Water is one of the earth's most abundant resources, which can be utilized to produce convenient forms of energy. The mechanical use of falling water to turn wheels of machinery has been known thousands of years ago. Since the end of 19th century hydropower, or the power of water, is also being exploited for the production of electricity. Today hydropower is an important renewable resource widely used to create electricity in the United States and all over the world. In 2009 about 3% of total US energy consumption came from hydroelectricity.


Water that moves quickly in a river or descends over a great distance possesses a large amount of usable kinetic energy. To harness it, the fast moving water can be sent through a pipe called a penstock. Inside the pipe, the water causes blades in a turbine to spin. The turbine's mechanical energy is then transferred through a drive shaft to the electric generator. In the generator, the rotational energy is transformed into electricity. Sometimes a penstock is added to a natural source of moving water, like a stream or waterfall. The water flow can be made artificially through dams that release it into the pipes when electricity is needed. The systems can be “run-of-river” without a reservoir, or can include reservoir storage capacity.


Other forms of renewable water energy are waves and tides. Winds and temperature differences caused by uneven heating of the ocean contribute to the formation of waves. Their movement must be transferred to some swinging system and converted to the mechanical energy of turbines or other hydraulic or pneumatic engines, which drive a generator. The waves can be focused into small channels in order to increase the amount of captured energy. Although the wave energy technology is still in an early stage, the first commercial wave farm has been already opened in Portugal in 2008.
Tides are caused by the interaction of the gravitational forces and the movement of the sun, moon and earth. The ocean moves toward the moon on the side facing the moon. This results in an up-and down movement of water along the coast. The seawater can be trapped with a dam in a bay at high tide. During low tide, it can be released from the bay to the ocean. As it falls, it can turn the turbine of an electric generator. Typical conversion efficiencies for tidal power are 10-25%.



An estimated levelized cost of electricity produced on new hydroelectric power plants is about $0.12 per kilowatt-hour. This is 50% times higher than the electricty cost on conventional combined cycle natural gas-fired plants and 20% higher than that of conventional coal plants. Note that these are US national average numbers: there are significant local variations depending on the markets and the availability of the water resources. Energy costs on large hydro plants can be as low as $0.03-0.05/kWh.