Renewable energy sources refer to those natural resources that are capable of providing clean energy and are self-replenishing. There are a number of different renewable energy sources that can be used around the world to meet energy needs for heating, cooling, and electric utilities, among others. Solar and wind energy are two of the most well known. Hydropower, geothermal power, and biomass power are others. Combined, the use of these renewable sources of energy can cut down on the amount of greenhouse gases released to the atmosphere, and can cut down the reliance on coal and fossil fuels such as oil and gas, the traditional sources of energy.
Wind and solar power are the most often discussed renewable energy sources. Wind turbines are installed on many ridgelines across the world, and as the turbines turn in the wind, the energy this creates is stored for later use. Solar power is harnessed through the installation of photovoltaic (PV) arrays, either on rooftops or in freestanding panels. These absorb the sun's energy throughout the day, storing the energy in batteries for later use, such as overnight and on cloudy days. Some people are able to take their homes entirely "off the grid" -- that is, not connected to any electric utility services -- by installing solar panels, though using a combination of solar and conventional power is more typical.
Hydropower is another one of the more common options for renewable energy sources. This often refers to turbines and dams built on rivers, which capture energy as the water flows through, rotating the turbines. Tide power is another example, in which turbines are installed in the oceans, and then turned as the tide goes in and out. This is still a relatively new concept, but it shows a lot of promise because it is less disruptive to ecosystems when compared to damns, which can significantly alter species composition in rivers. It is also less dependent on specific weather conditions, which means that it produces energy more consistently throughout the year.
Geothermal and biomass powers are two additional examples of renewable energy sources. Geothermal systems make use of the heat energy that is found deep in the earth, and can be used for the heating and cooling of homes or other structures, and is common in greenhouses and industrial buildings because it is so efficient. Biomass power typically refers to the burning of natural wastes, such as wood products that cannot be used for manufacture, or remain after the harvesting of certain crops such as corn. Biomass will likely be an extremely important source of renewable energy as technology improves and more sources can be used for this purpose.