Energy is the vital force powering our society, industry and eventually our day to day life. Broadly, energy sources can be classified as non-renewable and renewable sources. Non-renewable energy sources are those that can not be replenished (made again) in a short period of time. Petroleum (crude oil), coal, natural gas and Uranium are examples of non-renewable energy sources. Renewable energy sources can be replenished in a short period of time. For example, solar energy, wind, geothermal, hydropower and biomass are renewable energy sources.