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  • Wave power is the transport of energy by ocean surface waves, and the capture of that energy

  • to do useful workfor example, electricity generation, water desalination, or the pumping

  • of water. Machinery able to exploit wave power is generally known as a wave energy converter.

  • 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. In 2008, the first experimental

  • wave farm was opened in Portugal, at the Aguçadoura Wave Park. The major competitor of wave power

  • is offshore wind power.

  • Physical concepts

  • 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.

  • Wave height is determined by wind speed, the duration of time the wind has been blowing,

  • fetch and by the depth and topography of the seafloor. 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 near a reflecting coast,

  • wave energy is also present as pressure oscillations at great depth, producing microseisms. 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. Wave power formula

  • In deep water where the water depth is larger than half the wavelength, the wave energy

  • flux is

  • 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 per metre of wavefront length.

  • 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

  • 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:

  • where E is the mean wave energy density per unit horizontal area, the sum of kinetic and

  • potential energy density per unit horizontal area. The potential energy density is equal

  • to the kinetic energy, 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:

  • with cg the group velocity. 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: 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.

  • 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. 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. It appears that this was the first oscillating

  • water-column type of wave-energy device. From 1855 to 1973 there were already 340 patents

  • filed in the UK alone. Modern scientific pursuit of wave energy was

  • pioneered by Yoshio Masuda's experiments in the 1940s. 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.

  • 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, 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.

  • 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. 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 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.

  • 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, 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.

  • 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. 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. 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. 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. 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. 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. 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. 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. The table contains descriptions of some wave

  • power systems: A more complete list of wave energy developers

  • is maintained here: Wave energy developers Environmental Effects

  • 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 is an online knowledge

  • management system that provides the marine energy community with access to information

  • and scientific literature on environmental effects of marine energy developments.

  • Potential The worldwide resource of wave energy has

  • been estimated to be greater than 2 TW. 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.

  • 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 varies greatly. Other biophysical impacts

  • of scaling up the technology is being studied. In terms of socio-economic challenges, wave

  • farms can result in the displacement of commercial and recreational fishermen from productive