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Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work — including electricity generation, desalination, and the pumping of water (into reservoirs). Wave power is a form of renewable energy. Though often co-mingled, 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.

On December 18, 2007, Pacific Gas and Electric Company announced its support for plans to build America's first commercial wave power plant off the coast of Northern California.[1] The plant will consist of eight buoys, 2 1/2 miles offshore, each buoy generating electricity as it rises and falls with the waves. The plant is scheduled to begin operating in 2012, generating a maximum of 2 megawatt of electricity. Each megawatt can power about 750 homes.The world's first commercial wave farm is based in Portugal,[2] at the Aguçadora Wave Park, which consists of three 750 kilowatt Pelamis devices. Other plans for wave farms include a 3 megawatt array of four 750 kilowatt Pelamis devices in the Orkneys, off northern Scotland, and the 20MW Wave hub development off the north coast of Cornwall, England.

The north and south temperate zones have the best sites for capturing wave power. The prevailing westerlies in these zones blow strongest in winter. Waves are generated by wind passing over 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 most energetic 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 shear stress cause the growth of the waves.[3]

The wave height increases with increasing wind speed, duration since the wind started to blow, and of the fetch (the distance of open water that the wind has blown over), see Ocean surface wave.In general, large waves are more powerful. Specifically, wave power is determined by wave height, wave speed, wavelength, and water density.Wave size is determined by wind speed and 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. This limit is called a "fully developed sea."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.[3] 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). 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 twenty times the water depth, as found quite often near the coast, the group velocity is equal to the phase velocity.[10]
Modern Technology
Wave power devices are generally categorized by the method used to capture the energy of the waves. They can also be categorized by location and power take-off system. Method types are point absorber or buoy; surfacing following or attenuator; terminator, lining perpendicular to wave propagation; oscillating water column; and overtopping. Locations are shoreline, nearshore and offshore. Types of power take-off include: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine,[11] and linear electrical generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture.These are descriptions of some wave power systems:
  • In the United States, the Pacific Northwest Generating Cooperative[12] is funding the building of a commercial wave-power park at Reedsport, Oregon.[13] The project will utilize the PowerBuoy[14] technology which consists of modular, ocean-going buoys. The rising and falling of the waves moves the buoy-like structure creating mechanical energy which is converted into electricity and transmitted to shore over a submerged transmission line. A 40 kW buoy has a diameter of 12 feet (4 m) and is 52 feet (16 m) long, with approximately 13 feet of the unit rising above the ocean surface. Using the three-point mooring system, they are designed to be installed one to five miles (8 km) offshore in water 100 to 200 feet (60 m) deep.
  • A floating near shore device called the Energen Wave Power device has floating pontoons and multiple pivot arms. [1] This device converts ocean wave energy over a large surface area and utilises each wave repetitively until it passes through the device. [2]
  • An example of a surface following device is the Pelamis Wave Energy Converter. The sections of the device articulate with the movement of the waves, each resisting motion between it and the next section, creating pressurized oil to drive a hydraulic ram which drives a hydraulic motor. Two commercial projects utilizing Pelamis technology are under construction, one in Portugal the Aguçadora Wave Park near Póvoa de Varzim which will initially use three Pelamis P-750 machines generating 2.25 MW.[15] Funding for a 3 MW wave farm in Scotland was announced on February 20, 2007 and is projected to use four Pelamis machines.[16]
  • With the Wave Dragon wave energy converter large "arms" focus waves up a ramp into an offshore reservoir. The water returns to the ocean by the force of gravity via hydroelectric generators.
  • The AquaBuOY, made by Finavera Renewables Inc., wave energy device: Energy transfer takes place by converting the vertical component of wave kinetic energy into pressurized seawater by means of two-stroke hose pumps. Pressurized seawater is directed into a conversion system consisting of a turbine driving an electrical generator. The power is transmitted to shore by means of a secure, undersea transmission line. A commercial wave power production facility utilizing the AquaBuOY technology is beginning initial construction in Portugal.[17] The company has 250 MW of projects planned or under development on the west coast of North America.[18]
  • A device called CETO, currently being tested off Fremantle, Western Australia, consists of a single piston pump attached to the sea floor, with a float 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 desalination[19]
  •  
A device called Neo-AeroDynamic:[20] It is an airfoil base design to harness kinetic power of the fluid flow via an artificial current around its center. The device differentiates from others by its capability to directly transfer wave power into rotational torque to drive a generator without moving part. As the result of its high efficiency; it's not only applicable to wind but also to the variety of hydro electric including free-flow (rivers, creeks), tidal, oceanic currents and wave via ocean wave surface currents.
  • A point attenuating device called the Aegir Dynamo,[21] currently being developed by a UK based company called Ocean Navitas uses a direct mechanical conversion technique to produce rotational energy that can be converted to electricity in a similar way to wind turbine technology, and has a mechanical efficiency of 93%.
Challenges
These are some of the challenges to deploying wave power devices:
  • Efficiently converting wave motion into electricity; generally speaking, wave power is available in low-speed, high forces, and the motion of forces is not in a single direction. Most readily-available electric generators operate at higher speeds, and most readily-available turbines require a constant, steady flow.
  • Constructing devices that can survive storm damage and saltwater corrosion; likely sources of failure include seized bearings, broken welds, and snapped mooring lines. Knowing this, designers may create prototypes that are so overbuilt that materials costs prohibit affordable production.
  • High total cost of electricity; wave power will only be competitive when the total cost of generation is reduced. The total cost includes the primary converter, the power takeoff system, the mooring system, installation & maintenance cost, and electricity delivery costs.
Wave farms
Portugal has built the world's first commercial wave farm, the Aguçadora Wave Park near Póvoa de Varzim, installing three Pelamis P-750 machines generating 2.25 MW . Subject to successful operation, a further 70 million euro is likely to be invested before 2009 on a further 28 machines to generate 72.5 MW.[22]Funding for a 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 packages for marine power in Scotland. The farm will be the world's largest with a capacity of 3MW generated by four Pelamis machines.[23]Funding has also been announced for the development of a Wave hub off the north coast of Cornwall, England. 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 20MW of capacity to be connected with potential expansion to 40MW. Four device manufacturers have so far expressed interest in connecting to the Wave hub.The scientists have calculated that wave energy gathered by this generator will be enough to power up to 7,500 households. Savings that the Cornwall wave power generator will bring are significant: about 300,000 tons of carbon dioxide in the next 25 years.[24]
Potential
Good wave power locations have a flux of about 50 kilowatts per metre of shoreline. Capturing 20 percent of this, or 10 kilowatts per metre, is plausible. Assuming very large scale deployment of (and investment in) wave power technology, coverage of 5000 kilometres of shoreline (worldwide) is plausible. Therefore, the potential for shoreline-based wave power is about 50 gigawatts.[citation needed] Deep water wave power resources are truly enormous, but perhaps impractical to capture.
Discussion of Salter's Duck
While historic references to the power of waves do exist, the modern scientific pursuit of wave energy was begun in the 1970s by Professor Stephen Salter of the University of Edinburgh, Scotland in response to the Oil Crisis.His invention, Salter's Edinburgh Duck, continues to be the machine against which all others are measured. 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.[25] While it continues to represent the most efficient use of available material and wave resources, the machine has never gone to sea, primarily because its complex hydraulic system is not well suited to incremental implementation, and the costs and risks of a full-scale test would be high. Most of the designs being tested currently absorb far less of the available wave power, and as a result their Mass to Power Ratios remain far away from the theoretical maximum.According to sworn testimony before the House of Parliament, The UK Wave Energy program was shut down on March 19, 1982, in a closed meeting,[26] the details of which remain secret. The members of the meeting were recruited largely from the nuclear and fossil fuels industries, and the wave programme manager, Clive Grove-Palmer, was excluded.An analysis[27] of Salter's Duck resulted in a miscalculation of the estimated cost of energy production by a factor of 10, an error which was only recently identified. Some wave power advocates believe that this error, combined with a general lack of enthusiasm for renewable energy in the 1980s (after oil prices fell), hindered the advancement of wave power technology.[28] 
 
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