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Wave power

From Academic Kids

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Wavedragon_overtopping.jpg
Waves overtopping a test wave generator of the so-called Wave Dragon in Nissum-Bedding, Denmark. Image courtesy of Earth Vision.

Wave power refers to the capture of (ocean) wave energy to do useful work including electricity generation, desalination, and filling a reservoir with water. Wave Power is renewable energy and is a form of solar energy transferred to water by the wind. Though often co-mingled, wave power is physiologically distinct from the diurnal flux of tidal power and the steady gyre of ocean currents which are powered by the earth's rotation. Wave power generation is not a widely employed technology with only a few experimental sites in existence.

The potential wave power of a given coastline is expressed in Kilowatts per Meter and is dependant on the typical weather in the adjacent ocean, the distance over which the wind has excited the waves - called the fetch, and the topography and depth of the seafloor which can focus or disperse the energy of the waves. Generally waves are strongest near the poles, weakest near the equator, and the least energetic during the summer months. Wave motion is highest at the surface of the water, and diminishes exponentially with depth; however, wave energy is also present as pressure waves in deeper water.

Contents

State of the art

The fundamental challenges of wave power are first how to stop the wave motion, and second how to capture the energy released when the wave is stopped with the minimum amount of mass, complexity, and energy loss. Most of the many different designs developed thus far have been frustrated by the practical challenges of the sea, chiefly storm damage and saltwater corrosion.

Existing wave power devices are often categorized by the method used to attenuate the wave, by their intended location, and by power take-off. Method categories include Wave power point absorber which occupy a small area, Wave power attenuator which occupy a line parallel to the wave propogation, and Wave Power Terminator, which occupy a line perpendicular to the wave propogation. Location categories include shoreline, offshore, and deep water. Power take-off categories include Wedding Cake Variable-Speed Hydraulic Pump, Wiktionary:Hydraulic Ram, Elastomeric Hose Pump, Pump-to-Shore, Hydroelectric, Wells turbine, Linear generator, and Air turbine.

Categories of similar systems include the Oscillating Water Column (LIMPET, Azores, Mighty Whale, Energetech), Articulated Pontoons (Pelamis, McCabe Wave Pump), Anchored Buoys PowerBuoy, Fixed Buoys (SeaDog, Achimedes Wave Swing), Overtopping Reservoirs (TAPCHAN, WaveDragon, WavePlane, and Seawave SSG). Several of these designs incorporate parabolic reflectors as a means to increase the wave energy at the point of capture.

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 the wave motion and can convert 90% of that to electricity. While it continues to represent the most efficient use of available material and wave resources, Professor Salter's Duck has never gone to sea, primarily because the complex hydraulic system is not well suited for 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 have for this reason much higher Mass to Power Ratio than is theoretically possible.

Some types of wave power systems:

  • In one system, pontoons lying in the water are driven by wave action to push or pull a generator.
  • With another method, wave action compresses air in a tunnel which drives the vanes of the generator. The bigger the difference between wave top and wave valley, the more power potential there is.
  • A device called CETO, currently being tested off Fremantle, Western Australia consists of a seafloor pressure transducer coupled to a high-pressure hydraulic pump which pumps water to shore to drive generators and can also be forced through a reverse osmosis filter to provide desalinated water.
  • In yet another design, the overtopping of the waves are being led to a reservoir, the water, being pulled downwards through a tube by gravity, drives a generator.

Harnessing power from wave motion is a possibility which might yield much more energy than tides. The feasibility of this has been investigated, particularly in the UK. A 100-400 kW prototype shore based wave power generator is being constructed at Port Kembla in Australia, due for completion in January, 2005. The energy of waves crashing against the shore is absorbed by an air driven generator and converted to electricity. For countries with large coastlines and rough sea conditions the energy density of breaking waves offers the possibility of generating electricity in utility volumes. Excess capacity in periods of rough sea could be used to generate hydrogen.

History

While historic references to wave power exist, the modern scientific pursuit of wave energy began in the 1970's by Professor Steven Salter of the University of Edinburgh, Scotland in response to the Oil Crisis. Governmental support was later dropped in favor of atomic energy due primarily to a calculation error in an independant cost evaluation and a period of irrational and naive exhuberance over the safety and cost of splitting the atom prior to the sobering incidents at Three Mile Island and Chernobyl.

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