There are many different types of energy and many ways to save energy. Renewable energy sources can be replenished in a short period of time. The five renewable sources used most often are:
biomass - including wood and wood waste, municipal solid waste, landfill and biogas, ethanol, and biodiesel
The use of renewable energy is not new. More than 150 years ago, wood, which is one form of biomass, supplied up to 90 percent of our energy needs. As the use of coal, petroleum, and natural gas expanded, the United States became less reliant on wood as an energy source. Today, we are looking again at renewable resources to find new ways to use them to help meet our energy needs.
Uncertainty about the future of fusion research has increased the importance of "renewable" energy sources. Among the most important of these sources are hydroelectric, solar, and wind. As you will see, renewable energy sources' main assets are their environmental cleanliness and their virtual inexhaustability. Major drawbacks, however, are limited energy production (in most cases not suitable for large-scale power generation) as well as relative costliness to build and maintain. In light of diminishing fossil fuels, however, renewable energy may end up as the energy of choice for the 21st century.