According to the Natural Resources Defense Council, renewable energy comes from either sources that cannot be depleted or from sources that can replenish themselves on a human time scale. The former category includes wind, solar, geothermal and hydroelectric power, and an example of the latter is biofuel.
Solar energy can come from direct photoelectric generation using silicon panels, or it can come from harnessing the sun's heat to boil water and run an electric turbine. Wind power uses the wind's force to turn a propeller connected to a turbine, generating electricity from the motion. Hydroelectric systems use submerged propellers that turn as water flows through them, driving a turbine. Geothermal systems use Earth's heat to boil water, creating steam to run an electric turbine. Solar and geothermal energy can also be used for heating, either through passive building design or systems that actively capture and store heat.
Biofuels can be alcohol-based, meaning they are created by fermenting plant matter, or they can come from recycled plant and animal oils. In the United States, the ethanol added to automotive gas is an example of a fermented biofuel, while biodiesel comes from recycled materials. In either case, they are much easier to replenish than fossil fuels, which require millions of years of heat and pressure to convert decaying matter into petroleum.