Water

Hydropower systems capture the energy of flowing water and convert it to electric energy by use of propellers or turbine wheels. Of course the potential for hydro-electric systems is depending on the availability of a suitable water flow, but where this resource exists it will provide cheap and reliable electricity. Hydropower is divided into small and large hydropower systems. The division is arbitrary but generally hydropower stations of over 20 MW are not considered small. Small hydropower has subdivisions into small (up to 20 MW) mini (up to 5 MW) and micro (below 100 kW). However, small hydro power will have a significant contribution to future energy needs in wide areas of the world. It depends on proven and well tested technology and requires little maintenance. The water flows via a channel or pipe to a waterwheel or turbine where it strikes the bucket of the wheel, causing the shaft of the waterwheel or turbine to rotate. When generating electricity, the rotating shaft, which is connected to an alternator or generator, converts the motion of the shaft into electrical energy. This electrical energy may be used directly, stored in batteries, or inverted to produce utility-quality electricity. More: http://en.wikipedia.org/wiki/Hydropower or http://www.hydropower.org/