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Recently, due to concerns over pollution and the possibility that some sources of fuel might eventually run out or become uncomfortableuneconomicuncertainunfinancial uneconomic to obtain, there has been much greater support for renewable sources of power, such as wind or solar energy. Renewable energy sources constrainconstantconstituteconstitution constitute 25 percent of the electricity we use. Unlike burningfossilfossilizefoamy fossil fuels, they tend not to produce any waste or significantly add to global warming by producing gases.