The A to Z of the Energy Transition: S is for Storage
When we moved into our home, over a decade ago, I discovered that the previous owners had left a sack of coal in the garage, which I duly noted and left it hidden in a corner. I didn't return to it until tidying up the garage a couple of years ago. Unsure of how to dispose of the coal, I decided to burn it in our living room fire (please forgive me - it wasn't that much!). Beyond observing the different heating effect of coal versus wood, I was struck by long how a sack of coal can store its energy.
As we've transitioned to other forms of energy: oil, gas and electrons, storing energy has become increasingly difficult. Coal can be piled high and easily moved around by ship, train or a human with a shovel, and stored in a damp garage for a decade or more. Oil is a bit trickier. It needs to be contained in a tank or pipe and through time will release its more volatile components to the atmosphere and is more likely to ignite. Gas is even harder to store, due to its low density it needs to be stored in underground caverns, compressed in pipelines (line pack) or chilled to -162°C to form LNG. And as for those pesky electrons, well they are a real pain to store at scale and over long time periods!
"But what happens when the sun doesn't shine and the wind doesn't blow?"
I don't know how often I've heard the above phrase (often from those who have a particular agenda against renewables!), but with the increasing deployment of variable wind and solar, the need for energy storage is growing more rapidly than ever.
At relatively low levels of renewable deployment very little, if any, storage is required. As renewable penetration reaches around 50%, like in the UK, (depending on the overall mix of the grid) storage becomes far more important and it gets exponentially more critical as grids get closer to 100% renewable generation - particularly where there are large inter-seasonal variations, such as Northern Europe. Beyond just balancing supply and demand, storage may also provide other important services including frequency response and black-start capability.
I'll focus this edition on three key areas:
Battery storage
Pumped hydro
Long-duration energy storage (LDES): thermal, hydrogen and other novel technologies
1. Battery storage
Historically, lead-acid batteries dominated the world of rechargeable batteries. Relatively cheap, these were what powered thousands of milk floats on the UK's streets up until the 1980s. Lead-acid batteries are still widely used in the 12 Volt batteries used to power the electronics in most cars (and starter motors in petrol and diesel cars), but as anyone knows who's tried to lift one, they are incredibly heavy - which limited their prospects of being used in EVs, at least since 100+ years ago.
Arguably the largest breakthrough in battery technology over the last five decades is lithium ion. An ExxonMobil scientist, M Stanley Whittingham is credited with developing the first lithium-ion battery in 1972, with the first commercial applications in the 1990s in devices such as camcorders. Today lithium ion batteries dominate nearly all consumer electronics, from your phone, to your laptop and increasingly your car. Lithium-ion offers high energy density (making batteries lighter), high cycle life, high power output and input (for charging). The ubiquitous and modular nature of lithium-ion batteries has helped dramatically drive down costs - massively. As a relatively small proportion of the cost of a phone, the cost of batteries didn't matter too much. Where it really mattered was where batteries were used for EVs or stationary storage. As the chart shown below from BloombergNEF, prices have fallen by around 90% in a decade. It is reported that Chinese EV manufactures have now reached costs below $100/kWh.
Despite their advantages and falling costs, lithium-ion utility-scale batteries still suffer limitations, such as short duration (typically up to four hours), with some degradation over time, and supply chain concerns related to lithium and cobalt (which I covered in L is for Lithium and Other Critical Minerals). Whilst these factors may evolve, there are other chemistry developments which may be suited for stationary longer-duration applications, such as cheaper sodium-ion and redox flow batteries. A flow battery stores its energy in a liquid electrolyte - so in simple terms, to store more you just need a bigger tank and more electrolyte. Ultimately, different battery chemistries are a balance between weight, capacity, recharge time, cost and longevity. No doubt advances will be made across many different types of battery chemistry in the coming years.
I talked about recycling of battery materials in L is for Lithium and Other Critical Minerals. However, long before batteries are taken apart for recycling it's going to make a lot of sense to reuse them. Once degraded beyond practical use in an EV (or perhaps the increasingly likely outcome that the rest of the car reaches end of life), those batteries are likely to repurposed for stationary storage. AUDI AG and RWE have already put this test, building a storage site from 60 used EV batteries in Germany Second life for EV batteries: Audi and RWE build new type of energy storage system.
FWIW the Energy Institute recently issued the following technical guidance paper on the decommissioning and recycling of batteries: https://lnkd.in/p/ebNSXwtU
I'll be writing more on how we use batteries still attached to their four (or more) wheels as part of the solution to balancing the grid in a future edition. After all, around 80-90% of currently produced batteries are going into cars rather than stationary storage.
2. Pumped hydro
Pumped hydro energy storage is the world’s most established and widely deployed form of large-scale electricity storage - although battery storage is catching up fast. According to the International Hydropower Association (IHA) there is around 200 GW of installed pumped hydro capacity, versus 126 GW of utility-scale battery storage capacity (Energy Institute Statistical Review of World Energy).
Pumped hydro operates on a simple principle: using surplus electricity (often from renewables or off-peak generation) to pump water from a lower reservoir to a higher one. When electricity demand rises, the stored water is released back down through turbines, generating power as it returns to the lower reservoir.
Pumped hydro has a high round trip efficiency of up to 85%, it can be switched on in seconds and sustain output over many hours, or even days (depending how big the reservoirs are). Pumped hydro sites can operate for many decades, with relatively low operation and maintenance costs. There are two main types of pumped hydro: conventional (open-loop), which relies on natural bodies of water, and closed-loop, which uses artificial reservoirs and is less dependent on geography. There are also innovative start-ups look at fluids other than water. Using denser fluids offers the ability to generate more electricity, with a lower volume, but clearly comes at a cost plus the risks of environment damage should leakage occur.
To find out more on how pumped hydro works, here's an excellent animation from SSE plc SSE's Hydro Projects and Innovations
3. Long-duration energy storage (LDES): thermal, hydrogen and other novel technologies
Batteries are good for a few hours, pumped hydro, typically up to a few days. What about the weeks or even months that may be required in some climates? First, the good news. In a lot of regions there is reasonable inverse correlation between wind and solar. In Morocco this happens on a daily basis - it's very sunny during the day and then windy at night. To the point that Xlinks hopes to build a project with wind, solar and batteries to provide 24/7 firm power to Europe. In Northern Europe solar is clearly strongest from spring to summer and it tends to be windiest in winter. But... this is not always the cause. The German word, dunkleflaute, has become part of regular English language to describe a period where there is neither sun nor wind. At its extreme, a dunkleflaute may last a couple of weeks and it may cover much of Northern Europe. So here are a few of the ideas which may help bridge these much bigger gaps in renewable generation.
Thermal Energy Storage (TES): TES technologies store energy as heat or cold in materials such as molten salts, water, or phase-change materials. The stored thermal energy can be used directly for heating/cooling or converted back to electricity. At their simplest, a hot water tank in your home is a thermal energy storage system - for example a 150 litre hot water tank at 50°C is roughly equivalent to a 7 kWh battery. With an off-peak or time-of-use tariff and electrical heat, it can be 'charged' when electricity is cheap, storing the heat for up to 24 hours before the temperature falls significantly. In a home the hot water tank can only be used for, you guessed it, hot water. It's not practical to convert the thermal energy back into electricity.
At a more sophisticated level, UK start-up Caldera has created a modular thermal storage system for industrial applications such as breweries, paper manufacturing and heat networks. Find out more here: Caldera promo video
And at a much larger scale, Finish company Vantaan Energia Oy is currently building the world's largest inter-seasonal thermal storage system in Varanto. Water will be heated in a massive underground cavern and heated at pressure to 140°C to create a 90 GWh heat battery - enough to heat a medium-sized Finish town for a whole year! Read more here: Varanto - The Cavern Thermal Energy Storage - Vantaan Energia
Hydrogen storage: There has been huge hype about the role which hydrogen will or won't play in the energy transition. Much of this bubble has burst in the last year or two, but LDES may just be one application where hydrogen does play a role. It's even on the second row of Michael Liebreich's famous Hydrogen Ladder. Hydrogen Ladder Version 5.0 - by Michael Liebreich.
Green hydrogen from renewables or indeed blue hydrogen from gas and CCUS can be stored in tanks or underground caverns for relatively long durations, and then converted back into electricity (via gas turbines or fuel cells). The biggest challenges are the round trip efficiency, which can be as low as 25%, and the surmountable challenges of safely storing hydrogen without leakage. That efficiency factor means hydrogen, if it becomes more mainstream, will always be an expensive way of storing energy.
In H is for Hydrogen I also referred to naturally occurring white and gold hydrogen. There are several well-back start-up companies exploring the potential for producing such hydrogen , whether it will ever be practical or commercially viable (and cheaper than the alternative) will depend on many factors which few (including myself) have answers to yet.
Compressed Air Energy Storage (CAES): CAES systems use surplus electricity to compress air, which is stored in underground caverns or tanks. When electricity is needed, the compressed air is released, driving turbines to generate power. Advanced CAES designs, such as adiabatic and isothermal systems, improve efficiency by capturing and reusing heat generated during compression. CAES is well-suited for large-scale, long-duration storage, but requires suitable geological formations and careful management of thermal losses. To date most CAES systems are prototypes, and costs have yet to decline sufficiently to drive scale.
Liquid Air Energy Storage (LAES): Working on similar principles to CAES, LAES cools air to cryogenic temperatures, liquefying it for storage. When electricity demand rises, the liquid air is warmed, expanding rapidly to drive turbines. LAES offers high scalability, can be sited flexibly, and uses readily available materials. It is also still only being deployed in pilot projects.
Gravity-Based Storage: These systems store energy by lifting heavy masses (such as concrete blocks) again using surplus electricity. When energy is needed, the masses are lowered, converting potential energy back into electricity. This approach replicates the principles of pumped hydro. The challenge is cost and the size of such systems (and therefore the level of embedded CO2 from the required concrete and steel). EnergyVault is currently commissioning a 100 MWh / 25 MW gravity storage system in Rudong, China (the photo below gives a sense of the scale). I've not managed to get any data on the costs of such systems, particularly relative to utility-scale batteries. My hunch is that the declining costs of batteries, coupled with the additional services they offer, means that gravity storage systems may struggle to compete.
Conclusion
Energy storage is becoming a critical enabler for the energy transition. Utility-scale batteries offer speed and flexibility, pumped hydro provides scale and duration, and emerging technologies like hydrogen promise long-term and cross-sectoral solutions. The eventual mix of storage in any one country will depend on the overall energy mix, as well as factors such as the correlation of wind and solar, and of course demand patterns.
It is clear the batteries will dominate relatively short-duration storage, complementing the existing pumped hydro fleet. The longer-term answers are less clear. Ultimately it will come down to which solutions can scale and lower their costs the fastest. Thermodynamics and economics will prevail.
And of course, storage alone is not the only tool in the tool box. I'll be covering the role of transmission, distribution and interconnectors in a future edition. And increasing use of smart grid technologies and digital consumer engagement will help optimise grid demand and supply (which I covered in D is for Digitalisation).
Further reading
Further reading, as always, from the Energy Institute's New Energy World magazine - courtesy of Will Dalrymple
Battery storage
All aboard the battery express
US battery giant acquires all remaining Northvolt European assets
Storage industry’s $100bn investment in US-made grid batteries
Major battery swapping network aims to ‘transform mobility’ in India
Pumped hydro
Why pumped hydro is back in vogue for energy storage
Thermal and long-duration storage
World’s largest sand battery enters operation in Finland
Hot rocks could increase duration of thermal storage solutions
Weeding out the duds: what’s driving growth of long-duration energy storage in the UK (published only yesterday)
Chief Executive at Energy UK | Representing companies investing billions of pounds to secure our country’s current and future energy needs.
1dI can’t believe the series is at ‘S’ - I’ve enjoyed reading every fortnight, it’s been great Nick! You need to make it into a book or something at the end of the year!