Wave Power

Wave power refers to renewable energy from ocean surface waves and the capture of that energy for the following uses:

• electricity generation
• deslaination
• pumping of water (into reservoirs)

 Wave power generation is not widely used commercially. On December 18, 2007, however, 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. The plant will consist of eight buoys located 2 1/2 miles offshore. Each buoy will generate electricity as it rises and falls with the waves. The plant is scheduled to begin operating in 2012, and will generate a maximum of 2 megawatts (MW) of electricity. Each megawatt can power about 750 homes.

The world's first commercial wave farm is in Portugal at the Aguçadora Wave Park, which consists of three 750-kilowatt (kW) Pelamis devices. Other plans for wave farms include a 3-MW array of four 750-kW Pelamis devices in the Orkneys, off northern Scotland, and the 20-MW 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.

Physical concepts

Motion of a particle in an ocean wave.
A = At deep water. The orbital 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.

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. 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.

In general, large waves are more powerful.

Wave size is determined by the following factors:

• wind speed
• duration since the wind started to blow
• fetch (the distance over which the wind excites the waves)
• depth and topography of the seafloor (which can focus or disperse the energy of the waves)

Wave power is determined by the following factors:

• wave height
• wave speed
• wave length
• water density

Modern Technology

Wave power devices are generally categorized by the method used to capture the energy of the waves, location, and power take-off system.  The types of methods used to capture wave energy include the following:

• point absorber or buoy
• surfacing following or attenuator
• terminator
• lining perpendicular to wave propagation
• oscillating water column
• 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, and linear electrical generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture.

The following are examples of some wave power systems:

• In the United States, the Pacific Northwest Generating Cooperative is funding the building of a commercial wave-power park at Reedsport, Oregon. The project will use the PowerBuoy technology, which consists of modular, ocean-going buoys. The rising and falling of the waves moves the buoy, thus creating mechanical energy.  That energy 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. The buoys are designed to be installed one to five miles (8 km) offshore in water 100 to 200 feet (60 m) deep.
• The Energen Wave Power device is used near shore and has floating pontoons and multiple pivot arms. This device converts ocean wave energy over a large surface area and utilizes each wave repetitively until it passes through the device.
• The Pelamis Wave Energy Converter is an example of a surface-following device.  The sections of the device articulate with the movement of the waves, each resisting motion between it and the next section. This creates pressurized oil to drive a hydraulic ram, which in turn drives a hydraulic motor. The two aforementioned commercial projects using Pelamis technology--the one in Portugal, the other in Scotland--are under construction.
• The Wave Dragon uses large wave energy converter "arms" to focus waves up a ramp and 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.,  transfers energy  by converting the vertical component of wave kinetic energy into pressurized seawater, which is then pumped into a conversion system. The system consists of a turbine that drives an electrical generator. The power is transmitted to shore through a secure, undersea transmission line. A commercial wave power production facility using the AquaBuOY technology is being built in Portugal. The company is planning or developing 250 MW of projects on the west coast of North America.
• A device called CETO is currently being tested off Fremantle, Western Australia. CETO 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.  This in turn generates pressurized water, which is piped to an onshore facility to drive hydraulic generators or run reverse osmosis desalination.
• A device installed near Wollongong, New South Wales, Australia, uses a parabolic reflector to concentrate wave energy into an oscillating water column. This then drives air through a turbine that rotates in a constant direction in the oscillating airflow.

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  The Neo-AeroDynamic: is an airfoil base device that harnesses the kinetic power of water as it flows via an artificial current around its center. It differs from other devices because of its capability to directly transfer wave power into rotational torque to drive a generator without moving parts. As the result of its high efficiency, it's not only applicable to wind but also to a variety of hydroelectric forces including free-flow (such as rivers and creeks), tidal, oceanic currents, and wave via ocean wave surface currents.

• The U.K.-based Ocean Navitas is currently developing a point-attenuating device called the Aegir Dynamo.  This device uses direct mechanical conversion to produce rotational energy that can be converted to electricity in a manner similar to wind turbine technology. Mechanical efficiency is 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 and involves high forces.  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.
• Lowering (the already high) total cost of generating electricity from wave power to make it competitive. The total cost includes the primary converter, the power takeoff system, the mooring system, installation and maintenance costs, and electricity delivery costs.

Wave farms

The Scottish Executive announced  funding for a wave farm in Scotland on February 20, 2007. The cost is estimated at over 4 million pounds, as part of £13 million in funding for marine power in Scotland. The farm will be the world's largest with a capacity of 3 MW.  

A Wave hub will also be developed off the north coast of Cornwall, England. The hub will act as a 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.  This could be expanded to 40 MW. Four device manufacturers have so far expressed interest in connecting to the wave hub.

Scientists have calculated that wave energy gathered by this generator will be enough to power up to 7,500 households. The Cornwall wave power generator will bring savings of about 300,000 tons of carbon dioxide in the next 25 years.

Potential

Good wave power locations have a flux of about 50 kilowatts per meter of shoreline. Capturing 20 percent of this, or 10 kilowatts per meter, is plausible. Assuming very large-scale deployment of (and investment in) wave power technology, coverage of 5,000 kilometers of shoreline worldwide is plausible. Therefore, the potential for shoreline-based wave power is about 50 gigawatts. Deep-water wave power resources are truly enormous, but perhaps impractical to capture.

Discussion of Salter's Duck

Historic references to the power of waves do exist. However, Professor Stephen Salter of the University of Edinburgh, Scotland, began the modern, scientific pursuit of wave energy in the 1970s 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. Although it continues to represent the most efficient use of available material and wave resources, the machine has never gone to sea,. This is 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. As a result, their Mass to Power Ratios remain far away from the theoretical maximum.

An analysis^[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.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Wave power."