A review of wave energy converter technology

Abstract

Ocean waves are a huge, largely untapped energy resource, and the potential for extracting energy from waves is considerable. Research in this area is driven by the need to meet renewable energy targets, but is relatively immature compared to other renewable energy technologies. This review introduces the general status of wave energy and evaluates the device types that represent current wave energy converter (WEC) technology, particularly focusing on work being undertaken within the United Kingdom. The possible power take-off systems are identified, followed by a consideration of some of the control strategies to enhance the efficiency of point absorber-type WECs. There is a lack of convergence on the best method of extracting energy from the waves and, although previous innovation has generally focused on the concept and design of the primary interface, questions arise concerning how best to optimize the powertrain. This article concludes with some suggestions of future developments.

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DOI: 10.1243/09576509JPE782
2009 223: 887Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy
B Drew, A R Plummer and M N Sahinkaya
A review of wave energy converter technology
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REVIEW PAPER 887
A review of wave energy converter technology
BDrew
∗
, A R Plummer, and M N Sahinkaya
Department of Mechanical Engineering, University of Bath, Bath, UK
The manuscript was received on 26 March 2009 and was accepted after revision for publication on 16 June 2009.
DOI: 10.1243/09576509JPE782
Abstract: Ocean waves are a huge, largely untapped energy resource, and the potential for
extracting energy from waves is considerable. Research in this area is driven by the need to meet
renewable energy targets, but is relatively immature compared to other renewable energy tech-
nologies. This review introduces the general status of wave energy and evaluates the device types
that represent current wave energy converter (WEC) technology, particularly focusing on work
being undertaken within the United Kingdom. The possible power take-off systems are identified,
followed by a consideration of some of the control strategies to enhance the efficiency of point
absorber-typeWECs. There is a lack of convergence on the best method of extracting energy from
the waves and, although previous innovation has generally focused on the concept and design
of the primary interface, questions arise concerning how best to optimize the powertrain. This
article concludes with some suggestions of future developments.
Keywords: wave energy converter, power-take-off, wave power
1 INTRODUCTION
Despite being discussed in patents since the late 18th
century [1], modern research into harnessing energy
from waves was stimulated by the emerging oil crisis of
the 1970s (for example, see reference [2]). With global
attention now being drawn to climate change and the
rising level of CO
2
, the focus on generating electricity
from renewable sources is once again an important
area of research.
It is estimated that the potential worldwide wave
power resource is 2 TW [3], with the UK’s realistic
potential being 7–10 GW [4]. To put these figures into
perspective, the UK’s total grid capacity is 80 GW, with
peak demand stabilized at around 65 GW (George
Ulewande, nPower, 2008, personal communication).
As such, up to 15 per cent of current UK electricity
demand couldbe met by wave energy; when combined
with tidal stream generation, up to 20 per cent of the
UK demand could be met [5].
There are several reviews of wave energy converter
(WEC) concepts (for example, see references [2], [3],
[6], and [7]). These show that many wave energy
devices are being investigated, but many are at the
R&D stage, with only a small range of devices having
∗
Corresponding author: Department of Mechanical Engineering,
University of Bath, Claverton Down, Bath, BANES, BA2 7AY, UK.
email: b.drew@bath.ac.uk
been tested at large scale, deployed in the oceans. The
LIMPET shoreline oscillating water column (OWC),
installed at Islay, Scotland, in 2000 represents one sys-
tem that is currently producing power for the National
Grid [8]. In September 2008, another commercial wave
power system started operating in Northern Portugal.
It makes use of the Pelamis power generating device
built by Pelamis Wave (formerly OPD) in Scotland (see
section 2.4.1 for more information).
1.1 Benefits
Using waves as a source of renewable energy offers
significant advantages over other methods of energy
generation including the following:
1. Sea waves offer the highest energy density among
renewable energy sources [6]. Wa ves are gener-
ated by winds, which in turn are generated by
solar energy. Solar energy intensity of typically 0.1–
0.3 kW/m
2
horizontal surface is converted to an
average power flow intensity of 2–3 kW/m
2
of a ver-
tical plane perpendicular to the direction of wave
propagation just below the water surface [9].
2. Limited negative environmental impact in use.
Thorpe [3] details the potential impact and presents
an estimation of the life cycle emissions of a typical
nearshore device. In general, offshore devices have
the lowest potential impact.
JPE782 Proc. IMechE Vol. 223 Part A: J. Power and Energy

888 B Drew, A R Plummer, and M N Sahinkaya
3. Natural seasonal variability of wave energy, which
follows the electricity demand in temperate cli-
mates [6].
4. Waves can travel large distances with little energy
loss. Storms on the western side of the Atlantic
Ocean will travel to the western coast of Europe,
supported by prevailing westerly winds.
5. It is reported that wave power devices can generate
power up to 90 per cent of the time, compared to
∼20–30 per cent for wind and solar power devices
[10, 11].
1.2 Challenges
To realize the benefits listed above, there are a number
of technical challenges that need to be overcome to
increase the performance and hence the commercial
competitiveness of wave power devices in the global
energy market.
A significant challenge is the conversion of the slow
(∼0.1 Hz), random, and high-force oscillatory motion
into useful motion to drive a generator with output
quality acceptable to the utility network. As waves
vary in height and period, their respective power lev-
els vary accordingly. While gross average power levels
can be predicted in advance, this variable input has to
be converted into smooth electrical output and hence
usually necessitates some type of energy storage sys-
tem, or other means of compensation such as an array
of devices.
Additionally, in offshore locations, wave direction
is highly variable, and so wave devices have to align
themselves accordingly on compliant moorings, or be
symmetrical, in order to capture the energy of the
wave. The directions of waves near the shore can be
largely determined in advance owing to the natural
phenomena of refraction and reflection.
The challenge of efficiently capturing this irregular
motion also has an impact on the design of the device.
To operate efficiently, the device and corresponding
systems have to be rated for the most common wave
power levels. Around the British Isles and the western
coasts of Europe, the most common offshore waves
are around 30–70 kW/m [12]. However, the device also
has to withstand extreme wave conditions that occur
very rarely, but could have power levels in excess of
2000 kW/m. Not only does this pose difficult struc-
tural engineering challenges, but it also presents one of
the economic challenges as the normal output of the
device (and hence the revenue) are produced by the
most commonly occurring waves, yet the capital cost
of the device construction is driven by a need to with-
stand the high power level of the extreme, yet infre-
quent, waves [13]. There are also design challenges in
order to mitigate the highly corrosive environment of
devices operating at the water surface [6].
Lastly, the research focus is diverse. To date,
the focus of the wave energy developers and a
considerable amount of the published academic work
has been primarily on sea performance and survival,
as well as the design and concept of the primary wave
interface. However, the methods of using the motion of
the primary interface toproduce electricity are diverse.
More detailed evaluation of the complete system is
necessary if optimized, robust yet efficient systems are
to be developed
2 WAVE ENERGY CONVERTERS
There is a large number of concepts for wave energy
conversion; over 1000 wave energy conversion tech-
niques have been patented in Japan, North America,
and Europe [6]. Despite this large variation in design,
WECs are generally categorized by location and type.
2.1 Location
Shoreline devices have the advantage of being close
to the utility network, are easy to maintain, and as
waves are attenuated as they travel through shallow
water they have a reduced likelihood of being dam-
aged in extreme conditions. This leads to one of the
disadvantages of shore mounted devices, as shallow
water leads to lower wave power (this can be partially
compensated by natural energy concentrated loca-
tions [6]). Tidal range can also be an issue. In addition,
by nature of their location, there are generally site-
specific requirements including shoreline geometry
and geology, and preservation of coastal scenery, so
devices cannot be designed for mass manufacturing.
Nearshore devices are defined as devices that are
in relatively shallow water (there is a lack of consen-
sus of what defines ‘shallow’ water, but it has been
suggested that this could be a depth of less than one-
quarter wavelength [4]). Devices in this location are
often attached to the seabed, which gives a suitable
stationary base against which an oscillating body can
work. Like shoreline devices, a disadvantage is that
shallow water leads to waves with reduced power,
limiting the harvesting potential.
Offshore devices are generally in deep water
although again there is little agreement about what
constitutes ‘deep’ water. ‘Tens of metres’ is one
definition [5], with ‘greater than 40 m’ [4], and ‘a depth
exceeding one-third of the wavelength’ [9] being oth-
ers.The advantage of siting a WEC in deep water is that
it can harvest greater amounts of energy because of the
higher energy content in deep water waves [4]. How-
ever, offshore devices are more difficult to construct
and maintain, and because of the greater wave height
and energy content in the waves, need to be designed
to survive the more extreme conditions adding cost to
construction. Despite this, it is argued that with more
powerful waves, floating devices in deep water offer
greater structural economy [14].
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A review of wave energy converter technology 889
It is useful to note that wave energy occurs in
the movements of water near the surface of the sea
[5]. Up to 95 per cent of the energy in a wave is
located between the water surface and one-quarter of
a wavelength below it [4].
2.2 Type
Despite the large variation in designs and concepts,
WECs can be classified into three predominant types.
2.2.1 Attenuator (A)
Attenuators lie parallel to the predominant wave direc-
tion and ‘ride’ the waves. An example of an attenuator
WEC is the Pelamis, developed by Ocean Power Deliv-
ery Ltd (now known as Pelamis Wave Power [15]).
Figure 1 shows an artist’s impression of a Pelamis
wave farm. See section 2.4.1 for more details about
this particular WEC.
2.2.2 Point absorber (B)
A point absorber is a device that possesses small
dimensions relative to the incident wavelength. They
can be floating structure that heave up and down
on the surface of the water or submerged below the
surface relying on pressure differential. Because of
their small size, wave direction is not important for
these devices. There are numerous examples of point
absorbers, one of which is Ocean Power Technology’s
Powerbuoy [16]. Figure 2 shows an artist’s impression
of a wave farm using Powerbuoys.
2.2.3 Terminator (C)
Terminator devices have their principal axis parallel
to the wave front (perpendicular to the predominant
wave direction) and physically intercept waves. One
example of a terminator-typeWEC is the Salter’s Duck,
developed at the Unviersity of Edinburgh, as shown in
Fig. 3 (for more details, see section 2.4.2).
Fig. 1 Attenuator device: Pelamis wave farm [17]
Fig. 2 Point absorber device: OPT Powerbuoy [18]
Fig. 3 Terminator device: Salter’s Duck [19]
2.3 Modes of operation
Within the categories identified above, there is a
further level of classification of devices, determined
by their mode of operation. Some significant examples
are given below.
2.3.1 Submerged pressure differential
The submerged pressure differential device is a sub-
merged point absorber that uses the pressure dif-
ference above the device between wave crests and
troughs. It compr ises two main parts: a sea bed fixed
air-filled cylindrical chamber with a moveable upper
cylinder. As a crest passes over the device, the water
pressure above the device compresses the air within
the cylinder, moving the upper cylinder down. As a
trough passes over, the water pressure on the device
reduces and the upper cylinder rises. An advantage
of this device is that since it is fully submerged, it is
not exposed to the dangerous slamming forces experi-
enced by floating devices [20], and reduces the visual
impact of the device. Maintenance of the device is a
possible issue however. Owing to part of the device
being attached to the sea bed, these devices are typ-
ically located nearshore. An example of this device is
the Archimedes Wave Swing, an artist’s impression of
which is shown in Fig. 4.
JPE782 Proc. IMechE Vol. 223 Part A: J. Power and Energy

890 B Drew, A R Plummer, and M N Sahinkaya
Fig. 4 Submerged pressure differential: the Archimedes
Wave Swing [21]
2.3.2 Oscillating wave surge converter
An oscillating wave surge converter is generally com-
prised of a hinged deflector, positioned perpendicular
to the wave direction (a terminator), that moves back
and forth exploiting the horizontal particle velocity
of the wave. An example is the Aquamarine Power
Oyster [22], a nearshore device, where the top of the
deflector is above the water surface and is hinged
from the sea bed. A prototype of this device has been
constructed. Figure 5 illustrates the device.
2.3.3 Oscillating water column
An OWC consists of a chamber with an opening to the
sea below the waterline. As waves approach the device,
water is forced into the chamber, applying pressure on
the air within the chamber. This air escapes to atmo-
sphere through a turbine. As the water retreats, air is
then drawn in through the turbine. A low-pressure
Wells turbine is often used in this application as it
rotates in the same direction irrespective of the flow
Fig. 5 Oscillating wave surge converter: Aquamarine
Power Oyster [23]
direction, removing the need to rectify the airflow. It
has been suggested that one of the advantages of the
OWC concept is its simplicity and robustness [4].
There are examples of OWCs as point absorbers, as
well as being built into the shoreline, where it acts as a
terminator. An example of a shoreline mounted device
is the Wavegen Limpet. The device is installed on the
island of Islay, Western Scotland, and produces power
for the national grid. Figure 6 shows the design of the
Limpet. The OWC concept has also been proposed by
Oceanlinx, an Australian wave energy developer, in a
nearshore tethered device [24].
Fig. 6 OWC: the Limpet [25]
Fig. 7 Overtopping WEC: the Wave Dragon [26, 27]
Proc. IMechE Vol. 223 Part A: J. Power and Energy JPE782

A review of wave energy converter technology 891
2.3.4 Overtopping device
An overtopping device captures sea water of incident
waves in a reservoir above the sea level, then releases
the water back to sea through turbines. An example of
such a device is the Wave Dragon, which is shown in
Fig. 7. This device uses a pair of large curved reflectors
to gather waves into the central receiving part, where
they flow up a ramp and over the top into a raised
reservoir, from which the water is allowed to return to
the sea via a number of low-head turbines.
2.4 UK-based research and development
2.4.1 Commercial operations
There are many companies currently developing wave
power devices. UK-based companies are listed in
Table 1. A more exhaustive list of companies worldwide
can be found at the European Marine Energy Centre
(EMEC) website [28].
The Pelamis device deserves more details as it is the
offshore device closest to commercial operation (the
Wavegen Limpet, while it also operates commercially,
is a shoreline mounted OWC).The Pelamis is a floating
device comprised of cylindrical hollow steel segments
(diameter of 3.5 m) connected to each other by two
degree-of-freedom hinged joints. Each hinged joint is
similar to a universal joint, with the central unit of
each joint containing the complete power conversion
system. The wave-induced motion of these joints is
resisted by four hydraulic cylinders that accommodate
both horizontal and vertical motion. These cylinders
act as pumps, which drive fluid through a hydraulic
motor, which in turn drives an electrical generator.
Accumulators are used in the circuit to decouple the
primar y circuit (the pumps) with the secondary cir-
cuit (the motor), and aid in regulating the flow of fluid
to produce a more constant generation. The hydraulic
power take off (PTO) system uses only commercially
available components.
Each Pelamis is 120 m long, and contains three
power modules, each rated at 250 kW. It is designed
to operate in water depths of ∼50 m. The shape
and loose mooring of Pelamis lets it orient itself to
the predominant wave direction, and its length is
such that it automatically ‘detunes’ from the longer-
wavelength high-power waves, enhancing its surviv-
ability in storms [29]. A wave farm using Pelamis
technology was recently installed in Aguçadora Wave
Park, about 3 miles from Portugal’s northern coast,
near Pòvoa do Vorzina. This followed full-scale pro-
totype testing at the EMEC facility in Orkney [30]. The
wave farm initially uses three Pelamis P-750 machines
developing a total power of 2.25 MW.
2.4.2 University-based research
Working in parallel and sometimes collaboratively
with the wave energy developer companies listed in
Table 1, are universities and other research institutes.
The primary ones identified in this study are listed
below.
1. The University of Edinburgh: The Wave Power
Group at the University of Edinburgh [31], founded
in 1974, is very active inWEC development. Stephen
Salter, one of the early pioneers in wave energy
research, was based at Edinburgh, and thus much
work was focused on the Salter’s nodding ‘duck’
device. The Duck concept is reported to be theoret-
ically one of the most efficient devices, and the pri-
mary interface is able to absorb 100 per cent of the
energy contained in a wave [20]. The efforts to effi-
ciently convert motion from this device to electric-
ity have motivated research into efficient hydraulic
drives [32, 33]. The University of Edinburgh is also
involved in the EquiMar project (see below).
2. Lancaster University: There is a wide range of
wave energy projects in which the Lancaster
University Renewable Energy Group (LUREG) is
involved. They were involved in Supergen 1 and
Table 1 UK-based wave energy developers
Company Website Device type
Aquamarine Power www.aquamarinepower.com C
AWS Ocean Energy www.awsocean.com B
Checkmate Seaenergy (Anaconda) www.checkmateuk.com/seaenergy A
C-Wave www.cwavepower.com A/C
Embley Energy (Sperboy) www.sperboy.com B
Green Ocean Energy Ltd www.greenoceanenergy.com A
Neptune Renewable Energy Ltd www.neptunerenewableenergy.com C
Ocean Navitas www.oceannavitas.com B
Ocean Power Technology www.oceanpowertechnology.com B
Offshore Wave Energy Ltd www.owel.co.uk C
ORECon www.orecon.com B
Pelamis www.pelamiswave.com A
Trident Energy www.tridentenergy.co.uk B
Wave Dragon www.wavedragon.net C
Wavegen www.wavegen.com C
JPE782 Proc. IMechE Vol. 223 Part A: J. Power and Energy

892 B Drew, A R Plummer, and M N Sahinkaya
now Supergen 2 (see below), along with the Car-
bon Tr ust’s Marine Energy Accelerator Programme
and NaREC’s development test rig (see below for
more information on NaREC). They work in a wide
range of fields within wave energy, looking at new
device designs, numerical modelling and control,
PTO development (with a focus on electrical linear
generation), and device evaluation.
3. PRIMaRE (Universities of Plymouth and Exeter):
The Peninsular Research Institute for Marine
Renewable Energy is a collaboration between the
universties of Exeter and Plymouth to research
marine renewable energy [34]. This recently estab-
lished group undertakes inter-disciplinary research
and is directly linked to the SWERDA-funded Wave
Hub project [35] (see below for more details). The
University of Exeter is also involved in the EquiMar
project (see below).
2.4.3 Wave energy projects and organizations
The list below includes some of the wave energy
projects currently being undertaken, with links to
UK institutions and companies. The following have
technical content, but there are a number of other
wave energy organizations and projects that focus on
promotion, economics, marketing, and other aspects
to speed up the introduction of marine energy (for
example Waveplam [36] and WavEC [37]).
1. EMEC: The European Marine Energy Centre was
set up in 2003 with funding partly from the Car-
bon Trust and aims to stimulate and accelerate
the development of marine power devices, ini-
tially through the operation of a testing centre
in Orkney [30]. Similar to the planned Wave Hub
project, the facilities at EMEC are test berths, with
electrical connections enabling wave energy device
developers to test full-scale prototypes.
2. EquiMar: The Equimar Project (Equitable Test-
ing and Evaluation of Marine Energy Extraction
Devices in terms of performance, cost, and envi-
ronmental impact) is an FP7-funded (European
Commission) collaborative research and develop-
ment project that involved a consortium of 23
partners and will run for 3 years from 15 April 2008.
The aim of the EquiMar project is to ‘deliver a suite
of protocols for the equitable evaluation of marine
energy converters. These protocols will harmonize
testing and evaluation procedures across the wide
variety of devices’. These protocols will be used to
establish a base for future marine energy standards.
More details about this project can be found in
reference [38].
3. Marine Energy Accelerator: The Carbon Trust’s
current Marine Energy Accelerator is an initia-
tive to reduce the costs associated with marine
energy technologies, bringing forward the time
when marine energy devices can contribute to
emissions reduction [39]. It follows on from the
Marine Energy Challenge project [5], which sup-
ported development and understanding of wave
and tidal stream energy technologies.
4. SuperGen (Sustainable Power Generation and Sup-
ply): Engineering and Physical Sciences Research
Council supported research (in par tnership with
Biotechnology and Biological Sciences Research
Council, Economic and Social Research Coun-
cil, Natural Environment Research Council, and
the Carbon Trust) in sustainable power genera-
tion and supply, enabling the UK to meet envi-
ronmental emissions targets. The Marine Energy
Research Consoritum, led by Edinburgh Univer-
sity, and including LUREG (Lancaster), Heriot-Watt,
Robert Gordon University, Strathclyde and Queen’s
University Belfast, aims to increase the knowledge
and understanding of extraction of energy from the
sea. Funding is £2.6 million over 4 years. More infor-
mation about the SuperGen Marine, currently in its
second phase, can be found in reference [40].
5. Wave Hub: The aim of the Wave Hub project is to
construct a wave farm demonstration/evaluation
site off the nor thern coast of Cornwall [36]. The
budget of the project is £28 million, with £21.5
million coming from the South West of England
Regional Development Agency. The project will lay
a high-voltage cable 10 miles out to sea and con-
nect it to the National Grid. Companies will be able
to test their WECs in a leased and consented area of
sea. It is reported that the installation of Wave Hub
is planned for Spring 2010.
More details on other wave energy projects and
operations world wide can be found in references [3],
[6], [7], [9]to[11], and [41].
3 POWER TAKE OFF METHODS
The method of energy capture varies from device
to device, but with the exception of linear electri-
cal generation, discussed later, the general method of
producing electrical power is through conventional
high-speed rotary electrical generators [42]. One of
the major challenges of WECs is concerned with how
to drive these generators. Heaving- and nodding-type
devices are not directly compatible with conventional
rotary electrical machines, and a transmission system
is required to interface the WEC with the electrical
generator [20].
In this section, different types of rotar y generators
are briefly presented, followed by an overview of differ-
ent energy transfer methods. This starts with turbine
transfer, moving on to hydraulic conversion methods,
and then discussing direct electrical linear generators,
Proc. IMechE Vol. 223 Part A: J. Power and Energy JPE782

A review of wave energy converter technology 893
Fig. 8 Alternative PTO mechanisms
which could be considered as a competing technology.
These different PTO mechanisms are shown in Fig. 8.
3.1 Rotary generator types
Traditional power stations use on-line synchronous
generators (SGs), and are operated at a virtually
constant speed, matching the frequency of the grid
connection. Depending on the conversion system,
generators used for wave energy may have to cope
with variable speed. Four generator types are identi-
fied: doubly fed induction generators (DFIG), squirrel
cage induction generators, permanent magnet SGs,
and field would SGs.
O’Sullivan and Lewis [43] discuss these generator
options in terms of suitability for an OWC applica-
tion, by examining the advantages and disadvantages
in terms of environmental, electrical, and cost factors,
and by using a time-domain MATLAB model. The gen-
erators in OWC devices typically operate at variable
speed. There are similarities in this application with
the mature technologies currently used in wind tur-
bines. The favoured generators used in wind turbines
(DFIG driven via a gear box, and direct drive low-speed
SG with dedicated power electronics) are possible can-
didates for use in OWC WECs. O’Sullivan and Lewis’s
study concludes that the latter, the SG, are the pre-
ferred option due to its better energy yield, weight, and
controllability, despite the requirement for a full fre-
quency converter between the generator and the grid.
The significant disadvantage of the DFIG is its mainte-
nance requirement; DFIGs are not brushless machines
– a significant issue in offshore WECs.
Linear electrical generators are discussed in greater
detail in section 3.4.
3.2 Turbine transfer
‘Turbine transfer’ is the term used here to represent
the method employed in devices where the flow of
fluid (either sea water or air) drives a turbine, which
is directly coupled to a generator. The types of devices
using direct transfer include OWCs and overtopping
devices.
As discussed above, the requirements for genera-
tors in OWCs, such as variable speed input, are similar
to those of a wind turbine, and thus have been well
researched (for example, see references [44]to[46]).
The significant advantage of using sea water tur-
bines is that leakage of fluid causes no environmental
problems. The disadvantage is that sea water is a com-
plex fluid with various unpredictable constituents.
In addition, in nearshore devices, abrasive particles
could damage seals and valves. Cavitation could also
be a problem, unless the turbine is in deep water to
maintain positive pressure. In low-pressure situations,
experienced in overtopping devices, propeller-type
turbines are often used, such as the Kaplan design.
Using air as the working fluid has the advantage
of increasing the slow velocities of waves to high
air flow rate. The most popular air turbine design is
the Wells turbine, because of its ability to rotate in
the same direction, irrespective of airflow direction.
Inherent disadvantages include low efficiency (around
60–65 per cent [47, 48]), poor starting, high noise,
and high axial thrust when compared to traditional
turbines [49]. Pitch control of the turbine blades can
increase efficiency [50].
3.3 Hydraulics
Another method of converting the low-speed oscil-
lating motion of the primary WEC interface is to
employ a hydraulic system. Waves apply large forces
at slow speeds and hydraulic systems are well suited
to absorbing energy in these situations [51]. The use of
hydraulics operating at a pressure of 400 bar is a dis-
tinct advantage of some types of WEC where size and
weight are an issue [42], and the force created by these
pressures are considerably greater than those from the
best electrical machines.
Figure 9 is a circuit diagram for a basic hydraulic
PTO system for a WEC. The rod of the hydraulic cylin-
der is forced up and down by a floating buoy, which
forces fluid through check valves, rectifying the flow,
to a hydraulic motor. In this case, the generator could
be a constant speed device, and the hydraulic motor
has variable capacity, to drive the generator at close
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894 B Drew, A R Plummer, and M N Sahinkaya
Fig. 9 Typical hydraulic circuit for WEC
to constant speed despite a variable flowrate. The
control of the motor capacity could be based on mea-
sured or predicted sea states around the WEC, or fluid
flow measurements within the system. Additionally, a
throttling valve could alsobe used to control the flow to
the motor. Accumulators are included in the circuit to
provide energy storage and to maintain constant flow
to the hydraulic motor. In addition, the low-pressure
accumulator provides a small boost pressure to reduce
the risk of cavitation on the low-pressure side.
If the incident waves are close to sinusoidal, then
the flow from one port of the actuator is represented
in Fig. 10(a). Rectification through the check valves
results in the flow represented in Fig. 10(b). The accu-
mulator would then smooth this, with the variable
capacity motor driving the generator.
The hydraulic circuit employed in the Pelamis WEC
(see section 2.4.1 for further details) essentially follows
the design shown in Fig. 9. The accumulators provide
a decoupling between the hydraulic cylinders and the
motors, providing enough energy storage to supply the
pumps with a relatively constant energy supply. Active
valves are employed to rectify the flow, and are also
used to vary the reaction torque. In reference [51], it is
noted that losses in the primary transmission can be
kept below 20 per cent over a wide range of operating
conditions.
Fig. 10 Flow-time representations for hydraulic WEC
There are a number of challenges associated with
hydraulic conversion systems in WECs. These include
the following.
3.3.1 Fluid containment
Containment of hydraulic fluid is of great importance,
as is ingress of sea water. The UK Department of Trade
and Industry (DTI) Wave Energy Programme spon-
sored a report to review the current status of wave
energy technologies [52]. This report highlighted fluid
containment as an issue for performance and environ-
mental reasons, with a recommendation to investigate
the development of hydraulic systems based on water
or other environmentally acceptable fluid.The Carbon
Trust Guidelines on design and operation ofWECs [53]
indicate pressure containment as the primary consid-
eration for the hydraulic system. There are a number of
alternative biodegradable fluids, but each comes with
various benefits and disadvantages. In addition, the
compatibility of the fluids with the components and
seals must be ensured. Using sea water as the work-
ing fluid may be the most environmentally friendly
method, but there are limitations in terms of leakage,
sealing, temperature, pressure, speed, size, cycling,
deposition of solids, biological growth, lubrication,
and corrosion.
The Pelamis WEC uses hydraulic fluid that is
biodegradable in the marine environment (biodegrad-
able transformer fluid). In addition, there is reported to
be two levels of egress/ingress protection in the for m of
flexible rubber bellows that would have to fail to allow
water to ingress to a point where fluid could escape to
the outside environment [51].
3.3.2 Sealing
The issue of sealing is linked to fluid containment.
In addition, however, standard dynamic seals are
designed to operate at velocities lower than a typical
WEC [42]. The temperature rise caused by shear loss
and friction at the moving interface is a serious con-
tributor to seal wear; the life of the seal is inversely
related to the speed, distance, and length of its appli-
cation, which in turn has an impact on maintenance
requirements. It is argued that the lack of ‘land-based
demand’ for high-velocity seals, rather than any tech-
nical difficulty, is the reason for the high-speed issues
with seals [50].
3.3.3 Efficiency
The efficiency of the PTO system is vital to the abil-
ity to harness the energy of the device. Traditional
hydrostatic transmissions tend to use coupled vari-
able displacement pumps and motors, which have an
ideal operating point and a peak efficiency of around
80 per cent. Away from this ideal operating point,
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A review of wave energy converter technology 895
efficiency drops away; the part-load losses (including
coulomb and viscous friction, leakage, and compress-
ibility) are significant. Although the hydraulic system
may have a high rating, it is reasonable that the device
will spend most of the time operating at a fraction
of this rating, and therefore the system must have
the highest part-load efficiencies [54]. The DTI report
[52] also highlights the efficiency issue of hydraulics,
recommending that work should focus on the devel-
opment of dedicated hydraulic motors with low part-
load losses and high torque pumps. In addition, the
check valves used to rectify the flow, and the throttling
valve to control the flow, have pressure drops associ-
ated with their orifices, leading to a loss in power and
reduced efficiency.
Driven by the need for very high-efficiency high-
pressure bi-directional oil hydraulic transmissions
that could implement the advanced control algo-
rithms required to obtain the most energy out of
waves, a novel digital displacement pump–motor con-
cept was developed [32, 50, 55]. The basic structure
of the pump–motor is similar to a conventional
radial piston pump–motor, but includes electro-
magnetically operated poppet valves for each cylinder,
allowing the device to go from full output to zero
output in one revolution. Part-load efficiency is also
greatly increased due to the complete deactivation of
cylinders. A prototype digital displacement transmis-
sion is currently being developed for a wind turbine
application, obviating the need for heavy gearboxes
or full-power electronics and frequency converters
for low-speed electrical generators [56]. A similar
device is currently being developed for marine energy
applications [57, 58].
3.3.4 Maintenance
Carrying out maintenance in the marine environ-
ment is expensive, time-consuming, and poses many
risks. In a hydraulic conversion system, there are likely
to be several stages between the primary interface
and the electrical generator, each comprising mov-
ing parts, and thus may require maintenance. It is
important that the required maintenance is mini-
mized, preferably only requiring inspection annually
or less [52]. In addition, metal sur faces and compo-
nents must be protected from corrosion and erosion.
Ceramic coatings (such as Ceramax, manufactured
by Bosch Rexroth) offer a promising method of pro-
tecting the components in direct contact with sea
water.
One method that could be employed to minimize
maintenance costs (and potentially reduce the possi-
bility of leakage from hydraulic devices) would be to
position the hydraulic PTO system at the shore. This
has received limited interest due to the long, costly,
and inefficient pipework required to transport the fluid
from the offshore or nearshore device to shore, and the
associated significant power loss.
3.3.5 End-stop
The end-stop issue is not exclusive to devices employ-
ing a hydraulic PTO system; it applies to all moving
body converters with rigid connections to PTOs. The
problem arises from the oscillating interface exceed-
ing its design travel. With a hydraulic transfer system,
the oscillating interface could be connected to linear
hydraulic rams used to pump fluid to the motor. The
high forces and corresponding energy experienced
in extreme conditions cannot be suddenly absorbed
by hitting the end of cylinder stroke, damaging the
system. Mitigating this by employing high-stroke actu-
ators is compromised by their mass and expense, and
their stroke capability will not be exploited most of
the time. Buckling of extended stroke actuators may
also be an issue, particularly if side loads are present
at maximum extension.
Methods to mitigate end-stop issues with hydraulic
actuators include specific designs that mechanically
limit the stroke (Pelamis uses this technique), or are
based on rotation, in which case a radial piston pump
can be employed (the SEAREV [59] uses this method).
A winch mechanism could also be employed to drive
a rotary pump, and Salter has also examined a rotary
machine for use with the duck WEC [50].
There are other designs such as the inter project
service heaving buoy [21] (such as AquaBuOY [60])
that does not suffer from the end-stop issue. The inter
project service concept consists of a long tube, open at
both ends attached to a floating buoy. Within the tube
is a piston. As the buoy heaves, the water within the
tube forces the piston to move relative to the buoy. As
the tube is open at both ends, the concept does not
suffer from the ‘end-stop’ issue.
3.3.6 Energy storage
Some form of energy storage is usually incorporated
in a PTO system, as the fluctuations in absorbed wave
power will result in very variable electrical power
output, which is unsuitable for the grid [61]. Accu-
mulators can function as short-term energy storage
as part of the hydraulic system. By storing energy,
accumulators would help the system deal with the
high level of variance, reducing the capital cost and
power losses of all subsequent powertrain elements
[50]. The Pelamis hydraulic PTO system uses accu-
mulators to provide a ‘smooth’ flow to the hydraulic
motors, and are used to separate the primary transmis-
sion (hydraulic cylinders and their controls) from the
secondary transmission (hydraulic motors and elec-
tric generators). It is argued that this separation allows
for efficient absorption over a large range of incident
power; up to 80 per cent is reported [51].
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896 B Drew, A R Plummer, and M N Sahinkaya
3.4 Electrical linear generation
During early wave power research, the possibility of
using electrical linear generators was investigated. The
conclusions at this stage were that these machines
would be too heavy, inefficient, and expensive. New
magnetic materials and the reduced costs of frequency
converting electronics mean that this technology may
now be possible. It is argued that the increased com-
plexity of hydraulic or turbine systems introduce reli-
ability and maintenance issues, which are important
to minimize in offshore environments [20, 62].
A linear SG offers the possibility of directly convert-
ing mechanical energy into electrical energy. The elec-
trical direct drive PTO alternative, as shown previously
in Fig. 8, is much simpler than hydraulic systems, with
no intermediate steps between the primary interface
and the electrical machine.
Conventional electrical machines are designed to
be driven with high-speed rotary motion. The airgap
speed between the rotor and stator in these machines
can be high (upwards of 60 m/s) allowing for easy con-
version into a rapid change in flux. Linear oscillatory
motion from a WEC, however, is expected to have
a peak of around 2 m/s [43]. Developments for the
wind power industry have focused on direct drive gen-
erators (to replace unreliable and heavy gearboxes).
These direct drive generators have an airgap speed
of 5–6 m/s. The development of linear electrical gen-
erators requires continuing research into slow-speed
electrical machines.
The basic concept of a linear generator is to have
a translator (what would be the rotor in a rotary
machine) on which magnets are mounted with alter-
nating polarity directly coupled to a heaving buoy, with
the stator containing windings, mounted in a rela-
tively stationary structure (connected to a drag plate,
a large inertia, or fixed to the sea bed). As the heav-
ing buoy oscillates, an electric current will be induced
in the stator. The schematic of this device is shown
in Fig. 11.
Fig. 11 A schematic of a linear electricalgenerator based
on a permanent magnet generator
3.4.1 Linear generator types
The need for very low maintenance machines implies
the use of brushless generators. Baker and Mueller rule
out induction machines (induction machines have
been used in wind turbines, because of their ability
to cope with varying speeds), which require a large
minimum pole pitch to achieve sufficient flux density
and so are too large, and reluctance machines, owing
to their very small airgaps, which are difficult to main-
tain. The focus lies in permanent magnet machines
[20]. The development of high-energy density per-
manent magnets such as Neodymium–Iron–Boron
(Nd–Fe–B), which can produce high magnetomotive
force for a relatively small magnet height, has sig-
nificantly improved the power density of permanent
magnet machines.
Part of the SuperGen project, mentioned in section
2.4.3, covers PTO and conditioning, and confirms that
induction machines are not suitable because of the
low-speed motion of WECs. Permanent magnet gen-
erators exhibit high part load efficiencies and, while
they have been demonstrated at sea to a limited extent,
designs are not yet fully optimized [40].
There are three main topologies of linear electrical
generators:
(a) longitudinal flux permanent magnet generators;
(b) variable reluctance permanent magnet generators
(with transverse flux permanent magnet genera-
tors as a subset of these);
(c) tubular air-cored permanent magnet generators.
The design of electrical generators for direct drive
WECs was examined by Mueller [42], by comparing
the longitudinal flux permanent magnet machine with
the transverse flux permanent magnet machine. He
identified that the transverse flux machine as having
the best potential, owing to the design having higher
power density and efficiency, compared to the longi-
tudinal design. Despite the high shear stress offered
by transverse flux machines (up to 200 kN/m
2
[63]),
their topology requires str uctural support and they
suffer from low power factor requiring reactive power
compensation [64].
Baker et al. [65] discuss the permanent magnet
vernier hybrid machine (a type of variable reluc-
tance permanent magnet generator) and the air-cored
machine, both of which attempt to solve some of the
issues with the transverse flux machine. Although the
vernier hybrid machine offered a high shear stress,
it suffered from a low power factor, requiring a high
rating power electronic converter, and bearing issues
owing to large magnetic attraction forces. The tubular
air-cored machine, being developed at the Univer-
sity of Durham, has no attraction forces (and thus a
less complex suppor t structure) and low inductance,
which results in a high power factor (thus necessitating
a lower rated power electronic converter), but suffers
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A review of wave energy converter technology 897
from having significantly less shear stress. These two
topologies are also discussed in reference [66].
Leijon et al. [67] have conducted multiphysics simu-
lations of a three-phase linear permanent magnet gen-
erator (using the Archimedes Wave Swing as the target
device), with the results confirming its potential. This
work was followed by an experimental setup that suc-
cessfully verified the simulations [13]. The work also
briefly discussed the interconnection scheme when
dealing with an array of devices. Polinder et al. [68]
also discuss the linear permanent magnet generator
designed for use with the ArchimedesWave Swing. The
authors highlighted that such a machine was chosen
because of its high force density, reasonable efficiency
at low speeds, the availability of cheaper magnets with
high power density, and the lack of electrical contact
to the translator. Reasonable correlation between cal-
culated results from simulations and measured results
from experimental testing indicates that the generator
is appropriate [69].
3.4.2 Signal processing
One issue of linear electrical generators is converting
the signal to one appropriate for grid connection. If
the motion of the WEC is sinusoidal, the induced EMF
varies in amplitude and frequency during a wave cycle.
As the translator reciprocates, the speed is continu-
ously changing, resulting in a var ying frequency of the
induced voltage. Figure 12 from reference [20] shows a
typical EMF plot from a variable reluctance permanent
magnet machine excited by a sinusoidal displacement.
For grid connection, this waveform, which is vari-
able in both frequency and amplitude, must be recti-
fied before conversion into a sinusoidal fixed voltage
and frequency waveform using power electronics. The
rectification can be passive or active. A passive rectifier
can be a simple diode bridge, and this is characterized
by having a power factor of one. The active power can
Fig. 12 Typical EMF plot [20]
be increased if the power factor is not equal to one,
which can be accommodated by an active rectifier.
The method of converting the current through power
electronics is beyond the scope of this review, but is
covered in more detail by Baker et al. [65], Brooking
et al. [70], and Ran et al. [71].
4 CONTROL
In regular waves, energy is captured most efficiently
in a point-absorber-type WEC when the undamped
natural frequency of the device is close to the domi-
nant frequency of the incident wave [72]. At resonance,
the velocity of the oscillator is in phase with the
dynamic pressure (and hence force) of the incoming
wave, resulting in a substantial transfer of energy from
the wave to the oscillator [73]. The behaviour of the
device therefore is dependent on the damping. For
most power extraction, damping must be adjusted to
achieve maximum energy conversion efficiency. If the
damping is too high then the motions are limited and
little power is produced. If the damping is too light,
then the damper absorbs little power and little power
is taken off.With any PTO system, the correct damping
is vital for an efficient system.
Real seas, however, rarely exhibit regular conditions.
Instead, waves are continually changing in height and
frequency, and thus the requirement is a device that
can adapt to behave as if resonant over the wide range
of frequencies. It is noted that major improvements
in efficiency (and hence cost-effectiveness) of WECs
are possible with the implementation of active control
of the dynamics [14], along with potential improve-
ment in year-round productivity [74]. The level of
device tuning can range from adjusting parameters for
a particular sea state to wave-by-wave adaptation (also
known as fast tuning). This section covers some of the
techniques that can be employed to achieve efficient
energy conversion.
Salter et al. [50] review a range of different con-
trol strategies with varying degrees of sophistication.
Although many are suitable for specific incident wave
frequencies and amplitudes, the range of frequencies
and amplitudes that each can deal with gives the more
advanced strategies their advantage. Instead of restat-
ing the details in this review, more focus is given to
some selected strategies that have attracted greater
attention in the literature.
4.1 Latching control
Latching control was first examined by Budal and
Falnes in reference [75]. The objective behind latching
control is to stall (i.e. latch) the motion of the device at
the extremes of its movement (when velocity is zero),
and release it when the wave forces are in good phase
to maximize energy extraction. This control strategy
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898 B Drew, A R Plummer, and M N Sahinkaya
Fig. 13 Latching control
allows for a device whose natural frequency is higher
than the exciting wave frequency (and hence, may
have a smaller mass). Latching control is discrete,
highly non-linear, and by its nature sub-optimal. It
is illustrated in Fig. 13 (based on an illustration in
reference [76]).
In Fig. 13, curve a is the elevation of the water surface
caused by the incident wave; curve b is the vertical
displacement of a heaving buoy whose mass is so large
that its natural frequency matches that of the wave
(the ideal condition of resonance); and curve c is the
vertical displacement of a body with a smaller mass,
and hence a higher natural frequency, being latched
at the extremes of travel. The buoy would be released
when the wave force had built up to a suitable level,
so that its velocity would be nearly in phase with the
exciting force of the wave. The velocity of the buoy is
at its maximum at the wave crest or trough. A device
with negligible mass would follow curve a. Salter notes
that the latching system that holds and releases the
buoy has to react very quickly, a requirement easily
achievable with a hydraulic PTO system [50].
Latching control has been the subject of many simu-
lation studies. The challenge with the strategy is deter-
mining the optimum time to release the buoy from
the latched phase; this is the control variable. In regu-
lar waves, using half the difference between the wave
period and the natural period of the devicegives a good
approximation of the latching delay required [77].
Babarit et al. [78] conducted a study to examine
three different latching strategies in random seas.
Their simulation assumed a single degree of freedom
(DOF) heaving body, a simple PTO system, repre-
sented as a linear damper, and that the excitation force
of the future waves was known (a general assumption
in much of the literature); they acknowledge that pre-
diction algorithms exist. They found that the discrete
latching control significantly increases the amplitude
of the motion, and improves the efficiency of the sys-
tem over a uncontrolled heave motion by up to three
times. Later work by Babarit and Clément [79] applied
latching control to a four-DOFWEC (dubbed SEAREV).
With latching control, the mean absorbed power in
a random sea increased by a factor of two over the
uncontrolled result.
In reference [80], Falcão applies latching control to
a wave energy device with a high-pressure hydraulic
PTO system, as previously developed in reference [61].
He notes that the hydraulic PTO system provides
a natural method of achieving latching: the body
remains stationary for as long as the hydrodynamic
forces on its wetted surface are unable to overcome the
resisting force (gas pressure difference multiplied by
the cross-sectional area of the ram) of the PTO system.
The control strategy was effective, and demonstrated
in simulationa significant increase inabsorbed energy.
The system would need to be optimized through
experimental prototype testing, but such a system
could be implemented in a real WEC.
Korde [14] conducted an experimental study
of latching control, demonstrating an efficiency
improvement. With irregular waves, some method for
predicting the incoming wave profile is necessary.
He also notes that the only external force required
is to lock the actuator, resulting in easier practical
implementation than other control strategies.
In the same paper, Korde also discusses the diffi-
culty of predicting the future wave profile. In regular
waves, prediction is not an issue, but in irregular
waves, future oscillations cannot be known. Various
approaches are reviewed, but the article concludes
that attaining accurate and reliable techniques to pre-
dict incident waves or device oscillations remains a
challenge.
The use of an actively controlled platform has
advantages in deep sea offshore sites, as moor ing part
of the wave energy device on the sea bed is challeng-
ing. Korde investigates this in reference [81], followed
by an examination of latching control of aWEC with an
on-board, actively controlled motion-compensated
platform as a reference [82]. Comparisons are made
between this active reference and a sea-bottom fixed
reference. For the irregular wave case, the exciting
force is assumed to be known far enough into the
future. The absorbed energy using the active reference
was found to be less than latching using a sea-bottom
reference, but worthwhile pursuing in light of the
results presented.
The opposite of latching control, dubbed unlatching
or declutching is where the primary moving element
is allowed to move freely for part of the cycle, with the
PTO mechanism only being engaged at the desired
velocity. It is the subject of a paper by Babarit et al.
[59], who examined, in simulation, declutching con-
trol of the SEAREV device. In this device (which uses
a hydraulic PTO system), unlatching is achieved by
bypassing the pumps at certain moments, effectively
meaning that the PTO force is zero at these times.
These moments are determined by the optimal com-
mand theory. It is shown that efficiency is improved by
a factor of two for some wave conditions.
4.2 Reactive loading control
Reactive loading control is used to widen the efficiency
range of a WEC on either side of the resonant
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A review of wave energy converter technology 899
frequency [50]. This theoretically optimal control
strategy involves adjusting the dynamic parameters of
the primary converter, such as the spring constant,
inertia, and energy absorbing damping, to enable
maximum energy absorption at all frequencies. Korde
considers reactive control in reference [14], and found
that velocity feedback could be used to adjust the
damping coefficient provided by the PTO system to
balance the radiation damping of the device to enable
maximum permissible energy absorption. Optimal
power absorption requires that the primary converter
feels no reactive force (as at resonance) and that the
energy absorption rate (damping) equals the rate at
which kinetic energy is being radiated from the device.
Reactive loading introduces a phase shift into the
PTO force to cancel some of the undesirable stiff-
ness or inertia. Either side of the resonant frequency,
the wave force goes into deflecting the ‘spring’ of the
device (a semi-submerged float represents a spring),
or accelerating the inertia, reducing overall efficiency.
Maximum efficiency is achieved if the force is in phase
with the velocity of the device, as at resonance.
Through simulation, Korde studied reactive con-
trol of wave energy devices in irregular waves [74]
and later in reference [72], using a time history of
past velocity measurements to estimate future veloc-
ity of the primary energy converter. In reference [72],
two approaches are considered: where only the static
reactive components due to calm-water inertia and
hydrostatic spring are cancelled by the control force
at constant damping; and where, in addition, further
improvement is sought using estimates of future oscil-
lations (derived from past oscillations). Significant
efficiency gains were obser ved in the first approach,
with further gains with estimations of future oscil-
lations. A better prediction strategy or estimation
algorithm is needed.
Valér io et al. [83] compares reactive control, phase
and amplitude control (another theoretically optimal
strategy), latching control, and feedback lineariza-
tion control (two intrinsically sub-optimal strate-
gies), using the Archimedes Wave Swing as the target
device. Reactive control and phase and amplitude
control are examined, but since the implementa-
tion of these devices require approximations, and
both strategies rely on energy to be added to the
device (essentially supplying energy to the waves),
reducing overall efficiency, the strategies are ren-
dered sub-optimal. Latching is achieved through the
use of water dampers so as to prevent the floater
from moving. Feedback linearization control aims to
provide a control action chosen to cancel the non-
linear dynamics of the plant, so that the closed-loop
dynamics will be linear. Simulations were carried
out to test the strategies, with the assumption that
the behaviour of the incident waves is known. All
strategies improved the efficiency of the device, with
phase an amplitude providing the least improvement,
followed by reactive control, with latching increasing
the efficiency further, and feedback linearization pro-
viding the greatest improvement. Feedback lineariza-
tion, however, requires detailed knowledge of the
machine dynamics and characteristics, and as such
is not practical.
Another reason why, in practice, it is not possi-
ble for reactive control to be fully optimal, is due to
velocities that could become extremely high. As such,
constraints are necessary to safeguard against hazards
of mechanical/electrical overdriving [74].
4.3 Simulation for controller development
Some considerations are necessary concerning the
modelling of WECs and the assessment of control
strategies using these simulations. Yavuz et al. [84]
present a time-domain model of a single-DOF heav-
ing buoyWEC to investigate the effect on performance
of a dynamically changing sea wave frequency. A
time-domain simulation study is necessary, as single-
frequency-based mathematical models of WECs are
not suitable to predictthe performance of real systems,
as real sea waves are complicated and ever changing.
This need is confirmed by Falcão [61], who investigates
the modelling and control of a heaving buoyWEC with
a hydraulic PTO, by stating that the WEC examined
is highly non-linear, which requires a time-domain
model consisting of a set of coupled equations: (a)
an integral-differential equation that accounts for the
hydrodynamics of wave energy absorption and (b)
an ordinary differential equation that models various
aspects of the hydraulic system (including accumula-
tors, valves, flows, and viscosity effects). Josset et al.
[85] constructed a ‘wave to wire’ time-domain model
of the SEAREV WEC, including the PTO system, to
study the various elements of the complete device.
With respect to the hydrodynamics, many simu-
lation studies treat the incident waves using linear
models. It is generally agreed that this is suitable for
relatively calm seas, but in extreme conditions, this
linear approximation is not accurate, as non-linear
factors dominate (personal communication, Jun Zang,
University of Bath, 4 July 2008). Work is ongoing at the
University of Bath, and elsewhere, investigating the
modelling of these non-linear sea states (for example,
see reference [86]).
In terms of supplying models with an accurate
wave spectrum, many authors have used the Pierson–
Moskowitz spectrum [87], which accurately models
the behaviour of real sea waves [76].
5 CONCLUDING REMARKS
The potential for generating electricity from wave
energy is considerable. The ocean is a huge resource,
and har nessing the energy in ocean waves represents
JPE782 Proc. IMechE Vol. 223 Part A: J. Power and Energy

900 B Drew, A R Plummer, and M N Sahinkaya
an important step towards meeting renewable energy
targets.
This review introduces the current status of WEC
technology. The different device types are estab-
lished and evaluated. The institutions and companies
involved in WEC development, as well as collaborative
wave energy projects, are also identified.
The possible PTO systems are assessed and classi-
fied as hydraulic, linear electr ical generator, or turbine
based. A hydraulic PTO system is particularly well
suited to absorbing energy from a high force, slow
oscillatory motion and can facilitate the conversion of
reciprocating motion to rotary motion to drive a gen-
erator. There are, however, various design challenges
such as efficiency and reliability. A linear electri-
cal generator provides an alternative option, but the
technology is less mature.
The active control of a WEC can significantly
increase its efficiency, and hence cost effectiveness.
This research is currently ongoing with latching con-
trol being highlighted as a promising, simple method
of efficiently extracting energy.
Despite considerable research and development,
the concepts for converting a slow, high-force, recipro-
cating motion to one useful for generating electricity
show no signs of converging to a preferred solution.
Questions arise over which concept to use, how best
to optimize its performance, and how to control such
a system. Future research should take a systems engi-
neering approach, as the individual subsystems of a
WEC are all intimately related and any one should not
be optimized without considering the other subsys-
tems. Furthermore, individualWECs will often operate
as part of a wave farm, so future systems analysis must
include the interaction between devices.
© Authors 2009
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