Energy derived from sources that are not in widespread use or nontraditional sources, for example, compressed natural gas, solar, hydroelectric, and wind. Alternative energy sources replace more traditional fuel sources such as fossil fuels. The search for alternative energy sources is important since fossil fuels are a non-renewable resource and contribute to air pollution and climate change. Other examples of alternative energy sources include hydrogen, geothermal, cellulosic ethanol and biofuels made from algae.
The additional resource link will provide information on geothermal, solar, hydrogen, and other alternative energy sources.

Additional Resources
http://www.eere.energy.gov/