With the rise in concern about the environment in general since the 1960s, and the growing concerns about global warming over the past 20 years, governments, businesses and individuals all look for ways to reduce their environmental impact. Our use of fossil fuels to power our cars and create electricity creates the greenhouse gases that contribute to global warming. Using renewable energy sources---including water, one of the first power sources ever used by man---can help reduce our production of these gases. At its peak in the early 1940s, hydropower provided almost 40 percent of the electricity in the United States.
To use water to generate electricity, you first need to go to the source---generally, a river, ocean currents, or a large body of water, such as a lake or a man-made reservoir. The most common hydropower method generally involves building a dam across a river and creating a lake or reservoir behind it to act as a power source.
To begin generating electricity, the dam opens gates to allow water from the reservoir to flow through penstocks---large tubes inside the dam---where it turns the blades of turbines.
The turbines attach to generators with long shafts, which create electricity with the spinning motion caused by the water rushing through the penstocks.
The raw electricity created by the generators then travels to the utility company via transmission lines.
Though other types of hydropower exist, we have yet to develop them for practical use on a mass scale.
Tidal energy works much the same way as a dam, only the the tides, rather than a river or lake, act as the power source.
Wave energy generation works by using the motion of the waves to push air through a cylinder, turning a turbine, which then turns the generator.
Another experimental source, known as Ocean Thermal Energy Conversion (OTEC), works by using pressure to turn warm surface water into steam. The steam then spins a turbine, creating electricity.