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Oral Presentation
Theory and Applications of Wave Energy Converters: A Review
Baki KOYUNCU1*, Musaria MAHMOOD1, Indrit MYDERRIZI1
1Istanbul Gelisim University . Faculty of Engineering and Architecture, Department of Electrical and Electronic
Engineering ,İstanbul/Turkey
*Corresponding Author: bkoyuncu@gelisim.edu.tr
Abstract
Research on renewable energy sources increases due to the finite fossil fuel that meets a large
proportion of energy needs and leads to heavy environmental problems. Renewable energy
sources have become outstanding and alternative solutions to fossil fuels. Among these
sources of renewable energy, the wave energy is characterized by its large, unexploited
energy resources. The cost of electricity production by wave energy is higher than the
electricity generation from conventional fuels. One of the reasons is that the wave energy
converter technology is still in research and development phase. Many techniques for
transforming energy carried by ocean waves into usable energy have been exploited and
presented as future electricity market potentials. Most of them are stationed at offshore
locations and installed for several years in many locations on earth especially in Europe and
South America. Wave Energy Converters (WECs) are similar systems and they all share the
same characteristic of having low efficiency.
The review focuses on the wave energy principles that stand behind the development of the
production of energy from the sea and/or ocean waves. A classification of the existing
technologies based on the operational and technical properties of the wave energy converters
is presented. This review evaluates the wave energy potential of Turkish coasts and concludes
through propositions for future growth in this sector.
Keywords: Wave Energy, Converter, Power Take Off, Energy Efficiency.
Introduction
The world faces two major immediate problems related to energy consumption and
production. The first is the low production due to limited stocks of fuel causing prices to rise.
The second is the ecology and environmental problems associated with unregulated and
uncontrolled fuel consuming. The possibility of electrical production from the waves is
getting very important in recent years. Oceanic energy is an unlimited available source and
capturing this energy from waves performs a leading move in the direction of arriving at the
renewable energy objectives. Wind near the ocean surface activates a random movement of
the waves by means of energy transfer between wind and waves. Research in this area is
relatively elementary compared to the known renewable energy technologies (Drew et. al.,
2009). The difficulties that may get up from system structure, implementation, maintenance,
life cycle charges, recovery of expense, and ecologist matter are fully studied (Aderinto and
Hua, 2018). The benefits of wave energy are numerous such as being the infinite source of
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energy, reduction of environmental pollution, preservation of farmland, contribute to the
ecological balance, and the additivity of the system size according to the required energy
amount (Bedard, 2007). The disadvantages are the unpredictability of available wave energy,
the ecological impact of causing disturbances in the seafloor, the noise and visual pollution,
the hazard to ships, and the high cost of installation and maintenance (Meisen, and
Loiseau,2008).
Mathematically, wave energy is considered to be the derivative of solar energy. The most
important cause of the sea waves is wind activity. Wind-driven waves are created by wind
circulation on the sea as it passes near of water surface. The wind arrives with different speed
and direction and results in a large variance of waves with different characteristics (amplitude,
period, and direction) (Goda, 2010).
Wave Energy Converters (WEC) are designed by considering many factors such as the
mode of wave motion, electrical generating systems, and their location (onshore, or offshore)
(Carbon Trust, 2005). The performance of the WEC is measured with the energy conversion
efficiency between the wave energy power and the electric power generated (CZhang, and
Yin, 2015). It is evaluated via the load factor, which is equal to the output electrical power on
the wire divided by the input wave power estimation at the input of the system. WEC has a
variable efficiency regarding the wave state (Prendergast, and Sheng, 2018).
Wave Energy Theory
Ocean waves result from the superposition of a large number of waves which generate their
complex behavior. Many scientific studies show that ocean waves can be considered as the
algebraic sum of many sinusoidal shape waves on the ocean surface. This significant wave
type is based on the linear wave theory (Fois et. al., 2014).
A. Linear Wave Theory
Linear wave theory is the essential theory of sea and ocean surface waves used in naval
architecture and ocean engineering. The existence of solitary wave with a single crest,
propagating without change in the form or direction is mentioned in many studies. Many
prototype problems are resolved using the monochromatic waves based on the significant
wave theory (Goda, 2010). This method is one of the most used methods to analyze the
random behavior of sea and ocean waves. The wave energy analysis is based on a single
incident wave of sinusoidal shape. A simple harmonic function called "regular wave" is
produced as given in Fig. 1. The regular function propagates in x-axis direction, with , as
the significant wave high, is the wavelength, is the period, is the water depth, g is
the acceleration of gravity, is the water density, and k is the wavenumber. This wave is a
two-variable function given by (Abo Bakar et. al., 2011):
(1)
The velocity is defined by:
(2)
Replacing (2) in (1) gives:
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(3)
For two-dimensional waves:
(4)
For an incompressible fluid satisfies:
(5)
The velocity potential is:
, and , this yields to a Laplace equation, given as:
(6)
The motion of the fluid particles is described by (Sorensen, 2006):
(7)
The Bernoulli equation for small waves is:
(8)
The regular waves are described by (6), (7), and (8).
The group velocity in deep and shallow water is then:
(9)
(10)
Shallow water is considered when , whereas deep water is for ,
(Chenari, et al., 2014).
Figure 1: Significant wave parameters
B. Wave Energy Converter Efficiency
The power efficiency can be calculated as the ratio of the produced power level and the
available incident wave power at the system input:
(11)
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where, , is the system efficiency, is the output power, and is the power of the
incident wave at the WEC input, or the wave energy flux. The efficiency of wave energy
converter is usually small, for example, the maximum efficiency of Oscillating Water Column
(OWC) device is 25% (Martinelli, et al., 2009).
C. Wave Energy Evaluation
Generally, near-shore waves produce less energy, but have the advantages of having simple
energy converters in terms of installation and maintenance. The mean wave energy density
per unit horizontal area ( ) can be found in (Sağlam, et al., 2010):
(12)
Wave energy is transported along the wave direction as the wave propagates. As a result the
wave energy flux is equal to:
(13)
If is the wave wings aperture, then combining (9), (12), and (13) for deep water, yields:
(14)
If , , and , (14) becomes:
(15)
Combining (10), (12), and (13) for limited depth yields:
(16)
Wave energy potential can be estimated regarding the wave converter device locations in (15)
, (16).
Wave Energy Extractıon
A preliminary study of wave nature in the selected area of the system location is
necessary. Convenient WEC is deployed among many existing types according to wave’s
properties and environmental conditions.
A. Wave Energy Technologies
The study is focused onto the wave energy resource suitability of the location which is the
basic prerequisite for the design of wave energy devices. The design process begins with a
study of wave energy opportunity in the selected site as shown in (Rémouit, et al., 2018).
Wave devices are categorized by their deployment location, wave energy capture method and
by the power take-off systems (Polinder, and Scuotto, 2005). Locations are onshore,
nearshore, and offshore. All capture methods share the same principle of sensing the wave
movement but differ from the technology and procedures point of view.
Implemented systems can be fixed, submerged or floating. Additionally, the systems can be
used as independent generators or as part of breakwaters or harbor infrastructures (Rusu,
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2014). Evaluation of wave energy must distinguish between various WEC technologies. The
well-known technologies are given below (Kempener, and Neumann, 2014):
• Point Absorber Buoy (PAB): It is a floating structure or submerged below the surface
of the water relying on pressure differential (Drew et. al., 2009). It has a cylindrical
shape, and vertically installed in the water. Sea wave acts on the PAB producing a
vertical movement of the buoy followed by the moving part of the linear generator as
in Fig. 2. The generator is fixed to an underwater support, and the power is transferred
to the coast via cables (Al Mahfazur et. al., 2017). The piston-based linear generator is
used to produce energy from the continuous up-down movement of the buoy. The
advantages of PAB types are the non-dependency on the wave direction, simple
design, cost-effective manufacturing, installation and maintenance.
• Surface Attenuator: It is a mechanical WEC system with several floating bodies
connected by joints. The structure is operational in offshore location. The articulated
joints contain piston-cylinder arrangements whose relative movements pressurize
hydraulic fluid that drives a motor connected to a generator (Hodge, 2017). One of the
most known attenuator type operating WEC is the Pelamis device by Ocean Power
Delivery (Houhou et. al., 2018). The attenuator principle is illustrated in Fig. 3.
• Terminator Device: It has the principal axis parallel to the incident wave crest, situated
perpendicular to the direction of wave travel and terminates the wave. There are
commonly three types in terminator WEC: oscillating wave surge converter,
oscillation water column, and overtopping device.
• Oscillating Wave Surge Converter: It is generally comprised of a hinged deflector,
positioned perpendicular to the wave direction as shown in Fig. 4 (Drew et. al., 2009).
The movement of waves will act on the submerged deflector forward and backward.
This movement is used for a piston-based system for energy production.
• Oscillating Water Column (OWC): It consists of a chamber with an opening to the sea
below the waterline, enclosing a column of air on top of a column of water as in Fig. 5.
Wavegen Limpet mounted device is an example of the onshore system which consists
of two main parts: the air chamber and the power take off (PTO) subsystem. The air
chamber is opened to the seaside receiving wave energy and transforming this energy
into compressed air. Enclosed air will pass through the turbine in the PTO mechanism
Figure 2. Point absorber buoy Figure 3. Attenuator principle
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Figure 4. Oscillating wave surge Figure 5. Oscillating Water Column
which converts pneumatic energy into mechanical energy (Jebli, and Chagdali, 2018).
An OWC device can be installed onshore acting as a terminator of or as a PAB system
in the offshore location.
• Overtopping Device: It is the oldest strategy of using wave energy located in offshore
sites. The incident wave energy is captured by a water storage reservoir above the sea
level, and then releases the water back to sea through hydro turbines. This device is
similar to the hydroelectric power plant. The most important.
• Characteristic of this device is its high efficiency compared to other wave energy
converters. Wave Dragon, shown in Fig. 6, is the most well-known example of this
type.
Figure 6. Overtopping device
B. Power Take Off (PTO)
The mechanism converted the sensed wave energy by the WEC into power on the wire
is the power take-off system (PTO). At the PTO output, a usable power form is available.
WEC can be OWC, surface attenuator, or any convenient converter type (Pecher, and Kofoed,
2017). The PTO mechanisms can be based on high-pressure liquid hydraulic systems, air
turbines, or linear generator (Erselcan, and Kukner, 2014). The most used method of
electricity production is made after the conventional high-speed rotary electrical generators
(Drew et. al., 2009). Turbines must be designed according to the expected produced power by
the WEC. The complete PTO system block diagram consisting of sea wave energy extraction
device (WEC), air or liquid compressor, storage tank, turbine, and an electricity generator is
shown in the Fig. 7. Wave movement acts on the WEC device which enables the compressor
(piston-based system) to compress air and store it in the storage tank. The stored energy on
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the air or liquid under pressure is used to rotate a turbine and then, the generator for electricity
production.
A simple direct-driven PTO system is presented in (SZABÓ, et. al., 2008). A directly
connected buoy to a generator is used with no interposing of any mechanical subsystem. The
variation of wave high between a maximum and minimum values is used to activate the
generator. The active floating device obeys to the same water surface movement. The
generator is stabilized by a concrete structure on the sea bed and connected to the float
WEC
Turbine
Storage tank
Generator
Compressor
Sea Waves: Pwave
Electricity: Pout
Figure 7: PTO system block diagram
An improved method for the PTO is presented in (Farrok, et. al., 2018). The conventional
permanent magnet linear generators used for ocean wave energy conversion system suffers
from everlasting degradation in electrical power generation caused by the demagnetization.
This paper presents a new design that can be applied to various linear generators to avoid
demagnetization. The effectiveness of the proposed technique is tested by using the software
package ANSYS/ ANSOFT to simulate and test the Permanent Magnet Linear Generator
(PMLG).
The general objective of the work presented by (Martinelli, et al., 2009), is to examine, the
PTO efficiency under irregular wave conditions, for WECs with flow redirection. The
analysis is based on the experimental results of existing tests carried out in the 3D deep water
wave tank at Aalborg University, Denmark. The power at the modelled PTO is compared with
the available incident wave power in order to examine the overall system response in a scale
independent manner. The efficiency of WEC is usually small, for example 20% is the
maximum efficiency of OWC systems. The power take-off has a direct impact on the
efficiency, the overall dimensions, and the structural dynamics of the system (Pecher, and
Kofoed, 2017).
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Wave Energy Potentıal ın Turkish Coastline
Turkey's coastline is about 8,200 km excluding coasts of the Sea of Marmara. It is an
undeniable fact that Turkey has a great wave energy potential. The answer to the question of
the possibility to consolidate the current Turkish power plan by taking profit from the
availability of the opportunity of wave energy is presented in (Sağlam, et al., 2010).The wave
energy capability in the Turkish coasts is evaluated in various sites based on the existing data
in the “Wind and Deep-Water Wave Atlas of the Turkish Coast”. A value of 10 TWh /year is
a primary estimation for the available energy resources in Turkish coasts. The average wave
power estimation is about 4-17 kW/m. Comparison of these values with the actual
hydroelectric power generation in Turkey indicates that 7.8% of the currently generated hydro
power can be met by wave energy systems. The west of the Black Sea region and the
southwestern coasts of the Aegean Sea between Marmaris and Finike are founded to be the
best locations to promote the wave energy (Sağlam, et al., 2010). A prototype of a new
palette-type Sea Wave Energy Conversion (SWEC) device supported by buoys is proposed in
(Buldu, et. al., 2011). The SWEC under analysis is planned to be installed in the
Mediterranean Sea near the coast of Anamur in Turkey. The system is expected to generate
sustainable electrical energy even on an almost flat sea. It is an offshore device and more
favorable than other similar devices in terms of simple construction, environmental impacts
and energy output value. Both wave and tidal energies are transformed into mechanical
energy by WEC and then converted by the power-take-off into electrical energy. The
proposed PTO mechanical interface can be considered as a brand-new class based on a gear
wheel and a gearbox mechanism. It is expected that the system will have an efficiency near to
70%.
A study to determine the electrical energy potential obtainable from the wave energy
converters in Turkey is proposed in (Kukner, et. al., 2016). Many types of wave energy
conversion systems have been investigated and as a result of the study, the OWC, is the most
effective energy converter due to a suitable power generation system. The results have shown
that the Igneada region is the best location within the five selected regions to generate
electrical energy from the absorber type wave energy converters. The paper concludes that the
selection of the best type of wave converter for a location depends on many variables,
including wave power, water depth, region, and climate. Seasonality is also a factor which has
an impact on the wave energy system feasibility. The Igneada region's wave energy potential
is analyzed for one OWC prototype where the value of generated energy is found to meet the
needs of the small community of around 30 houses. A higher electricity power can be
accomplished by providing a wave energy farm system.
The obstacles against the use of wave energy in Turkey are outlined in (Yeşilyurt, et. al.,
2017). The complex structure of wave energy conversion systems, marine conditions,
mechanical difficulties and high initial investment costs are the most important causes. The
selection of WEC type is directly related to the wave properties, marine conditions and the sea
weather. A good selected WEC for a site may not perform as good in another location with
different marine behavior. The features of Baltic Sea appear to be conform to Turkish coasts,
hence systems designed for that region can be perfect candidates for analysis in Turkey. The
work concludes that the most suitable sites for WEC installation and energy production are
reported to be in the Black Sea and southwest Mediterranean region.
The possibility of various renewable energy types near the Turkish coasts, have been
examined in (Sener, and Aytac, 2017), . The study is focused on finding an appropriate site
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for nearshore multi-type renewable energy and implement a platform and create a notion of
such design. Depending on renewable energy possibility, the regions with the highest level of
opportunity in wave, solar, current, and wind resources are determined. Finally, a relatively
small-size prototype system, situated close to the coastline, is proposed.
A study to determine the exploitation rate of wave energy in Turkey is presented in (Gur, et.
al., 2018). According to this study, the most suitable place to take advantage of wave energy
is the sea between Izmir and Antalya due to the reception of highest wave intensities. . The
regional average wave densities for Turkish coastline are as follows; Black Sea (1.96 – 4.22
kWh/m), Marmara Sea (0.31 – 0.69 kWh/m), Aegean Sea (286 – 8.75 kWh/m), Izmir -
Antalya (3.91 - 12.05 kWh/m).
Conclusions
Ocean and sea waves are among the richest new forms of renewable energy resources. A
huge clean energy resource for generating electricity is provided for humanity. The
development of wave energy systems is motivated by unlimited, sustainable, and
environmentally friendly resources.
This review presents a profile for the current wave energy technologies and their
mechanisms. Six main WEC technologies are evaluated and currently most popularly used
PTO systems are emphasized. The potential of wave energy in Turkey is highlighted through
several research papers including statistical analyses.
Although there are many studies on wave energy technologies, there are still several areas that
need to be improved, such as efficiency and reliability optimization. Accordingly, future
research can evaluate the effect of the WEC floating device shape with respect to system’s
reliability and efficiency. Additionally, the impact of the coast wall shape on system
efficiency can also be examined.
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