what are the types of renewable energy alternative?

Question

what are some challenges with using and managing these alternative, renewable energy resources?

Answer 1

http://en.wikipedia.org/wiki/Renewable_energy

http://www.nrel.gov/

http://www.eia.doe.gov/kids/energyfacts/sources/renewable/renewable.html

http://www.sciencedaily.com/news/earth_climate/renewable_energy/