The need for use of alternative energy sources due the increasingly devastated state of our environment is obvious. Reliance on nuclear power, oil, gas and coal all produce a great deal of waste product which pollutes the environment. Oil, natural gas and coal produce toxic smoke which pollutes the air and affects global warming, while nuclear produces radioactive waste and can give rise to environmental disasters. It is a necessity to shift to alternative energy sources in order to maintain a 
sustainable ecosystem.
Renewable energy is derived from natural processes that are replenished constantly, such as sunlight, wind, rain, tides & geothermal heat. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and biofuels and hydrogen derived from renewable resources.
Renewable energy sources are generally sustainable in the sense that they cannot "run out" as well as in the sense that their environmental and social impacts are generally more benign than those of fossil.