 109 3.6.4 Cavitation 109 3.6.5 Predictability 110 3.6.6 Bidirectional flow 110 3.6.7 Limited flow field 110 3.6.8 Floating debris 111 3.7 Axial flow turbines 112 3.8 Crossflow turbines 112 3.9 Ducts and diffusers 113 3.10 ‘Flying’ turbines 115 3.11 Oscillating foils 115 3.12 Vortex shedding 116 3.13 Tidal sails 116 3.14 Arrays 117 3.14.1 Tidal farm layout 117 3.14.2 Mixing and wake recovery – streamwise and lateral spacing 118 viii Renewable energy from the oceans
- 14. 3.15 Economics 118 3.16 Actual progress to date on large-scale grid-connected hydrokinetic power 119 3.17 Case study – development of GEM, a tidal stream energy system 120 3.17.1 GEM system configuration 120 3.17.2 Turbine and diffuser experimental tests (small-scale models) 121 3.17.3 Tests on full model in small scale 128 3.17.4 Full-scale prototype tests 134 Symbols used 139 References 140 4 Thermal and salinity gradient systems 145 Gianfranco Rizzo and Francesco Antonio Tiano 4.1 Energy resources 145 4.1.1 Oceanic thermal gradients 145 4.1.2 Salinity gradients 149 4.2 Ocean thermal energy conversion (OTEC) plants 154 4.2.1 Open-cycle plants 154 4.2.2 Closed-cycle plants 159 4.2.3 Hybrid-cycle plants 164 4.2.4 Feed-pump removal technique 168 4.2.5 OTEC and solar pond 169 4.2.6 OTEC systems characteristics 170 4.2.7 Environmental impact 171 4.2.8 Economic aspects 172 4.3 Salinity gradient energy (SGE) plants 176 4.3.1 Pressure-retarded osmosis membrane (PRO) plants 178 4.3.2 Reverse electrodialysis (RED) plants 180 4.3.3 Electric Double-Layer Capacitors plants 184 4.3.4 Faradaic pseudo-capacitor plants 185 4.3.5 SGE systems characteristics 187 4.3.6 Environmental impact 188 4.3.7 Economic aspects 189 Acknowledgments 190 List of acronyms 190 References 190 5 Offshore wind energy 195 Graeme McCann 5.1 Offshore wind characterisation 197 5.1.1 The random nature of wind 199 5.1.2 Long-term offshore wind speed characteristics 200 Contents ix
- 15. 5.1.3 Turbulence 201 5.1.4 Wake flow effects 203 5.1.5 Other offshore-specific conditions 205 5.2 Wind turbine technology development: a historical perspective 206 5.3 Basic principles of wind turbine operation 207 5.3.1 Energy conversion and concentration 207 5.3.2 One-dimensional momentum and Betz 208 5.3.3 1D momentum with rotational wake 212 5.3.4 Basic aerofoil principles 213 5.3.5 BEM theory 215 5.3.6 Basic corrections to BEM 216 5.4 Offshore wind turbine control systems 219 5.4.1 Control system fundamentals 219 5.4.2 Steady-state control 221 5.4.3 Dynamic control 224 5.4.4 Advanced control for load suppression 226 5.5 The future of offshore wind turbine technology 230 Acknowledgements 233 References 233 6 Marine solar energy 235 Luciano Mule’ Stagno 6.1 Solar cell technology 236 6.1.1 Semiconductor properties and growth 236 6.1.2 Semiconductor properties: the P–N junction 241 6.1.3 Crystalline solar cells 243 6.1.4 Thin film solar cells 246 6.2 Solar systems 247 6.2.1 Solar panels 247 6.3 Floating solar systems 248 6.3.1 Motivation 248 6.3.2 Components of a floating system 249 6.3.3 Chronology 249 6.3.4 Advantages and disadvantages 256 6.3.5 Systems at sea: motivation and special challenges 257 6.3.6 Systems at sea: current situation 258 6.3.7 Offshore solar: the future 264 References 265 7 Offshore support structure design 271 Erin E. Bachynski and Maurizio Collu 7.1 Offshore support structures 271 7.1.1 Bottom-fixed support structures 272 7.1.2 Floating support structures 276 x Renewable energy from the oceans
- 16. 7.2 Support structure design 282 7.2.1 Initial design 282 7.2.2 Loads and load effects 290 7.2.3 Short-term and long-term design analysis 301 7.2.4 Design standards, guidelines, and other considerations 310 References 312 8 Electrical power transmission and grid integration 321 Elisabetta Tedeschi and Abel A. Taffese 8.1 Introduction 321 8.2 Implications of the grid-side converter topology on the grid integration of MECs 321 8.3 Impact of MECs’ integration into power distribution systems 324 8.3.1 Power quality issues in marine energy installations 324 8.3.2 System impact of marine energy installations 327 8.3.3 Case study 329 8.4 Impact of MECs’ integration into power transmission systems 332 8.4.1 Additional ancillary services that can be provided by marine energy installations 332 8.4.2 Transmission technologies 333 8.4.3 Case study 336 8.4.4 Hybrid HVAC/DC systems and expansion planning 338 References 338 9 Offshore energy storage 345 Seamus D. Garvey and Rupp Carriveau 9.1 Underwater compressed air energy storage 346 9.1.1 How much exergy is stored per unit volume of air containment 349 9.1.2 Corrections for air density and non-ideal gas behaviour 350 9.1.3 Structural capacity and its relevance to energy storage 351 9.1.4 Exergy versus structural capacity for underwater containments 352 9.1.5 The air ducts 355 9.1.6 Using thermal storage in conjunction with air storage 357 9.1.7 An example system design 359 9.1.8 Sites available for UWCAES 360 9.2 Offshore pumped hydro 362 9.2.1 Exergy storage density for UWPH 363 9.2.2 Key distinctions between UWPH and UWCAES 363 9.2.3 The EC2SC ratio for UWPH 365 9.3 Buoyancy energy storage systems 366 9.4 Offshore thermal energy storage systems 368 9.5 Other concepts 371 9.6 Integrating offshore energy storage with generation 372 References 373 Contents xi
- 17. 10 Multipurpose platforms 377 Maurizio Collu and Erin E. Bachynski 10.1 Introduction 377 10.1.1 Context 377 10.1.2 Why multipurpose platforms? 378 10.2 Multipurpose platform projects and concepts 381 10.2.1 EU projects 381 10.3 Design and analysis of multipurpose platforms 387 10.3.1 Multidisciplinary design methodology 387 10.3.2 Resource assessment: combined wind-wave resources 388 10.3.3 Modelling and analysis 390 10.4 Conclusions 392 References 393 11 Installation, operation and maintenance of offshore renewables 397 Vincenzo Nava, Pablo Ruiz-Minguela, Germán Pérez-Morán, Raúl Rodrı́guez-Arias, Joseba Lopez-Mendia, and José-Luis Villate-Martı́nez 11.1 Introduction 397 11.1.1 Impact of installation, operation and maintenance activities in offshore renewable systems 397 11.1.2 Functional decomposition of offshore renewable systems 399 11.1.3 Concepts of reliability and failure analysis 401 11.2 Life cycle activities for offshore energy systems 404 11.2.1 Installation phase 404 11.2.2 Operation and maintenance phase 408 11.2.3 Decommissioning phase 409 11.2.4 Vessels and equipment 410 11.3 Planning the operations 411 11.3.1 Strategies for planning the operations 411 11.3.2 Weather windows 414 11.3.3 Estimation of the delay time 416 11.3.4 Offshore standards and technical recommendations for operations 417 11.4 Economic modelling of installation, operation and maintenance 418 References 422 12 Challenges and future research 425 Neil Kermode 12.1 Challenge one – proving it works 425 12.2 Challenge two – keeping it working 427 xii Renewable energy from the oceans
- 18. 12.3 Challenge three – technical improvements 429 12.3.1 Servicing 429 12.3.2 Access 430 12.3.3 Data 430 12.3.4 Materials 431 12.4 Challenge four – environmental acceptability 432 12.5 Challenge five – social acceptability 433 12.6 Challenge six – making it work commercially 434 12.7 Challenge seven – getting the price down 436 12.8 Challenge eight – public support required 438 12.8.1 Financing arrays 438 12.8.2 Market pull 439 12.9 Challenge nine – market development 440 12.10 Challenge ten – making it happen 442 References 444 Index 447 Contents xiii
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- 20. Preface Energy is essential to humanity, and our society cannot operate without it. Energy has been the prime mover of the industrial revolution to develop the world econ- omy as we know it today. Yet, our energy supply system, which is still heavily based on fossil fuels for both electricity generation and transport, has now become a threat to mankind and the global ecosystem we live in. Massive amounts of carbon dioxide emitted into the atmosphere every day are changing our climate. Scientific evidence has confirmed that the induced greenhouse effect is driving a rise in air and water temperatures at a rate that is much larger than that observed in the pre-industrial era, leading to the destruction of natural habitats, loss of sea ice and ice sheets, higher sea levels, drought and extreme weather events. While decarbonising our present energy supply system has emerged to be one of the largest challenges ever faced by mankind, the availability of natural and clean energy resources that are large enough to meet the demand of the world populations is definitely not an issue. The amount of solar energy alone that reaches Earth every day, in the form of light as well as in the form heat that in turn creates wind, sea waves and marine thermoclines, is already orders of magnitude larger than mankind’s energy needs. With over 71 percent of the Earth’s surface covered by water, our oceans offer by far the major contribution of clean energy through various renewable forms: offshore wind, wave, tidal and current, and thermal and salinity gradient energy. It is worth noting that the majority of the people in the world live in close proximity to the sea, in coastal cities and islands. Limited space on land in these often densely populated areas and accessibility to onshore space that is large enough to meet the requested energy demand through renewable energy exploitation are primary fac- tors driving coastal populations to exploiting energy resources at sea. The exploitation of renewable energy at sea brings about a number of engineering challenges making the design of marine systems more difficult as compared to those on land. The structurally integrity requirements offshore are significantly more demand- ing as a result of more extreme winds and the rough waves experienced over the typical design lifetime of 20 to 30 years. The corrosive marine environment also dictates more drastic measures to protect the marine systems against material degradation. Other practical considerations, such as the presence of marine growth, moving ice, accidental impacts by sea vessels and scour induced by sea currents, imply the need for more rigours engineering analysis during the design process. Installation, operation and maintenance procedures at sea are logistically more complex than on land not only because of site accessibility that is often impaired by stormy weather conditions but also by the need of good port facilities and adequate sea vessels.
- 21. This book presents an overview of the development of ocean energy technol- ogies, renewable energy resources and latest emerging trends. It presents a broad perspective, covering important aspects from energy conversion, installation, operation and planning to grid connection and storage. Latest literature sources in various relevant fields are presented together with theoretical fundamentals for system engineering analysis. The book is organised in 12 chapters: Chapter 1 reviews the progress achieved in ocean energies and the levels of technology maturity reached by the different tech- nologies. The following three chapters, Chapter 2 to 4, are dedicated to ocean energy technologies designed to extract energy directly from the sea water. Chapters 2 and 3 deal with the extraction of potential and kinetic energy available in sea waves and current flows. Chapter 4 is dedicated to energy extraction from temperature and salinity differences present in the oceans. The next two chapters discuss two tech- nologies that utilise marine space, yet they only exploit the renewable above the sea surface: offshore wind and marine solar energy. Chapter 7 covers offshore support structure design, including both bottom-mounted and floating platform concepts, that is necessary for safely supporting different renewable energy systems in the harsh marine environment. Chapters 8 and 9 are dedicated to two equally important aspects for renewable energy systems at sea: electrical power transmission and offshore energy storage. Ocean energy conversion systems may be located well offshore, at large distances from the nearest grid connection point onshore. In such cases, power transmission to shore and connectivity to the electricity networks on land become critical aspects affecting both the overall efficiency of energy conversion and the cost of generation. Furthermore, although renewable energy technologies have made significant pro- gress, many natural energy sources remain intermittent and will pose important challenges when integrated into electricity grids at high penetration levels. Countries nowadays relying on large amounts offshore wind capacity are already known to experience low and often unsustainable electricity prices due to congestion problems encountered in the electricity grids during periods of high wind availability and low energy demand. Energy storage is now regarded as the missing link for solving such problems and to enable a fully decarbonised energy supply system. Given its strategic importance nowadays, we believed that a whole chapter in this book should be solely dedicated to offshore energy storage concepts. Chapter 10 discusses multi-purpose platforms integrating different forms of renewable energy generation and other activities, such as aquaculture, to improve the overall economic viability of offshore-based environmentally sustainable activities. Chapter 11 addresses installation, operational and maintenance aspects of offshore renewables. Finally, Chapter 12 discusses challenges in the offshore renewable energy sector and future expectations. We would like to thank Ing. Gaetano Gaudiosi who served as the president of the OWEMES1 society for many years and who had encouraged us to take-up the task of compiling this book. This work would not have been possible without the 1 Offshore wind and other marine renewable energies in Mediterranean and European Seas. xvi Renewable energy from the oceans
- 22. contribution of many authors, experts in various fields of offshore renewables and who have brought together a wealth of knowledge and experience for the benefit of our readers. We worked closely with them to edit the book in a coherent manner. We are indebted for their hard work and dedication in preparing their chapters in a timely manner. Finally, we would like to express our sincere gratitude to the Institution of Engineering and Technology (IET) of the UK, especially the editorial and production teams, for their continuous guidance, support and meticulous work to produce the book on time. Domenico P. Coiro and Tonio Sant Preface xvii
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- 24. List of contributing authors Chapter 1 – A review of progress in ocean energies Neil Kermode, The European Marine Energy Center, UK Chapter 2 – Wave energy Gianmaria Sannino, ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy Adriana Carillo, ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy Arne Vogler, University of the Highlands and Islands – Lews Castle College, Stornoway, Isle of Lewis, UK Giuliana Mattiazzo, Politecnico di Torino, Torino, Italy Giovanni Bracco, Politecnico di Torino, Torino, Italy Diego Vicinanza, Inter-University National Consortium for Marine Sciences (CoNISMa), Italy Pasquale Contestabile, University of Campania Luigi Vanvitelli, Aversa (Caserta), Italy Domenico P. Coiro, University of Naples Federico II, Naples, Italy Giancarlo Troise, SeaPower Scarl – Research Consortium, Naples, Italy Luca Castellini, UMBRAGROUP S.p.A., Foligno, Italy John Ringwood, National University of Ireland, Maynooth, UK Chapter 3 – Tidal and current energy Brian Kirke, University of Australia, Barbara Hardy Institute, Australia Domenico P. Coiro, University of Naples Federico II, Italy Giancarlo Troise, Seapower Scrl – Research Consortium, Italy Chapter 4 – Thermal and salinity gradient systems Gianfranco Rizzo, University of Salerno, Fisciano, Italy Francesco A. Tiano, University of Salerno, Fisciano, Italy Chapter 5 – Offshore wind energy Graeme McCann, Goldwind Denmark A/S, Bristol, UK
- 25. Chapter 6 – Marine solar energy Luciano Mule’ Stagno, University of Malta, Malta Chapter 7 – Offshore support structure design Erin Bachynski, Norwegian University of Science and Technology, Trondheim, Norway Maurizio Collu, University of Strathclyde, Glasgow, UK Chapter 8 – Electrical power transmission and grid integration Elisabetta Tedeschi, Norwegian University of Science and Technology, NTNU, Norway Abel A. Taffese, Norwegian University of Science and Technology, NTNU, Norway Chapter 9 – Offshore energy storage Seamus D. Garvey, University of Nottingham, UK Rupp Carriveau, University of Windsor, Canada Chapter 10 – Multipurpose platforms Maurizio Collu, University of Strathclyde, Glasgow, UK Erin Bachynski, Norwegian University of Science and Technology, Trondheim, Norway Chapter 11 – Installation, operation and maintenance of offshore renewables Vincenzo Nava, Tecnalia Research and Innovation, Energy and Environment Division, Derio, Spain / Basque Centre for Applied Mathematics, CFD-MS Group, Bilbao, Spain Pablo Ruiz-Minguela, Tecnalia Research and Innovation, Energy and Environment Division, Derio, Spain Germán Pérez-Morán, Tecnalia Research and Innovation, Energy and Environ- ment Division, Derio, Spain Raúl Rodrı́guez-Arias, Tecnalia Research and Innovation, Energy and Environ- ment Division, Derio, Spain Joseba Lopez-Mendia, Tecnalia Research and Innovation, Energy and Environ- ment Division, Derio, Spain José-Luis Villate-Martı́nez, Tecnalia Research and Innovation, Energy and Envir- onment Division, Derio, Spain Chapter 12 – Challenges and future research Neil Kermode, The European Marine Energy Center, UK xx Renewable energy from the oceans
- 26. Chapter 1 A review of progress on ocean energies Neil Kermodea 1.1 A risky business Writing the history of marine energy is a risky process. It is a task that is fraught with the dangers of personal bias and limited experience, and because there is no presently accepted historical narrative, it is a space that is open to debate and discovery. As such, this is a subjective analysis, and, of course, I am viewing the world through a particular geographical lens: I have lived most of my life in the UK and have spent the last 10 years in Orkney, witnessing the birth and growth of the European Marine Energy Centre (EMEC). So, much of my most recent experience has been in the commercialisation of wave and tidal stream technologies – and therefore I will have a natural bias to talk about that which I know best. In reviewing the progress of ocean energies, wave energy is a good place to start. The idea of harnessing energy from waves has been around for centuries, yet today it remains a major engineering challenge to do this successfully at scale. As such, wave energy ‘brackets’ our story nicely. Here in the UK, it is popularly accepted that Professor Stephen Salter is the ‘father of wave energy’, and that his 1970s ‘Nodding Ducks’ were the world’s first wave energy machine. However, this ignores the facts that Stephen was not the first to think of this energy source, nor was he alone at the time when he did his sterling work. And as soon as you say ‘this is where it started’, you find precedents that prove you wrong. At best you can be very specific about the claim such as ‘Pelamis was the world’s first floating deep-water wave converter to generate into the National Grid’, but it is less elegant than ‘it started here’. So, as we journey through the history of marine energy, there will need to be occasional digressions and eddies to the narrative. Often these may be due to par- tially told stories and may provide clues for further research. However, the intent is to show as complete a record of the development journey as it appears to me in late a The European Marine Energy Centre, Orkney, UK
- 27. 2018, secure in the knowledge that the sector will continue to progress across many fronts even before the ink in this chapter has had time to dry. 1.2 In the beginning So rather than ask ‘where did it all start’, it is more illuminating to ask ‘why did it all start’? Energy. Society needs energy to produce or process material into more valu- able goods. This has always been the case and over history energy has been pro- duced by the manual labours of humans, or their domesticated animals. For most of human history, the most valuable commodity was food and nearly all energy harvesting systems went into this. In most cases, some natural energy flow was harvested, initially the wind and in due course the flow of rivers. All of these techniques require technology to turn flowing fluids into useful work and most relied upon grinding or processing plant material into food stuffs. This gives the first example of a tidal site I have encountered: Eling Mill in Hampshire. The mill is believed to have Roman foundations. It has been built across the river at Eling where it enters Southampton Water and the sea. The gates of the mill are left open as the tide rises so allowing the mill runs releasing pond to be filled with the incoming tide. At high tide the gates are closed, so impounding the tidal water in the mill pond. Once the tide has fallen outside the pond, the mill is free to run by releasing the impounded sea through its mill wheels. It has been doing this for nearly 2,000 years. There must be other sites where this has been done but forgotten. Indeed, there are multiple tidal mills shown on maps in Wales and Cornwall, but the dates of them are often unclear. However, they never achieved the ubiquity that wind mills achieved, so they have often been ‘the first of a kind’ when the idea was re-discovered again and again during history. However, as an idea they were eclipsed by the bigger and more powerful forces of fossil fuels. There are practically no examples of new tidal schemes being built, commissioned and successfully run for any period of time once the technology to burn fossils was developed. Where there are examples then they tended to be commissioned at times of crisis when oil supplies were looking fragile, or when the costs looked to be rising uncontrollably. Another driver for the development of renewables seems to have been the development of the means to generate and use electricity. This occurred around 1880 when shaft power could be turned into kilowatts and gave rise to a flurry of schemes. This leads to my earliest wave scheme: Herring Cove, Nova Scotia. The Par- sons Ocean Power Company was incorporated in 1922 [1] and by 1925 had built a machine which successfully generated power, until its later destruction in a storm. This scheme relied upon capturing the movement of a tethered float. The move- ment of the float caused machinery to rotate by way of a lever arm and gear mechanism and generated sufficient electricity to light ‘several 60 watt bulbs’. The Pacific coast also seemed to attract those who saw the opportunity and there are several examples of around a dozen schemes such as the oscillating water column at Parallel Point, San Francisco. 2 Renewable energy from the oceans
- 28. Schemes like Herring Cove aimed to generate electricity for the local area, but the advent of the electricity supply grid generally marked the end of their useful- ness, or certainly their competitiveness. In the space of a few short decades, a new era of coal, steam and electric power, together with the creation of an electricity grid, disconnected the need to co- locate generation and the users of energy and heralded a long hiatus in the need for, or an interest in, renewable power. Industrial economies became used to plentiful and relatively cheap electricity provided through increasingly large fossil fuel power plants streaming electrical energy to people’s homes. Motor transport grew on the back of plentiful, cheap oil. This meant that, with very few exceptions, there was little need to consider renewables. Yet in some places renewables found a role. In France, La Rance Tidal Power Station was opened in 1966 as the world’s first tidal power station, feeding power into the country’s grid. It was for decades the largest tidal power station in the world. Whilst in the Highlands of Scotland – geographically remote and poorly con- nected – massive hydro schemes were built throughout the middle of the twentieth century to ‘power the glens’ and provide economic stimulus to what was until then a socio-economic backwater. Likewise in the USA, where, by 1920, 40 per cent of all electricity came from hydro [2]. Similarly, other one-off renewable projects could be found around the world. But by and large, there was limited interest or need to consider new forms of power until, like the industrial revolution, another tectonic shift occurred. 1.3 Shocks to the system The first OPEC oil price shock in 1973–74 [3], which saw the world oil price quadrupled in less than a year (from $3 to $12 per barrel), was a major blow to the world’s economy and meant – for the first time, that politicians, policymakers and politicians would need to consider new forms of power. This shock sparked a new interest in alternative sources of energy, with wave power leading the charge for ocean technologies. A further shock in 1979 saw the price double again (to nearly $40 a barrel), whilst by the 1980s, a growing recognition of climate change demanded action on the world’s stage. Interest in onshore wind began to grow. By 1987 the Montreal Protocol was agreed, restricting chemicals that damage the ozone layer, then in 1989 the UK Prime Minister Margaret Thatcher – possessor of a chemistry degree – warned in a speech to the UN that We are seeing a vast increase in the amount of carbon dioxide reaching the atmosphere... The result is that change in future is likely to be more fun- damental and more widespread than anything we have known hitherto. It has been this recognition – and the resulting global treaties on climate change – that has spurred action around the world and has sparked the massive growth in renewables we see today. A review of progress on ocean energies 3
- 29. The industrial revolution, the growth of grids and the recognition of climate change have all resulted in radical shifts in how we make and consume energy. At a political level, nations states must decide – for political and social ends – how energy is created and used, and the plain numbers in $ per kWh must somehow or other stack up. And these decisions are apt to change. In the last decade alone two further shocks – the Fukushima nuclear disaster and the U.S. shale gas revolution – have marked ground-breaking shifts in the ways in which different countries view energy and energy security. Mature economies including Japan and Germany have decided to move away from nuclear energy, whilst China – now living with the very real health and cli- mate risks caused by an over-reliance on coal – has become the world’s largest developer of onshore wind. Looking ahead it is clear we cannot know what future changes we might see. Climate change continues to accelerate, the move towards electric vehicles can only grow and we must all find new ways to lighten our load on our crowded planet. Other ‘unknown unknowns’ – to paraphrase Donald Rumsfeld – inevitably lurk around the bend. Renewable energy is making an incredible contribution having gone from practically zero in the 1940s to now being the dominant electricity source in several countries such as Norway and Scotland. In 2016, cumulative grid-connected onshore wind capacity reached 451 gigawatts (GW) and wind power accounted for almost 4 per cent of global electricity generation [4]. Onshore wind capacity is now expected to reach almost 750 GW by 2022 – and although ocean energy technologies lag significantly behind, once each sector develops a commercially available product, we can with some confidence expect similar long-term exponential growth. 1.4 The challenge of ocean energy More than 70 per cent of our planet is covered in ocean, and whilst in many areas our seas are already heavily exploited, for food, oil and gas – they still represent a massive bounty of renewable energy waiting to be harvested. Waves, tides, wind, salinity, temperature gradients and the sun all offer tempting new sources of power – power which can often be generated offshore, with little or no conflict with other sea users. In some cases, these technologies are already in the market – indeed some have been around for more than a century, and in others they are not quite there yet. But what is clear is that – in an ever-changing world – we cannot afford to shut out alternative new sources of energy, whatever they may be. The challenge – as will become clear in this book – is not only to develop new technologies that work, but also to develop new technologies that work com- mercially. And to understand this, we need a brief exploration of technology readiness. 4 Renewable energy from the oceans
- 30. 1.5 Technology Readiness Levels Trying to describe how ready a technology is for deployment has always been difficult. Every inventor is enthusiastic about their project, often overly so, and this tends to lead to wild claims. Fortunately, NASA came up with a Technology Readiness Levels (TRL) scale to measure the readiness of any technology and this has now been widely adopted as a means of comparison and categorisation. Technological and commercial maturation occurs over several phases, from concept design to commercial deployment at sea. Each technology project is evaluated against the parameters for each technology level and is then assigned a TRL rating based on the project’s progress. Moving from one phase to the next requires increased deployment, leading to technological and hence economic improvements. There are nine technology readiness levels. TRL 1 is the lowest and TRL 9 is the highest. NASA defines TRLs as follows [5]: When a technology is at TRL 1, scientific research is beginning, and those results are being translated into future research and development. TRL 2 occurs once the basic principles have been studied and practical applications can be applied to those initial findings. TRL 2 technology is very speculative, as there is little to no experimental proof of concept for the technology. When active research and design begin, a technology is elevated to TRL 3. Generally, both analytical and laboratory studies are required at this level to see if a technology is viable and ready to proceed further through the development process. Often during TRL 3, a proof-of-concept model is constructed. Once the proof-of-concept technology is ready, the technology advances to TRL 4. During TRL 4, multiple component pieces are tested with one another. TRL 5 is a continuation of TRL 4; however, a technology that is at 5 is identified as a breadboard technology and must undergo more rigorous testing than technology that is only at TRL 4. Simulations should be run in environments that are as close to realistic as possible. A TRL 6 technology has a fully functional prototype or representational model. TRL 7 technology requires that the working model or prototype be demon- strated in a space environment. TRL 8 technology has been tested and ‘flight qualified’ and it’s ready for implementation into an already existing technology or technology system. Once a technology has been ‘flight proven’ during a successful mission, it can be called TRL 9. To look specifically at ocean energy, the EU has described a number of stages for the development of ocean energy technologies as they pass through these levels (Figure 1.1). A crucial question is how quickly any given technology will progress through these stages. It is essential to understand that any timeline is highly dependent on overcoming the barriers faced by ocean energy developers and the level of public support offered in the short and medium terms by national and regional govern- ments and international institutions such as the EU. A review of progress on ocean energies 5
- 31. To put above in context, if we take the example of onshore wind, it took 40 years from the first experiments until early rollout (Figure 1.2) – and the speed of rollout was highly dependent on the public policy framework. One should also note that it was not the first inventors (Scotland and the USA) which capitalised on the industrial rollout and economic benefit which ensued that prize went to Denmark. They put in place the specific tariffs, investment vehicles and consents processes that gave the fledgling technology somewhere to take flight. The lesson here is the benefit went to the country which commercialised the technology, not the one that invented it, and there are parallels here for ocean energy policymakers [6]. Wave Energy Scotland, an organisation established to bring wave energy to the point of commercialisation, has taken the TRL model and matched it to its own four-stage progression. EMEC provides an overview of the technology develop- ment pathway described here (Figure 1.3). Applying this discipline to marine energy is useful. It provides a clear pathway from the lab to the sea, and all of the steps that need to be taken in between. It is not, however, the end of the story. As in the example of the onshore wind industry, it is not just about inventing a technology that works, it is about finding a path to make a technology work commercially and this can take many years. This is a theme I will return to in the closing chapter. In the last decade, a number of ocean energy technologies have made tre- mendous strides towards TRL 9 and here we will look at them all in turn. R&D • Small-scale device validated in lab • Component testing and validation • Small-/medium-scale pilots Prototype • Representative single-scale devices with full-scale components • Deployed in relevant sea conditions • Ability to evidence energy generation Demonstration • Series or small array of full-scale devices • Deployed in relevant sea conditions • Ability to evidence power generation to Grid • For OTEC and salinity gradient: full functionality down- scaled power plant Pre-Commercial • Medium-scale array of full-scale devices experiencing interactions • Grid connected to a hub or substation (array) • Deployed in relevant/ operational sea conditions • For OTEC and salinity gradient: scalable Industrial Roll-Out • Full-scale commercial ocean energy power plant or farms • Deployed in operati- onal real sea conditions • Mass production of off-the-shelf components and devices TRLs 1–4 TRLs 3–6 TRLs 5–7 TRLs 6–8 TRLs 7–9 Figure 1.1 From EU Ocean Energy Forum Strategic Roadmap 10% EU power demand Average onshore turbine size = 1 MW Average offshore turbine size = 2 MW Offshore farm 450-kW turbines (Denmark) Renewable obligation in USA 3-bladed 200-kW turbine (Denmark) 12-kW turbines (Scotland & Ohio) Wind 2014 2002 2001 Prototype to industrial roll-out: 40+ years 1991 1978 1956 1887 Figure 1.2 Development of wind turbines, from early experiments to industrial rollout, Ocean Energy Europe 6 Renewable energy from the oceans
- 32. 1.6 Wave energy – TRL 7 Wave energy remains one of the world’s last great untapped sources of renewable energy – with a total theoretical wave energy potential of 32 PWh/year, roughly twice the global electricity supply [7]. But thus far, it has proved the most chal- lenging to develop. Today there are an estimated 218 active wave energy developers around the globe [8], ranging from start-ups and small-to-medium enterprises, to utilities, original equipment manufacturers and industrial engineering firms, each seeking to develop one of at least nine different types of wave energy converter. Yet we still have not seen a dominant technology emerge at full-scale TRL 8, with various wave energy concepts in development. A full explanation of the different types of wave energy converters can be found on the EMEC website at the following link: www.emec.org.uk/marine- energy/wave-devices/. In Europe, it is estimated ocean energy could meet 10 per cent of EU electrical demand by 2050 [9], whilst worldwide it is recognised that ‘blue growth’ has the potential to regenerate peripheral areas affected by declining traditional indus- tries [10]. This is particularly true for wave energy, where areas of high resource are closely aligned with remote and economically fragile areas – places where new, high value jobs have a disproportionate impact. I can vouch personally for the massive impact the industry has had in Orkney, where it is estimated that EMEC has spent over £16 million in Orkney with around 200 people currently employed in the marine renewables sector [11]. An economic impact assessment commissioned by government agency Highlands and Islands Enterprise estimates that EMEC has generated a gross value added to the wider UK economy of £284.7 million, with 4,224 full-time equivalent (FTE) job years so far [12]. Figure 1.3 EMEC pathway to commercialisation A review of progress on ocean energies 7
- 33. In the modern era, I consider the wave energy story began in earnest in the 1970s in the UK at the Wave Power Department at the University of Edinburgh, led by Prof Stephen Salter whose invention – known as the Edinburgh or Salter Duck – excited considerable international attention and showed that the words ‘wave’ and ‘energy’ do actually fit together. However, his work was not done in isolation as he gathered a team of researchers and there were others such as hovercraft inventor Sir Christopher Cockrell also working on schemes in parallel. By the late seventies however, the UK had struck oil in the North Sea and public support for wave energy waned. It is regrettable that Salter’s work was undermined by inaccurate reporting by UK government’s advisors of the time which led to the programme’s cancellation. And although wave energy remained a significant topic at an R&D level, it was not until the 2000s that we saw a sig- nificant amount of progress – again led by a Scottish concern. The rise of Pelamis in the early years of this century was a tremendous story of endeavour, and one of the firsts in the world. It dovetailed seamlessly with the establishment of EMEC in 2003 and enabled the Edinburgh pioneers to success- fully pilot the world’s floating first grid-connected device. For the next decade, Scotland and the UK gave tremendous support to the sector until – by 2014, the technological and financial challenges in creating energy from waves overwhelmed the leaders in the sector – with the high-profile collapses of Pelamis and, shortly thereafter, Aquamarine Power. What these financial collapses, or as I would prefer to term them lessons, have shown us is that some technological challenges are too big or too complex for single private firms to tackle alone, even with public support. 1.7 Tidal and current energy 1.7.1 Tidal range – TRL 9 Whilst wave energy continues to demonstrate its ability to generate electricity, tidal range power is already well proven. As I mentioned earlier, Eling Mill in Hampshire has been utilising tidal range power for over 2,000 years, whilst the 240-MW la Rance Tidal Power Station in France has been operating since 1966. In Canada, the 20-MW Annapolis Royal Generating Station Opened in 1984, whilst the 254-MW Sihwa Lake Tidal Power Station in South Korea has been operational since 2011. Tidal range technology shares characteristics similar to hydropower, essen- tially utilising the height difference of two bodies of water created by a dam or barrier in order to produce electricity. Its main advantage is that is it very pre- dictable. However, its capacity factor of 25 per cent is not as high as offshore wind due to the nature of tidal cycles and turbine efficiency [13]. In recent years, tidal range has evolved with the concept of tidal lagoons: these are artificial basins built in bays and estuaries and are considered less intrusive than tidal barrages (which tend to span entire estuaries and alter the salinity and 8 Renewable energy from the oceans
- 34. ecology). In the UK, the 320-MW Swansea Bay Lagoon project in Wales has (at the time of writing) failed to secure the government support it requires to proceed. This was intended to be a pathfinder project, leading to five larger proposed tidal lagoon projects in the United Kingdom: Cardiff (3,000 MW), Newport (1,800 MW), Colwyn Bay, West Cumbria and Bridgwater Bay. One recent review [14] estimates that 5,792 terawatt-hours (TWh, around a third of the total global electricity supply) could be produced by tidal range power plants. However, 90 per cent of the resource is distributed across just five countries, with both the UK and France having a significant share of that resource. In common with other forms of energy, it is not the technology per se that is an obstacle to progress – tidal range clearly works. The issue appears to be its uncompetitiveness versus unsustainable and incumbent fossil-based fuels, or heavily subsidised nuclear generation. However, the technology does require a very specific set of physical and geographical circumstances to make a scheme viable – and the necessary revenue stream or public support to offset the high initial capital costs. 1.7.2 Tidal stream – TRL 8 In contrast to wave energy, the tidal stream sector has made significant advances towards commercialisation in recent years. Orkney technology firm Orbital Marine Power has developed a floating turbine already providing significant power to Orkney Mainland, whist in the Pentland Firth the MeyGen scheme, owned by SIMEC Atlantis, has the world’s largest operational array, powered by Andritz Hydro Hammerfest and Atlantis Resources turbines. Tidal stream generators draw energy from water currents in much the same way as wind turbines draw energy from air currents. However, the higher density of water relative to air (water is 800 denser) means that tidal generators have smaller blades than wind turbines and operate at lower velocities. Given that power varies with the density of medium and the cube of velocity, water speeds of nearly one-tenth the speed of wind provide the same power for the same size of turbine system; however, this limits the application in practice to places where tide speed is at least 2 knots (1 m/s) even close to neap tides. In Europe, the majority of high resource tidal stream sites which meet this criteria lie around the coasts of the UK, Ireland and France with some hotspots in the Mediterranean. Worldwide other tidal hotspots include Nova Scotia, Chile, the Philippines and Indonesia. In the modern era, the development of tidal energy was effectively led by Dr Peter Frankel when he installed his 15-kW machine in Loch Linnhe, Scotland, in 1994–95 [15]. This two-bladed rotor project by IT Power had been sponsored by Scottish Nuclear Electric and was the forerunner of the 300-kW SeaFlow project later installed off Lynmouth in Devon in 2003 by Peter’s company Marine Current Turbines (MCT). That device was not grid connected but provided invaluable information on the practicalities of a stand-alone generating structure at sea. It led directly to the iconic A review of progress on ocean energies 9
- 35. twin rotor 1.2-MW SeaGen device at Strangford Loch in Northern Ireland installed in 2008. This turbine operated successfully for several years and generated over 350 MWh into the grid until it was shut down in 2017. In parallel with MCT’s work, a number of other companies also began development on their own tidal turbines, with several notable examples. In the Straits of Messina, Ponte de Archimede developed a 150-kW vertical axis turbine (KOBOLD). This largely forgotten device was the first to generate into the grid in Europe in 2001 [16]. The company was owned by ship owner Dr Elio Matacena and used his experience of the Voith Schneider vertical propellers that powered his car ferries to come up with the design. In Hammerfest, Norway, a fully submerged horizontal axis turbine was installed in 2003 [17]. This 300-kW machine generated into the local grid and led to the development of the 1-MW horizontal axis turbine installed at EMEC in 2011. Herbert Williams in Florida developed a rim generator in the 1990s to work in the Gulf Stream [18] which eventually became Irish-owned OpenHydro. They installed a test rig at EMEC in 2006 and gird-connected their first 6-m diameter open-centred turbine in 2008. They narrowly beat MCT to the accolade of the first to generate to the GB National Grid and continued to test at EMEC until the company’s demise in 2018. OpenHydro had gone on to install a 10-m diameter machine in the Bay of Fundy in Canada in 2009 before installing a 16-m diameter machine at a nearby site and also two machines at Roche Blanchard in France [19]. In the middle of all this Tidal Generation Limited, a company made up in part of ex-MCT employees, developed a buoyant sub-sea turbine. This initially 500-kW machine led to a 1-MW machine through Rolls Royce’s support and eventual ownership of the company before its sale to Alstom. In more recent times, Scotland has seen three most notable successes. In 2016, Nova Innovation deployed the world’s first fully operational, grid- connected offshore tidal energy array, at Bluemull Sound in Shetland. The first Nova M100 turbine was deployed at the site in March 2016, and the second was deployed in August 2016, making this the first offshore tidal array in the world to deliver electricity to the grid. A third turbine was added to the array in early 2017. The first phase of the MeyGen project passed the 8-GWh generation milestone in July 2018 [20] through its use of two Hammerfest turbines and an Atlantis machine – both companies having undertaken testing work at EMEC in the pre- vious years. This scheme is set to expand its site with the next phase being planned at the time of writing. And finally, the device of which Orkney is most proud is the locally envisaged, designed, managed and installed SR1-2000 made by Scotrenewables (now known as Orbital Marine Power), which repeatedly generated around 7 per cent of Orkney’s electricity through much of 2018. This machine is the first pre- commercial prototype of the company and follows years of development by the local team. The first open water tests of a large-scale model were undertaken within sight of EMEC’s offices and led to the 250-kW machine being developed and connected in 2012 [21]. The success of that device led to the 2-MW machine that effectively powered Orkney for a day a fortnight in 2018 [22]. 10 Renewable energy from the oceans
- 36. And there have been many other successes and firsts achieved in this period that are too numerous to mention here with both devices built and tested around the world, and also the experience of some of the work above being taken to Asia and South America to join with their local initiatives. Now after decades of dreaming and sporadic one-off projects, we have reached the point where several developers have built more than one device. This enables the essential ‘learning by doing’ to begin to take place. Up to this point, most schemes were the first (and often last) of a kind. Now we have some serious learning being undertaken. We will return to this point in the final chapter. So in the last 17 years, we have gone from the first grid-connected machine, to the delivery of industrially useful and dependable electricity day in and day out. This is remarkable progress. However, we have to recognise that it is just the start. 1.8 Thermal and salinity gradient systems 1.8.1 Ocean thermal energy conversion – TRL 8 Ocean thermal energy conversion (OTEC) is unusual in that its first recorded presence is in fiction. In 1870, Jules Verne introduced the concept in Twenty Thousand Leagues Under the Sea: ‘I owe it all to the ocean; it produces electricity, and electricity gives heat, light, motion, and, in a word, life to the Nautilus.’ His idea – which relied upon utilising the different temperatures of water in the sea – was spot on. The potential for ocean thermal energy arises from the tem- perature difference between near-tropical surface seawater, which may be more than 20 C hotter than the temperature of bathyal (1,000 m) ocean water, which is relatively constant at about 4 C. Bringing large quantities of this cold seawater to the surface enables a heat exchange process with the warmer surface waters, from which energy can be extracted. By the 1880s American, French and Italian scientists are all believed to have been working on the concept. But it is the Frenchman, physicist Jacques-Arsene d’Arsonval, who is generally credited as the father of OTEC to create power. One of the students of D’Arsonval, Georges Claude, built the first working OTEC power plant in 1930 in Cuba, which produced 22 kW of electricity. This led to an onshore open cycle plant, with a pipe extending out to sea and he went on in 1935 to construct another plant, this time aboard a 10,000-ton cargo vessel moored off the coast of Brazil. Unfortunately, both plants were destroyed by weather before they could become operational and French research continued in earnest through the 1940s and into the 1950s. Research also began in California in the 1940s. In all cases, work was slowed or halted by cheaper alternatives to power generation. In the 1960s, J. Hilbert Anderson and his son James Anderson designed a closed-cycle OTEC power plant, aimed to be more practical, compact and eco- nomic. This cycle pumps warm surface water through heat exchangers to boil a working fluid into a vapour. The vapour expands to power turbines and drive A review of progress on ocean energies 11
- 37. generators. Cold water pumped from the deep ocean condenses the vapour back into its liquid state. As in the story of wave energy, the Arab oil embargo and skyrocketing oil prices in the mid-1970s drove away interest in the Andersons’ and other OTEC models. Today, with a much more visible threat posed by climate change, and with island communities threatened by rising sea levels, we see a renewed interest in OTEC with a handful of state-sponsored projects now taking place around the world. These include the following. Hawaii, USA In 2015 Hawaii became the first state in the USA to generate electricity using their 105-kW OTEC plant on Big Island. The plant cost was $5 million and was funded and developed through a collaboration between the U.S. Office of Naval Research and Hawaii’s Makai Ocean Engineering. Okinawa, Japan This 2013 scheme aims to combine power production from OTEC with the use of deep seawater for other uses to improve the economics of the system as a whole. The project has been built in the Okinawa Prefecture Deep Sea Water Research Institute and utilises an existing OTEC plant which has been expanded with the construction of post-OTEC seawater pipelines in 2017. The worldwide potential of ocean thermal power conversion has been con- servatively estimated at 44,000 TWh/year across the Tropics – however, the chal- lenge lies in realising this potential. This hurdle has been summarised neatly by Gérard C. Nihous, an OTEC expert at the University of Hawaii who said: ‘The technology is simple to understand but very difficult to implement in the field. There are engineering challenges, but most of the reasons for its incomplete development are economic.’ 1.8.2 Salinity gradient – TRL 4 The idea of exploiting the osmotic pressure which arises when freshwater and saltwater meet was first conceived in the 1970s. However, a quarter of a century had to pass before market conditions made several independent public and private enterprises take up the idea and start developing the technology further. Seawater is approximately 200 times more saline than freshwater, and this relatively high salinity establishes a chemical pressure potential with fresh river water, which can be used to generate electricity. The two leading methods for generating power from salinity gradients are pressure-retarded osmosis (PRO) and reverse electrodialysis (RED). Other tech- nologies such as hydrocratic generators, solar ponds (vapour compression) and osmotic heat engines have also been proposed. PRO uses a membrane to separate a concentrated salt solution (like seawater) from freshwater. The freshwater flows through a semipermeable membrane towards the seawater, which increases the pressure within the seawater chamber. A turbine is spun as the pressure is compensated and electricity is generated. 12 Renewable energy from the oceans
- 38. RED uses the transport of (salt) ions through membranes. RED consists of a stack of alternating cathode and anode exchanging permselective membranes. The compartments between the membranes are alternately filled with seawater and freshwater. The salinity gradient difference is the driving force in transporting ions that results in an electric potential, which is then converted to electricity. Two main applications exist: as stand-alone plants in estuaries where freshwater rivers run into the sea and as hybrid energy generation processes recovering energy from high-salinity waste streams, which could be, for example, brine from desali- nation or salt mining. A possible third application is salinity gradient technologies applied to land-based saltwater lakes or other types of saltwater reserves [23]. The total technical potential for salinity gradient power is estimated to be around 647 GW globally; however, despite this high theoretical potential, chal- lenges both in proving the technology and in finding a route to commercialisation means current salinity gradient schemes are currently on hold. Norwegian hydro utility Statkraft was an early champion of the sector and developed its first PRO power plant at Tofte on Hurum, near Oslo in 2009. It used 2,000 m2 of flat sheet membranes and could theoretically produce 10 kW, although the actual production was around 5 kW and powered an electric kettle for demon- stration purposes. Yet by 2013, Statkraft had decided to withdraw from the sector, suggesting that the technology would not be sufficiently developed to become competitive within other technologies. Making his assessment of the technology, Statkraft department manager Stein Erik Skilhagen said: We have proven that the technology works and have achieved substantial improvements through our efforts. Our main challenge has been to make the technology efficient enough to achieve energy production costs on par with competing technologies. With the current market conditions, we see that we cannot achieve this in the foreseeable future. There are other technologies which have developed enormously in recent years. These are more competitive and relevant investments for us in the future [24]. In 2013, Dutch firm REDstack officially opened 50-kW RED pilot, situated on the sea defence causeway, which separates relatively clean freshwater on one side from relatively clean seawater present in the Wadden Sea/North Sea [25]. This pro- ject is still operational, although has not published plans for any further development. 1.9 Offshore wind – TRL 8 Unique amongst the other forms of ocean energy explored in this book, offshore wind is already a highly successful competitive product, fully proven at TRL 8 and close to TRL 9. Compared to other forms of power, offshore wind remains a relative newcomer – but already this decade deployment of offshore wind has more than quintupled from A review of progress on ocean energies 13
- 39. 3.2 GW in 2010 to 18.7 GW in 2017 by which time it contributed some 56 TWh or 0.3 per cent of global electricity generation [26]. The key factor behind the rise of the offshore wind market is a concerted series of public/private initiatives undertaken by countries bordering the North Sea in Europe. More than 80 per cent of global offshore wind capacity is located in Europe, of which the United Kingdom with installed capacity of 6.8 GW and Germany with 5.4 GW are the two largest countries. Beyond Europe, only the People’s Republic of China has large-scale offshore wind capacity installed, at 2.7 GW, while smaller offshore wind facilities are located in the United States, Korea and Japan. Although it uses a fundamentally similar technology to onshore wind, offshore wind enjoys some distinctive advantages: the main ones are that offshore installa- tions are able to tap more consistent and higher winds speeds, and there are fewer restrictions on ground area and height. As a result, project sizes and turbines are typically larger and performance is higher. Costs relative to other forms of renewables are higher. However, recent energy auctions across Europe have seen offshore wind energy prices fall to below £58 per MWh [27]. And despite the vast resource potential that could be tapped with fixed-bottom configurations (i.e. turbines with foundations on the seabed), studies have found that 80 per cent of estimated offshore wind resources in Europe are located in waters at depths greater than 60 m, with the corresponding figure being 80 per cent in Japan and 60 per cent in the United States [28]. This has sparked a new wave of innovation – to site offshore wind farms in ever-deeper waters – with a growing appetite to surmount any technological chal- lenges. Already there is a vast supply chain engaged in the development of huge offshore wind parks, and the ever-present pressure to reduce cost and exploit new resources will continue to spur new advances. In Japan, they are successfully testing a number of floating turbine concepts at scale at the Fukushima Floating Offshore Wind Farm [29], whilst in late 2017 Statoil switched on the world’s first commercial floating wind farm – Hywind – off Peterhead in Scotland. This comprises five 6-MW floating turbines anchored by means of three suction anchors in 95–120 m of water. Industry voices expect we will see ever larger turbines – potentially 20 MW or more by the middle years of the next decade – and it is clear offshore wind is here to stay. 1.10 Marine solar Marine solar on the other hand is a new concept. It relies upon the open and unfettered expanse of water providing a flat and unshaded area to place floating solar panels. This technology may find a better home on lakes and reservoirs where the added benefit of reducing evaporation may strengthen water resources in arid areas. 14 Renewable energy from the oceans
- 40. It is a technology to watch and will clearly owe much to the technical devel- opments of terrestrial solar and its application is expected to centre in more equa- torial regions. 1.11 Enabling technologies and actions For the purposes of this opening chapter, I do not propose to go into detail in what I have termed ‘enabling technologies and actions’ each of which merits their own section within this book, including the following: ● offshore support structure design ● electrical power transmission and grid integration ● offshore energy storage ● multi-purpose platforms ● installation, operation and maintenance of offshore renewable. What I do observe, however, is that all of these topics – and they are major ones – involve what we do with ocean energy technologies at sea. How do we build structures that can withstand the extraordinary forces they will experience in winter storms? How do we harness new technologies such as High-Voltage Direct Current (HVDC) to bring power ashore? (Or can we, indeed store energy at sea?) Can we reduce cost through multi-purpose platforms, and how can we install, maintain and operate these new technologies in real ocean conditions? For someone starting off on their journey of creating energy from the oceans with a radical new idea, these enabling technologies and actions can appear far away from the early days spent in the laboratory. But as soon as a technology crosses the shoreline in earnest, and aims to go to sea, then these are the areas which present some of the biggest challenges we face. In order to operate at sea, each technology must be able to operate – and operate reliably – whilst experiencing forces far beyond the average sea state. Robustness and reliability are key, and they do not come cheap. So, whilst in the ocean the first challenge is to prove ‘it works’ the next challenge is to show it works reliably. Then we must show there is at least a route to making it work commercially – given whatever support mechanisms may be in place. This is a topic I will return to in the closing chapter – where themes such as ‘learning by doing’ and serial production will come to the fore. Meanwhile, dip in, and enjoy exploring the fascinating journey to realise renewable energy from our oceans. References [1] https://otcns.ca/news/2012-06-01/ocean-wave-energy-a-nova-scotia-first-, accessed on 5 March 2019. A review of progress on ocean energies 15
- 41. [2] ‘History of Hydropower’. U.S. Department of Energy, https://www.energy. gov/eere/water/history-hydropower accessed on 5 March 2019. [3] https://en.wikipedia.org/wiki/1973_oil_crisis, accessed on 5 March 2019. [4] https://www.iea.org/topics/renewables/wind/, accessed on 5 March 2019. [5] https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_ accordion1.html, accessed on 5 March 2019. [6] The Danish wind industry 1980–2010: Lessons for the British marine energy industry, Kyle Smith, International Journal of the Society for Underwater Technology, Vol. 30, No. 1, pp. 27–33, 2011. [7] https://www.worldenergy.org/wp-content/uploads/2017/03/WEResources_ Marine_2016.pdf, accessed on 5 March 2019. [8] EMEC. http://www.emec.org.uk/services/ [9] https://webgate.ec.europa.eu/maritimeforum/sites/maritimeforum/files/ OceanEnergyForum_Roadmap_Online_Version_08Nov2016.pdf, accessed on 5 March 2019. [10] https://ec.europa.eu/maritimeaffairs/policy/ocean_energy_en, accessed on 5 March 2019. [11] http://www.hie.co.uk/growth-sectors/energy/marine-energy.html, accessed on 5 March 2019. [12] Atlantic Marine Energy Test Site (AMETS) Economic Impact Case Study on EMEC in Orkney, December 2017. [13] WEC, 2016. https://www.worldenergy.org/wp-content/uploads/2016/10/World- Energy-Resources-Full-report-2016.10.03.pdf [14] https://phys.org/news/2018-05-tidal-range-power-potential-electricity.html#jCp, accessed on 5 March 2019. [15] https://www.supergen-marine.org.uk/sites/supergen-marine.org.uk/files/ publications/taiwan_2009_mct.pdf, accessed on 5 March 2019. [16] https://tethys.pnnl.gov/annex-iv-sites/enermar-project, accessed on 5 March 2019. [17] https://www.powerengineeringint.com/articles/print/volume-19/issue-1/features/ orkney-tidal-power-a-hotbed-on-the-seabed.html, accessed on 5 March 2019. [18] https://www.bloomberg.com/news/articles/2016-06-09/the-ex-con-inventor- disrupting-underwater-energy, accessed on 5 March 2019. [19] https://www.rpsgroup.com/images/UK/PDFs/MARITIME_TIDAL_161012. aspx, accessed on 3 February 2019. [20] https://simecatlantis.com/2018/07/11/meygen-phase-1a-operational-update/, accessed on 5 March 2019. [21] http://www.scotrenewables.com/technology-development/sr250, accessed on 5 March 2019. [22] http://www.scotrenewables.com/technology-development/sr2000, accessed on 5 March 2019. [23] http://www.irena.org/documentdownloads/publications/salinity_energy_v4_ web.pdf, accessed on 5 March 2019. [24] https://www.statkraft.com/media/news/News-archive/2013/Statkraft-halts- osmotic-power-investments/, accessed on 5 March 2019. 16 Renewable energy from the oceans
- 42. [25] http://www.redstack.nl/en/home, accessed on 5 March 2019. [26] https://www.iea.org/publications/freepublications/publication/WEO2017 Special_Report_OffshoreEnergyOutlook.pdf, accessed on 5 March 2019. [27] https://www.gov.uk/government/publications/contracts-for-difference-cfd- second-allocation-round-results, accessed on 5 March 2019. [28] Carbon Trust, 2015. https://www.carbontrust.com/resources/reports/technology/ floating-offshore-wind-market-technology-review/ [29] https://www.4coffshore.com/windfarms/project-dates-for-fukushima-floating- offshore-wind-farm-demonstration-project-(forward)-phase-2-jp13.html, accessed on 5 March 2019. A review of progress on ocean energies 17
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- 44. Chapter 2 Wave energy Gianmaria Sanninoa , Adriana Carilloa , Arne Voglerb , Giovanni Braccoc , Giuliana Mattiazzoc , Diego Vicinanzad , Pasquale Contestabilee , Domenico P. Coirof , Giancarlo Troiseg , Luca Castellinih , and John V. Ringwoodi Wave energy has an estimated global potential of 3.7 TW, almost double that of current world electrical energy consumption, and has, to date, remained virtually untapped [1]. Wave also has a relatively low correlation with, for example, wind and solar [2], meaning that a balance of wave with other renewable technologies can offer greater consistency of supply. Despite these attractive features, the drive to make wave energy economic has made relatively poor progress, due to a com- bination of the following factors: the ocean environment is hostile, wave energy is reciprocating (rather than unidirectional, therefore requiring rectification) and variable, and the technology is still in its relative infancy. These difficulties are manifest in the current unattractively high levelized cost of energy (LCoE) for wave energy, currently estimated at €225/MW h compared to the LCoE for off- shore wind, for example, at €165/MW h [3]. This chapter examines the various facets of wave energy, including resource quantification and wave measurement (Section 2.1), the wide variety of onshore and offshore wave energy devices (Section 2.2), the variety of power take-off (PTO) mechanisms which convert wave power into other useful forms (Section 2.3) and concluding with some insight to how wave energy devices are modelled and controlled (Section 2.4). a ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Develop- ment, Rome, Italy b Department of Marine Energy Research, University of the Highlands and Islands – Lews Castle College, Stornoway, Isle of Lewis, UK c Dipartimento di Ingegneria Meccanica e Aerospaziale, Politecnico di Torino, Torino, Italy d Inter-University National Consortium for Marine Sciences (CoNISMa), Italy e Department of Engineering, University of Campania Luigi Vanvitelli, Aversa (Caserta), Italy f Department of Industrial Engineering – Aerospace Section, University of Naples Federico II, Naples, Italy g SeaPower Scarl – Research Consortium, Naples, Italy h UMBRAGROUP S.p.A., Foligno, Italy i Centre for Ocean Energy Research, Maynooth University, Ireland
- 45. While this chapter can only give a limited overview of the challenges and some potential solutions to successfully harness wave energy, it belies the truly vast and diverse array of prototype wave energy converter (WEC) devices [4]. This diversity demonstrates the lack of technology convergence in WEC design and, combined with the multitude of PTO modalities described in Section 2.3, the potential variety of wave energy systems is truly broad. Nevertheless, from the overview and examples presented in this chapter, it is hoped that the reader will gain a better understanding of the challenges, and potential benefits, of adding wave energy to the current mix of commercial renewable energy technologies. 2.1 The wave resource 2.1.1 Wave resource assessment1 Waves are generated from the transfer of energy from wind blowing over the oceans and can travel thousands of kilometres from the generation area. During their propagation, the wave characteristics change and waves become progressively more regular and increase their wavelength, becoming the so-called swell waves. Due to the superposition of wind and swell waves, the distribution of wave heights does not coincide with that of winds. The highest latitudes both in the Northern and in the Southern Hemisphere are characterized by the largest values of wave height. Monthly mean values over 5 m are reached in both hemispheres during winter but, while there is a strong seasonality in the Northern hemisphere, values are more constant over the year in the Southern hemisphere [5]. Wave energy is proportional to the square of height and to the mean period. Due to the difference in frequency characteristics of wind waves and swell waves also the relative occurrence of the two types of waves has been investigated at a global scale [6]. Wave energy atlases have been created both on global scale and on particular regions (e.g. [1]). They are based on wave measurements obtained from buoys, satellite data and output from model simulations. In the last decades, satellite alti- meters have been launched and are providing significant wave height data covering the entire ocean surface at constantly increasing resolution. This data is com- plemented by a variety of wave models that are actually running over different domains and at different resolutions. These models simulate numerically the growth, propagation and decay of waves forced by surface winds. They produce homogeneous data over long periods of time and provide a series of wave variables permitting a detailed characterization of the waves. In situ wave measurements from wave buoys represent the reference for the validation of models, recording very high temporal resolution and good quality data. Even if a few centres exist worldwide that collect and maintain buoys networks, observations are sparse and the maintenance of instruments expensive. Time series from wave buoys describe wave climate only locally and often present large data gaps, caused by temporary failure or by routine maintenance operations. 1 Written by Gianmaria Sannino and Adriana Carillo. 20 Renewable energy from the oceans
- 46. 2.1.1.1 Methods Prior to the installation of a device in a specific site, the assessment of the wave energy resource has to be computed using available data. Due to the variability of waves, local climatology has to be computed over a period of at least a decade, considering seasonal and inter-annual mean values of the principal wave char- acteristics such as significant wave height, mean direction and period. Preliminary analysis can be based on large-scale models using data at inter- mediate and deep water. Then local wave conditions should be evaluated by means of high-resolution models directly validated at the site by ad hoc installed instruments. Surface elevation of waves can be represented as the sum of a large number of harmonic waves, statistically independent. Actually, most of the sea wave models are spectral and are based on the observation that sea surface is composed of a high number of random waves of different frequency and length, forced by the irregular wind. The models compute the propagation of the variance density spectrum E( f ) that is related to the variance of the sea surface elevation by the equation: hh2 i ¼ C 0 ð Þ ¼ ð1 0 E f ð Þdf (2.1) where E(f) is the Fourier transform of the autocovariance (C(t)) of the surface elevation, and t represents the time lag. The energy density spectrum can also be expressed as a function of the wave directions q, with E f ð Þ ¼ ð2p 0 E f ; q ð Þdq (2.2) The integral wave parameters can be expressed as the m-order moment of the energy density spectrum: mn ¼ ð1 0 f n E f ð Þdf (2.3) In particular, the significant wave height (in m) is defined HS ¼ 4 ffiffiffiffiffiffi m0 p (2.4) The peak period is the period corresponding to the maximum in the energy spec- trum and the mean wave energy period (in s) as Te ¼ m1 m0 (2.5) Wave energy flux is the rate of propagation of the energy density across a plane perpendicular to the propagation direction and extending from the surface down to the bottom. In the linear theory, wave energy flux is the product of the energy density times the group velocity. Wave energy 21
- 47. In deep water, the available wave energy flux per meter of wave crest in Watt can be expressed as J ¼ rg2 64p TeHs 2 (2.6) where g is the gravity acceleration, r the sea water density assumed to be r ¼ 1,025 kg/m3 . Energy matrices are one of the most useful means to synthetize the wave characteristics over a long time period in order to assess the productivity of a specific wave converter. All the data collected are divided into bins of equal period and height, and then the energy in each bin is computed using the previous formula. The productivity of a device in a specific site can be determined multiplying the distribution of occurrences of the different sea states as a function of Hs and Te and the power matrix of the device. 2.1.1.2 Mediterranean wave assessment Global wave energy atlases lack the spatial resolution required to correctly describe the wave energy distribution in small and semi-enclosed basins as the Mediterranean Sea. In such regions, wind waves dominate as the fetch is limited. Wave height and period show substantial spatial variations due to wind variability and to the presence of complex topography. In these regions, specific high-resolution wave models represent the most important tool to assess wave-energy distribution. Different wave energy climatologies have been realized for the Mediterranean Sea in the last years. They are based on data collected from the Italian Wave Buoys Network, operating since 1989 (e.g. [7]), or from model simulations (e.g. [8]). The wave energy climatology for the entire Mediterranean Sea presented in [9] covers the period 2001–10. The wave model used is a parallel version of WAM (WAve Model) Cycle 4.5.3 at a horizontal resolution of 1/16 [10]. The model has been forced with six-hourly wind fields obtained from ECMWF operational ana- lysis at 1/4 spatial resolution. The main integral wave parameters, wave height (Hs), mean wave period (Tm), significant wave period (Te) and mean direction (qm) have been collected over the entire model domain, every 3 h. An accurate validation of the wave parameters obtained from the model simulation has been performed against available buoys data, from the Italian Wave Buoy Network, managed by ISPRA; wave heights have also been compared against satellite radar altimeters data. Both the comparisons have shown very good statis- tical agreement. Maps of the available seasonal wave power flux per unit crest averaged over the entire 10 years of the Mediterranean simulation are shown in Figure 2.1. In the Mediterranean basin, the winter season is the most energetic, followed by autumn, while summer is characterized by very low values almost everywhere. The most productive area is located in the Western Mediterranean, between the Balearic Islands and the western coast of Sardinia where an average energy flux of around 24 kW/m is reached in a large area, during winter. 22 Renewable energy from the oceans
- 48. 46°N 44°N 42°N 40°N 38°N 36°N 34°N 32°N 30°N 46°N 44°N 42°N 40°N 38°N 36°N 34°N 32°N 30°N 46°N 44°N 42°N 40°N 38°N 36°N 34°N 32°N 30°N 0° 10°E 20°E Energy flux (kW/m) Spring Summer Winter 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 12 14 16 18 20 22 24 30°E 0° 10°E 20°E Energy flux (kW/m) 30°E 0° 10°E 20°E Energy flux (kW/m) 30°E Figure 2.1 Seasonal distribution of average power per unit crest in the Mediterranean Sea. Averages are calculated seasonally for the entire 10-year simulation Wave energy 23
- 49. Other areas characterized by high levels of wave energy in most of the seasons are the north-western coast of Sicily and the Central Mediterranean. Values of energy lower than 9 kW/m are observed in summer everywhere. 2.1.1.3 Wave assessment at Pantelleria Results from the model covering the entire Mediterranean Sea have shown that the Sicily Channel represents a promising area for wave-energy production. The area around the island of Pantelleria (located in the Sicily Channel) has been selected to perform a higher resolution study, which can be used in support for the installation of a specific device [11]. Data from the Mediterranean simulation performed using WAM have been used to laterally force a higher resolution version (1/120 corresponding to about 1 km) of the same model. The nested model grid is centred on the Island of Pan- telleria and has the same discretization of the directional wave energy density function of the coarse model, and the same surface forcing. The map of the energy averaged over the 10-year model is shown in Figure 2.2. The main contribution to wave energy comes from waves propagating from North-Northwest. The sheltering effect of the island is evident in the decrease of energy in the Southern coast. The distribution of wave energy among different sea states has been analysed in detail at a near-shore site on the northern coast. The scatter plot in Figure 2.3 represents the distribution of the yearly average energy in terms of Te and Hs, evaluated over the 10-year period. The contribution to the total energy given by individual sea states are lumped in 0.25 s wide intervals of Te and 0.25 m wide intervals of Hs. Rectangles are coloured according to the total energy computed over the 3-h sea states extracted from the model. Lines of constant power are drawn on the scatter plot. The maximum amount of energy is available for significant wave heights between 2 and 3 m and for mean periods between 6 and 7 s. 46°N 44°N 42°N 40°N 38°N 36°N 34°N 32°N 30°N 0 2 4 6 8 10 12 14 16 18 20 22 24 Autumn 0° 10°E 20°E Energy flux (kW/m) 30°E Figure 2.1 (Continued ) 24 Renewable energy from the oceans
- 50. 37°N 36°55'N 36°50'N 36°45'N 36°40'N 36°35'N 36°30'N 11°50'E 12°10'E Energy flux (kW/m) 0 1 2 3 4 5 6 7 8 9 10 12°E Figure 2.2 Wave energy flux per unit crest around Pantelleria, averaged over the entire high-resolution 10-year simulation Energy (kW h/m) 0 10 8 6 4 2 H s (m) 0 2 4 6 8 Mean period (s) 10 200 kW/m 100 kW/m 50 kW/m 20 kW/m 10 kW/m 5 kW/m 2 kW/m 12 500 1,000 1,500 2,000 Figure 2.3 Distribution of wave energy among sea states computed over the entire simulation period at the site indicated by a star in the previous figure
- 51. 2.1.1.4 Conclusions The Mediterranean is a semi-enclosed sea and it is characterized by lower values of wave energy with respect to the major oceans. Nevertheless, even under these con- ditions, the conversion of wave energy can represent an economically profitable resource if ad hoc designed energy converters are developed. In these lower energy areas, in fact, devices of reduced size can be more suitable to extract energy. To implement efficient wave energy conversion solutions, accurate high-resolution projections of the resource are essential, as well as devices specifically designed for the harvesting of energy in the range typical of the area. 2.1.2 Wave measurements2 To ensure both safe shipping and the integrity of coastal structures such as break- waters, ports and navigational aids, the observation and measurement of waves has always been an important consideration for seafarers, naval architects and coastal engineers. Initially based on visual observation only, which are limited in the number of distinguishable parameters and accuracy, advances in technology in the second half of the twentieth century have resulted in a range of sensors capable of recording and transmitting wave parameters with a considerably higher level of detail and within specified error boundaries. Early visual observations on waves and sea states were collated from the eighteenth century onwards, based on passage reports from ships’ logbooks, and these resulted in the production of sailing directions for coastal and ocean voyages, together with the so-called pilot charts showing seasonally and spatially discretized probability of occurrence of relevant wind and wave parameters. However, such reports were generally confined to standard shipping routes only, as often insuffi- cient data was available for areas less frequented by ships to allow establishing generically applicable sea state information. The development of improved wave-sensing capabilities for the shipping, engineering and meteorological science sectors experienced various step changes in recent decades. Visual observations were initially supplemented with rigidly mounted staff poles or lines marked at breakwaters to allow readings of the water level, but also of height and period from passing waves near the coastline. Although this method was later enhanced with the additional installation of electronic sensors such as capacitive or resistance probes to allow continuous logging of the waves without maintaining visual contact, the requirement to rigidly mount wave staff gauges on the seafloor or manmade structures with a pole extending to above the water surface generally allowed for shallow water installations only. This limitation was to some extend overcome with the introduction of submerged pressure sensors for wave measurements, where a clear relation of excursive pressure readings and the wave variable sea surface elevation could be observed. With an ability of pressure sensors to accurately register passing waves at considerably deeper water 2 Written by Arne Vogler. 26 Renewable energy from the oceans
- 52. than what was previously possible with staff gauges, a reduced ability to capture shorter period waves with an increased deployment depth was also observed. Significant advances in wave measurement technology were made in the 1960s with the use of accelerometers mounted in ships or floating buoys. Through the process of double integration, the accelerometer output can be converted to dis- tance, thus providing continuous readings of vertical wave displacement. Limita- tions to this system were found in the inertia and size of the carrier platform, which prevented accurate readings to be taken for shorter wavelengths and periods as in wind waves. This limitation was to some extent overcome by installing accel- erometers in smaller size buoys with a reduced mass, and thus providing an increased ability to follow the surface motion related to smaller short period waves. Another constraint in the case of floating buoys was due to the mooring connection, which limits the ability of a floating body to follow the sea surface freely. By deploying buoys free floating, which provides accurate wave data in a non- stationary way as the buoy moves around with the wind, wave and current drift, or more often by using a flexible and damped mooring configuration, this constraint was largely eliminated. A shared shortcoming of the methods described in the above section was the non-ability to capture wave directionality or multimodality for situations with waves transiting the sensors from more than one direction. An initial solution to resolving directional wave data was found by installing sensors in an array con- figuration, often triangular, which allowed the tracing of individual waves through the array, and subsequent calculation of direction and wave speed. By fitting out a buoy with multiple accelerometers along three perpendicularly aligned axes, two in the horizontal pane and the third one vertically aligned, a breakthrough was achieved in the 1980s, as it was now possible for the first time to gather wave directional data from a single instrument by combining horizontal and vertical motions [12]. Around a similar time, successful results on measuring wave spectra were also obtained from an acoustic sensor system utilizing the Doppler effect [13], and this has now become a standard feature of many acoustic Doppler current profilers (ADCPs) available on the market. Generally, the use of buoys and acoustic profilers to measure waves is confined to providing time series or time- averaged spectral data for single-spot locations. Additional modern wave-sensing technologies and approaches include the use of radar and satellite imagery to also allow continuous wave monitoring across wider spatial areas, and a more detailed introduction into the different sensor types is given in Section 2.1.2.2. 2.1.2.1 Wave characterization for energy applications To provide annual energy production estimates of WEC farms, a sound under- standing of the wave resource is imperative. Such understanding and estimates are an essential part of project design and development and are not only required to match and refine WEC technology against a local wave resource, but also to increase confidence in cash flow projections, thus supporting the investment case. In addition to the requirement for detailed long-term wave resource assessment during the project development stages, ongoing wave forecasting and monitoring is Wave energy 27
- 53. also important throughout the construction and operational phases of WEC farms. Offshore installation works such as lifting of heavy loads during anchor handling or tasks that involve crew transfer from support vessels onto individual WECs, e.g. to undertake electrical connections between WECs and offshore hubs, can only be safely undertaken during moderate or calm sea states and thus careful weather windowing is required. During operations, the constant tuning of energy converters to the impacting sea states allows an increased energy production by operating the WECs close to their resonance frequencies. A further improvement to the sea state dependent tuning of energy converters is suggested through the implementation of a fast adaptive control approach, based on an individual wave-by-wave input that combines instantaneous measured wave data a short distance up-wave of a WEC with a short-term wave forecasting algorithm [14,15]. Wave monitoring during the operational phase at an energy site is further required to implement mitigation and survival strategies during wave conditions exceeding the operational design win- dow of the deployed technology. The requirement to use measured wave data for the development of wave resource assessments is defined in standard IEC 62600-101:215 published by the International Electrotechnical Commission [16]. Although the primary standard tools for wave resource assessments are numerical spectral wave models such as SWAN or DHI Mike 21 SW, typically applied for a 10-year hindcasting period, access to measured data for a number of locations within the model domain is considered essential for the calibration and validation of model outputs. The rele- vant IEC standard prescribes that ‘all numerical modelling shall be validated using measured wave data. Whenever possible the numerical model output should be validated using data from one or more locations close to where wave energy con- verters might realistically be deployed’ [16, p. 16]. The same standard furthermore recommends measured field data be used covering a minimum of a consecutive year to avoid any seasonal bias in the model outputs. In the context of wave resource assessments, the importance of using measured wave data for model calibration is visualized in Figure 2.4 for an area off the Isle of Lewis, north-west Scotland. Shown is the distribution of significant wave height of a prospective WEC development site, based on a Mike21 spectral wave model, together with the locations of two acoustic wave and current profilers (AWAC 1 and AWAC 2) deployed at a spacing of 600 m between sensors. A considerable difference in wave heights between both sensor locations was found with an energy hotspot at location AWAC 1. By calibrating against the sensor data, it was possible to replicate the hotspot in the numerical model, thus increasing model confidence and applicability for the wider area. This example also highlights the importance to deploy multiple sensors at a development site during the resource assessment process to capture a wider range of localized phenomena. To this end, it is helpful to set up an initial numerical model to identify potential anomalies prior to the deployment of sensors. Once initial results are obtained from such a model, appropriate data acquisition locations can be selected to confirm and help improve initial model outputs. In addition to the two AWACs shown in Figure 2.4, addi- tional three-wave measurement buoys were deployed at the same time slightly 28 Renewable energy from the oceans
- 54. further offshore to help inform boundary conditions for the localized model, and also as additional calibration points for a larger model domain. Where the calibration of phase averaged wave models requires spectral wave data, i.e. data related to sea state parameters derived from the wave spectrum and generally with directional energy flux discretized across a range of frequencies, the analysis of statistical parameters from wave time series data is relevant to some other applications. Although standard wave models allow for extreme value ana- lysis including assessment of a 50-year return periods to assess probabilities of occurrence of severe sea states, model outputs are limited to the assessment of wave geometry, such as differences between back and front wave steepness, maximum individual wave heights or detection of rogue waves across the model domain. If the interest is on assessment of individual wave-by-wave parameters, this is best facilitated through directional time series data from in situ sensors followed by statistical processing. However, the decomposition of spectral data following theoretical models such as Jonswap [18] or [19] wave spectra offers an alternative route for such analysis if no time series data is available. An overview of different wave sensor types suitable for measurement of time series or spectral data at single spot locations or across wider domains is given in the following section. 2.1.2.2 Instrumentation for wave measurement Where the earliest method to parameterize sea states relied on visual observations by trained, often ship based, observers, a range of technological solutions is now available to provide vastly improved detail and accuracy. Although it is widely accepted that a visual observer is well able to estimate the significant wave height (Hs), defined as the average of the highest third of waves during an observation period, this method provides poor results with view to detection of wave period, Sign.wave height (m) AWAC 2 1 km AWAC 1 Above 3.00 2.80 – 3.00 2.60 – 2.80 2.30 – 2.60 2.10 – 2.30 1.90 – 2.10 1.70 – 1.90 1.50 – 1.70 1.30 – 1.50 1.00 – 1.30 0.85 – 1.00 0.65 – 0.85 0.45 – 0.65 0.20 – 0.45 0.00 – 0.20 Below 0.00 Figure 2.4 DHI Mike 21 SW model at 200 m mesh size. Close up of the AWAC location clearly confirms the energy hot spot 600 m SW of AWAC2 at AWAC1. An additional hotspot is visible 3.5 km to the NNE of the AWACs (adapted from [17]) Wave energy 29
- 55. Random documents with unrelated content Scribd suggests to you:
- 59. The Project Gutenberg eBook of Ariel Dances
- 60. This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online at www.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook. Title: Ariel Dances Author: Ethel Cook Eliot Release date: December 5, 2018 [eBook #58412] Language: English Credits: Produced by Stephen Hutcheson and the Online Distributed Proofreading Team at http://www.pgdp.net *** START OF THE PROJECT GUTENBERG EBOOK ARIEL DANCES ***
- 61. Ariel Dances by Ethel Cook Eliot Boston Little, Brown, and Company 1931 Copyright, 1931, BY ETHEL COOK ELIOT All rights reserved Published February, 1931 Reprinted February, 1931 (three times) PRINTED IN THE UNITED STATES OF AMERICA FOR MY MOTHER
- 62. 3 Ariel Dances
- 63. 4 Chapter I Ariel, quiet but alert, lay in her steamer chair, one of the most inconspicuous of the several hundred passengers the Bermuda was bringing to New York. No one would be likely to look at her twice or give her a second thought, as she crouched away from the March wind, insufficiently protected from the cold by her nondescript tweed coat, and carelessly, casually bare-headed. All about her on the deck were people of outstanding, vivid types. The thing that had impressed Ariel about these fellow passengers during the two days of the voyage was their apparent self-sufficiency,—a gay, bright assurance of their own significance, and the reasonableness, even the inevitableness, of their being what and where they were. The very children appeared to take it quite as a matter of course that they should come skimming over the Atlantic in a mammoth boat- hotel while they played their games, read their books and ate their meals,—just like that. Ariel took nothing as a matter of course, and she never had from the minute of earliest memory. Her proclivity to wonder and to delight was as organic as her proclivity to breathe. But now it was neither delight nor wonder but an aching suspense that quivered at the back of her mind. She thought, “If Father were here! If it weren’t alone, this adventure! New York Harbor at
- 64. 5 last! I—Ariel! But it isn’t real. There’s no substance. It was to have happened and been wonderful, but this is paler than our imagining of it. The shadow of our imagining. Oh, it’s I who have died and not Father. Where he is, whatever he is doing, it’s still real with him. With Father it would be always real,—alive.” A steward came up the deck, carrying rugs and a book for the woman who had occupied the chair next to Ariel’s during the two days’ voyage. Two children with their nurse trailed behind. Ariel’s glance barely touched the group and returned to New York’s terraced, dream- world sky line. But she was glad that these people had come up on deck and would be near her during the little while left of ship life. It did not matter that they would remain unaware of her until the very end. It was more interesting, being interested in them, than having them interested in her. And there was no reason on earth why they should be interested in her. It never entered Ariel’s head that there was. Joan Nevin, the woman, was tall, copper haired and eyelashed, and graceful with a lithe, body-conscious kind of gracefulness, of fashion, perhaps, more than of nature. Her sleek fur coat, her high-heeled, elegant pumps—even the close dark hat, flaring back from her copper eyebrows—these seemed to motivate her gait and her postures. She was, perhaps, more pliable to them than they to her. But Ariel did not mind this, although she realized it. It was wonderful, in its way, fascinating by strangeness. To tell the truth, Mrs. Nevin interested her more at the moment than the unknown, beautiful harbor at which she appeared to be gazing. And no aching longing for her father’s sharing of this interest could turn it
- 65. 6 dreamlike, for her father could never share it, alive or dead. Fashionable women, even at a distance, bored him. But how did a woman like that feel, Ariel wondered, about her so finished and catered-to beauty, and her easy self-sufficiency? And how did it feel to have two burnished, curled children that were one’s very own, to love, to live for, to play with? How wonderful if Ariel herself had had children of her own to play with and dance with on their beach, while her father was alive and she could still have gloried in them, before the sense of unreality had settled like a thin dust over unshared happiness! The Nevins and the nurse had come the length of the deck now, and were standing near her, but not taking their chairs, and oddly silent. Still, she would not look directly at them to discover the reason. If she looked into their faces she might become visible to them. So far, these two past days, Ariel had kept herself wrapped in a cloud of invisibility, she felt, merely by not meeting other eyes. She was shy of contacts, ever since her father’s death; and the aching, hurting suspense at the back of her mind, which was caused by dread of the near approaching meeting with her father’s friend, had only intensified her desire for invisibility. As for Mrs. Nevin, until this instant she had been nearly as unaware of Ariel as Ariel supposed her to be. She had looked at her once or twice in the beginning, to wonder whether it was a child, a girl or a woman who occupied the neighboring chair, but quickly decided that such speculation was waste of time since the one thing certain was that Ariel’s age didn’t matter, since she was obviously—nobody. From that decision she had returned to social obliviousness, lying back for hours at a time, wrapped up preciously by her eager cabin steward in
- 66. two fur-lined rugs, which could not have been hired for the passage but must be her own expensive property, following with absorption the fine print of a thick novel by some one named Aldous Huxley. Now and then she would lift languid but brilliant eyes and gaze for a while at the flying sea. That was all, for after the first half hour on board she had not thought it worth her while to waste that brilliant languid gaze on any other fellow- passenger more than on Ariel. But now she remained standing by Ariel’s chair, as though with some intention, and Ariel had finally to look up and meet, for the first time, in a direct exchange of glance, those brilliant, mahogany-colored eyes set wide apart under their strongly arched coppery brows, and it was, without doubt, a breathtaking moment. But it was the steward who was speaking, and his tone was seriously accusatory. “You are occupying the lady’s chair.” He was right. In the excitement of at last being almost in, so near the landing, Ariel had neglected to make sure of her own name—Ariel Clare—on the slip of pink cardboard stuck into the holder on the chair’s back. “I’m sorry,” she muttered, rose and was off like a bird. The steward’s eyelids just flickered as she brushed past him in exquisite, smooth flight. But the flicker was not because the steward had recognized that the nondescript, pale, young girl had turned exquisite with motion. He blinked merely because her decision to depart and the departure had been so strangely, almost weirdly, simultaneous. “Tuck it in at the foot more, please. Very well. That will do. Thank you.” Ariel, out by the deck rail, heard Mrs. Nevin’s low, but carrying voice directing and dismissing
- 67. 7 her eager slave. “It was unkind and perfectly needless,” she thought. “Any chair would have done her just as well for the next few minutes until we land. It doesn’t matter, though. I won’t care.” But she decided to go for a last time up to the sun deck. She could watch the boat docking from there just as well—better than from here—and discover her father’s friend among the crowds on the dock just as easily. She was through with deck chairs and pink cards and haughty neighbors, for this voyage, anyway. But she wished she could wipe out from her memory forever those brilliant, indifferent eyes. She found the sun deck surprisingly clear of passengers. The deck chairs there had been almost all gathered up and were now being stacked into corners to wait for the return voyage and new voyagers. Ariel crossed to the rail and began to search, eyes narrowed against the cold sunlight glinting from cold waves, for her father’s friend in the dark mass at the edge of the pier over there, which only now was beginning to show itself as separate individuals waiting for the docking of the Bermuda. “When I care so much that just a stranger scorns me and finds me in the way, how am I going to help caring terribly if the Weymans don’t like me?” she asked herself, baffled that by no act of will could she slow the beating of her excited heart or cool the fire she felt in her cheeks. “Hugh’s so tall I must soon make him out, if he’s really come to meet me. I’ll wave when he catches sight of me.... Forget myself.... Wave for Father.... Pretend it’s Father seeing Hugh after all these years, and not I. I will not be strange and shy.”
- 68. 8 She imagined her father in her place, leaning on the rail, —blond, blue-eyed, chuckling softly and searching with anticipatory eagerness for the high-held dark head of his friend which would stand out any minute now above the crowd of people. And Gregory Clare was so living, so vibrant with life and joy in life, that when the people on the pier, looking up, first caught sight of him, not a soul of them but would ask himself “Who’s that rather wonderful-looking person?” and an involuntary light, a contagion of life, would ripple answeringly in the lifted faces. The wind whipped a strand of Ariel’s hair smartingly across her eyes. She shut them against the pain for an instant, and when she opened them again her father had gone. She was alone. She was only herself now, shy, trivial, pale,—a worm that wondered about the impression she was going to make on her father’s friend and his family. And all the time there was New York’s sky line to glory in. Well, even though she was so mean a person, so little and mean in her hidden self, perhaps she could do something to improve the outward girl. She could at least put on her hat, stand straight—not flattened against the rail like a weak piece of straw in the wind,— hold her chin up—her chin that was like her father’s, pointed, but firm. She pulled out the hat from one of the pockets of the tweed coat, pushed her blown hair up under its brim and pulled it well down on her head. It was a notable hat, once well on, and whatever it did for the inner girl, it certainly changed the whole air of the outer, visible girl. It was French felt of an exceptionally fine quality, and green, the shade of Bermuda waters when they are stillest. Her father had bought it for her one day in St. George’s. He said he
- 69. 9 had got it for a song at a stupid sale. It was one of the very few hats of her life, as it happened, because her father thought hats in general ridiculous and more suitable for monkeys than for men and women. But this hat was different. He realized that, when he caught it from the corner of his eye, passing the shop window. It sang Ariel. And he had got it for a “song.” But not the feather that was tacked to the brim, ruffling jewel notes in the wind. That had dropped from a song, not been bought at all. He had picked it up on the beach almost at their door as he came back one afternoon, not many weeks ago, from what was to prove his last swim. No bird from which this feather could have dropped had ever been seen on the island, so far as any ornithologist knew. But here was the feather, in spite of that. It was magic, then. And it magic’d the hat. It pointed the fact that Ariel’s eyes, rather narrow, but nice friendly eyes, and free as the day from the malice that one sometimes detects even in the pleasantest children’s eyes, were as green as itself,—as green as Bermuda waters. Now those eyes had discerned one head that did top all the other heads on the approaching pier, and it very probably was Hugh’s. But she had decided last night, or early this morning—she had slept very little—that she would begin, at least, by calling him “Mr. Weyman.” For it was five years and a few months over since they had seen each other. His father too had died, since that far- away time, and he had left law school to become the support of his mother and younger brother and sister. At twenty-five, still a student without responsibilities, when they had entertained him at the studio, he had seemed a boy. But at thirty now, and having, as she had, encountered death, could he be the same at all, any more than she was the same fourteen-year-old girl that he must be remembering? She thought not; and
- 70. 10 whether she was shaking with chill from the March wind or from apprehension of change in her father’s friend, she did not know. But she was shaking, miserably, and a strand of hair had escaped again and was stinging her eyes.
- 71. Chapter II He had been in Bermuda that time for part of his Christmas holidays, along with his mother and young sister. But the mother and sister had never appeared on the Clares’ beach, never come with Hugh to the studio. Hugh’s own arrival there was the merest accident. One mid-morning he came pushing his rented bicycle across the fields to their beach, which he had glimpsed from a high spot on the road to St. George’s, intending a solitary swim in the shadow of their rocks. Only he did not know that they were their rocks or that there was a house at all, hidden away on the slope of purple cedars. He passed within a few yards of the studio, without sensing its presence, and went coolly down to the beach with the intention of undressing for his swim in the very seclusion where Gregory Clare was at the moment in the middle of painting a picture. The artist, hearing the careless approach to the sacred privacy of his working place, rose wrathfully to drive the intruder away. But it turned out that he did not resume his brushes and his palette again until he had joined the young man in a noon-hour swim in the emerald waters. For Hugh had succeeded in doing more that morning than blunder on to private property and interrupt the creation of a picture; he had blundered into a friendship with Gregory Clare, the artist, Ariel’s father.
- 72. 11 12 The sudden friend knew next to nothing about painting. That was evidenced by his awkward silences once he had come into the studio and stood looking with unconcealed bewilderment at the dozens of canvases stacked around the walls and against the chairs and tables. But the young man’s ignorance did not hinder Gregory Clare from talking art to him. He dragged forward the canvases, one after another, making rapid and brilliant criticisms of them himself in the face of Hugh’s blank silences, propounding exactly what it was that made each picture’s strength or weakness in its stab at beauty. And all the while Hugh looked from the artist to his paintings and listened, dark head slightly bent, but with a hawklike alertness in its poise that gave Clare, and even Ariel, watching, a sense of balanced keenness. Ariel and her father prepared the studio meals by turns, and this day of Hugh’s appearance happened to be Ariel’s day as cook. Hugh was more articulate about food, it soon transpired, than about art, and had intelligent praise for pungent soup and crisp salad. But though that was what he was at ease about and could speak of, his real interest was, Ariel saw, all in Gregory Clare and his rushing passionate talk concerning the paintings. He seemed scarcely conscious of Ariel, the lanky young girl in a faded green smock, with hair a pale wave on her shoulders, who had cooked the luncheon and soon so quietly cleared the table and then disappeared, dissolving, so far as he was concerned, perhaps, into the white, hot Bermuda afternoon. She knew that he was glad to be left alone with her wonderful father. After that, for the remaining days of his vacation on the island, Hugh was constantly at the studio. He must have
- 73. entirely deserted his mother and sister, and he never bothered to speak of them again, after his first mention of the fact that there were such persons with him at the hotel in Hamilton. Even the morning that his boat was to sail he appeared at the studio, inviting himself to breakfast with the Clares, in spite of having had a farewell dinner with them the night before. And that morning, at last, he commented on Gregory Clare’s work, or at least on one of his canvases. It was time for him to go, they had told him, if he was to make his boat; but he delayed. And suddenly, in an embarrassed manner he turned back from the door, when they really thought he was off, and standing in front of an easel with a just finished painting on it blurted, “I really like this one, ‘Noon,’ the best of the lot, Clare, if you don’t mind my saying so. It’s the light that makes it so extraordinary, isn’t it? It beats out on you. Makes you squint. It’s the first time I ever saw light, or even felt it; I’m sure of that. Your picture has taught me what the sun hasn’t!” He laughed, self-depreciatively, and added almost defiantly, “It’s great stuff, I think!” Ariel’s father said nothing. He stood by the table in the wide window where they had just breakfasted, jingling some coin in the pockets of his white duck trousers, and kept a smiling silence. Ariel wanted to cry, “Oh, do go; hurry, Hugh, now, or you’ll miss your boat!” But Hugh seemed to be waiting for something, wanting to say more, and she kept still. After a minute he got it out, “I’d like awfully to take this picture home with me, Clare. Now. I’ve written out a check for a thousand dollars—did it last night—just on the chance you’d sell. I don’t know anything, of course, about the prices you put on your stuff. But this is exactly one quarter of my year’s allowance, and all the actual cash I can put my
- 74. 13 hands on now. If you will sell, and the price is higher— and you can wait for the rest—” Hugh was not looking at the artist or at Ariel or even at the picture by this time. His abashed gaze was toward the sea, while he waited for Gregory Clare to answer. The painting was the one that Hugh’s intrusion on their beach had interrupted. It was a bit of a corner of the beach seen at high noon. Everything was sun-stilled, even the water, except for the figure of Ariel herself, who was dancing in the violet heat-glow above the rocks. But although it was Clare’s daughter, the artist had not seen her as human, since he placed her dancing feet on air, not earth. And the faded smock— the smock she was wearing the day Hugh had first come to the studio—in the painting had found its vanished color at the same time that the hot sunlight struck all color from her partly averted face. Gregory Clare might have called this painting “Ariel Dances,” but instead he called it “Noon.” And it was Noon, actually. Ariel was only the heart-pulse at the center of the otherwise still, white light. But one thousand dollars! The listening girl was stunned, strangely taken aback. Her father, however, did not show even surprise. He merely chuckled and jingled the coins in his pockets like music. “I congratulate you, Hugh,” he murmured, after a minute. “You show your taste. ‘Noon’ is my best, quite easily my best, so far. I’m awfully glad that you see it. I’ve felt all along, though, that you were seeing an awful lot, really. And to sacrifice one fourth of your year’s income to beauty won’t hurt you. Indeed, it might very well happen to save your soul. Even so, I advise you to
- 75. 14 take more time. Think it over. Write me. I can always ship you the thing. I won’t part with it for less than the thousand, though.” But the fledgling art connoisseur was not to be put off. Until now he had been in regard to the studio, the people in it, and the paintings, the soaring, silent hawk. This, however, was his instant of darting and seizing. He had carried ‘Noon’ off with him, under his arm, unwrapped, and made the boat without a second to lose. And amazingly soon thereafter Gregory Clare and his daughter had got themselves to Europe, which meant Paris; and once in Paris, Gregory swept Ariel straight to the Louvre, where she sat or promenaded with him as long as Hugh’s thousand dollars lasted, gazing on cold, dim old pictures, but with her father’s warm, vibrant artist’s hand often on hers. It had been Ariel’s one adventure beyond Bermuda, until this present adventure: alone, and her father dead. Hugh had never come back to Bermuda and his letters were infrequent. Gregory Clare’s own letters were, from the beginning, almost non-existent, because that was his casual way with friends. One of Hugh’s first letters told them of the sudden death of his father, and that Hugh’s plan for making himself a lawyer was frustrated by the necessity of getting as quickly as was possible into his father’s niche in the business world. But Hugh did not use the term “frustration,” and there was, indeed, no touch of bitterness in the communication. The hint of a real grief was there, and a suggestion, somehow, that his father could not have been so exceptional in business capacity as in personality and character, since at the time of his death he had pretty well gone through his inheritance and was leaving his family little but a name. The name, however, was not
- 76. 15 clouded by his purely financial inability and was now of invaluable assistance to Hugh, who was being quite spoiled—according to his own account—by Wall Street associates of his father who had taken him into a big bond house on a floor several stories removed from the bottom. After that the studio heard from Hugh Weyman, bond salesman, at longer and longer intervals. Clare was afraid that his friend was absorbed by business, a dire calamity to befall a young man who had once been rejoiced to spend one fourth of his year’s income on the pigment splashed on a four foot by three foot bit of canvas. And now, for a year past, no word of any sort had come from Hugh, until the morning of the artist’s death. And although her father seemed actually to have held his death at bay those last few days, merely in the hope of that last letter, he did not show it to Ariel. But he explained to her, faintly and with an odd, smiling satisfaction, after he had read it to himself, and she had carefully burned it under his direction in the studio fireplace, that it was an answer to a letter from himself written within the week. His letter had told Hugh that he was near death, and asked him to invite Ariel to visit the Weymans for the latter part of the winter, while Charlie Frye, a young disciple of Clare’s, who had spent the last few months in Bermuda working with him, was arranging for an exhibition and sale of Clare’s paintings in New York. Ariel was being left only a very few hundred dollars, but the sale of the pictures ought to carry her through any number of farther years, until, in any case, she should either have married or have prepared herself for some profession. Their doctor, here in Bermuda, would be Ariel’s actual guardian in law. Charlie Frye would be her
- 77. 16 business manager in a practical sense. Would Hugh make himself her host and friend for the coming difficult period? Neither the kindly doctor, nor the young and enthusiastic Frye seemed to Clare quite the man to do precisely this for his girl. That was the substance of the artist’s letter as told to Ariel, and Hugh’s reply had been an instant promise to receive Ariel and with his mother’s help do anything for her that was in his power. Gregory could rely on his friend. Only, the doctor must keep him informed of his patient’s health, and it had better be the doctor who should arrange for Ariel’s coming to New York if the end that Clare had prophesied did transpire. That was the substance of Hugh’s letter. And Gregory Clare had finished explaining it all to Ariel as she stood watching the last scraps of it curl into charred blackness in the grate. “You mustn’t worry, darling,” he gasped, when her silence had become prolonged, “for when you remember that the only picture I ever even thought of selling brought us one thousand dollars ... and now there are two hundred of them soon to be up for sale in New York ... where there’s so much wealth ... I’ve marked those Charlie’s to drown out beyond the reef to- morrow—the ones that aren’t really good enough, you know—and it leaves, even at that, two hundred pictures. Suppose they only bring half the price of the first one each.... Why, even that is wealth, my dear....” “Oh, don’t, Father! What does it matter?” She was dismayed that his last strength was being given to such trivialities.
- 78. 17 But he struggled on, with harshly drawn breaths. “Funny why I’m trusting you to Hugh, beyond every one else! I suppose it’s because he saw that ‘Noon’ was the best of the lot.... He did see, remember? And he sacrificed something for that seeing. A quarter of his income, wise boy! He understood ‘Noon’—so he’ll understand you, Ariel, darling, my dearest—sweetest. He may have changed, but hardly so much—for ‘... Fortunate they Who, though once only and then far away, Have heard her massive sandal set on stone.’ Beauty’s sandal, that was. Do you remember the sonnet? Well—Hugh’s one of those Fortunate.... I’ve never seen in any one else’s face what I saw in his that morning when he stood, looking at ‘Noon’ and saying it showed him what the sun hadn’t....” “Oh, Father! Hush! Don’t try to speak any more. Rest!” Ariel was kneeling by his bed, pressing his hands, hot with her tears for all their waning life, against her cheeks. “Everything will be all right. There is nothing, nothing at all to worry about. Only never forget me. Don’t go so far that you forget me. Don’t go far. Not far....” He understood all that she meant, all that was beyond saying, and he promised with a gesture never to let death’s freedom intrigue him into adventure that would leave the memory and the love of his girl out. But he looked over her head at the doctor who had been standing all these minutes in the window, and the doctor nodded. The nod seemed a signal for something the two men had previously agreed on, as it was. And Gregory Clare, acting on the signal, which had come
- 79. 18 finally and at last, said to Ariel in the voice of authority which he so seldom had used during their life together, “Now, beloved, it is time you went away. Go down to the beach, please. Give my love and my farewell to the light, to earth light, and to our beach. I shall be gone when you come back, and you are not to see me die.” Ariel rose to obey. There was no question about obedience for it was the voice of Death itself which had commanded her. But at the door her father spoke again, and she had thought never to hear him speak again, and it was the voice of Life. “No— No. I was wrong. We made a mistake, Doctor. A woman is bound to have plenty to do with pain—before she’s through. I think, Ariel, we’ll have this pain together.... If you like—darling. I won’t send you out of it. Doctor, I want to be with my girl when she bears her first anguish—which will be my agony, as it happens. It’s yourself, Friend, I want away. No more need of you till it’s over. Ariel will help me. Your arm under my shoulder, dear. That’s—that’s—right....” But he had not sent the doctor with his love and his farewell to their beach and the earth light, for not every one can take such a message, and Ariel would do it later. The doctor sat down in the loggia, within hearing if Ariel should cry out for him. He smoked cigarettes for an hour, throwing their stubs angrily one after another out into the roses, and did not approve; for Ariel seemed only a child to him, and this was terrible. Perhaps she had been a child when he, the doctor, had been made to leave her face to face with physical agony and final death in the studio. But when, at last, he saw her coming out into the strong white sunlight and knew that she brought with her the stark word he waited, she was
- 80. 19 a woman. The doctor would have been blind not to have recognized the mark of that maturity on her face. And this forced and sudden growth had happened to the girl because of her father’s colossal selfishness, he believed, stumbling forward to his feet and reaching both his hands for hers. But when they were close in his, those young, live hands, the doctor knew nothing for certain any more about the business; it might be imagination in Clare—colossal imagination—that had made him act so, not a grain of selfishness in it. For to his amazed relief the slight hands he held were steadier, stronger, at the moment, than his own.
- 81. 20 Chapter III She would certainly call him Mr. Weyman, not Hugh. And the first thing she would say would be a “thank you” for his invitation to visit him; for she had not written the note of acceptance herself but left it to Doctor Hazzard. And now she thought that if only she had written herself, it would somehow have prepared the way better for the instant, almost reached now, when the boat would be close enough to the pier for the tall man to discern her, to meet her eyes, and for her to wave a greeting. And then, suddenly, she woke to the fact that that was not Hugh at all. The sun on the water had dazzled her. It was an older man, heavily bearded, foreign looking. He was taller, and certainly much broader than Hugh would ever be. She had never seen any one, except perhaps her father, stand out from a crowd as this man was standing out from it. Even from a distance his personality had reached her, impressed itself, and this had nothing to do with his unusual bulk and height. No, it was personality, bodiless, that reached across the water, and absorbed her attention. The big man had pushed his way through the crowd and soon stood right out at the edge of the pier, his head thrown back, eagerly scanning the Bermuda’s
- 82. decks. Then, as the ship sidled a few yards nearer, he raised his big, long arms straight above his head in sudden cyclonic greeting, and laughed up a big laugh of gleaming white teeth almost into Ariel’s face. But it couldn’t be herself he was so ardently saluting, and she turned quickly to see who was near her, here on the sun deck. It was Mrs. Nevin again. She was there, with her children, almost at Ariel’s shoulder. And she was smiling down at the bearded man. But the children were looking at Ariel. She had so plainly refrained from inviting their acquaintance during the voyage that they had not once tried to force a contact. She had seemed to their sensitive child perceptions to be out with the flying fish and the dip of the waves, more than in her steamer chair beside their mother, for that was where her gaze had lived. But the small green feather, which fluttered its down incessantly against the brim of her hat, had all the while had a life, they felt, quite apart from its wearer’s. It had been a veritable fairy flag, waving recognition and good will to them whenever their play brought them near. And now Ariel had turned so quickly that she had caught the children’s glances of camaraderie with the feather. And suddenly she took in their magic, realized it, as they had from the very first recognized and taken in the magic of the feather her father had found and given her. She was aware of the children—really aware—at last. That was all that it needed. They saw her face lose its abstraction, come as alive as the wind-dancing feather. Ariel’s eyes and lips smiled. Everything went golden. The children’s hearts fluttered as though they were magic feathers.
- 83. 21 But even now when Ariel’s smile had taught them all that there was to know about her the children did not rush upon her. They came slowly, with sensitive delicacy, as children will,—but for all the delicacy, with an air of deep, almost frightening assurance. Each child, taking one of Ariel’s cold, ungloved hands, pressed close. “We’ll be in, in another minute,” Ariel faltered, tremulously and almost beneath her breath, as if to warn them of the unreasonableness of this sudden, overwhelming intimacy which must be lost almost as soon as consummated. “Look. There goes the gangplank. And there’s some one—some one I know.” Suddenly, and when she had really forgotten his very existence, she had seen Hugh. To her relief this first sight assured her that he had not changed in the five years. He was the same Hugh, her father’s eager, quiet friend of the hawklike dark head, poised, alert, on shoulders that for all their breadth had an indefinable air of elegance about them. In his darkness and poise he was in direct contrast to the blond-bearded person gesticulating to Mrs. Nevin. Hugh stood beside this giant, looking up at the decks of the Bermuda as he was looking up, but with a difference. Without excitement, but rapidly, his eyes were traveling along the tiers of decks and the bending faces. In another minute he would get to the last deck and find what he sought, Ariel. Their eyes would meet and in the meeting remember everything of that sunlit week of five years ago. Under one arm she saw that he was carrying, tucked there as though it might be any ordinary parcel, a big bunch of English violets. They were for her, of course. So why had she ever been shy, afraid? She had forgotten the children and was bending
- 84. 22 forward over the rail, waiting with genuine gayety now the moment of his recognition. But just before his glance, in its methodical journey, came to her deck, she had her first sense of change in him. After all, he was different, a little, from the Bermuda days. There was a moody hunger in his eyes, and something gaunt, unfed, in the face that she had remembered only as keen, without shadows. But his face would light up in the old way when he discovered her. This might be his look when alone and unaware of friends near. The light, however, when it came, was not for Ariel. It was Mrs. Nevin his searching glance was halted by, and the glory that transfigured the dark, uplifted face took away Ariel’s breath. Mrs. Nevin laughed down a greeting, and murmured above her breath, so that Ariel caught the words, “Now how’d he know I was coming?” It flashed through Ariel’s mind that much reading of Aldous Huxley during the voyage, if that was the author’s name, must have dulled Mrs. Nevin’s perceptions, if she did not see that it had needed surprise as well as joy, so to shatter Hugh’s reserve. Mrs. Nevin called to her children, who still pressed against Ariel, holding her hands, “There’s Uncle Hugh, darlings. Wave to him. See, he has found us. Isn’t it nice of him to meet our boat!” Hugh returned the children’s obedient salutes, but the light was gone. Was it merely habitual reserve returning to duty, or had the sudden delight really as suddenly died? Ariel knew instantly and intuitively that these
- 85. 23 children were not related to Hugh, although Mrs. Nevin had called him uncle. Now he had to see herself, wedged in between the children. She tried to smile down at him, to help him to his recognition, but her lips were as cold as the wind in her face. She could not smile. His glance was passing her by as casually as it had passed a hundred other bending faces above the deck rails. After a little farther search it returned to Mrs. Nevin who bent forward, held out her gloved hands, and called down, “Toss, Hugh! Toss! I can catch!”—laughing. For just an instant Hugh appeared puzzled. Then he remembered the violets jammed under his arm, and tossed them up to the waiting hands. It was an expert toss, and Ariel remembered how her father had once drawn her attention to the fact that all Hugh’s motions were expert, effective. The smell of the violets, so near now, was dizzying her with nostalgia. She wanted to cry out, “They are mine, not yours. He brought them for me. He never even knew you were on the boat!” But instead, she loosened the children’s hands from hers and turned her back to the pier. Through the darkness of tears she moved away toward the stairs, with the intention of making sure that her baggage had left her stateroom. It would be time enough to identify herself to Hugh, who had forgotten her, when she came off the ship. She was almost the last person down the gangway. Hugh was there at the foot, looking anxious, for he had begun to be afraid he had missed Ariel Clare in the disembarking crowd. But even when she stopped by him and with head back, so that he might see her face plainly under the brim of her green hat, said, “I’m Ariel, Mr. Weyman. It’s kind of you to have me and to meet me,” he looked doubtful.
- 86. 24 “You!” he murmured, obviously taken aback and surprised. “Why, I thought you were the twins’ nurse!” But even as he spoke he saw that it was indeed Ariel, standing with the look that she used to wear sometimes before vanishing away into hot, white sunlight, years and years ago when he was young and she was an unreal fairy creature, hovering almost unnoticed somewhere on the edges of his first deep experience of friendship. Of course this was she; how hadn’t he known? “But the twins were clinging to you like burrs, weren’t they!” he insisted, explaining his stupidity. “It looked, you know, as if you belonged, body and soul, to Persis and Nicky. But of course it’s you.” Yet even now when he was at last shaking hands with her Hugh was looking over her head at a group of people a few yards away, with Mrs. Nevin at its center. The big man, the foreign-looking, bearded personage who had come to meet Mrs. Nevin, was beside her, his hand on her arm. He was possessive in his bearing, and openly exuberant that the lady had landed and was for the moment, at least, under his protection. And now a great sheaf of yellow roses in Mrs. Nevin’s arms quite obscured the violets, if, indeed, she still had them. Ariel was conscious that Hugh returned his attention to herself with an almost painful effort. “Your luggage will be under C,” he unnecessarily informed her, and then added with a sudden access of responsibility, “This is the way. We’ll do our best to speed things up in spite of the unlucky popularity of your letter. We’ll grab tea somewhere then, and get right along to Wild Acres, where Mother and Anne are waiting for us. They would have come in to meet you with me—Anne would, anyway—but we’ve got another visitor with us—Prescott Enderly, the novelist. Know his
- 87. 25 stuff?” And all the while he was skillfully guiding her through a milling crowd of over-anxious people.
- 88. 26 Chapter IV The younger Weymans had been skiing most of that afternoon with their guest, Prescott Enderly. Although Enderly was Glenn Weyman’s intimate at Yale and only a year or so older, he was a novelist of some notoriety. He had written only one novel, it is true, but during the past summer—the book was published in the spring—it had skyrocketed to fame. Its publishers described it in their advertising as an honest and fearless description of the private life of almost any averagely intelligent college man. Its author was now—except for the necessity of doing some classwork if he were to graduate this year, and taking time out for being a lion— working on a second novel. It was late in the afternoon when they returned home from their skiing in the snowy country around the Weymans’ estate on the Hudson. Glenn went up to his room to lounge and read until dinner time, but Anne staggered with an exaggerated air of fatigue into the library, and Enderly followed her. A fire, recently lighted, blazed its invitation from the far end of the long room, and although it was not yet quite dark outside, the heavy velvet curtains had already been drawn across the windows and several table lamps were glowing through rich, soft-colored shades. Enderly, without asking Anne’s leave, went the round of the lamps,
- 89. turning off their lights. But even without the lamps the freshly lighted fire kept the room alive and awake. Anne threw herself into the exact center of the deep divan which was drawn up before the fireplace, and Enderly, without hesitation or a word, settled himself close at her side. She leaned her head against the back of the divan, shut her eyes, and murmured “Hello. Where’d you come from?” as though already half asleep. Her voice was oddly, boyishly deep, but with a slight catch in it which turned it thrillingly feminine. Enderly liked Anne’s voice: it was the thing that had attracted him to her in the beginning, when he had met her at a house party in New Haven. “Why, I’ve been tobogganing, darling.” “So’ve I. Funny. There was a creature along with us,— name of Prescott Enderly. Thinks he’s a novelist and quite important, you know. Perhaps he can write, but he’s not so good in the snow.” “Really? Well, darling, you are magnificent in the snow, so it doesn’t matter about me. You were a gorgeous red bird, always flying somewhere ahead in the face of a dead, white world. Beautiful!” Anne opened her eyes and glanced down at her flannel skirt, ruby in the firelight. “But yesterday, Pressy, you insisted I was a flame. I’d really rather be a flame than a bird. Aren’t I more a flame? Say, ‘yes’!” He laid his hand over her two hands which were clasped on her crossed knees. But he laid it casually, looking into the fire. Her eyelids flickered at the contact, but her hands did not stir or tremble. “You’re a flame in the
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