http://en.wikipedia.org/wiki/Renewable_energy
Renewable energy is energy that comes from natural resources like sunlight, wind, rain, tides, and geothermal heat. These are all renewable.

Main forms of renewable energy:


Wind power- the power productivity of a turbine is a function of the cube of the wind speed, so as wind speed increases, power output increases. Areas where winds are stronger and more stable are better locations for wind farms.
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Hydropower- usually set aside for large hydroelectric dams.
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Solar energy- the energy that comes from the sun through solar radiation. Solar powered electrical generation relies on photovoltaics and heat engines. Solar technologies are distinguished as passive solar or active solar.
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Biomass- a renewable energy source because the energy it contains comes from the sun. The plants take hold of the suns energy through photosynthesis. When the plants are burned, they release the sun's energy they contain.

Geothermal energy- energy gained by tapping the heat of the earth itself, both from kilometers deep into the Earth's crust in some places of the globe or from some meters in geothermal heat pump in all the places of the planet. Three types of power plants are used to generate power from geothermal energy: dry steam, flash, and binary. Dry steam plants take steam out of fractures in the ground and use it to put a turbine that spins a generator. Flash plants take hot water out of the ground, and allows it to boil as it rises to the surface. It then separates the steam phase in steam/water separators and then runs the steam through a turbine. In binary plants the hot water flows through heat exchangers, boiling an organic fluid that spins the turbine.
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