A Direct Drive Permanent Magnet Generator Design for a Tidal

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A Direct Drive Permanent Magnet Generator
Design for a Tidal Current Turbine(SeaGen)
Ozan Keysan∗ , Alasdair S. McDonald† , Markus Mueller†
∗† School
of Engineering, Institute for Energy Systems
University of Edinburgh, King’s Buildings, Edinburgh EH9 3JL United Kingdom
∗ Email: o.keysan@ed.ac.uk
Abstract—In this study, the feasibility of a direct-drive permanent magnet generator for a tidal turbine power take-off
system, namely MCT’s SeaGen –the world’s first full scale
commercial tidal turbine– has been investigated. The investigated
PM generator topology is called C-GEN which is an air-cored
axial-flux generator developed in the University of Edinburgh.
The C-GEN is prior to conventional PM generators by absence
of magnetic attraction forces between rotor and stator, absence
of cogging torque, ease of manufacturing, modularity and high
fault-toleration [1].
Firstly, the integrated analytical design tool that couples
electromagnetic, structural and thermal aspects of the generator
has been introduced. Then, an optimization tool based on genetic
algorithm has been used to maximize the annual electricity
generation and to minimize the initial cost of the generator.
The optimized generator is validated using FEA tools and the
specifications of the generator has been presented.
I. I NTRODUCTION
The contribution of marine renewables to the world’s electricity generation is expected to increase in the next decades
[2]. In the last years, UK government has introduced subsidies
to wave and tidal energy companies to increase the marine
renewable energy production. UK has abundant wave and tidal
energy resources, especially, off the west coast of Scotland it
has been estimated that 10 percent of the UK’s electricity may
be provided by tidal energy [2].
The tidal currents are caused by the relative motion of the
sun and the moon. The tides are predictable but tide velocity
is varying through day and month [3]. There are roughly four
current velocity peaks daily and one spring and neap periods
monthly as given in Fig. 1. The tidal stream energy is the
kinetic energy contained in the mass of moving water. The
tidal power can be similarly expressed to wind energy as [4];
1
3
Cp ρAVtide
(1)
2
where Cp is the conversion ratio which is expected to
be 0.3–0.4 for tidal turbines. The density of water is much
higher than the density of air, so the same power can be
extracted from a much smaller sweep area compared to wind
turbines. As it can be observed from (1), the power of the
tidal currents is proportional to velocity cubed. Thus, it is
very important to find locations with the highest possible flow
velocities if energy is to be extracted as cost-effectively as
possible [5]. Compared to wind turbines, suitable locations
for tidal turbines are more limited. An economically feasible
P =
location should have peak tidal current velocities higher than
2–2.5 m/sec which can only be found at pinch points, such as
straits between islands and the mainland and shallows around
headlands [6]. The advantages of tidal current energy have
been listed in [5] as follows;
•
•
•
•
The tidal currents can be predicted precisely, which
allows contractable energy.
Compared to wave energy converters, reduced extreme
conditions make it easier to manufacture economically
feasible devices.
Environmental impact (i.e. threat of impact on marine
wild-life, conflicts with other users of sea, pollution) is
expected to be low.
Energy Return on Energy Invested for a tidal turbine is
predicted to be higher than for most marine renewable
technologies.
At present, there are at least three different tidal energy
converter technology solutions: horizontal axis turbine, vertical
axis rotating device and an oscillating hydroplane device [7].
Tidal energy device developers currently focus on the design
of the prime mover, but they usually prefer to use off the shelf
components for power take-off system which usually features
an induction generator coupled with a gearbox or a hydraulic
pump. These intermediate stages decrease the efficiency and
more importantly decrease the reliability of the overall system,
which is by far the most important issue for marine renewable
devices. A few developers have built direct-drive tidal turbines
such as: Open Hydro, Ireland [8], Clean Current, Canada [9].
Also, Swan Turbines [10] has proposed a commercial machine
rated at 1 MW of unknown diameter [11] which will be the
first MW scale direct-drive tidal turbine.
A novel air-cored permanent magnet generator topology
has been developed in the University of Edinburgh [1]. The
principal benefits of this technology in direct-drive systems
are; absence of cogging torque, reduced structural mass, ease
of manufacturing and fault tolerance owing to its stacked
configuration. A 20 kW prototype test rig and a 15 kW
machine for a wind turbine has been manufactured and tested
previously. In addition to wind turbine, C-GEN technology
can also be applied to direct-drive marine energy converters.
In this study, the results of a direct-drive C-GEN permanent
magnet generator power take-off system that is designed for
SeaGen will be presented.
00:00
06:00
12:00
Time of Day
18:00
00:00
Coal
14%
Fig. 2. The power output during a spring and neap tide.
% max output
Fig. 4. The installed plant mix for the Irish system in 2008.
1.00
0.66
0.33
0
5
10
15
Days
Fig. 3. The power output from a tidal device over a 15 day period.
Fig. 1.
The power output from a tidal device over a 15 day period [3]
addition, as shown in Fig. 3, the maximum tidal output varies
throughout the month with the spring neap cycle.
A. Marine Current Turbines-SeaGen
Energy extraction from tidal currents in UK is being led by
3. CaseCurrent
study Turbines (MCT). By the end of 1990’s, the
Marine
company gained the support of European Commission for the
In order to quantify the costs and benefits of tidal generation a
‘Seaflow
Project‘, to develop the world’s first full-scale (300
real electricity system was taken as a case study. Ireland is the
kW)
offshore
tidalforturbine
whichsince
wasit successfully
installed
case study chosen
this analysis
is an island electricity
and
has
exceeded
the
expected
power
under
favourable
flow
system with a potentially rich tidal energy resource. In addition,
Ireland has
conditions
[6].very limited interconnection to other systems
allowing
for a controlled
of tidal
generation.
However,
After Seaflow,
MCT study
initiated
a £10
million
projecttheto
issues that are raised here are not unique to the case system and
manufacture
and install a twin rotor system rated at 1.2 MW,
are likely to be relevant in other systems considering tidal
called
SeaGen and it was installed in Strangford Narrows,
generation.
NorthTheIreland
2008 historically
[11]. The device
is mounted
to seabed
Irish in
system
consisted
of two separately
operated
but interconnected
systems,
the Republic
of
with
a monopole
structure. There
areone
twoin counter
rotating
Ireland
one inasNorthern
However,
in 2004
blades
in and
the device
shown inIreland.
Fig. 2(a).
Each rotor
is 16anm
the electricity regulators in the
inagreement
diameter was
and reached
drives abetween
600 kW
power-train. The operation
Republic and in the North to establish a single ‘all-island’ market
range
of
SeaGen
is
shown
in
Fig.
2(b).
The figure also shows
for electricity. This new ‘all-island’ Single Electricity Market (SEM)
the
expected
operation
range
of
first-generation
tidalSEM
turbines
was launched in November 2007 (SEMO, 2007). The
is a
with
varyinggross
rotorpool
diameter
mandatory
market[2].
with centralised commitment of
units.
The marginalof
generator
sets the of
system
marginal price
for
The power-train
MCT consists
a multi-stage
gearbox
all generators in the gross pool market. In addition, to the gross
and
a induction generator designed as a sub-sea motor for the
pool market there is a separate capacity payment mechanism.
oilThus
andgenerators’
gas industry
[5]. The power-train is submersible, with
bids should consist of their marginal and start
a costs
pair of
planetary
gearboxes
are designed
by Orbital2
only. This paper examineswhich
this ‘all-island’
electricity
system,
and
manufactured
by ofWikov
theNorthern
Czech Ireland
Republic
[5]. The
covering
the Republic
Irelandinand
(referred
to
jointly in
this is
paper
‘Ireland’).
gearbox
ratio
70:1asand
the rated speed of the generator is
currently
approximately
of installed
capacity.
1000Ireland
rpm when
thehas
blade
is rotating9 GW
at 14.3
rpm. The
total
The generation plant mix was traditionally based on large coal and
mass of the drive-train is 27 tonnes [12].
The blades have full-span pitch control with carbon/glass
fibre composite rotor blades [5]. The pitch control system can
also be used to prevent power spikes to limit the power output
and to ensure optimum performance during both the ebb and
flood tides [11]. The blades and power take-off system can be
raised above the sea surface for maintenance as shown in Fig.
2(c).
II. I NTEGRATED D ESIGN OF C-GEN
In the design of conventional electrical machines, firstly
electromagnetic aspects of the generator are designed and
optimized. Then, the structural parts are added to the generator
design. On the other side, for large diameter direct-drive
machines, the structural mass is dominant and structural issues
should be carefully addressed. Better machines can be obtained
with a coupled structural and electromagnetic design tool.
Another issue is the thermal design of the generator for tidal
turbines. A well-designed generator can benefit from improved
oil fired generation plant with a small number of peat plants and
old thermal gas generators. Since 1990 however, the share of high
carbon content fuels such as coal has fallen in Ireland due to a
large increase in the use of natural gas combined cycle plants
(CCGTs). Gas fired generation now accounts for over 50% of the
generation in Ireland (Deloitte, 2005). Ireland has one pumped
storage station and a small number of hydropower plants. In
addition, Ireland has one 500 MW interconnector to Scotland. The
installed plant mix for the Irish electricity system as of March
2008 is illustrated in Fig. 4.
Bryans et al. (2005b) determined the resource for tidal energy
around Ireland using a 2 dimensional tidal model to simulate the
tidal flows for the waters surrounding the entire island with a
(a)
405 m by 405 m grid. They found that the resource currently
accessible to the MCT tidal device (as shown in Fig. 1) is 374 MW
around Ireland. However, it is predicted that into the future, TED
development will lead to larger turbines which will be financially
viable at greater depths and lower spring current velocities. Based
on the predictions by Bryans et al. (2004), a tidal resource of up to
560 MW is investigated here, representing 6% of installed
generation capacity.
4. Methodology
During the design process of the Single Electricity Market in
Ireland software from Energy Exemplar, known as PLEXOS for
Power Systems, was used by the market design team to model the
likely operation and prices in the new market (PLEXOS, 2006).
The purpose of this modelling work was to assist industry
participants in developing a greater
understanding of the new
(b)
electricity market arrangements and to provide quantitative
support in assessing the potential impacts of the arrangements
on both the industry and the final customer (AIP, 2008).
The PLEXOS tool is a sophisticated modelling technique which
uses mixed integer optimisation to determine the unit commitment decisions and accounts for generator constraints such as
minimum and maximum operation, ramp rates, start times and
costs, maintenance schedules and transmission constraints. The
optimisation also co-optimises for reserve provision and includes
energy limited cascade constraints for the operation of hydrostations and genuine optimisation of the pumped hydro stations
(PLEXOS, 2006).
(c)
Fig. 2. a) SeaGen operational mode (Courtesy of MCT), b) Extractable
power from marine currents of a given velocity at average 30% efficiency
and operation range of SeaGen [2], c) SeaGen maintenance mode and drivetrain(Photo:ISA [13]).
heat transfer conditions while operating under water. To benefit
from this advantage in SeaGen design, a coupled thermal
model has been included in the analytical model. The multiphysics optimization design tool used in this study, ensures
that the most appropriate generator design is produced taking
into account the operating conditions of SeaGen. The design
tool comprises of the following parts; electromagnetic design,
structural design and thermal design.
A. Electromagnetic Design
The C-GEN machine takes its name from the C-shaped
magnetic core structure. The permanent magnets are placed
on each limb of the core as shown in Fig. 3. The armature
composed of air-cored concentrated coils, reinforced with
epoxy filling. One limitation of the air-cored windings is the
decreased magnetic performance due to higher reluctance. On
the other side, using air-cored windings magnetic attraction
forces between rotor and stator are completely eliminated
which reduces the mechanical load on structure.
NI
NI
SPM
Sag
SPM
Ssp
Sl
A
NI
Sl
Ssp
SPM
Sag
NI
SPM
A
Ssp
Sl NI
Sl
SspNI
SPM
Sag
SPM
(a)
φPM
A
(a)
Leakage
Flux
NI
SPM
Sag
PM
PM
φst
Linking Flux
B
Sst
d
A
PM
NI
φPM SPM
A
d
PM
hm
(b)
Fig. 3. a) Reluctance network for three C-core modules seen end on, b)
Intra-module reluctance network seen side on, common point labelled A
The electromagnetic circuit of the generator is modelled
using the lumped parameter method. The details of the calculations can be found in [14]. The magneto-motive force of
the permanent magnets can be expressed as N I, which is
the equivalent number of turns and current flow required to
achieve the same magneto-motive force, as given in (2) and
(3). Lm , hm , wm represents the length, the height and the
width of the magnet. Br is the remanence flux density, µr is
the relative permeability of the permanent magnet.
N I = Φ.R
hm
Br hm
N I = Br Lm wm
=
Lm wm µ0 µr
µ0 µr
(b)
Fig. 4. a) Reluctance network for three C-Core modules seen end on, b)
Intra-module reluctance network seen side on, common point labelled A
magnetic potential difference is constant (2N I), but the length
of the leakage path is varying. The leakage flux on part B can
be approximated as in (6). The equivalent leakage reluctance
can be defined as the magnetic potential difference between
two magnets divided by the total leakage flux as given in (7).
R=
L
µ.A
(4)
(2)
R
Zout Zhm
(3)
In the magnetic circuit, Sag is the airgap reluctance, SP M is
the permanent magnet reluctance, Ssp is the spacer reluctance,
Sweb is the web reluctance and Sl is the leakage path reluctance. The reluctance calculations for each element is straightforward and can be defined by using the well-known formula
given in (4). On the other side, to increase the accuracy of the
analytical model the leakage flux reluctance (Sl ) should be
carefully estimated. Leakage flux travels between neighbouring magnets without crossing the airgap as seen on Fig. 4(b).
The amount of leakage flux increases as the distance between
magnets decreases. Assume the angle between magnets is θ in
radians as given in Fig. 4(a) and the magnets lay between Rin
and Rout . The height of the magnet is hm and the vertical gap
between opposing pole magnets is 2d. The leakage flux can
be divided into two parts; part A is the flux travelling between
side walls of the magnets, part B is the flux travelling from top
surfaces of the magnets. Then, ΦA can be calculated taking
the double integral over magnet height and magnet length as
in (5). The flux density is higher at upper levels of side wall
due to increased magnetic potential difference. For ΦB , the
ΦA =
Rin
0
2N I.x
hm
= N Iµ0 hm (ln(Rout ) − ln(Rin ))
θr
µ0 .dx.dr
(5)
R
ZoutZd
2N I
ΦB =
Rin 0
2(
π
2x
µ0 .dx.dr
Sl =
)+(
2N I
ΦA + ΦB
θr
)
µ0 .dx.dr
(6)
(7)
One important advantage of the C-GEN design is the
paralleled design option, i.e. several machines can be stacked
in the axial direction to reduce the overall active material
mass. More over, these stacks can be independently controlled
to increase fault-toleration of the generator. In Fig. 5(a) the
magnetic flux direction in a single C-core for eight stacked
machines is shown. The magnetic flux travels axially to the
end of machine then to the neighbouring magnets as can be
seen in Fig. 5(b). In order to verify the analytical model,
simulations are performed using the Opera 3D FEA software.
The maximum flux density in the airgap obtained from the
qM axwell =
(a)
2
B̂ag
2µ0
N/m2
(8)
Another important issue is the deflection and torsion of the
rotor torque-arm structure. The rotor structure is modelled
using a six-arm hollow rectangle structure [17]. The rotor
structure should cope with short-circuit torques (which may
be up-to six times of the rated torque) and any axial loads
than can arise during maintenance or transportation. During
fast tidal currents, the tidal turbine structure has to cope with
very large axial thrust forces. But, these thrust forces should
be isolated from the generator using the main turbine bearing.
C. Thermal Design
(b)
Fig. 5. a) Reluctance network for three C-Core modules seen end on, b)
Intra-module reluctance network seen side on, common point labelled A
analytical method is compared with the result obtained from
the FEA software. For SeaGen case, the analytical model
estimated the maximum airgap flux density as 0.61 T where
FEA simulation result is 0.62 T. The analytical model are
also compared with varying dimensions of, all of which are
consistent with the FEA results and the error is less than 5%
for all cases which is acceptable.
B. Structural Design
In a conventional permanent magnet machine, the main
structural load is the magnetic attraction force between rotor
and stator. Thus, the rotor structure should be stiff enough
to keep the airgap clearance with certain tolerance. Owing to
its air-cored armature structure, there is no attraction force
between rotor and stator in the C-GEN generator, which
simplifies the rotor structure, especially for large diameters.
But, the attraction force between C-core limbs that try to close
the airgap still exists. The Maxwell stress in the C-core can
be calculated using (8). For the SeaGen design, the maximum
airgap flux density magnitude is 0.61 T which gives a Maxwell
stress of 148 kN/m2 . The Maxwell stress is used as an input
to the beam structure model which is presented in detail in
[15]. In an axial flux machine, the magnets can be trapezoidal
shaped. In this case, the deflection can be modelled with a
trapezoidal force applied on a beam with one side is fixed as
shown in [16].
The conventional generators used in tidal turbines do not
benefit from the improved heat transfer paths by water unless
they are specially designed for submerged operation. A better
design with either higher efficiency or smaller mass can be
achieved by utilizing the thermal advantage of submerged
operation and by considering thermal aspects of the generator
during design process [18]. In order to compare different
options, thermal simulations are performed using the conventional, submerged (but airgap is not filled water), flooded
(submerged and airgap is filled with water) generator types.
Using the same dimensions and a current density of 6.8
A/mm2 , the maximum temperatures on the armature coil has
been compared. The conventional air-cooled generator has a
working temperature of 116 ◦ C, where for submerged machine
it is 88 ◦ C, and for flooded generator it is 58 ◦ C. It is clear
that, the flooded generator has the best thermal performance,
but flooding the generator introduces many other challenges.
Some of the most critical ones can be listed as: corrosion
protection of the NdFeB permanent magnets, increased fluid
drag losses due to water-filled airgap, electrical insulation
problems, necessity to modify the bearing design. Due to
these difficulties a flooded generator is not preferred, instead a
submerged generator configuration has been used for SeaGen.
In the submerged generator, heat conduction from armature
coils in radial direction (from outer surface to tidal currents)
can be increased using thermally conductive structural parts.
For the optimization process, a lumped parameter thermal
model is used to estimate the temperature of the armature
windings and permanent magnets. The heat input to thermal
model is the total copper loss and eddy current loss. The
thermal model requires an iterative process since the coil
resistance increases with temperature which means an increase
in copper losses for the same amount of current (I 2 R). Once
the thermal model converges, the output can be used to
calculate the efficiency of the generator for that operation
point. Dynamic analysis is not required, since the tidal currents
do not vary as quickly as wind and highly predictable. The
output of the thermal model is also used to define the power
output limit and to identify any de-magnetization risk for
permanent magnets.
III. O PTIMIZATION FOR S EAG EN C ASE
In order to get the most suitable solution for the SeaGen,
electromagnetic, structural and thermal models should be
coupled with each other. The coupled model creates a multivariable model with discontinuities. The inputs of the model
are mainly the dimensions of the C-GEN and the material
properties e.g. the remanence flux density of the PM. The
optimized generator should match the torque characteristic of
the turbine for each operating speed.
There are several optimization methods. In this study, the
genetic algorithm (GA) was chosen since it does not require
any derivative information and has a minimum chance of
trapping in a local minima.
A. Objective Function
For variable speed generators as in tidal turbines, the
optimization process should not only be performed at the
rated speed but for the whole operating range. The objective
function of the optimization process should be thoroughly
defined. Objective functions that consists of only material cost
or efficiency may not result in the most optimum solution. For
example, in [19] a very detailed description of the optimization
of an axial flux generator has been given, but the machine has
been optimized for minimum PM material cost, and then for
maximum efficiency. The objective function should include
the electricity generation income. In [20], Jung et al. coupled
GA optimization with FEA software to get the best designs
for annual energy production, but they neglected the initial
cost of the generator. The objective function should include
the electricity generation income as well as the manufacturing
cost of the generator; any other manufacturing constraints (e.g.
maximum diameter)should also be included.
The objective function in this study has been defined in 10
discrete operating conditions as given in (9), where x is the
input vector that defines generator’s dimensions [16].
Fcost (x)
=
+
k.fmaterial (x)
10
X
X
(−pi fincome (x) +
wi fpenalty (x))(9)
i=1
fmaterial (x) represents the material and the manufacturing
cost of the generator, details of which have been given
in [16]. Firstly, the permanent magnet mass, copper mass,
iron and structural steel mass are separately calculated and
multiplied by material prices as follows; Pmagnet =£35/kg,
Pcopper =£10/kg, Psteel =£3/kg. However, the material prices
change continuously, this ratio gives a good approximation of
the contribution of each material to total generator cost. Then,
the total material cost is multiplied by a factor (k), which is
assumed as 2.5, to estimate the manufacturing and assembly
cost.
fincome (x) is the electricity generation income over the
generator’s lifetime. pi is the probability density of each
operating point. The rated speed of the SeaGen is 14.3 rpm
[12], but due to non-disclosure agreement the full operating
Fig. 6. Optimization outputs with varying diameter for 600 kW SeaGen
generator
(a)
(b)
Fig. 7.
a) Optimised 600 kW direct-drive axial-flux C-GEN machine.
b)Cross-section view of the generator (8 parallel machines)
characteristics of the generator cannot be published. The
availability of the generator and the losses in power electronics
and transformers should be also included. The kWh price
of the electricity generated from renewable sources changes
according to Renewables Obligation Certificate (ROC) [21].
For SeaGen, it is estimated as £14.4/W over ten years of
period.
fpenalty (x) is used to define the required constraints on
the design such as; maximum phase voltage, maximum flux
density in the core, temperature limits of the armature coils
and magnets, structure deflection limits. Each constraint is
multiplied by a weighting factor wi to adjust the priority of
each constraint.
B. Optimized Design
An axial-flux C-GEN machine for SeaGen has been designed using the defined cost function and GA. Ambient temperature is assumed to be 10◦ C. Firstly, GA has been applied
with different diameter limits. Fig. 6 shows the variation of
efficiency, total mass, axial length and material cost with
TABLE I
S PECIFICATIONS OF THE OPTIMIZED GENERATOR FOR S EAG EN
Rating
Number of Stacks
Peak Efficiency
Average Efficiency
Outer Diameter
Axial Length
Generator Weight
Number of Poles
Airgap Flux Density
600 kW
8 x 75 kW
94.1 %
88.8 %
3.2 m
0.75 m
17.5 t
120
0.61 T
diameter. It can be observed that the maximum efficiency
of the generator converges to 94% for diameters larger than
3 m. The generators with small diameter are heavy due to
active material mass requirements (i.e. magnet mass) which
can also be observed in the soared material cost. As the
diameter is increased the mass reduces to a minimum value of
14 tonnes when the diameter is around 4 m. As the diameter is
further increased, the mass of the generator starts to increase
again. But this time, total mass is increased due to increase
in structural materials which can be observed from the nearly
constant material cost.
Secondly, the optimization has been performed without any
limits on the diameter. The GA has been initiated with a
population of 1500, and then has been run for 60 generations
with a population of 300. The output of the optimization
is a generator with 3.2 m diameter. The generator consists
of eight parallel generators stacked axially. In Fig. 7(a), the
optimised generator is shown with a human figure for scaling.
The cross-section view of the generator is given in Fig. 7(b).
Full specifications of the generator are presented in Table I.
IV. C ONCLUSION
In this paper, an axial flux PM direct drive generator for
MCT’s SeaGen tidal turbine has been designed. It is showed
that the C-GEN topology is feasible for tidal turbine applications. It is easier to obtain the most suitable design over all
operating conditions using the GA optimization with a coupled
multi-physics analytical design tool. The most important issue
in the electrical machine optimization is a realistic definition of
the objective function. In this study, the electricity generation
income as well as the material cost have been included in the
objective function to find the most cost-effective solution.
The optimized 600 kW direct-drive generator for the
SeaGen weights 17.5 tonnes, which is comparable with the
actual power-train of the device. The efficiency of the generator can be increased by better utilization of tidal currents as
a passive cooling medium. The removal of gearbox decreased
the moving parts in the power take-off increasing the robustness of the system. Also paralleled structure of the C-GEN
generator provides a fault-proof system. The generator is able
to continue to operate at reduced rating even one of the stages
had to be turned off due to a fault. All these features show
that, the C-GEN direct-drive generator is a feasible option for
tidal energy converters.
ACKNOWLEDGMENT
This research is funded by NPower Juice. The authors would
like to thank Peter Fraenkel and Marine Current Turbines for
their support and collaboration.
R EFERENCES
[1] M. Mueller and A. McDonald, “A lightweight low-speed permanent
magnet electrical generator for direct-drive wind turbines,” Wind Energy,
vol. 12, no. 8, pp. 768–780, 2009.
[2] E. Commission, “The Exploitation of Tidal Marine Currents (Report:
EUR 16683 EN),” IT Power Ltd, 1996.
[3] E. Denny, “The economics of tidal energy,” Energy Policy, vol. 37, no. 5,
pp. 1914–1924, May 2009.
[4] S. Elghali, Ben Eddine, M. E. H. Benbouzid, T. Ahmed-Ali, and J. F.
Charpentier, “High-Order Sliding Mode Control of a Marine Current
Turbine Driven Doubly-Fed Induction Generator,” IEEE Journal of
Oceanic Engineering, vol. 35, no. 2, pp. 402–411, Apr. 2010.
[5] P. Fraenkel, “Marine current turbines: pioneering the development of
marine kinetic energy converters,” Proc.of the Inst.of Mechanical Engineers, Part A: Journal of Power and Energy, vol. 221, no. 2, pp.
159–169, Jan. 2007.
[6] P. Frankel, “Tidal Current Energy Technologies,” Ibis, vol. 148, pp. 145–
151, Mar. 2006.
[7] M. Mueller and N. Baker, “Direct drive electrical power take-off for
offshore marine energy converters,” Proc.of the Inst.of Mechanical
Engineers, Part A: Journal of Power and Energy, vol. 219, no. 3, pp.
223–234, Jan. 2005.
[8] “OpenHydro,
UK.”
[Online].
Available:
http://www.openhydro.com/home.html
[9] “Clean
Current,
Canada.”
[Online].
Available:
http://www.cleancurrent.com/index.htm
[10] “Swanturbines,
UK.”
[Online].
Available:
http://www.swanturbines.co.uk/
[11] J. King and T. Tryfonas, “Tidal stream power technology - state of the
art,” Oceans 2009-Europe, no. 2, pp. 1–8, May 2009.
[12] “Seagen
Facts.”
[Online].
Available:
http://www.seageneration.co.uk/downloads/recent/General
Documents/Seagen Facts EXTERNAL 2.pdf
[13] “Island
Environmental
Accountancy.”
[Online].
Available:
http://ieaci.com/photos for web/Sea-Gen-2.jpg
[14] A. McDonald, “Structural analysis of low speed, high torque electrical
generators for direct drive renewable energy converters,” PhD Thesis,
University of Edinburgh, UK, 2008.
[15] A. McDonald, M. Mueller, R. Crozier, S. Caraher, and J. P. Chick,
“Integrated design of direct-drive linear generators for wave energy
converters,” 2009 International Conference on Sustainable Power Generation and Supply, pp. 1–7, 2009.
[16] O. Keysan, A. McDonald, and M. Mueller, “Integrated Design and
Optimization of a Direct Drive Axial Flux Permanent Magnet Generator
for a Tidal Turbine,” in International Conference on Renewable Energies
and Power Quality - ICREPQ’10, Granada, 2010.
[17] O. Keysan, A. McDonald, M. Mueller, R. Doherty, and M. Hamilton, “CGEN, a lightweight direct drive generator for marine energy converters,”
5th IET International Conference on Power Electronics, Machines and
Drives (PEMD 2010), pp. 1–6, 2010.
[18] M. Mueller, N. Hodgins, W. Tease, and D. Staton, “Measurement and
modelling of induction generator performance in an oscillating water
column wave energy converter,” 4th IET International Conference on
Power Electronics, Machines and Drives (PEMD 2008), no. 1, pp. 76–
80, 2008.
[19] W. Rong-Jie, K. Van Der Westhuizen, M. Kamper, and J. F. Gieras, “Optimal design of a coreless stator axial flux permanent-magnet generator,”
IEEE Transactions on Magnetics, vol. 41, no. 1, pp. 55–64, 2005.
[20] S.-Y. Jung, H. Jung, S.-C. Hahn, H.-K. Jung, and C.-G. Lee, “Optimal
Design of Direct-Driven PM Wind Generator for Maximum Annual
Energy Production,” IEEE Trans. on Magnetics, vol. 44, no. 6, pp. 1062–
1065, Jun. 2008.
[21] “The
Office
of
Gas
and
Electricity
Markets
Renewables
Obligation.”
[Online].
Available:
http://www.ofgem.gov.uk/sustainability/environment/renewablobl
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