Potential for power generation from ocean wave renewable energy source: A comprehensive review on state of the art technology and future prospects

Abstract

This study presents a comprehensive review of the ocean wave technology and prospects of the wave energy penetration to cater to clean global energy demand. An ocean wave is a remarkable energy resource, but it presents a very small share in the global energy mix because of various challenges and limitations encountered to unleash its potential. This study evaluates intensively the complex barriers to the ocean energy technology deployment. The existing and prospective major wave energy projects are extensively examined to identify the learned lessons and optimise possible technological solutions to close the gap in the energy market. Furthermore, limiting and motivating factors to foster the global wave energy potential growth are deeply discussed to ignite new research directions and promising solutions. In particular, the wave energy converters as the underpinning enabling technology are fully investigated regarding their technical readiness, reliability, competitiveness and critical challenges. To complete the power equation, possible energy conversion stages, grid connection and integration issues are dealt with in a broad view of the wave energy power system. Eventually, this study aims at providing an updated ocean wave technology review and progress while introducing new research gap to fast-track contributions in the global energy mix.

Full text

IET Renewable Power Generation
Review Article
Potential for power generation from ocean
wave renewable energy source: a
comprehensive review on state-of-the-art
technology and future prospects
ISSN 1752-1416
Received on 19th June 2018
Revised 2nd October 2018
Accepted on 3rd December 2018
E-First on 17th January 2019
doi: 10.1049/iet-rpg.2018.5456
www.ietdl.org
Francis Mwasilu1,2, Jin-Woo Jung1
1Division of Electronics and Electrical Engineering, Dongguk University-Seoul, 30, Pildong-ro 1-gil, Jung-gu, Seoul 04620, Republic of Korea
2Department of Electrical Engineering, University of Dar es Salaam, P.O. Box 35131, Dar es Salaam, Tanzania
E-mail: jinwjung@dongguk.edu
Abstract: This study presents a comprehensive review of the ocean wave technology and prospects of the wave energy
penetration to cater to clean global energy demand. An ocean wave is a remarkable energy resource, but it presents a very
small share in the global energy mix because of various challenges and limitations encountered to unleash its potential. This
study evaluates intensively the complex barriers to the ocean energy technology deployment. The existing and prospective
major wave energy projects are extensively examined to identify the learned lessons and optimise possible technological
solutions to close the gap in the energy market. Furthermore, limiting and motivating factors to foster the global wave energy
potential growth are deeply discussed to ignite new research directions and promising solutions. In particular, the wave energy
converters as the underpinning enabling technology are fully investigated regarding their technical readiness, reliability,
competitiveness and critical challenges. To complete the power equation, possible energy conversion stages, grid connection
and integration issues are dealt with in a broad view of the wave energy power system. Eventually, this study aims at providing
an updated ocean wave technology review and progress while introducing new research gap to fast-track contributions in the
global energy mix.
1 Introduction
Various factors such as growing of global energy consumption,
demand for low carbon economy, combat against world climate
change, depletion of fossil fuel and geopolitics of oil economy
have escalated the interests in finding alternative energy sources
for power generation [1, 2]. On this note, renewable energy sources
(RESs), which are vastly available in the world, have proven to be
a promising solution to the global energy demand crisis. In this
case, wind and solar renewable energy sources have been
intensively researched as a consequence numerous commercial
power plants have already been put in full-scale operation [3–5].
According to the renewable 2016 global status report, only wind
and solar energy sources account for 77% of the annual increase in
global power generation capacity [6]. Then, it recapitulates that
these RESs have matured and have been extensively studied to
maximise their potential in the energy market. Moreover, it iterates
the growing interest and high expected sustainability of the RESs
for electric power generation.
However, some challenges exist such as difficulties in the
weather forecast, sustainable continuous operation and complex
system development along with the rapid increase in global
population and urbanisation that could enormously raise energy
demand [7–9]. The World energy outlook 2016 projects a 30%
increase of the global energy demand in 2040. Furthermore, a 60%
projection of the power generation in 2040 is estimated to originate
from the RESs to meet the current global constraints such as
weather climate change mitigations [10, 11]. It is apparent that the
available energy sources including fossil fuels, in particular, cannot
guarantee to supply the energy demand by at least 2040 [12, 13].
Therefore, finding alternative and reliable energy sources to close
this gap has been an ongoing process in the global energy context.
On the other hand, the ocean wave energy which has been less
harnessed to date, is one of the most reliable (i.e. high accuracy in
energy prediction and little energy loss because wave propagates
over a long distance with), powerful (i.e. higher density of sea
water compared to air/wind) and very attractive renewable energy
sources (i.e. availability and forecast-ability) [14–16]. Wave energy
possesses far greater power intensity (2–3 kW/m2) compared to
solar (0.1–0.2 kW/m2) and wind energies (0.4–0.6 kW/m2) [17]. It
is useful to point out the classification of different wave energy
sources by their scale, which can be identified as follows [18]: (i)
waves generated by local winds that are frequently referred to as
wind-sea waves (typical periods 2–5 s or more specific with
periods of <8 s); (ii) waves generated by remote storms that are
under the name of swells (typical periods of 10–20 s); (iii) waves
generated by tides (periods of about 12 h or 24 h). Different waves
have very different energy scales and time constants. In this
context, the different types of wave energy converters (WECs)
technologies should be applied for the effective wave energy
harvest.
The current literature [19, 20] show that the wave energy is
hugely available in various coasts with the potential of >100 kW/m
average annual power density. Many researchers, academic
institutions and energy firms have embarked on finding effective
ways to capture this untapped powerful renewable energy source
for electricity power generation applications [21–24]. The wave
energy is converted into useful electric power by using various s
and power take-off (PTO) technologies. Recently, there is a notable
increase of activities about the wave energy power generation such
as pilot projects, ocean wave resource assessments, patents [21–23]
and deployment of new WECs technologies [24–27]. Besides, large
utility companies such as EDF (i.e. Électricité de France ==>
French national utility company), Iberdrola (i.e. Spanish national
utility company) and renowned engineering firms like Lockheed
Martin, ABB Group and Mitsubishi Heavy Industries have recently
entered the wave energy market [28]. To this end, the ocean wave
energy is attaining a pinnacle stage after more than three decades
of the exhaustive research and development.
Several works have been reported, which pertain to ocean wave
energy technology for electric power generation in the past [29–
31]. López et al. [29] present a review detailing the wave energy
technology and various technological concepts for PTO and WEC
systems considering both near shore and offshore locations. In this
work [28], the emphasis is put on general technical reviews and
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363

various fundamental WECs concepts. A deep review on the
evolution and first-generation practices of the WECs is reported by
Falcao [30]. These literatures [29, 30] delivered a conceptual
analysis of the wave energy technology and assessment of the past
generation. On the other hand, the review of electrical control
techniques for different WEC devices is reported in [31]. It was
observed that the control tools might have significant impacts on
design complexity and energy capture of the WEC devices.
It is worth to stress that temporal and spatial variation studies of
the wave energy resources are deemed highly imperative to
identify potential locations with high wave energy capture [32].
Also, this knowledge gives a clue to the design features and
capacity of the WECs that could be suitably deployed at the
particular sea locations. Note that the effective wave power density
(kW/m) is directly proportional to the square of the significant
wave amplitude and periodic duration of the incident waves [33]. It
implies that the waves with large amplitude and longer periodic
motion may result in yielding a higher wave power density [34].
The numerical wave models are commonly applied in these studies
such as the SWAN (Simulating WAveNearshore), WAM (WAve
Model) and WAVEWATCH III (WW3) using hindcast and buoy
data [35–37]. To this end, the assessment of the wave energy
resources using WW3 model in Central Chile is reported in [34].
This study is based on the near shore location and it concluded that
there is a potential wave energy power of maximum 5 MW per
year with an energetic wave height of 3.5 m within a period of 11 s.
Wang et al. [36] examine five-year seasonal variations and average
power of the wave resources in the Bohai Sea by applying an
improved SWAN model. It is found that the largest wave height is
2.5 m with energy period of 5 s and the maximum average power is
3.5 kW/m. In [37], the spatial and temporal variations of wave
energy in the Caspian Sea are investigated using a SWAN wave
model. The study assesses the near shore inter-annual variations of
average wave power that shows significant sustainability. In [38],
the wave energy variation characters such as temporal, seasonal,
monthly and inter-annual are evaluated under the near shore
location in the Gulf of man. The same studies are reported for other
countries such as Canada [39] and Australia [40]. It is noted that
the wave resource assessment is scarcely reported about the
offshore locations which prompt more focus in this area because
huge wave amplitudes with longer periods are found in these
locations. Also, a unified study model is needed to allow wave
energy projections to be reliable and widely acceptable. In this
case, a correlation study of all standard wave models and variations
instincts would be interestingly reported. Knowing precisely the
amount of ocean wave energy resources will significantly increase
the sustainability of technological development and the flow of
investments.
Nevertheless, the technology development in this area keeps
escalating and some researchers have taken a stake in finding the
wave energy potential in their regions recently. In [41, 42], the
wave energy potential and technological development are reported
considering Korea and Iran water bodies, respectively. A huge
shifting trend from oil dependence to renewable energy has been
observed in power consumption (e.g. increase in renewable energy
consumption by 177% between 1998 and 2009. Also, plans have
been set to increase >11% from the current state by 2030 in Korea
[41]) and various projects for WECs development are
demonstrated. China as the second largest energy consumer (with
>60% from coal) is also taking steps in exploiting ocean energy
[43]. A global wave energy status shows that most developments
are in early stages with some challenges such as high cost, low
efficiency and little flow of investments (project funding). Also,
there are some challenges and difficulties in the integration of
WEC devices into large-scale wave farms to achieve mega-watt
class power capacity due to the existence of small-scale devices
compared with other RESs like wind and PV solar energy systems.
Therefore, extra efforts are required to close this gap to exploit the
huge available wave energy [44–46]. The current ocean energy
estimates range from 20,000 to 80,000 TWh of electricity per year,
which accounts for 100–400% of the existing global power demand
[47].
To extensively identify and potentially utilise this large
untapped power resource from the ocean, there is a need for a deep
understanding of an integrated marine energy framework and
challenges that are facing the development of wave energy
technology. In this case, effective solutions can be readily
developed to meet the energy market and industrial demands.
Therefore, this paper extensively reviews the state-of-the-art ocean
wave technology and ascertains the prospects to the share in the
global energy mix. In the precise contexts, the WECs as the
underlying technology is deeply examined regarding their technical
readiness, reliability, competitiveness and critical challenges. Grid
connection and wave farm integration issues are also discussed in
the wave energy power system structure viewpoint. Particularly, a
rigorous investigation is presented based on the wave arrays and
wave farms configuration, techniques and grid connection to
pinpoint the existing challenges and set new research directions.
Finally, the various limitations and motivations are extensively
discussed to unleash the potential of the ocean wave energy that
would fast-track the participation in the energy market and
eventually mitigate the global power capacity deficit.
2 State-of-the-art wave energy technology and
challenges
Wave energy is an emerging industry with most technologies being
at an immature stage. The technology development faces
challenges from the fact that ocean environment (especially at
offshore) is uncertain to work in. On the other hand, policy
frameworks and governmental priorities regarding funding hinder
the fast growth of this industry [15]. Lack of enough projected
funding and investments in the ocean wave energy industry
decelerates the exploration of solutions to break through the WECs
technology difficulties [17]. For example, the Ocean Energy
Systems (OES) assessments based on the technical potential
criterion project the worldwide wave energy installation capacity of
337 GW by 2050 whereas the International Energy Agency (IEA)
estimates a 63 GW installation capacity by considering technical
potential and policy frameworks criteria for the same year 2050
[48]. It can be observed that the 81.3% difference demonstrates
how the above-mentioned factors seriously influence the
investments and development flows in this industry. Numerous
WECs are in different development stages that meet full-scale
commercial deployment to contribute to the global energy mix [19,
29, 30]. Fig. 1 reveals the wave energy installed and planned power
capacity as of 2016 [28]. It is worth mentioning that Japan has a
huge planned capacity of 350 MW, while UK and Norway each
expects to install 40 MW. Note that these data (for Japan, UK and
Norway) have not been included in Fig. 1 to increase resolution
and give clear, comparable data with other countries.
2.1 Wave energy technology status and impacts to global
energy
Note that the west coastal regions such as those in Europe,
Australia and US are the ones with high wave energy resource and
most of the activities have been cantered in these coastlines to
exploit the wave energy potential [49, 50]. In this case, wave
energy is an exceedingly promising renewable source to cater for
the future green power demand. A number of WEC prototypes
have been patented and developed to reveal the future potential of
the wave energy power generation [22, 28]. Confirmation of a huge
wave energy resource around the world recently influences marine
technology to abate global energy deficit. Numerous wave
assessments report the optimism of wave energy to tackle the
excessive electricity demands in different countries in the world.
For example, it is estimated that wave energy would meet ∼60, 15–
25 and 33% of the total electricity demands in the US, UK and
Denmark, respectively [15]. Similarly, the estimated 15% of the
electricity demand in Europe alone could be supplied by the wave
power generation. It is clear that the interest and potential of this
energy source are vividly supported by many countries such as UK,
US, Spain, Australia, Portugal, Denmark, Japan, Korea and so on,
through energy policy enforcement and direct involvement in
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technology development of the WEC devices [50]. In turn, the
wave energy technology will have a huge impact on global power
production with clean and renewable energy sources by 2040 if all
these efforts come into maturity.
In the past decades, the growth of wave energy technology was
increased slowly and more interest started to be drawn into this
sector recently [16, 17, 30, 31]. The WEC technology progress
follows a systematic development trend (i.e. from project inception
to a prototype demonstration) to assure that all the challenges are
taken aboard prior to the full-scale device manufacturing. This is
because the wave energy resource is so hostile and powerful that
the WEC device survivability features are always put into a high
priority in the development equation. It is not a standard that all
WEC devices should follow the systematic development
procedures (i.e. technology readiness level) however, it is a
recommended practice proven to work effectively in other mature
industries especially aerospace engineering [15]. This systematic
development trend is categorised as technology readiness level
(TRL) indexed 1 to 9 as clearly described in [22]. Fig. 2 depicts the
development of a WEC technology with technical readiness
perspective [15, 22], which depicts the development cycle and
requirement deemed at each stage.
2.2 WECs deployment and challenges
The deployment of WECs faces many challenges and difficulties in
a broad technical and non-technical perspective. As it will be clear
later in this paper, it is optimistic that most of the technical issues
such as wave-model-to-wire efficiency would be met by
considering the intensive research and development [46]. However,
a great number of non-technical issues such as policy frameworks
and law enforcements are so critical that they require much more
attention in advancing this emerging energy [15].
3 New wave technology developments and trends
The current emerging technology could help to increase the
penetration and commercial debut of the WEC systems in the
energy market. As it can be apparently observed, the capital
expenditure (CAPEX) and operational expenditure (OPEX) should
be reduced to enable the WEC devices to fairly compete with other
renewable energy technologies such as those in the wind and solar.
Reducing these costs would mean a significant impact on the
levelised cost of electricity/energy (LCOE) for the wave energy
[51]. In particular, the size-density of the WECs system is one of
the factors to be envisaged by the current technology development.
Recently, the triboelectric nano-generators (TENGs) have been
invented, which can produce an average power output of 1.15 MW
from 1 km2 surface area [27]. The TENG technology features light
weight, cost-effectiveness and easy implementation. It is a
compelling innovative and effective technology approach for the
large-scale ocean wave energy harvesting [25, 27]. Fig. 3 illustrates
the triboelectric nano-generators technology deployed in the sea for
large-scale wave energy capture [27]. The TENG technology
occupies little space but results in huge energy capture. With a new
emerging technology like TENG, it would be possible to deploy a
small wave farm with a large power output capacity that also
entails an increase in efficiency.
On the other hand, a number of technologies have emerged in
the power system industry that could help influence the
Fig. 1 Wave energy installed capacity and planned power capacity as of 2016 [28]
(a) Installed capacity, (b) Planned capacity
Fig. 2 Development of WEC technology with technical readiness perspective [15, 22]
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technological challenges in the wave energy conversion systems.
One of the challenges of the WECs system is a regular
maintenance practice which leads to low reliability and high
investment cost [46, 51]. In this case, the self-healing and
resilience WEC systems are attractive solutions to encompass less
frequent or no maintenance activities during the entire operating
window [46]. The OPEX due to the maintenance of the WEC
devices is ∼10% of the total investment cost [51]. The cyber-
physical system (CPS) is another emerging technological solution
that would help predictive maintenance as the WEC systems are
not suitable for preventive maintenance [52]. Also, due to synergy
operation between wave and wind energy farms [53] or interactions
between WECs devices within a particular wave farm or arrays
[54, 55], the CPS technology would help smooth coordinated
control of these devices to complement each other [56]. Since the
CPS technology is gaining interest and attention in power system
lately [57], a clear correlation and synergy can be established to
influence the reliability, availability and efficiency of the WEC
devices as well as wave farms.
Similarly, the machine learning and its concomitant artificial
intelligence (AI) techniques are game changers technologies in the
green power system contexts. The AI techniques are superb in
predicting the wave energy and garnering the wave resources
assessment datasets [58, 59]. With machine learning the WEC
devices can be trained to adapt to its environment accordingly
depending on its hydrodynamic non-linearities and other wave
datasets instantly available [60]. Extensive exploration of the AI
techniques to the WECs technology development could
significantly enhance the WECs design, availability, survivability
and energy capture efficiency [58–60]. For example, Carnegie
Clean Energy Ltd with their CETO WEC technology is working on
the application of machine learning to the CETO for improving the
operating performance at large [61]. To this end, the integrated
framework and exploration of new technology would be a key
stepping stone to massive deployment of WEC farms in the
practical (real) grid systems. More research and development
activities are deemed compulsory in these new interdisciplinary
technologies to abate the rapidly growing global energy demand in
the context of wave farms.
3.1 WECs, ocean arrays and wave farm
Wave energy capture is accomplished from the shoreline along the
way to the deep sea (offshore) locations. This is one of the
deterministic factors for the various WECs design and deployment
mechanism choice. The slow wave motion (i.e. typically ∼1 Hz) is
transformed into electric energy by using different PTO
technologies such as air turbines, electric generators and hydraulic
turbines [29, 30]. Most WECs produce electricity as a single unit in
a small amount <1 MW [46]. To have a useful and efficient
utilisation of the wave energy resource in a commercial scale,
several WEC units are aggregated into multiple scales of
interconnected units to form wave arrays or farm and collectively
generate electric energy in multi-megawatt scales [54, 55].
It is observed in many kinds of literature that the aggregation of
the wave arrays significantly reduces the impact of wave power
output variability by considering various factors such as WEC
device spacing within particular arrays [62–64]. There is no
standard yet as to what size and configuration should be used to
integrate WEC devices into large farms or arrays because most
technologies are still under premature stages (i.e. at technology
validation) with almost no significant share in the commercial
electricity market. However, the rule of thumb would be the
optimal maximum power absorption required (i.e. installed
capacity) about the WEC arrays layout and deployment location
(i.e. offshore or near shore) [55].
3.1.1 WECs configuration: WEC is an enabling technology for
harvesting wave energy into useful electric power. Different WEC
technologies exist depending on these locations, incident wave and
WEC operating characteristics. Therefore, WECs are classified
based on ocean water-depth location, device size relative to
incident wave and their principle operating modes [29].
Furthermore, three categories can be found based on the ocean
water-depth location: onshore, near-shore and offshore WECs [30].
In particular, onshore WECs are designed and constructed to
operate at shoreline locations. The attractive features in this
location are less severe catastrophic sea environment, easy access
to grid network resources and fewer maintenance efforts (i.e. easily
reachable from the shore) [15, 16]. However, less powerful ocean
waves are available in this location, which leads to lower electric
power harvest. Nearshore WECs are installed in the shallow water
of water-depth 10–25 m with the advantages of gaining more
power capture compared to the shoreline WECs [17]. Besides,
offshore WECs are designed to operate in the deep water-depth
>40 m with powerful waves. These are very difficult devices to
operate and maintain due to the harsh environment in the deep-sea
location [46]. Due to the prevailing vast energetic waves, the
offshore WECs have to withstand the much heavy force that calls
for high reliability and complicated design requirements [14, 15].
On the other hand, the WEC devices can be classified
intuitively depending on their basic principle of operation to
convert the hydrokinetic wave energy into electricity. Three main
categories can be considered in this classification: Oscillating
water column (OWC), oscillating bodies and overtopping [16, 17,
30]. In the OWC, air flow is trapped in the column chamber that
changes air pressure depending on the variations of ocean waves
(crest/trough movements). As a result, a column of air pressure
(low/high) is produced inside the chamber, which can drive an air
turbine linked to the generator set for power generation [17].
Oceanilix, Multi Resonant Chamber (MRC) and Limpet WEC
devices use this technology. The oscillating bodies capture ocean
wave energy by using heaving motion that compresses fluid linked
to the hydraulic motors or turbines, which drive the generator set to
produce electric power [30]. The existing WEC devices found in
this category are as follows: Pelamis, WaveRoller, Archimedes
Wave Swing (AWS) and PowerBuoy. Apart from that, the
overtopping WEC devices extract ocean wave energy by
intercepting ocean waves and collecting water into a raised
reservoir and then return water to the sea through a low-head hydro
turbine by which electricity is produced. WavePlane and
WaveDragon are mature existing WEC devices' technology found
in this category. Fig. 4 depicts the classification of different WECs
designs about their working principle and deployment location. It
can be observed that all WEC devices can be floating or submerged
in the sea while located in any of the three locations (e.g. onshore,
nearshore or offshore) depending on the principle governing the
hydrokinetic conversion process [16].
3.1.2 Ocean arrays and wave farm deployment: The wave
farm can be formed by optimising the positioning and size
(geometry) of the WECs in the array structure. Numerous
engineering constraints exist during the deployment of wave arrays
that are supplemented with the WEC device geometry, electrical
interconnections among WEC units (also power grid), mooring
systems, control techniques and hydrodynamic interactions among
WEC units [17, 29, 54, 55, 65]. Equally, the design of the power
system structure in the near shore or offshore wave farms is
complicated and difficult due to apprehensive access to power grid
connection. Besides, the interaction between WEC devices may be
too high to reduce performances [45]. In this case, a well-thought
design has to be set to allow an optimal performance in the sense of
high-power wave energy capture and reliability [16].
Fig. 3 TENGs technology deployed in sea for large-scale wave energy
capture [27]
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In [66], the maximisation of the electrical networks for the
WEC arrays is investigated based on the capacity factor and
CAPEX about the marine cable design. It is observed that
increasing the capacity factor from 30 to 60% reduces the cost by
20–30%. Also, since WECs arrays are not fully utilised all the time
(i.e. 100% rating for 3.2% of the total time versus 83% rating for
6.2% of total time), underrating the cable size would reduce
CAPEX. Sjolte et al. [67] investigate the integration of the wave
farm with 48 WEC devices (60% capacity factor) and their effects
on the downstream power system. In this study [67], the orientation
of WECs in relation to incident waves (geometry and spacing
between WECs) is considered to compensate for the fluctuations
that influence the power quality. It is noted that large spacing
between WECs significantly reduces the hydrodynamic interaction.
Furthermore, the operational impacts for deploying large-scale
wave farm (500 MW installed capacity) with a capacity factor
range between 30 and 35% and 1 MW WEC device at the Pacific
Northwest coast are reported in [64]. It is noted that there is a
chance to reduce deployment cost, optimise power output and
increase utilisation factor of the wave farm. To this end, the
important factors to be considered with respect to the wave energy
capture are the hydrodynamics, wave arrays orientation (i.e.
spacing and geometry) and incident angle [55, 56, 65, 67]. Owing
to the premature technology in the marine power generation, there
is little experience on the operation and deployment of the wave
farms or WEC arrays. However, the WEC arrays in the form of the
wave farms would be necessary for the optimal and economic
power generation (i.e. multi-megawatt) using ocean wave energy
[46, 54, 55]. Fig. 5 illustrates two possible configurations (i.e.
radial-connected type WEC arrays and star-connected type WEC
arrays) of the wave arrays found in most studies. The
configurations reflect the real architecture of the WEC devices in
the wave farms deployment. It unveils the WEC device interactions
about their geometrical design and space while echoing the cost of
installation regarding complexity, structure design requirements
and cable routing. Depending on the particular project each
configuration suits the purpose. For example, the star configuration
has redundancy as opposed to the radial scheme. However, during
wave array deployment a trade-off between technical requirements
and installation costs should be sought [46, 54, 55].
3.2 WEC stages and electric generators configuration
WEC may use the existing fundamental electric generator concepts
for the hydrokinetic energy transformation. The most common
technologies involved are permanent magnet synchronous
generators (PMSGs), doubly-fed induction generators (DFIGs) and
squirrel cage induction generators (SCIGs) [16, 19, 29, 46].
However, the intermediate stages are required to convert the slow
wave motion to smoothly match the high-speed generator
operations [17].
3.2.1 WEC stages and configuration: Direct or indirect
conversion mechanism is employed to harvest wave energy
depending on the WEC device configuration. The direct conversion
transforms the heaving motion of the incident wave to electric
energy without any intermediate device mechanism. This
phenomenon usually employs linear generators to convert the
heaving wave motion of the WEC device into useful electric
energy [19, 29]. In a current state, the PMSGs are a preferred
choice due to their high efficiency, reliability and fallen market
price of the rare earth magnet [68].
On the contrary, the indirect conversion involves transforming
slow heaving wave motion through intermediate stages such as air
turbines, pneumatics, hydraulic turbines and hydraulic motors to
couple high-speed rotary electric generators [16, 30]. Fig. 6 shows
the conversion stages and configurations of wave energy
conversion systems. Fig. 6a illustrates the direct conversion system
configuration with linear electric generators while Fig. 6b depicts
the indirect conversion system configuration with various
intermediate stages such as hydraulic and turbine systems. The
design and deployment of WECs devices face some challenges
such as corrosive environment, endurance to extreme large wave
amplitudes, the variability of the incident wave (i.e. wave power
variability) and low-frequency motion (∼1 Hz) [18, 43]. The WEC
device at the design stage should take into account and overcome
all these difficulties to meet sustainable electric power delivery.
3.2.2 Electric generators technology and deployment: The
advancement of the electric generators technology is a very
deterministic instinct for the future progress in the wave energy
industry. This is because the wave energy transformation to electric
power is quite different from the conventional power conversion
systems such as wind energy systems. Deployment mechanism of
the WEC devices (e.g. submerged or floating structure) poses a
Fig. 4 Classification of different WECs designs in relation to their
working principle and deployment location
(a) WEC classification by location, (b) WEC device classification by working
principle
Fig. 5 Possible configurations of the wave arrays found in most studies
(a) Radial-connected type WEC arrays, (b) Star-connected type WEC arrays
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significant challenge in the choice of generator technology because
it highly influences the reliability and efficiency among other
factors [16, 46]. Mueller [69] investigates the feasibility and
suitability of various direct drive permanent magnet generator
(PMG) design technologies such as longitudinal and transverse
flux machines. In [46, 70, 71], the DFIGs, SCIGs and PMSGs have
been examined for the wave energy conversion applications. It is
observed that DFIG and brushed synchronous generator (SG)
would not be good candidates due to high maintenance cost and
low reliability [69, 71]. Linear PMG (LPMG) technology is
envisaged as a suitable candidate for the direct conversion stage
WECs such as point absorbers (e.g. AWS) [70]. However, this
concept is not off-the-shelf technology. In this case, some
improvements are being made at the research and development
stages to enhance efficient and smooth energy conversion into
electricity [72–75]. In [73], the power density and distance of
relative movement are increased by designing a LPMG with the
outer permanent magnet for the direct drive WECs application.
Linear permanent magnet Vernier machines have also been
considered for the direct drive WECs to improve the torque density
and thermal stability features [74]. Gargov and Zobaa [75]
proposed an air-cored LPMG to tackle the magnetic attraction
problems between permanent magnet and magnetic core. More
research is required to investigate and optimise the performance
and operation of the linear PMSG for the WECs applications [46].
On the other hand, owing to the direct connection of the DFIG
stator to the grid, it is also prone to grid faults and interferences
that have been already investigated in wind energy technology
development; however, the effects are magnified furthermore by
the WECs configurations [76]. Similarly, the power converter
interface with the grid through the rotor slip ring brushes
exacerbates the maintenance difficulties because of the uneasy
access to WEC devices for more frequent maintenance activities
[46]. Table 1 shows the different electric generator technologies
that have been successfully deployed in potential existing WECs
devices.
Wave energy after transformation into useful electrical energy is
interconnected to the existing power grid so that it reaches the
intended end-user. A different mechanism is utilised to accomplish
this task depending on the wave farm location (i.e. onshore, near
shore or offshore) [44, 77–80]. Near shore and offshore WECs
arrays put forward a great challenge on grid connection regarding
Fig. 6 Conversion stages and configurations of wave energy conversion systems
(a) Direct conversion system configuration, (b) Indirect conversion system configuration
Table 1 Different electric generator technologies successfully deployed in potential existing WECs devices
Electric generator WEC device technology Remark(s)
DFIG OWC, e.g. Limpet high speed PTO, generator driven with high speed air turbine
PMSG overtopping, e.g. WaveDragon, WavePlane low speed PTO, generator driven with low-head water turbine
LPMG oscillating bodies, e.g. AWS direct drive generator, low speed PTO
SCIG oscillating bodies, e.g. Pelamis, SEAREV oscillating surge converter, generator driven with hydraulic motor
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cost and cable installation layout. In extreme cases, an offshore
substation is required to facilitate grid connection [77, 78, 81].
3.3 Grid connection and wave farm integration issues
On the other hand, shoreline WEC devices are advantageous to the
access of the power grid resources. In all cases, electric power is
transmitted from the wave farm station to the onshore grid network
using high-voltage DC (HVDC) or AC (HVAC) facilities [44, 79].
At some point, the underwater substation is required to boost
voltage level and reduce transmission losses [46, 77, 78]. This
occurs for example with LPMG (e.g. AWS device), which
produces a too low voltage to meet optimal power transmission
constraints. Fig. 7 illustrates the grid connection configuration of
the wave energy conversion systems (wave farm) with underwater
substation [46].
Installation cost, installed capacity requirements and
transmission distance are among the deterministic factors to decide
an optimal choice for the grid connection. With the prevailing
offshore wind facilities, both technologies are feasible [80]. The
extensive studies based on the Pacific Northwest coast found that
the integration cost of the wave energy arrays into the power grid
would be lower than other alternative energy sources such as wind
and solar energy [64]. For convenient operations, WECs have to
comply with most grid codes to be viable in the energy market.
The International Electrotechnical Commission (IEC) under the
Technical Committee 114 (TC114) has prepared a draft on the
standards to govern all marine energy devices for power generation
especially wave, tidal and other water current converters. Similarly,
the IEC Technical Specifications (TS) for the marine energy
converters (IEC 62600) stipulates different standards such as power
quality and grid connection requirement issues for the WECs [82].
In [83], the grid integration and power quality testing issues with
the emphasis on the fault-ride through capability of the marine
energy converters are reported while considering the IEC 62600. It
was observed that the interactions between WEC devices and
active power fluctuations within wave farms are the most crucial
parameters that need much attention during the grid integration
process. On the contrary, the reactive power support in grid
integration context is observed to be easily handled [83].
3.4 Research gap and future trends
To ensure that the WECs deployment enters a competitive energy
market and increases confidence to investors, the wave energy
industry should focus on methodologies which will help reach an
effective, affordable cost for the grid connection [15, 46]. Intensive
researches on WECs devices are modelling and control (wave
energy capture) are underway with promising results when
compared to the competitive cost reductions for the same devices
[17, 31, 67, 84]. There is still a gap of knowledge on how to
establish cost factors (i.e. especially for the OPEX) to reduce
LCOE because most patented WEC devices are in the premature
stages (within TRL 1–3) for commercialisation [22, 51]. However,
using the experience of the current deployed wave farms [49–51], a
clear relationship and fact-figures can be examined.
Furthermore, the arrangement and aggregation of the wave
farms will be different in various regions because of different WEC
sizes, (i.e. power and components), structural design (i.e. WEC
geometry) and configuration (i.e. operating principle) compared to
the existing wind or PV solar farms [21, 22]. To have an effective
wave energy harvest and utilisation, the layout of WEC arrays
within a wave farm would better be optimised. Some studies have
already been presented to address the array optimisation using a
genetic algorithm [85] and other computational intelligence
techniques [58]. However, a precise understanding and consensus
of the input parameters in the optimisation problem are missing to
establish an effective performance index. For example, taking into
account that a reasonable wave model is available, what significant
ingredients are considered in an optimisation problem formulation:
cost factors (both CAPEX and OPEX), the spacing between WECs,
power conversion rates or other array structural design parameters?
Advanced control schemes for regulating WEC devices in the
context of wave array or wave farm are required. It is known that
the WEC devices behave very differently about the existing plants
like wind turbines, PV solar or thermal power plants. The high
nonlinear nature due to variable incident waves and interaction
sensitivity due to the interdependence of the WEC arrays increase
the control design complexity [16, 46]. There is a need for
extensive studies on the advanced coordinated control schemes to
maximise the device performance within the wave farm layout
contexts. Little progress has been made in this area recently
because there are very few researches that have been reported in
the past [17, 84]. Nevertheless, there are enormous prospects to
design high-performance coordinated control by combining the
insights from the emerging technology in the control theory, digital
control implementation and computer communications
perspectives.
4 Feasibility of wave energy farms, limitations
and prospects
Various WECs have already been deployed to the sea sites, which
are in different stages depending on technological readiness indices
succeeding to expect commercial debut. This is a clear sign that
wave energy would be feasible to address the growing green
electric power demand [15, 29, 45]. The arrangement of the WECs
integrated to form a wave farm is a very crucial aspect because it
influences marine cable layout and overall investment cost [54, 55,
85]. Focus on the significant cost reduction for both OPEX and
CAPEX by reducing the WEC device size and associated
engineering components would increase opportunity and self-
Fig. 7 Grid connection configuration of wave energy conversion systems with underwater substation [46]
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assurance to reach prospects of the wave energy technology (i.e.
massive market uptake).
4.1 Existing wave projects, facilities and learned lessons
The first WECs to be deployed were the 750 kW Pelamis based on
oscillating bodies (terminator) concept by the Pelamis Wave Power
and the 250 kW AWS based on oscillating bodies (point absorber)
concept installed in Portugal in 2004. The Aquamarine Power
Company has deployed the first generation 315 kW Oyster and its
second generation 800 kW grid-connected Oyster in the Orkney
(European Marine Energy Center (EMEC)) completed in 2009 and
2012, respectively [21, 22]. The AW-Energy company has installed
near-shore 300 kW grid-connected WaveRoller units (three 100
kW WEC units) in Piniche, Portugal and construction of the first
commercial WaveRoller energy converter has been under way
since mid-2016 [86]. The Basque Energy Board opened the first
commercially-operated wave energy power plant in Mutriku by
deploying the OWC wave energy conversion system of 18.5 kW
each with a total installed capacity of 296 kW (i.e. 16 WEC units)
[17, 21, 22]. The Pico plant (employing OWC technology) with
installed capacity 400 kW in the Azores, Portugal has been in full-
scale operation for more than a decade now [29]. Figs. 8 and 9
exhibit in detail some of the existing WEC technologies deployed
in various projects worldwide by 2018. According to these figures
it can be observed that numerous WECs technologies such as
WaveRoller, CETO 5&6 and PowerBuoy have already reached
substantial milestones. However, to reach the commercialisation
stage, the development cost should be significantly reduced [21].
4.1.1 Testing sites and next-generation prospects: Wave
energy testing centres are pillars to the technological development
and sustainability of WECs that would lead to significant growth of
this emerging industry. The WECs have to pass through different
prototype development levels (i.e. device scales) to reach full-scale
deployment at sea. The testing centres played a great role in
fostering and influencing the performance improvements of WEC
arrays [50, 89], which are an instrumental pivot for enabling
collection of practical data on control, operation, maintenance,
impacts and survivability of the WEC prototypes. It can be directly
conceived that most of the WEC prototype devices are qualified at
the testing centres to escalate the manufacturing readiness levels
and evaluate their respective TRLs before a commercial debut [21,
22]. In this case, wave test sites are very significant resources for
providing WEC device developers with another chance to correct
and their proposed technology. Unlike other renewable energy
conversion devices (especially wind), the WEC device has to
undergo rigorous tests in both laboratory and a real sea site for the
final technology verifications and commercial certification [15].
The sea site tests are more expensive, intensive and complex to
accomplish than the laboratory tests. Considering the prematurity
level and low penetration of the wave energy technology into the
existing energy market, the wave energy technological experience
is mostly garnered at these marine energy centres [17, 29].
The EMEC in Scotland, possesses fourteen full-scale grid-
connected test berths that have contained a good number of the
wave energy devices from different manufacturers around the
world with over 14 years of experience (since 2004) [90] whereas
the Wave Hub test site in Cornwall, UK also has several such
berths with 48 MW grid-connected facilities [50]. The United
States Navy's Wave Energy Test Site (WETS) in Hawaii, has three
full-scale grid-connected test berths. As can be observed there is a
growing need for constructing more new sea site test centres to
facilitate numerous WEC prototypes, which are under
development. It takes a long time (at least 1 year) for one prototype
WEC device to successfully complete the full-scale ocean test [91].
Based on the particular test site, various data measurements
such as power output profiles, social and environmental effects,
mooring and anchoring, survivability and bio-fouling are
conducted before grid-connected verification is successfully
granted [49, 50]. This duration might delay many other WEC
devices to pursue sea tests that would result in limiting the market
uptake timing and device certifications. In [92], an environment
impact assessment was performed considering the wave energy
activities at the European testing centres such as EMEC and Wave
Hub to enlighten the experiences and facts on this emerging
industry.
Next generation prospects would be to construct more wave
energy testing facilities in different locations worldwide with
advanced technology to meet high demands of the WEC devices
under development (i.e. TRL 6–9) [22]. The next generation WEC
devices are expected to meet cost-competitive energy market as the
LCOE for the PV solar and wind farms is keeping on decreasing
due to various investment stimuli [2, 6]. In this context, PV solar
modules cost has been reduced by 80% today compared to the
prices in 2009. Correspondingly, the cost of wind turbine
generation systems has declined by 50% within a similar period
[93].
Further researches to meet these challenges depend much on the
growth and advanced technology in the wave energy testing centres
to facilitate verification of the cost–benefit analysis and evaluation
of the technology performance levels (TPLs) [94]. Different from
the TRL that assesses the technological readiness of the WEC
device, the TPL evaluates how best a particular WEC technology
performs regarding capital expenditure and operational
expenditure, system availability and acceptability, power
absorption, conversion and delivery capability within a project
lifecycle [15, 95].
4.1.2 Wave energy farms practices and prospects: Little
scientific information is available on what are the actual practices,
impacts and experiences of operating full-scale wave farms due to
a few significant facilities under commercial operation. However,
Fig. 8 Existing WECs technology in various projects by 2018 [28]
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the environmental and life cycle assessments are most direct
conceivable impacts that have been studied and many projections
demonstrate little effects by the ocean wave energy compared to
other marine energies such as tidal energy [96]. In [32], an
investigation of wave energy practices beyond technology is
reported. It is concluded that most environmental impacts depend
on the size of the wave energy conversion units and resource
locations. On the other hand, the impacts could range from marine
ecosystems and sediment dynamics to marine fisheries’ activities.
It has been observed that the marine spatial planning would
significantly improve and smooth these constraints on aquatic
ecosystems concerning the wave energy harvesting [32, 96].
4.2 Global wave energy perspective and potential growth
The concept of harvesting wave energy for the electricity
generation is a more feasible and promising alternative solution to
produce power as investigated previously. It is already noted that
there is huge wave energy available in various coasts potentially to
contribute to the global renewable energy mix and electricity
demands mitigation shortly [19, 23]. In particular, the prevailing
westerlies phenomenon causes tremendous wave energy resources
in the west coastal regions around the world and their respective
poles. In this case, most west coastal regions such as Europe (e.g.
British Isles), Canada, US (e.g. Pacific Northwest) and Australia
have been enormously involved with wave energy activities [50,
64]. Also, the southern (southwest) and northern (northwest) parts
of the world especially between 30° and 60° latitude experience
large wave energy due to the same phenomenon [29]. In the global
perspective, the wave energy policies and many working-groups
have been set to meet both technological and non-technological
challenges [15].
4.2.1 Marine life, social and legal issues: Intensive activities
into the ocean space directly affect the marine species and original
marine structures. This may include the sediment dynamics and
mortality of marine species due to the interactions of operating
WEC devices and installation or decommissioning activities of
these devices [97, 98]. Boehlert and Gill [98] discussed in detail
the environmental and ecological effects of ocean renewable
energy development. It was noted that the new structures installed
due to the marine renewable energy harvesting activities (i.e.
WECs devices) would result in the fundamental change to the
habitat. The study concluded that the effects prevail
notwithstanding the WECs position (i.e. installed position
concerning sea level) both above sea level with WECs device like
Oceanilix or below sea level with WECs like bioWave, which is a
fully submerged device.
On the other hand, the most notable positive socio-economic
impact of wave energy harvest activities along the sea is job
creation that has been advocated by numerous literatures recently
[21, 46, 92]. On the other side of the coin, to cater to the global
electricity demand by using WECs technologies can be conceived
as a positive socio-economic impact at present and shortly
perspective. To put this into context, the increasing access to
energy (especially electricity) would equally improve the quality
life of a particular community and similarly reprieve energy
insecurity [21]. There are high possibilities of improved social
services at remote islands such as quality health care systems and
quality fresh water through WEC-incorporated desalination plants
under wave farm microgrid system or self-enabled WEC device
under reverse osmosis technique [61, 99, 100]. However, to meet
this level adequately, a rigorous technological breakthrough in this
sector should be sought. The coexistence of marine fisheries’
activities, marine energy capture systems (WEC device in this
case) and marine species should be maximised and optimised to
meet the sustainable development for all. The rigorous research
outputs in this field shall satisfactorily respond to the answers to
this call.
Furthermore, the oceanic space is wide with various species and
natural resource heritage at some point. As noted in the previous
sections particularly as shown in Fig. 4, the WEC arrays can be
installed on the shoreline, nearshore and offshore localities [21,
46]. Specifically, intensive wave energy exploitation activities
might create some interference with fishing activities or aquatic
species’ movements and ecological environment [97, 98]. If these
activities are not legally endorsed, they might jeopardise aquatic or
marine environmental sustainability in the enshrined economic
development with enormous green energy harvest call. Therefore,
geopolitics and geo-economics coexist in associations with marine
renewable energy harvest activities [101]. In this case, marine
governance and their respective legal obligations set forth to
smooth marine energy exploitations activities.
About marine renewable energy (wave energy inclusively),
extensive marine governance issues have been articulately
discussed in [102]. Resource management rights and ownerships of
Fig. 9 Available WECs technology employed in different projects by 2018
[28]
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the marine spaces have been dealt with immensely. It was observed
that the legal rights and clear marine governance paradigm should
be enacted to seek a clear jurisdiction on discharging these matters
(i.e. marine activities). In [103], the legal landscape of wave energy
pilot projects on the Oregon coast has been reported with
regulation in Oregon's territorial sea put into consideration.
Eventually, the integrated framework that will encapsulate the
marine life, social and legal issues is of paramount for the massive
wave energy farms commercialisation in the near future.
4.2.2 Motivating factors and future trends: Other wave energy
harvesting technological concepts have matured enough and there
is a clear sign for near commercial deployment. The OWC, which
is a mature technology has been employed for many years [30] and
is still showing interesting results through existing projects as
illustrated previously (e.g. Mutriku and Pico wave plants) [21, 22].
Besides, the WaveRoller with an oscillating surge concept has
recently demonstrated good results following its rigorous sea test
of 350 kW at EMEC test site, in 2016. Also, construction of
commercial WaveRoller units started in the mid-2016 and it is
expected to deploy 1.5 MW capacity in Bretagne, France in 2018.
AW-Energy Oy in collaboration with other partners in Europe is
expecting to start development of PTO for the 1 MW WEC device
scale in 2018 [28]. On the other hand, numerous other WEC
devices are under rigorous development and construction stages to
meet commercial deployment constraints such as survivability,
reliability and cost-effectiveness [16, 44, 46, 104–110]. In
particular, the WaveStar Energy is expecting to bring a full-scale 1
MW WEC device into a market [104]. Researchers at Plymouth
University have been improving and testing the performance of the
SEARASER WEC device that could harvest up to 1 MW of energy
per day. Similarly, the scientists in the same institution are working
on the WEC device concept ‘The Penguin’ that could undergo sea
test at Wave Hub [105].
Equally, there is a huge hope to advance wave energy
technologies by effectively exploiting the experience and off-the-
shelf marine energy technologies (e.g. offshore wind energy). For
example, technologies such as mooring systems, electrical
technologies (i.e. electric generators, marine cables, power
transmission, and offshore substation), hydrodynamics and other
hydrokinetics mitigation have been successfully utilised to fast-
track the development of WEC facilities [17, 32, 67, 85]. Some
countries have special initiatives to foster the development of wave
energy industry [48, 50, 101]. For instance, Denmark under
Energynet.dk program supports the technology developments of
WaveDragon. Similarly, the Spanish government under PSE-Mar
project supports the development of PipoWEC prototype [15, 91,
110]. In the US, the Water Power Program under the Department of
Energy in collaboration with various national laboratories supports
and nurtures the wave energy development efforts [50]. Intensive
wave research development activities have been conducted at the
Pacific Northwest Coast (in the US) supported by The Oregon
Wave Energy Trust (OWET) in collaboration with the Oregon State
University (OSU) researchers and other marine energy
stakeholders in the region since 2007. Equally, the Northwest
National Marine Renewable Energy Centre (NNMREC) has been
supporting the wave energy technology research activities (i.e.
including wave energy testing facilities) in partnership with the
OSU since 2008 [111, 112]. Table 2 depicts several WEC devices
with their technical readiness status by 2017. Similarly, Table 3
shows various WECs devices with their TRL stages under
development around the globe by 2017. It can be observed that
from this dataset most of the WEC devices are still deployed in the
commercial demonstration stage with very few of them completely
connected to the grid network [21, 113].
Apart from that, there are numerous wave energy testing centres
under development (i.e. with full-scale grid-connected facility) that
will cater for the need of the WEC devices’ sea testing facilities
and escalate a number of pending technologies certification [50,
92, 113]. The California Wave Energy Test Centre (CalWave) is an
envisioned grid-connected sea test facility expected to commence
providing services in 2022. The construction of the wave energy
test site of 5 MW capacity incorporating five berths commenced in
2016 in Korea [50]. In the late 2016, the Pacific Marine Energy
Centre South Energy Test Site (PMEC-SETS) received a grant to
construct an advanced grid-connected testing facility that is
scheduled to be completed in 2020 [50, 110]. If this trend continues
successfully in the next coming 10–20 years, it will not only
increase test site flexibilities and options but also reflect positively
on the WEC devices’ cost competitiveness and sea test
Table 2 Technical readiness status of various WECs concepts around the globe by 2017 [21, 22, 113]
WEC device name (company
name)
WEC technology TRL
stage(s)
Remark(s)
bioWAVE (BioPower Systems,
Australia)
oscillating bodies (surge
converter)
6/7 under commercial demonstration projects
WaveRoller (AW-Energy, Finland) oscillating bodies (surge
converter)
5/7 under development for commercial demonstration projects,
MW grid connection license available to Portugal national
electric agency
PowerBuoy (Ocean Power
Technologies, Inc.,USA)
oscillating bodies (submerged
point absorber)
6/7 under development for commercial demonstration projects,
provide UPS functions with an on-board battery energy
system
Archimedes Wave Swing (AWS
Ocean Energy, UK)
oscillating bodies (fully
submerged point absorber)
5/6 use direct drive linear generator PTO, under wave energy
Scotland project to develop an advanced AWS 1:20 scale
model
Table 3 Various WECs devices with their TRL stages under development around the globe by 2017 [21, 22, 113]
WEC device name (company
name)
WEC technology TRL stage(s) Remark(s)
CETO (Carnegie Wave Energy,
Australia)
oscillating bodies (fully submerged
point absorber)
6/7 under commercial demonstration projects
Searaser (Ecotricity, UK) oscillating bodies (floating point
absorber)
4/5 undergoing an advanced technology development
ahead of actual sea tests
WaveStar (Wave Star Energy, Denmark) oscillating bodies (multiple floating
point absorber)
5/6 under H2020 project to develop full-scale 1 MW for
commercial demonstration
Multi-Resonant Chamber (OREcon, UK) OWC 5/6 full scaled deployment in Portugal in 2011
SINN Power wave energy converter
(SINN Power GmbH, Germany)
oscillating bodies (multiple floating
point absorber)
4/6 first sea test commenced in 2015 at Greek island
Crete
Oyester (Aquamarine Power, UK) oscillating bodies (surge converter) 5/6 under advanced technology development
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commencing timing while ascertaining high diversities in the WEC
testing practices in a very near future [110].
Furthermore, the European Commission sets a target to reduce
the carbon emission by 20% that should abide by all its member
states by 2020, which projects that 67% of the power generation
should originate from the renewable energy sources by 2050 [4, 7].
The wave energy industry is inclusive in this renewable energy
package following the grant awards and supports in many ways
such as Horizon 2020 grants [49]. Nevertheless, the Paris
Agreement signed by >120 countries in 2016 to combat the global
climate change is another shot that would inevitably contribute to
more penetration of the renewable energy mix (with wave energy
inclusively) [6]. All these efforts and supports are green light to the
development and sustainability of the WEC technology to reach
commercialisation stages in a very near future. Also, other
initiatives such as InnovFin Energy Demo Projects for the
WaveRoller in 2016 [50, 114] and EU Horizon 2020 Research and
Innovation programme act as a quick bridge [93] to link the
demonstration prototypes to full-scale commercial product that
would mean extensive exploitation and utilisation of the wave
energy resource in a very near future.
4.2.3 Limiting factors and prospective solutions: Various
technical (e.g. complex design and operation of WECs) and non-
technical (e.g. policy frameworks, public and private funding,
investments etc.) issues highly impede the vertical growth of wave
energy sector [15–19]. Most of the WEC projects worldwide highly
depend on the public grants and supports with very few private
investors involvements [28, 49, 108]. For example, some of the
WEC arrays (devices) are decommissioned following a serious
collapse of the investments from the potential funders or after
meeting catastrophic challenges during the actual deployment at
the sea level [115, 116]. The Oceanlinx Energy Company with their
commercial second-generation Oceanus (greenWAVE device) of 1
MW capacity collapsed in the Australian Southern Coast during the
deployment process in 2014 and there is an ongoing effort to
reconstruct the new device to meet energy market in a near future
[115].
In contrast, other WEC devices such as MRC (OREcon) and
Pelamis have completed their full-scale prototype sea tests, but
lack of project funds and high production cost constrain their
scopes to successfully reach the commercialisation stage [116]. In
one hand, there is no choice other than putting more efforts to
improve and address economies of scale especially at the
production level. On the other hand, government energy policies
could have introduced some incentives such as feed-in tariff
schemes for those WEC array prototypes that have successfully
met the full-scale test performance. This would act as a stimulus to
strike a balance between production cost and profit margins that
would leapfrog the commercialisation of the wave energy sector
[50, 110].
4.2.4 Synergy and competitiveness: The existence of marine
energy activities especially offshore wind farms can play a great
role in fostering the penetration of wave energy conversion
systems. On the other hand, as the wave energy variability is very
minimum compared to wind (accurately predictable over 48 h
ahead), this phenomenon attracts synergies between wave and
offshore wind farms to complement each other for the sustainable
power generation [63]. Various authors have reported the
possibility of coexistence between WEC devices and wind turbines
[53, 64].
It is envisioned that WEC devices would be aggregated in
arrays to efficiently harvest electric energy [46, 54, 55]. To meet a
competitive market in the future, the WEC devices in a modular
structure would be much more attractive to allow easy maintenance
and sustainable operation. It means that the malfunctioning of one
unit would not affect the operation of the other units, however only
a reduction of power can be experienced and tolerated. In this case,
smart energy storage systems can be deployed to act as an
uninterruptible power supply (UPS) to compensate for the power
deficit [72, 84, 117]. Also, the wave arrays and farms would
benefit from the modular structure regarding design and
interoperability.
According to temporal variations in wave energy resource
assessments, it is observed that the wave power can be enormously
harvested during winter season compared to summer season [35–
38]. This seasonal trend is an excellent phenomenon that can be
efficiently utilised to offset high power demand during winter
periods and relieve load demands to the existing power grid. An
early study on this synergetic operation between the existing power
grid and wave farms has been studied in Korea [41]. More of the
same studies are encouraged to be conducted with a focus on the
economies of scale perspectives that would increase the potential
of the wave energy in the future electricity market.
Another interesting study is reported in [63, 64] on the analysis
of the utility reserve requirements for the large-scale renewable
energy grid integration with wave, wind and solar synergetic
perspectives. It is concluded that through energy mix, these
variable and non-dispatchable renewable energy sources can highly
lessen the reserve demand and congestions of the electric grid
generation. In [34], the possibility of integrating WEC devices and
offshore wind energy systems in a single unit to fully maximise
utilisation of the offshore ocean resources is reported whereas in
[118], the synergetic operation of the wave and wind energy
systems is investigated with electricity market perspectives.
Stoutenburg and Jacobson [119] found that there is a great chance
of reducing offshore transmission requirements by integrating
offshore wind and wave energy farms. All these researches are
potential insights of the significant contributions that wave energy
could bring after fully exploitation of this renewable energy
resource.
5 Conclusion and final remarks
This paper has presented an extensive review regarding the state-
of-the-art and future potential of the wave energy power
generation. The challenges facing the rapid growth of this
emerging technology were identified and potential solutions to
reach high share in the energy market have been given. From this
study, it is clear that the leverage of the potential wave energy
resource is a cross-disciplinary target with the underlying WEC
technology. The complex marine ecosystem cannot be ignored to
maximise the potential of the wave energy. Several issues including
legal landscape, social-welfare, marine life and technology should
move together to unfold the development barriers of the wave
energy sector. Marine governance and legal enforcement should
strike a balance between social development and environmental
sustainability.
As discussed in this paper, most of the WEC devices’
technologies are under low TRL indexing scales. Specifically, the
WEC devices have not yet been matured enough to meet the
contemporary or near future competitive energy market. It has been
revealed that the daunting cost of WEC development would be
mitigated by the rapid growth of technology in this sector. It means
that the WEC developers and other stakeholders in this field should
adapt new emerging technologies to address the WEC design and
operating performance. For instance, the emerging electric
generator designs, mooring systems and PTO techniques should act
as a focal point to drive these changes in attaining the cost-
effective WEC devices. It has also been observed that there are
huge potential and opportunity for the ocean wave energy in
synergetic operation perspectives that could relieve electric grid
challenges and enhance renewable energy mix.
On the other hand, the amount of funding for the wave energy
projects is very modest compared to other renewable energy
projects especially wind and solar. The growth of WEC technology
to reach the commercialisation stage highly depends on these
public funding schemes. Therefore, energy policy makers have to
consider incrementing funds consistently to meet the blue growth
vision in the very near future by adapting the learning curves from
the wind energy industry pioneered by countries such as Denmark,
Germany, US, UK, China, Japan and Spain.
IET Renew. Power Gener., 2019, Vol. 13 Iss. 3, pp. 363-375
© The Institution of Engineering and Technology 2018
373

6 Acknowledgments
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education (no.
2018R1D1A1B07046873).
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