Wave power

This article is about transport and the capture of energy in ocean waves. For other aspects of waves in the ocean, see Wind wave. For other uses of wave or waves, see Wave (disambiguation).

Pelamis Wave Energy Converter on site at the European Marine Energy Centre (EMEC), in 2008

Azura at the US Navy’s Wave Energy Test Site (WETS) on Oahu

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'Wave power is the transport of energy by ocean surface waves, and the capture of that energy to do useful work – for example, electricity generation, water desalination, or the pumping of water (into reservoirs). A machine able to exploit wave power is generally known as a wave energy converter (WEC).

Wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave-power generation is not currently a widely employed commercial technology, although there have been attempts to use it since at least 1890.^[1] In 2008, the first experimental wave farm was opened in Portugal, at the Aguçadoura Wave Park.^[2]

Physical concepts

When an object bobs up and down on a ripple in a pond, it follows approximately an elliptical trajectory.

Motion of a particle in an ocean wave.
A = At deep water. The elliptical motion of fluid particles decreases rapidly with increasing depth below the surface.
B = At shallow water (ocean floor is now at B). The elliptical movement of a fluid particle flattens with decreasing depth.
1 = Propagation direction.
2 = Wave crest.
3 = Wave trough.

Photograph of the elliptical trajectories of water particles under a – progressive and periodic – surface gravity wave in a wave flume. The wave conditions are: mean water depth d = 2.50 ft (0.76 m), wave height H = 0.339 ft (0.103 m), wavelength λ = 6.42 ft (1.96 m), period T = 1.12 s.^[3]

See energy, power, and work for more information on these important physical concepts. See wind wave for more information on ocean waves.

Waves are generated by wind passing over the surface of the sea. As long as the waves propagate slower than the wind speed just above the waves, there is an energy transfer from the wind to the waves. Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind, making the water to go into the shear stress causes the growth of the waves.^[4]

Wave height is determined by wind speed, the duration of time the wind has been blowing, fetch (the distance over which the wind excites the waves) and by the depth and topography of the seafloor (which can focus or disperse the energy of the waves). A given wind speed has a matching practical limit over which time or distance will not produce larger waves. When this limit has been reached the sea is said to be "fully developed".

In general, larger waves are more powerful but wave power is also determined by wave speed, wavelength, and water density.

Oscillatory motion is highest at the surface and diminishes exponentially with depth. However, for standing waves (clapotis) near a reflecting coast, wave energy is also present as pressure oscillations at great depth, producing microseisms.^[4] These pressure fluctuations at greater depth are too small to be interesting from the point of view of wave power.

The waves propagate on the ocean surface, and the wave energy is also transported horizontally with the group velocity. The mean transport rate of the wave energy through a vertical plane of unit width, parallel to a wave crest, is called the wave energy flux (or wave power, which must not be confused with the actual power generated by a wave power device).

Wave power formula

In deep water where the water depth is larger than half the wavelength, the wave energy flux is^{[lower-alpha 1]}

P = \frac{\rho g^2}{64\pi} H_{m0}^2 T_e \approx \left(0.5 \frac{\text{kW}}{\text{m}^3 \cdot \text{s}} \right) H_{m0}^2\; T_e,

with P the wave energy flux per unit of wave-crest length, H_m0 the significant wave height, T_e the wave energy period, ρ the water density and g the acceleration by gravity. The above formula states that wave power is proportional to the wave energy period and to the square of the wave height. When the significant wave height is given in metres, and the wave period in seconds, the result is the wave power in kilowatts (kW) per metre of wavefront length.^[5]^[6]^[7]^[8]

Example: Consider moderate ocean swells, in deep water, a few km off a coastline, with a wave height of 3 m and a wave energy period of 8 seconds. Using the formula to solve for power, we get

P \approx 0.5 \frac{\text{kW}}{\text{m}^3 \cdot \text{s}} (3 \cdot \text{m})^2 (8 \cdot \text{s}) \approx 36 \frac{\text{kW}}{\text{m}},

meaning there are 36 kilowatts of power potential per meter of wave crest.

In major storms, the largest waves offshore are about 15 meters high and have a period of about 15 seconds. According to the above formula, such waves carry about 1.7 MW of power across each metre of wavefront.

An effective wave power device captures as much as possible of the wave energy flux. As a result, the waves will be of lower height in the region behind the wave power device.

Wave energy and wave-energy flux

In a sea state, the average(mean) energy density per unit area of gravity waves on the water surface is proportional to the wave height squared, according to linear wave theory:^[4]^[9]

E=\frac{1}{8}\rho g H_{m0}^2,

^{[lower-alpha 2]}^[10]

where E is the mean wave energy density per unit horizontal area (J/m²), the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy,^[4] both contributing half to the wave energy density E, as can be expected from the equipartition theorem. In ocean waves, surface tension effects are negligible for wavelengths above a few decimetres.

As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the wave energy flux, through a vertical plane of unit width perpendicular to the wave propagation direction, is equal to:^[11]^[4]

P = E\, c_g, \, \

with c_g the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ, or equivalently, on the wave period T. Further, the dispersion relation is a function of the water depth h. As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:^[4]^[9]

Properties of gravity waves on the surface of deep water, shallow water and at intermediate depth, according to linear wave theory
quantity	symbol	units	deep water ( h > ½ λ )	shallow water ( h < 0.05 λ )	intermediate depth ( all λ and h )
phase velocity	$\displaystyle c_p=\frac{\lambda}{T}=\frac{\omega}{k}$	m / s	$\frac{g}{2\pi} T$	$\sqrt{g h}$	$\sqrt{\frac{g\lambda}{2\pi}\tanh\left(\frac{2\pi h}{\lambda}\right)}$
group velocity^{[lower-alpha 3]}	$\displaystyle c_g= c_p^2 \frac{\partial\left(\lambda/c_p\right)}{\partial\lambda}=\frac{\partial\omega}{\partial k}$	m / s	$\frac{g}{4\pi} T$	$\sqrt{g h}$	$\frac{1}{2} c_p \left( 1 + \frac{4\pi h}{\lambda}\frac{1}{\sinh\left(\displaystyle \frac{4\pi h}{\lambda}\right)} \right)$
ratio	$\displaystyle \frac{c_g}{c_p}$	–	$\displaystyle\frac{1}{2}$	$\displaystyle 1$	$\frac{1}{2} \left( 1 + \frac{4\pi h}{\lambda}\frac{1}{\sinh\left(\displaystyle \frac{4\pi h}{\lambda}\right)} \right)$
wavelength	$\displaystyle\lambda$	m	$\frac{g}{2\pi} T^2$	$T \sqrt{g h}$	for given period T, the solution of: $\displaystyle \left(\frac{2\pi}{T}\right)^2=\frac{2\pi g}{\lambda}\tanh\left(\frac{2\pi h}{\lambda}\right)$
general
wave energy density	$\displaystyle E$	J / m²	$\frac{1}{16} \rho g H_{m0}^2$
wave energy flux	$\displaystyle P$	W / m	$\displaystyle E\;c_g$
angular frequency	$\displaystyle \omega$	rad / s	$\frac{2\pi}{T}$
wavenumber	$\displaystyle k$	rad / m	$\frac{2\pi}{\lambda}$

Deep-water characteristics and opportunities

Deep water corresponds with a water depth larger than half the wavelength, which is the common situation in the sea and ocean. In deep water, longer-period waves propagate faster and transport their energy faster. The deep-water group velocity is half the phase velocity. In shallow water, for wavelengths larger than about twenty times the water depth, as found quite often near the coast, the group velocity is equal to the phase velocity.^[12]

History

The first known patent to use energy from ocean waves dates back to 1799, and was filed in Paris by Girard and his son.^[13] An early application of wave power was a device constructed around 1910 by Bochaux-Praceique to light and power his house at Royan, near Bordeaux in France.^[14] It appears that this was the first oscillating water-column type of wave-energy device.^[15] From 1855 to 1973 there were already 340 patents filed in the UK alone.^[13]

Modern scientific pursuit of wave energy was pioneered by Yoshio Masuda's experiments in the 1940s.^[16] He has tested various concepts of wave-energy devices at sea, with several hundred units used to power navigation lights. Among these was the concept of extracting power from the angular motion at the joints of an articulated raft, which was proposed in the 1950s by Masuda.^[17]

A renewed interest in wave energy was motivated by the oil crisis in 1973. A number of university researchers re-examined the potential to generate energy from ocean waves, among whom notably were Stephen Salter from the University of Edinburgh, Kjell Budal and Johannes Falnes from Norwegian Institute of Technology (now merged into Norwegian University of Science and Technology), Michael E. McCormick from U.S. Naval Academy, David Evans from Bristol University, Michael French from University of Lancaster, Nick Newman and C. C. Mei from MIT.

Stephen Salter's 1974 invention became known as Salter's duck or nodding duck, although it was officially referred to as the Edinburgh Duck. In small scale controlled tests, the Duck's curved cam-like body can stop 90% of wave motion and can convert 90% of that to electricity giving 81% efficiency.^[18]

In the 1980s, as the oil price went down, wave-energy funding was drastically reduced. Nevertheless, a few first-generation prototypes were tested at sea. More recently, following the issue of climate change, there is again a growing interest worldwide for renewable energy, including wave energy.^[19]

The world's first marine energy test facility was established in 2003 to kick start the development of a wave and tidal energy industry in the UK. Based in Orkney, Scotland, the European Marine Energy Centre (EMEC) has supported the deployment of more wave and tidal energy devices than at any other single site in the world. EMEC provides a variety of test sites in real sea conditions. It's grid connected wave test site is situated at Billia Croo, on the western edge of the Orkney mainland, and is subject to the full force of the Atlantic Ocean with seas as high as 19 metres recorded at the site. Wave energy developers currently testing at the centre include Aquamarine Power, Pelamis Wave Power, ScottishPower Renewables and Wello.^[20]

Modern technology

Wave power devices are generally categorized by the method used to capture the energy of the waves, by location and by the power take-off system. Locations are shoreline, nearshore and offshore. Types of power take-off include: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine,^[21] and linear electrical generator. When evaluating wave energy as a technology type, it is important to distinguish between the four most common approaches: point absorber buoys, surface attenuators, oscillating water columns, and overtopping devices.

Generic wave energy concepts: 1. Point absorber, 2. Attenuator, 3. Oscillating wave surge converter, 4. Oscillating water column, 5. Overtopping device, 6. Submerged pressure differential

Point absorber buoy

This device floats on the surface of the water, held in place by cables connected to the seabed. Buoys use the rise and fall of swells to drive hydraulic pumps and generate electricity. EMF generated by electrical transmission cables and acoustic of these devices may be a concern for marine organisms. The presence of the buoys may affect fish, marine mammals, and birds as potential minor collision risk and roosting sites. Potential also exists for entanglement in mooring lines. Energy removed from the waves may also affect the shoreline, resulting in a recommendation that sites remain a considerable distance from the shore.^[22]

Surface attenuator

These devices act similarly to point absorber buoys, with multiple floating segments connected to one another and are oriented perpendicular to incoming waves. A flexing motion is created by swells that drive hydraulic pumps to generate electricity. Environmental effects are similar to those of point absorber buoys, with an additional concern that organisms could be pinched in the joints.^[22]

Oscillating wave surge converter

These devices typically have one end fixed to a structure or the seabed while the other end is free to move. Energy is collected from the relative motion of the body compared to the fixed point. Oscillating wave surge converters often come in the form of floats, flaps, or membranes. Environmental concerns include minor risk of collision, artificial reefing near the fixed point, EMF effects from subsea cables, and energy removal effecting sediment transport.^[22] Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture. These capture systems use the rise and fall motion of waves to capture energy.^[23] Once the wave energy is captured at a wave source, power must be carried to the point of use or to a connection to the electrical grid by transmission power cables.^[24]

Oscillating water column

Oscillating water column devices can be located on shore or in deeper waters offshore. With an air chamber integrated into the device, swells compress air in the chambers forcing air through an air turbine to create electricity.^[25] Significant noise is produced as air is pushed through the turbines, potentially affecting birds and other marine organisms within the vicinity of the device. There is also concern about marine organisms getting trapped or entangled within the air chambers.^[22]

Overtopping device

Overtopping devices are long structures that use wave velocity to fill a reservoir to a greater water level than the surrounding ocean. The potential energy in the reservoir height is then captured with low-head turbines. Devices can be either on shore or floating offshore. Floating devices will have environmental concerns about the mooring system affecting benthic organisms, organisms becoming entangled, or EMF effects produced from subsea cables. There is also some concern regarding low levels of turbine noise and wave energy removal affecting the nearfield habitat.^[22]

List of devices

The table contains descriptions of proposed wave power systems, for those implemented see List of wave power stations

Device	Proponent	Country of origin	Capture method	Location	Power take off	Year	Notes
Albatern WaveNET	Albatern	Scotland, UK	Multi Point Absorber array	Offshore	Hydraulic / electric / DC	2010	Albatern are working with their third iteration devices with a 14-week deployment on a Scottish fishfarm site in 2014,^[26] and a 6 unit array deployment for full characterisation at Kishorn Port in 2015.^[27] Initially working with smaller devices and arrays, the company is targeting off grid markets where diesel generation is presently used in offshore fish farms, coastal communities and long endurance scientific platforms. Demonstration projects are under development for fishfarm sites and an island community.^[28]
Anaconda Wave Energy Converter	Checkmate SeaEnergy.[25]	UK	Surface-following attenuator	Offshore	Hydroelectric turbine	2008	In the early stages of development, the device is a 200 metres (660 ft) long rubber tube which is tethered underwater. Passing waves will instigate a wave inside the tube, which will then propagates down its walls, driving a turbine at the far end.^[29]^[30]
AquaBuOY	Finavera Wind Energy, later SSE Renewables Limited	Ireland-Canada-Scotland	Buoy	Offshore	Hydroelectric turbine	2003	In 2009 Finavera Renewables surrendered its wave energy permits from FERC.[27] In July 2010 Finavera announced that it had entered into a definitive agreement to sell all assets and intellectual property related to the AquaBuOY wave energy technology.^[31]^[32]^[33]^[34]
AWS-iii	AWS Ocean Energy	UK (Scotland)	Surface-following attenuator?	Offshore	Air turbine	2010	The AWS-III is a floating toroidal vessel. It has rubber membranes on the outer faces which deform as waves pass, moving air inside chambers which in turn drive air-turbines to generate electricity. AWS Ocean tested a 1/9 scale model in Loch Ness in 2010, and are now working on a full sized version which will be 60m across and should generate 2.5 MW. It is envisage these will be installed in offshore farms moored in around 100m depth of water.^[35]^[36]^[37]^[38]
CCell	Zyba Renewables	United Kingdom	Oscillating wave surge converter	Nearshore & offshore	Hydraulic	2015	CCell is a directional WEC consisting of a curved flap operating mainly in the surge direction of wave propagation. Being curved gives the device two advantages over flat paddle oscillating wave surge converters: the energy is dissipated over a long arc reducing the wave height, and the shape cuts through the waves which reduces turbulence on the boundaries. In addition, unlike other oscillating wave surge converters, the latest version of CCell is designed to float just under the water surface, maximising the available wave energy. The developers claim this makes CCell the world's most efficient wave energy device.^[39]
CETO Wave Power	Carnegie	Australia	Buoy	Offshore	Pump-to-shore	1999	As of 2008, the device is being tested off Fremantle, Western Australia,[35] the device consists of a single piston pump attached to the sea floor with a float (buoy) tethered to the piston. Waves cause the float to rise and fall, generating pressurized water, which is piped to an onshore facility to drive hydraulic generators or run reverse osmosis water desalination.^[40]^[41]
Crestwing	Crestwing ApS	Denmark	Surface-following attenuator	Offshore	Mechanical	2011	The device consists of two floats connected by a hinge. It uses atmospheric pressure acting on its large area to stick to the ocean surface. This allows it to follow the waves. Motion of the two floats relative to each other is transferred to electricity by a mechanical power take-off system. As of 2014, there is a 1:5 scale prototype that has been tested in the sea near Frederikshavn.^[42]
Cycloidal Wave Energy Converter	Atargis Energy Corporation	USA	Fully Submerged Wave Termination Device	Offshore	Direct Drive Generator	2006	In the tank testing stage of development, the device is a 20 metres (66 ft) diameter fully submerged rotor with two hydrofoils. Numerical studies have shown greater than 99% wave power termination capabilities.^[43] These were confirmed by experiments in a small 2D wave flume^[44] as well as a large offshore wave basin.
FlanSea (Flanders Electricity from the Sea)	FlanSea	Belgium	Buoy	Offshore	Hydroelectric turbine	2010	A point absorber buoy developed for use in the southern North Sea conditions.[31][32][33] It works by means of a cable that due to the bobbing effect of the buoy, generates electricity.^[45]^[46]^[47]
Islay LIMPET	Islay LIMPET	Scotland	oscillating water column	Onshore	Air turbine	1991	500 kW shoreline device uses an oscillating water column to drive air in and out of a pressure chamber through a Wells turbine.^[48]^[49]^[50]
Lysekil Project	Uppsala University	Sweden	Buoy	Offshore	Linear generator	2002	Direct driven linear generator placed on the seabed, connected to a buoy at the surface via a line. The movements of the buoy will drive the translator in the generator.^[51]^[52]
Oceanlinx	Oceanlinx	Australia	OWC	Nearshore & Offshore	air turbine	1997	Wave energy is captured with an Oscillating Water Column and electricity is generated by air flowing through a turbine. The third medium scale demonstration unit near Port Kembla, NSW, Australia, a medium scale system that was grid connected in early 2010.^[53] In May 2010, the wave energy generator snapped from its mooring lines in extreme seas and sank on Port Kembla's eastern breakwater.^[54] A full scale commercial nearshore unit, greenWAVE, with a capacity of 1MW will be installed off Port MacDonnell in South Australia before the end of 2013.^[55]
Oceanus 2	Seatricity Ltd	UK	Buoy	Nearshore and Offshore	Pump-to-shore	2007	The Oceanus 2 device is the first and only device yet to have been deployed and tested at the UK's WaveHub test site as a full-scale prototype (2014-2016). The 3rd generation device consists of a single piston patented pump mounted on a gimbal and supported by an aluminium 12m diameter buoy/float. The pump is then tethered to the seabed. Vertical wave motion is used to pump seawater to hydraulic pressures which is then piped to an onshore facility to drive hydraulic generators or run reverse osmosis water desalination. Multiple devices deployed in arrays provide modularity, resilience and redundancy.
OE buoy	Ocean Energy	Ireland	Buoy	Offshore	Air turbine	2006	In September 2009 completed a 2-year sea trial in one quarter scale form. The OE buoy has only one moving part.^[56]
OWEL	Ocean Wave Energy Ltd	UK	Wave Surge Converter	Offshore	Air turbine	2013	The surging motion of long period waves compresses air in a tapered duct which is then used to drive an air turbine mounted on top of the floating vessel.^[57] The design of a full scale demonstration project was completed in Spring 2013, ready for fabrication.^[58]
Oyster wave energy converter	Aquamarine Power	UK (Scots-Irish)	Oscillating wave surge converter	Nearshore	Pump-to-shore (hydro-electric turbine)	2005	A hinged mechanical flap attached to the seabed captures the energy of nearshore waves. It drives hydraulic pistons to deliver high pressure water to an onshore turbine which generates electricity. In November 2009, the first full-scale demonstrator Oyster began producing power at the European Marine Energy Centre's wave test site at Billia Croo in Orkney. In 2015, Aquamarine entered adminisration.^[59]
Pelamis Wave Energy Converter	Pelamis Wave Power	UK (Scottish)	Surface-following attenuator	Offshore	Hydraulic	1998	As waves pass along a series of semi-submerged cylindrical sections linked by hinged joints, the sections move relative to one another. This motion activates hydraulic cylinders which pump high pressure oil through hydraulic motors which drive electrical generators.^[60] The first working Pelamis machine was installed in 2004 at the European Marine Energy Center (EMEC) in Orkney. Here, it became the world's first offshore wave energy device to generate electricity into a national grid anywhere in the world.^[61] The later P2, owned by E.ON, started grid connected tests off Orkney in 2010.^[62] Agucadoura Wave Farm in Portugal, first commercial application of the Pelamis design (2008)
PowerBuoy	Ocean Power Technologies	US	Buoy	Offshore	Hydroelectric turbine	1997	The Pacific Northwest Generating Cooperative is funding construction of a commercial wave-power park at Reedsport, Oregon using buoys.^[63] The rise and fall of the waves moves a rack and pinion within the buoy and spins a generator.^[64] The electricity is transmitted by a submerged transmission line. The buoys are designed to be installed one to five miles (8 km) offshore in water 100 to 200 feet (60 m) deep.^[65] PB150 PowerBuoy with peak-rated power output of 150 kW
R38/50 kW, R115/150 kW	40South Energy	UK	Underwater attenuator	Offshore	Electrical conversion	2010	These machines work by extracting energy from the relative motion between one Upper Member and one Lower Member, following an innovative method which earned the company one UKTI Research & Development Award in 2011.^[66] A first generation full-scale prototype for this solution was tested offshore in 2010,^[67]^[68]^[69] and a second generation full-scale prototype was tested offshore during 2011.^[70] In 2012 the first units were sold to clients in various countries, for delivery within the year.^[71]^[72] The first reduced scale prototypes were tested offshore during 2007, but the company decided to remain in a "stealth mode" until May 2010^[73] and is now recognized as one of the technological innovators in the sector.^[74] The company initially considered installing at Wave Hub in 2012,^[75] but that project is on hold for now. The R38/50 kW is rated at 50 kW while the R115/150 kW is rated at 150 kW.
Sea Power (company)	Seapower Ltd.	Ireland	Surface-following attenuator	Offshore or Nearshore	RO Plant or Direct Drive	2008	Sea Power carry out ongoing tank testing and development. Currently reducing LCOE targets further.^[76]^[77]\|
SDE Sea Waves Power Plant	SDE Energy Ltd.	Israel	Buoy	Nearshore	Hydraulic ram	2010	A breakwater-based wave machine, this device is close to the shore and utilizes the vertical pumping motion of the buoys for operating hydraulic rams, thereby powering generators. One version ran from 2008 to 2010, at peak producing 40KWh.^[78]
Seabased	Seabased AB.	Sweden	Buoy	Offshore	Linear generator on seabed	2015	Seabased Industry AB in cooperation with Fortum and the Swedish Energy Agency is developing its first wave power park, northwest of Smögen on the Swedish West coast. The first phase of the wave power park was deployed during the week commencing 23 March 2015 and comprises 36 wave energy converters and one substation.r.^[76]^[79]
SeaRaser	Alvin Smith (Dartmouth Wave Energy)\Ecotricity	UK	Buoy	Nearshore	Hydraulic ram	2008	Consisting of a piston pump(s) attached to the sea floor with a float (buoy) tethered to the piston. Waves cause the float to rise and fall, generating pressurized water, which is piped to resoviors onshore which then drive hydraulic generators.^[80]^[81] It is currently "undergoing extensive modelling ahead of a sea trial" ^[82]
Unnamed Ocean Wave-Powered Generator	SRI International	US	Buoy	Offshore	Electroactive polymer artificial muscle	2004	A type of wave buoys, built using special polymers, is being developed by SRI International.^[83]^[84]
Wavebob	Wavebob	Ireland	Buoy	Offshore	Direct Drive Power Take off	1999	Wavebob have conducted some ocean trials, as well as extensive tank tests. It is an ocean-going heaving buoy, with a submerged tank which captures additional mass of seawater for added power and tunability, and as a safety feature (Tank "Venting")
WaveEL	Waves4Power	Sweden	Buoy	Offshore	Hydroelectric turbine	2010	Waves4Power is a developer of buoy based OWEC (Offshore Wave Energy Converter) systems. There are plans to install a demonstration plant in 2015 at Runde test site (Norway). This will be connected via subsea cable to the shore based power grid.^[85]^[86]
Wavepiston	Wavepiston ApS	Denmark	Oscillating wave surge converter	Nearshore	Pump-to-shore (hydro-electric turbine)	2013	The idea behind this concept is to reduce the mooring means for wave energy structures. Wavepiston systems use vertical plates to exploit the horizontal movement in ocean waves. By attaching several plates in parallel on a single structure the forces applied on the structure by the plates will tend to neutralize each other. This neutralization reduces the required mooring means. “Force cancellation” is the term used by the inventors of the technology to describe the neutralization of forces. Test and numerical models prove that force cancellation reduces the means for mooring and structure to 1/10. The structure is a steel wire stretched between two mooring points. The wire is a strong and flexible structure well suited for off shore use. The mooring is slack mooring. When the vertical plates move back and forth they produce pressurized water. The pressurized water is transported to a turbine through PE pipes. A central turbine station then converts it to electric power. Calculations on the current design show capital cost of EUR 0,89 per installed watt.
Wave Dragon	Erik Friis-Madsen	Denmark	Overtopping device	Offshore	Hydroelectric turbine	2003	With the Wave Dragon wave energy converter large wing reflectors focus waves up a ramp into an offshore reservoir. The water returns to the ocean by the force of gravity via hydroelectric generators. Wave Dragon seen from reflector, prototype 1:4½
WaveRoller	AW-Energy Oy	Finland	Oscillating wave surge converter	Nearshore	Hydraulic	1994	The WaveRoller is a plate anchored on the sea bottom by its lower part. The back and forth movement of surge moves the plate. The kinetic energy transferred to this plate is collected by a piston pump. Full-scale demonstration project built off Portugal in 2009.^[87]^[88] WaveRoller farm installation in Peniche, Portugal. August 2012
Wave Star	Wave Star A/S	Denmark	Multi-point absorber	Offshore	Hydroelectric turbine	2000	The Wavestar machine draws energy from wave power with floats that rise and fall with the up and down motion of waves. The floats are attached by arms to a platform that stands on legs secured to the sea floor. The motion of the floats is transferred via hydraulics into the rotation of a generator, producing electricity. Wave Star has been testing a 1:10 machine since 2005 in Nissum Bredning, Denmark, it was taken out of duty in November 2011. A 1:2 Wave Star machine is in place in Hanstholm which has produced electricity to the grid since September 2009.^[89]

A more complete list of wave energy developers is maintained here: Wave energy developers^[76]

Environmental Effects

Further information: Environmental impact of electricity generation § Wave

Common environmental concerns associated with marine energy developments include:

The risk of marine mammals and fish being struck by tidal turbine blades;
The effects of EMF and underwater noise emitted from operating marine energy devices;
The physical presence of marine energy projects and their potential to alter the behavior of marine mammals, fish, and seabirds with attraction or avoidance;
The potential effect on nearfield and farfield marine environment and processes such as sediment transport and water quality.

The Tethys database provides access to scientific literature and general information on the potential environmental effects of wave energy.^[90]

Potential

The worldwide resource of wave energy has been estimated to be greater than 2 TW.^[91] Locations with the most potential for wave power include the western seaboard of Europe, the northern coast of the UK, and the Pacific coastlines of North and South America, Southern Africa, Australia, and New Zealand. The north and south temperate zones have the best sites for capturing wave power. The prevailing westerlies in these zones blow strongest in winter.

World wave energy resource map

Challenges

There is a potential impact on the marine environment. Noise pollution, for example, could have negative impact if not monitored, although the noise and visible impact of each design vary greatly.^[7] Other biophysical impacts (flora and fauna, sediment regimes and water column structure and flows) of scaling up the technology is being studied.^[92] In terms of socio-economic challenges, wave farms can result in the displacement of commercial and recreational fishermen from productive fishing grounds, can change the pattern of beach sand nourishment, and may represent hazards to safe navigation.^[93] Waves generate about 2,700 gigawatts of power. Of those 2,700 gigawatts, only about 500 gigawatts can be captured with the current technology.^[23]

Wave farms

Main article: Wave farm

Portugal

The Aguçadoura Wave Farm was the world's first wave farm. It was located 5 km (3 mi) offshore near Póvoa de Varzim, north of Porto, Portugal. The farm was designed to use three Pelamis wave energy converters to convert the motion of the ocean surface waves into electricity, totalling to 2.25 MW in total installed capacity. The farm first generated electricity in July 2008^[94] and was officially opened on September 23, 2008, by the Portuguese Minister of Economy.^[95]^[96] The wave farm was shut down two months after the official opening in November 2008 as a result of the financial collapse of Babcock & Brown due to the global economic crisis. The machines were off-site at this time due to technical problems, and although resolved have not returned to site and were subsequently scrapped in 2011 as the technology had moved on to the P2 variant as supplied to Eon and Scottish Power Renewables.^[97] A second phase of the project planned to increase the installed capacity to 21 MW using a further 25 Pelamis machines^[98] is in doubt following Babcock's financial collapse.

United Kingdom

Funding for a 3 MW wave farm in Scotland was announced on February 20, 2007, by the Scottish Executive, at a cost of over 4 million pounds, as part of a £13 million funding package for marine power in Scotland. The first machine was launched in May 2010.^[99]
A facility known as Wave hub has been constructed off the north coast of Cornwall, England, to facilitate wave energy development. The Wave hub will act as giant extension cable, allowing arrays of wave energy generating devices to be connected to the electricity grid. The Wave hub will initially allow 20 MW of capacity to be connected, with potential expansion to 40 MW. Four device manufacturers have so far expressed interest in connecting to the Wave hub.^[100]^[101] The scientists have calculated that wave energy gathered at Wave Hub will be enough to power up to 7,500 households. The site has the potential to save greenhouse gas emissions of about 300,000 tons of carbon dioxide in the next 25 years.^[102]

Australia

A CETO wave farm off the coast of Western Australia has been operating to prove commercial viability and, after preliminary environmental approval, underwent further development.^[103]^[104] In early 2015 a $100 million, multi megawatt system was connected to the grid, with all the electricity being bought to power HMAS Stirling naval base. Two fully submerged buoys which are anchored to the seabed, transmit the energy from the ocean swell through hydraulic pressure onshore; to drive a generator for electricity, and also to produce fresh water. As of 2015 a third buoy is planned for installation.^[105]^[106]
Ocean Power Technologies (OPT Australasia Pty Ltd) is developing a wave farm connected to the grid near Portland, Victoria through a 19 MW wave power station. The project has received an AU $66.46 million grant from the Federal Government of Australia.^[107]
Oceanlinx will deploy a commercial scale demonstrator off the coast of South Australia at Port MacDonnell before the end of 2013. This device, the greenWAVE, has a rated electrical capacity of 1MW. This project has been supported by ARENA through the Emerging Renewables Program. The greenWAVE device is a bottom standing gravity structure, that does not require anchoring or seabed preparation and with no moving parts below the surface of the water.^[55]

United States

Reedsport, Oregon – a commercial wave park on the west coast of the United States located 2.5 miles offshore near Reedsport, Oregon. The first phase of this project is for ten PB150 PowerBuoys, or 1.5 megawatts.^[108]^[109] The Reedsport wave farm was scheduled for installation spring 2013.^[110] In 2013, the project has ground to a halt because of legal and technical problems.^[111]
Kaneohe Bay Oahu, Hawai - Navy’s Wave Energy Test Site (WETS) currently testing the Azura wave power device^[112]

Patents

U.S. Patent 8,806,865 — 2011 Ocean wave energy harnessing device – Pelamis/Salter's Duck Hybrid patent
U.S. Patent 3,928,967 — 1974 Apparatus and method of extracting wave energy – The original "Salter's Duck" patent
U.S. Patent 4,134,023 — 1977 Apparatus for use in the extraction of energy from waves on water – Salter's method for improving "duck" efficiency
U.S. Patent 6,194,815 — 1999 Piezoelectric rotary electrical energy generator
US application 20,040,217,597 — 2004 Wave energy converters utilizing pressure differences^[113]

Notes

↑ The energy flux is $P = \tfrac{1}{16} \rho g H_{m0}^2 c_g,$ with $c_g$ the group velocity, see Herbich, John B. (2000). Handbook of coastal engineering. McGraw-Hill Professional. A.117, Eq. (12). ISBN 978-0-07-134402-9. The group velocity is $c_g=\tfrac{g}{4\pi}T$ , see the collapsed table "Properties of gravity waves on the surface of deep water, shallow water and at intermediate depth, according to linear wave theory" in the section "Wave energy and wave energy flux" below.
↑ Here, the factor for random waves is ¹⁄₁₆, as opposed to ¹⁄₈ for periodic waves – as explained hereafter. For a small-amplitude sinusoidal wave $\scriptstyle \eta=a\,\cos\, 2\pi\left(\frac{x}{\lambda}-\frac{t}{T}\right)$ with wave amplitude $\scriptstyle a,\,$ the wave energy density per unit horizontal area is $\scriptstyle E=\frac{1}{2}\rho g a^2,$ or $\scriptstyle E=\frac{1}{8}\rho g H^2$ using the wave height $\scriptstyle H\,=\,2\,a\,$ for sinusoidal waves. In terms of the variance of the surface elevation $\scriptstyle m_0=\sigma_\eta^2=\overline{(\eta-\bar\eta)^2}=\frac{1}{2}a^2,$ the energy density is $\scriptstyle E=\rho g m_0\,$ . Turning to random waves, the last formulation of the wave energy equation in terms of $\scriptstyle m_0\,$ is also valid (Holthuijsen, 2007, p. 40), due to Parseval's theorem. Further, the significant wave height is defined as $\scriptstyle H_{m0}=4\sqrt{m_0}$ , leading to the factor ¹⁄₁₆ in the wave energy density per unit horizontal area.
↑ For determining the group velocity the angular frequency ω is considered as a function of the wavenumber k, or equivalently, the period T as a function of the wavelength λ.

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↑ Prototype Testing Could Help Prove a Promising Energy Source
↑ FreePatentsoOline.com Wave energy converters utilizing pressure differences, 11 April 2004

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Wave energy converters(a list of main types of wave energy converters, including animations)
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"Ocean waves – our new electricity supplier" Archived 7 January 2010 at the Wayback Machine (Uppsala university 2010)
Kate Galbraith (September 22, 2008). "Power From the Restless Sea Stirs the Imagination". New York Times. Retrieved 2008-10-09.
"Wave Power: The Coming Wave" from the Economist, June 5, 2008
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"The untimely death of Salter's Duck"
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