energy, alternative
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Geothermal energy is derived from the natural heat present below the surface of the Earth. Cool water is pumped down where it is heated up in large underground reservoirs before being pumped back to the surface.
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An old tower windmill in Skagen, at the northernmost tip of the Danish mainland. The tower windmill was first used in Europe in the 14th century, for grinding grain and pumping water for irrigation.
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Control arms of a tower windmill. The turret at the top of the windmill revolves so that the sails can be pointed into the prevailing wind. The sails turn a mill for grinding corn or pumping water.
Energy from sources that are renewable and ecologically safe, as opposed to sources that are non-renewable with toxic by-products, such as coal, oil, or gas (fossil fuels), and uranium (for nuclear power). The most important alternative energy source is flowing water, harnessed as hydroelectric power. Other sources include the oceans' tides and waves (see tidal power station and wave power), wind power (harnessed by windmills and wind turbines), the Sun (solar energy), and the heat trapped in the Earth's crust (geothermal energy) (see also cold fusion).
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