Portable Pico Linear Generator Design with Different Magnet Shapes for Wave Energy Conversion System

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 8, No. 1, March 2017, pp. 360~366
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v8i1.pp360-366  360
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Portable Pico Linear Generator Design with Different Magnet
Shapes for Wave Energy Conversion System
Nursyamimi Shahabudin1
, Taib Ibrahim2
, Naily Akmal Mohd Zamri3
1,2,3
Department of Electrical and Electronic Engineering, Universiti Teknologi PETRONAS, Perak, Malaysia
Article Info ABSTRACT
Article history:
Received Oct 24, 2016
Revised Dec 30, 2016
Accepted Jan 09, 2017
There are abundant of wave energy converter technologies available to
convert wave energy into useable energy. However, most of them are huge
and suitable for large application. Thus, this paper aimed to propose portable
pico generator designs for small scale application. Investigation on the
performance of designs with varying halbach magnet shapes was mainly
focused and discussed. Two designs of different magnet shape i.e. triangular
and trapezoid were proposed. Open-circuit simulation and optimization
results were obtained using Finite Element Method. From the results, it was
found out that Trapezoid Magnet Design produced better performance and
lower material cost compared to another proposed design, Triangular Magnet
as well as conventional Rectangular Magnet shape.
Keyword:
Halbach magnet
Linear generator
Portable generator
Wave energy conversion
Copyright © 2017 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Taib Ibrahim,
Department of Electrical and Electronic Engineering,
Universiti Teknologi PETRONAS,
Seri Iskandar, 32610, Perak, Malaysia.
Email: taibib@petronas.com.my
1. INTRODUCTION
Discussion on ocean wave energy conversion into useable energy started centuries ago. The first
patent for the conversion techniques was introduced in year 1799 [1]. At present, several countries have
started to develop and implement wave energy power plant in generating electricity. Up to date, high
numbers of technologies for wave energy converter (WEC) are available [2],[3]. Generally, WEC system
consists of several subsystems; primary interface, secondary interface, generation and control system.
Primary interface is responsible to capture the moving ocean wave and it can be categorized as oscillating
water column (OWC), overtopping devices and floating body [3]. Next, the captured mechanical energy will
be transferred by secondary interface or transmission system to generation part of the system. There are
several types of secondary interface for WEC which are hydraulic, turbine and direct drive [4]. The
mechanical energy will then be converted into electricity by generator which can be either conventional
rotary generator or linear generator.
Among the notable WEC designs is Pelamis WEC which was founded in year 1998 and the first
installation was done in wave test site in year 2010 [5]. The technology is a floating device and it uses
hydraulic transmission system [4]. Second generation Pelamis WEC, P2 was designed to give output power
of 750 kW and the device is 180 m long with weight approximately 1350 tonnes [5]. Another established
WEC technology is Oyster WEC that was developed by Aquamarine Power [4]. First generation Osyter has
rated output power of 315 kW and weighted 200 tonnees [6]. The device was deployed and installed in year
2009 and bigger second generation Osyter is currently being developed [7]. There are many other WEC
devices that have been developed and some was installed on site such as OWC based Wavegen Limpet [8]
and OPT Power Buoy [9]. However, the available technologies are mostly designed for big scale application
and thus the size of the structure is also huge.

IJPEDS ISSN: 2088-8694 
Portable Pico Linear Generator Design with Different Magnet Shapes for .... (Nursyamimi Shahabudin)
361
Wave energy can also be targeted for small scale application for example fishing activities or near
shore activities. For these types of usage, the system must have smaller output power with reduced size and
lesser weight. Therefore, WEC system that suits these criteria was targeted to be developed. This research
was mainly focus on generator part of the system that plays role in energy conversion into electricity. The
proposed generator designs was targeted to weight around 25 kg with the output power of 100 W. The
objectives of the research are;
a. To propose generator designs that is portable and small in size
b. To conduct simulation and optimization on the proposed designs using finite element method
c. To perform comparison on the designs’ performance and choose the best design
2. RESEARCH METHOD
Two proposed designs were introduced based on suitable linear generator topology. The proposed
designs were simulated and optimized using Finite Element Method. Instead of just comparing the proposed
designs with each other, the proposed designs were also being compared with conventional design to observe
for any improvement.
2.1. WEC Technology and Proposed Design
Floating body technology was chosen to be used in this research because it can be used in small
scale application and simpler power take-off (PTO) can be implemented with this technology which is direct
drive [4]. Direct drive linear generator has lesser mechanical part than conventional rotary generator and thus
reduce the construction and maintenance cost [10]. Figure 1 shows the general concept of floating body
technology with linear generator.
Topology of linear electrical machine can be categorized into several group which are; 1) tubular or
planar structure, 2) slotted or slotless stator, and 3) iron-cored or air-cored. Oprea et al. studied the structures
of permanent magnet linear generator which are tubular and planar four-sided structures. The performance
for both structures were the same when considering the induced voltage and coil magnetic flux. However, the
construction of tubular is more difficult compared to the four-sided structure. In contrast, the weight of the
four-sided structure is heavier for the same dimensions and specifications as the tubular structure and thus
causes larger loss [11]. Tubular topolgy using permanent magnet is implemented for this research as it will
maximize power density, magnetic flux density and efficiency as well as able to minimize power loss
compared to the four-sided structure [12].
Bizzozero et al. proposed two topologies of tubular linear generator for sea wave energy production
which are the slotted and slotless permanent magnet generators [13]. They concluded that the usage of the
slotted configuration will give rise to more construction difficulties and by extension, the cost. In addition,
when comparing in term of the force acting on the coil, the magnitude of force for slotless is lower than the
magnitude of slotted generator due to high cogging and end effects. Moreover, slotted generator generates
more reactive power but less active power compared to slotless generator. Nevertheless, slotted topology
produces high flux densities and sheer stress compared to the slotless one [13],[14]. Thus, slotless stator was
chosen to be used in order to obtain minimum cogging force. This will prolong the lifespan of the permanent
magnet because the motion of piston is even and constant [15].
The design utilizes halbach magnet configuration instead of axial and radial configuration because it
is expected to produce higher force with equal volume of magnet. Halbach configuration will eliminate one
side of the magnetic flux while the other side will be increased by √2 [16]. The usual halbach magnet
configuration has rectangular magnet shape as shown in Figure 2.
Figure 1. Floating Body Technology with Linear Generator [3]

 ISSN: 2088-8694
IJPEDS Vol. 8, No. 1, March 2017 : 360 – 366
362
Figure 2. Conventional Rectangular Halbach Magnet Configuration
Based on the selection, the proposed designs will be tubular slotless linear generator with halbach
magnet configuration. Two designs were proposed in which the manipulating aspect of these designs is the
shape of the magnet. Instead of utilizing conventional rectangular magnet shape, different magnet shapes
which are triangular and trapezoid shape were implemented in the proposed designs. The dimensions of the
proposed designs are as shown in Table 1 while the proposed designs are illustrated in Figure 3. It is expected
that different magnet shapes will produce better output as the number of horizontal edges of the proposed
magnet shapes is more than the usual shape. This is because as flux density is mostly concentrated at the
edges of magnet, more edges present in the design will increase the amount of flux linkage.
Table 1. Dimension of the Proposed Designs
Dimension Value Dimension Value
Length of stator, ls 200 mm Total turns of coil winding 1000 turns
Height of back iron, hbi 5 mm Air gap diameter, g 0.8 mm
Length of winding, wwd 200 mm Length of magnet assembly, lm 400 mm
Height of winding, hwd 18 mm Height of magnet, hm 5 mm
Outer radius of stator, re 100 mm Number of magnetic pole 1
Outer radius of magnet, rm 76.2 mm
(a) (b)
Figure 3. 2D Image of Proposed Designs (a) Triangular Magnet Shape Design and (b) Trapezoid Magnet
Shape Design
2.2. Finite Element Analysis and Optimization
The proposed designs were simulated using Finite Element Analysis ANSYS Maxwell software.
Conventional rectangular magnet shape was also simulated to serve as reference for comparison. Air gap
flux density was being observed for static conditions. Open circuit test was conducted in dynamic simulation
to obtain flux linkage and induced voltage produced by the generator designs. The parameters for open-
circuit simulation are tabulated in Table 2.
Table 2. Parameters for open circuit test
Parameter Value
Velocity 1 m/s
Moving distance 100 mm
Stop time 0.05 s
Step size 0.05 s

363
Optimization was conducted to increase the performance of the design. Two values were optimized
which are pitch ratio, Tmr / Tp and split ratio, Rm/Re. Pitch ratio optimization was executed by altering axial
length of radically magnetized magnet while maintaing pole pitch. Induced voltage was observed during the
process. For split ratio optimization, the radius of magnet was modified and at the same time the outer radius
of stator core was kept constant. The result for split ratio was in term of efficiency of the designs.
3. RESULTS AND ANALYSIS
3.1. Air-gap Flux Density Results
Figure 4 shows the flux density along air gap of the three generator designs. From the graph, it can
be observed that Triangular Magnet Shape design has the same density peaks as reference design. These
peaks are formed at the edges of the magnets where most flux can be found. For Trapezoid Magnet Shape
design, other than the peaks that are located at the same spots as other designs, it has additional four peaks.
These additional peaks are due to the shape of trapezoid magnet which is not in 900
at the sides and thus
having more edges that are at different horizontal spots than each other. The average and maxiumum flux
density values are tabulated in Table 3. From the data, Triangular magnet shape has the highest value of flux
density.
3.2. Flux Linkage and Induced Voltage Results
Figure 5 shows the result of flux linkage of the designs while the average and maximum values are
tabulated in Table 3. From the tabulated data, Trapezoid Magnet Shape design has the highest maximum and
average value of flux line, while the reference Rectangular Magnet Shape design has the lowest flux linkage
value. This result was as expected in which proposed designs that have more horizontal magnet edges will
produce better flux linkage. Flux linkage graph of the three designs are having the same pattern, however the
gradient of the graphs differ. This difference will reflect in the induced voltage produced by the generator.
Induced voltage result of the designs is illustrated in Figure 6 and tabulated in Table 3. The result
shows that Trapezoid Magnet Shape design produced highest maximum and average value of induced
voltage. These induced voltage results are corresponding to flux linkage results in which design with high
gradient of flux linkage graph is having high value of induced voltage. Based on flux linkage results, it can
be observed that Trapezoid Magnet Shape design has steepest graph line compared to others. This high flux
linkage and induced voltage of Trapezoid Magnet Shape design is due to the surface area of axial magnet
which is larger compared to other two magnet shape designs. It can be concluded that from the induced
voltage results, both proposed magnet shape designs which are triangular and trapezoid have higher induced
voltage value compared to the reference design which is Rectangular Magnet Shape design.
Figure 4. Air-gap Flux Density Figure 5. Flux Linkage of the Designs
0
100
200
300
400
500
600
700
800
0 50 100 150 200
Flux
density
(mTesla)
Distance (mm)
Rectangle
Triangle
Trapezoid
0
1
2
3
4
5
0 10 20 30 40 50 60 70 80 90 100
Flux
Linkage
(Wb)
Time (ms)
Rectangle
Triangle
Trapezoid

 ISSN: 2088-8694
364
Figure 6. Induced Voltage of the Designs
Table 3. Average and Maximum Values of Flux Density, Flux Linkage and Induced Voltage
Design
Rectangular
Magnet (Ref.)
Triangular
Magnet
Trapezoid Magnet
Flux Density
Average Value (mT) 139.611 144.001 111.649
Maximum Value (mT) 393.794 730.145 178.479
Flux Linkage
Average Value (wb) 2.903 3.192 3.387
Maximum Value (wb) 4.420 4.446 4.591
Induced Voltage
Average Value (V) 45.03 45.54 47.77
Maximum Value (V) 84.20 108.23 111.26
3.3. Optimization Results
3.3.1. Pitch Ratio Optimization
Induced voltage value is observed in pitch ratio optimization. The result for pitch ratio optimization
is illustrated and tabulated in Figure 7 and Table 4 respectively. Based on the result, it is shown that the
optimal pitch ratio in Rectangular, Triangular and Trapezoid Magnet Shape design are at different point.
Trapezoid Magnet Shape design produced the highest optimal induced voltage with the increment of 2%
from original value while reference design; Rectangular Magnet Shape recorded the lowest value.
3.3.2. Split Ratio Optimization
The result of split ratio optimization is as shown in Figure 8 and Table 4. As will be observed, the
optimal split ratio value for Triangular and Trapezoid Magnet Shape design are the same at 0.742 while for
reference design, the optimal value is slightly differ. Trapezoid Magnet Shape design produced highest
efficiency for split ratio optimization followed by Rectangular and Traingular Magnet Shape designs. This
dissimilar value of efficiency is due to copper loss for each design is different at different point of Rm/Re.
Figure 7. Pitch Ratio Optimization Figure 8. Split Ratio Optimization
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80 90
Induced
Voltage
(V)
Time (ms)
Rectangle
Triangle
Trapezoid
44.5
45.0
45.5
46.0
46.5
47.0
47.5
48.0
48.5
0.57 0.59 0.61 0.63
Induced
voltage
(V)
Tmr/Tp
Rectangle Triangle Trapezoid
74
76
78
80
82
84
86
88
90
0.72 0.745 0.77 0.795
Efficiency
(%)
Rm/Re
Rectangle Triangle Trapezoid

365
Table 4. Induced Voltage and Efficiency of Optimal Pitch and Split Ratio Value
Design
Rectangular
Magnet (Ref.)
Triangular
Magnet
Trapezoid
Magnet
Pitch Ratio
Optimal Pitch Ratio 0.5915 0.5915 0.6115
Average Induced Voltage (V) 45.13 45.67 48.32
Split Ratio
Optimal Pitch Ratio 0.762 0.742 0.742
Efficiency (%) 84.40 80.58 89.20
3.4. Material Cost Calculation
Total material cost is estimated based on the volume of a specific material required by the designs.
Table 5 shows the total weight and material cost of each design.
Table 5. Designs’ Total Weight and Material Cost
Design
Volume of material (cm3
)
Total Weight (kg)
Total Material Cost
(RM)
Copper Steel Magnet Iron
Rectangular Magnet (Ref.) 1950 611 926 613 32.58 2,391.48
Triangular Magnet 1950 595 890 613 32.19 2,331.94
Trapezoid Magnet 1950 596 890 613 32.19 2,331.94
As shown in Table 5, the price of three designs are almost the same. However, the proposed designs
are slightly cheaper compared to the reference conventional design, Rectagular Magnet Shape. Nonetheless,
the cost considered does not include the construction cost and it is only the cost of the materials for the
generator. The total weight of the designs are heavier than the targeted weight by approximately 7 kg or 28 %
in order to obtain the desired output power of 100 W.
From the optimization and material cost calculation, the best generator design is Trapezoid Magnet
Shape design which has better efficiency than Triangular Magnet by 11 %. In addition, the design also
improve the performance of conventional magnet shape design by 5 % and the material cost is also cheaper.
Even though the weight of the design is slightly heavier than the targeted weight, the 7 kg difference is still in
acceptable range.
4. CONCLUSION
Two generator designs which are Triangular and Trapezoid Magnet Shape design were proposed for
the utilization with small scale wave energy conversion system. The designs differ in term of the shape of the
permanent magnets while maintaining other dimensions. From the simulation and analysis, it shows that
Trapezoid Magnet Shape design has the best performance. The induced voltage produced and efficiency of
Trapezoid Magnet Shape design is the highest among the simulated designs. Furthermore, the material cost
calculation for Trapezoid Magnet Shape design is slightly lower compared to conventional Rectangular
Magnet Shape design. It can also be concluded that different magnet shape designs able to slightly improve
the performance of the conventional shape. It is recommended that further study to be conducted in
fabricating the design so that experimental testing can be done. From experimental results, the simulation
data can then be validated.
ACKNOWLEDGEMENTS
The authors would like to thank Universiti Teknologi PETRONAS and Ministry of Higher
Education for the awarded Fundamental Research Grant Scheme (FRGS).
REFERENCES
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Songkhla, Thailand, 2016.
[4] B. P. Drew, et al., ―A review of wave energy converter technology,‖ in Proceedings of the Institution of
Mechanical Engineers, Part A: Journal of Power and Energy, vol/issue: 223(8), pp. 887-902, 2009.

 ISSN: 2088-8694
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[5] ―Pelamis Wave Power,‖ The European Marine Energy Center Ltd. [Online]. Available:
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BIOGRAPHIES OF AUTHORS
Nursyamimi Shahabudin was born in Kedah, Malaysia on October 1993. She obtained her
Bachelor degree in Electrical and Electronic Engineering from Universiti Teknologi
PETRONAS in year 2016. She conducted research on portable linear electrical machine
design for her Final Year Project (FYP). She is currently working as Information Technology
Officer at MAMPU, Malaysia Prime Minister Office.
Taib Ibrahim obtained his Bachelor degree in Electrical and Electronic Engineering from
Coventry University, UK in 1996 and MSc in Electrical Power Engineering from University
of Strathclyde, UK in 2001. He was then awarded Ph.D in Electrical Machine Design from
University of Sheffield, UK in 2009. He has published more than 20 journal papers on
various aspects of electrical machine design and energy. At present, he is working as
Associate Professor in Universiti Teknologi PETRONAS with 16 years of teaching
experience. His current research interests are on linear and rotary electrical machine design,
renewable energy (hydro and wave), power electronic converter and electrical power
protection system.
Naily Akmal Mohd Zamri was born in Selangor, Malaysia in 1992. She obtained her
Bachelor degree in Electrical and Electronic Engineering from Universiti Teknologi
PETRONAS, Malaysia in year 2015. She has published one international journal paper and 2
papers for international conference. She is currently an MSc student in Universiti Teknologi
PETRONAS. Her interest areas include linear electrical machine design, renewable energy
(wave) and power electronics.

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