Wave energy conversion is still in an early development stage and it is impossible to gain a realistic idea of costs. However UK estimates have suggested that wave power may be able to produce electricity for between €0.06/kWh and €0.12/kWh. This may make wave power more economical for remote coastal and island communities that currently rely on diesel generation. Capital costs range from €800/kW to €40 000/kW though the former must be considered optimistic at this stage while the latter refers to a technology in an early stage of development. Commercial wave power plants are unlikely to be deployed before 2010.
The movement of water within oceans and seas is the basis for ocean current energy conversion. This movement may be caused by tidal ebb and flow. In this case the water movement will follow a sinusoidal variation in speed and direction, the latter normally reversing twice every 24 h.
Other currents are caused by thermal gradients. The most prominent of these is the Gulf stream which moves around 80 million m3 of water each second9 but there are other, lesser currents in many parts of the world. These currents usually flow in one direction only, and are relatively constant in strength.
The conversion of ocean currents into electricity involves similar considerations and technology to that employed by wind power plants. The main difference is that the energy density contained in a current of water is much higher than that of air. As a result turbines can be much smaller. For a tidal current which varies regularly a current of around 1.5 m/s is considered sufficient to exploit. Where the current is continuous in a single direction, a flow speed of 1m/s is exploitable. The latter also offers a higher-capacity factor, around 80% whereas a tidal current will provide a capacity factor of 40-50%.
Sea currents hold one further advantage over wind; they are predictable. Thus whereas a wind farm cannot guarantee its output, the output from an ocean current power plant should be entirely predictable. This has significant implications for network operation and dispatching since a reliable source of electricity is much more valuable than an unpredictable source.
As with wind turbines, there are two different configurations of tidal stream turbine, a horizontal axis and a vertical axis turbine. Both types are under development.
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