National Renewable Energy Laboratory
The United States currently relies heavily on coal, oil, and natural gas for its energy. Fossil fuels are nonrenewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. In contrast, renewable energy resources—such as wind and solar energy—are constantly replenished and will never run out.
Solar
Most renewable energy comes either directly or indirectly from the sun. Sunlight, or solar energy, can be used directly for heating and lighting homes and other buildings, for generating electricity, and for hot water heating, solar cooling, and a variety of commercial and industrial uses.
Wind
The sun's heat also drives the winds, whose energy is captured with wind turbines. The Earth's rotation also contributes to the winds, particularly through the Coriolis Effect(科里奥利效应).
Biomass
Along with the rain and snow, sunlight causes plants to grow. The organic matter that makes up those plants is known as biomass. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass for any of these purposes is called biomass energy.
Geothermal
Not all renewable energy resources come from the sun. Geothermal energy taps the Earth's internal heat for a variety of uses, including electric power production and the heating and cooling of buildings.
Hydrogen
Hydrogen also can be found in many organic compounds, as well as water. It's the most abundant element on the Earth. But it doesn't occur naturally as a gas. It's always combined with other elements, such as with oxygen to make water. Once separated from another element, hydrogen can be burned as a fuel or converted into electricity. Because energy is always needed to produce hydrogen, hydrogen is not in itself an energy source, but rather a way to store and transport energy, so it is often referred to as an energy carrier.
Ocean
The ocean can produce thermal energy from the sun's heat and mechanical energy from the tides and waves. Tides are driven by the gravitational pull of the moon and sun upon the Earth, while waves are driven by winds blowing over the ocean's surface. See the U.S. Department of Energy's Energy Savers for basic information on ocean energy.
Hydropower
Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower. For more information on hydroelectric power, see the hydropower basics from the U.S. Department of Energy's Water Power Program.
Come from http://www.nrel.gov/learning/