Renewable energy sources can be replenished in a short period of time.  The five renewable sources used most often are: o biomass o water (hydropower), o geothermal, o wind, o solar.

Many important events have occurred during the history of using renewable sources. The use of renewable energy is not new. Five generations (125 years) ago, wood supplied up to 90 percent of our energy needs. Due to the convenience and low prices of fossil fuels, wood use has fallen in the United States. Now, some biomass that would normally be taken to the dump is converted into electricity (e.g., manufacturing wastes, rice hulls, and black liquor from paper production).
Overall consumption from renewable sources in the United States totaled 6.8 quads (quadrillion Btu) in 2006, or about 7 percent of all energy used nationally. Consumption from renewable sources was at its highest point in 1997, at about 7.2 quads.
Historically, low fossil fuel prices, especially for natural gas, have made growth difficult for renewable fuels. A number of State and Federal Government incentives, including the Energy Policy Acts of 2002 and 2005, have encouraged the production and use of renewable fuels. Even with these incentives, the renewables share of the ""energy pie"" in the United States is not expected to change much over the next 25 years because we will also be using more non-renewable fuels.
The use of renewable sources is also limited by the fact that they are not always available (for example, cloudy days reduce solar energy, calm days mean no wind blows to drive wind turbines, droughts reduce water availability to produce hydroelectricity). Despite these limitations, renewable energy plays an important role in the supply of energy. When renewable energy sources are used, the demand for fossil fuels is reduced. Unlike fossil fuels, most renewable sources do not directly emit greenhouse gases. 
Each of the energy sources we use is measured, purchased, and sold in a different form. Many units of measurement are used to measure the energy we use each day.  Learn more about converting energy units in the Units of Measurement section.
------------------------- Energy from wind
Wind is simple air in motion. It is caused by the uneven heating of the earth’s surface by the sun. Since the earth’s surface is made of very different types of land and water, it absorbs the sun’s heat at different rates.
During the day, the air above the land heats up more quickly than the air over water. The warm air over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating winds. At night, the winds are reversed because the air cools more rapidly over land than over water.
In the same way, the large atmospheric winds that circle the earth are created because the land near the earth's equator is heated more by the sun than the land near the North and South Poles.
Today, wind energy is mainly used to generate electricity. Wind is called a renewable energy source because the wind will blow as long as the sun shines.
(To be contd.)