Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC). Contact online >>
Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC).
Waves are generated primarily by wind passing over the sea''s surface and also by tidal forces, temperature variations, and other factors. As long as the waves propagate slower than the wind speed just above, energy is transferred from the wind to the waves. Air pressure differences between the windward and leeward sides of a wave crest and surface friction from the wind cause shear stress and wave growth.[1]
Wave power as a descriptive term is different from tidal power, which seeks to primarily capture the energy of the current caused by the gravitational pull of the Sun and Moon. However, wave power and tidal power are not fundamentally distinct and have significant cross-over in technology and implementation. Other forces can create currents, including breaking waves, wind, the Coriolis effect, cabbeling, and temperature and salinity differences.
As of 2023, wave power is not widely employed for commercial applications, after a long series of trial projects. Attempts to use this energy began in 1890 or earlier,[2] mainly due to its high power density. Just below the ocean''s water surface the wave energy flow, in time-average, is typically five times denser than the wind energy flow 20 m above the sea surface, and 10 to 30 times denser than the solar energy flow.[3]
In 2000 the world''s first commercial wave power device, the Islay LIMPET was installed on the coast of Islay in Scotland and connected to the UK national grid.[4] In 2008, the first experimental multi-generator wave farm was opened in Portugal at the Aguçadoura wave park.[5] Both projects have since ended. For a list of other wave power stations see List of wave power stations.
Wave energy converters can be classified based on their working principle as either:[6][7]
The first known patent to extract energy from ocean waves was in 1799, filed in Paris by Pierre-Simon Girard and his son.[8] An early device was constructed around 1910 by Bochaux-Praceique to power his house in Royan, France.[9] It appears that this was the first oscillating water-column type of wave-energy device.[10] From 1855 to 1973 there were 340 patents filed in the UK alone.[8]
Modern pursuit of wave energy was pioneered by Yoshio Masuda''s 1940s experiments.[11] He tested various concepts, constructing hundreds of 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 Masuda proposed in the 1950s.[12]
The oil crisis in 1973 renewed interest in wave energy. Substantial wave-energy development programmes were launched by governments in several countries, in particular in the UK, Norway and Sweden.[3] Researchers re-examined waves'' potential to extract energy, notably Stephen Salter, Johannes Falnes, Kjell Budal, Michael E. McCormick, David Evans, Michael French, Nick Newman, and C. C. Mei.
Salter''s 1974 invention became known as Salter''s duck or nodding duck, officially the Edinburgh Duck. In small-scale 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.[13] In the 1980s, several other first-generation prototypes were tested, but as oil prices ebbed, wave-energy funding shrank. Climate change later reenergized the field.[14][3]
The world''s first wave energy test facility was established in Orkney, Scotland in 2003 to kick-start the development of a wave and tidal energy industry. The European Marine Energy Centre(EMEC) has supported the deployment of more wave and tidal energy devices than any other single site.[15] Subsequent to its establishment test facilities occurred also in many other countries around the world, providing services and infrastructure for device testing.[16]
The £10 million Saltire prize challenge was to be awarded to the first to be able to generate 100 GWh from wave power over a continuous two-year period by 2017 (about 5.7 MW average).[17] The prize was never awarded. A 2017 study by Strathclyde University and Imperial College focused on the failure to develop "market ready" wave energy devices – despite a UK government investment of over £200 million over 15 years.[18]
Public bodies have continued and in many countries stepped up the research and development funding for wave energy during the 2010s. This includes both EU, US and UK where the annual allocation has typically been in the range 5-50 million USD.[19][20][21][22][23] Combined with private funding, this has led to a large number of ongoing wave energy projects (see List of wave power projects).
In situations relevant for energy harvesting from ocean waves these assumptions are usually valid.
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.[1] Pressure fluctuations at greater depth are too small to be interesting for wave power conversion.
The behavior of Airy waves offers two interesting regimes: water deeper than half the wavelength, as is common in the sea and ocean, and shallow water, with wavelengths larger than about twenty times the water depth. Deep waves are dispersionful: Waves of long wavelengths propagate faster and tend to outpace those with shorter wavelengths. Deep-water group velocity is half the phase velocity. Shallow water waves are dispersionless: group velocity is equal to phase velocity, and wavetrains propagate undisturbed.[1][25][26]
The following table summarizes the behavior of waves in the various regimes:
In deep water where the water depth is larger than half the wavelength, the wave energy flux is[b]
with P the wave energy flux per unit of wave-crest length, Hm0 the significant wave height, Te 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.[28][29][30][31]
For 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 s. Solving for power produces
In major storms, the largest offshore sea states have significant wave height of about 15 meters and energy period of about 15 seconds. According to the above formula, such waves carry about 1.7 MW of power across each meter of wavefront.
An effective wave power device captures a significant portion of the wave energy flux. As a result, wave heights diminish in the region behind the device.
In a sea state, the 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:[1][26]
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,[1] both contributing half to the wave energy density E, as can be expected from the equipartition theorem.
The waves propagate on the surface, where crests travel with the phase velocity while the energy is 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 the energy flux (or wave power, not to be confused with the output produced by a device), and is equal to:[33][1]
Due to the dispersion relation for waves under gravity, the group velocity depends on the wavelength λ, or equivalently, on the wave period T.
Wave height is determined by wind speed, the length of time the wind has been blowing, fetch (the distance over which the wind excites the waves) and by the bathymetry (which can focus or disperse the energy of the waves). A given wind speed has a matching practical limit over which time or distance do not increase wave size. At this limit the waves are said to be "fully developed". In general, larger waves are more powerful but wave power is also determined by wavelength, water density, water depth and acceleration of gravity.
Wave energy converters (WECs) are generally categorized by the method, 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,[34] and linear electrical generator.
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