Biomass wastes, including forest residues, lumber and paper mill waste, crop wastes, garbage, and landfill and sewage gas, can be used to produce heat, transportation fuels, and electricity, while at the same time reducing environmental burdens. Solar energy , power from the sun, is free and inexhaustible. Converting sunlight into useful forms is not free, but the fuel is. Sunlight has been used by humans for drying crops and heating water and buildings for millennia. A twentieth-century technology is photovoltaics, which turns sunlight directly into electricity.
Wind power is another ancient energy source that has moved into the modern era.
Advanced aerodynamics research has developed wind turbines that can produce electricity at a lower cost than power from polluting coal plants. Geothermal energy taps into the heat under the earth's crust to boil water. The hot water is then used to drive electric turbines and heat buildings. Hydroelectric power uses the force of moving water to produce electricity. Hydropower is one of the main suppliers of electricity in the world, but most often in the form of large dams that disrupt habitats and displace people.