Source: Energy Information Administration, April 2007 Monthly Review
*-- Biomass resources exist in three forms: wood, waste and plant-derived alcohol fuels. Wood chips or harvested wood can be burned to produce energy. Waste energy comes from industrial waste, landfill gases or plant and animal waste. Ethanol is the main form of alcohol fuel.
^-- Geothermal energy comes from natural steam or hot water reservoirs in the Earth's crust. Steam turbines produce energy for low-to-moderate demand use.