Nov 23 2006
Renewable energy
Renewable energy (sources) or RES capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, flowing water, biological processes, and geothermal heat flows. The most common definition is that renewable energy is from an energy resource that is replaced rapidly by a natural process such as power generated from the sun or from the wind.

Most renewable forms of energy, other than geothermal and tidal power, ultimately come from the sun. Some forms are stored solar energy such as rainfall and wind power which are considered short-term solar-energy storage, whereas the energy in biomass is accumulated over a period of months, as in straw, or through many years as in wood. Capturing renewable energy by plants, animals and humans does not permanently deplete the resource. Fossil fuels, while theoretically renewable on a very long time-scale, are exploited at rates that may deplete these resources in the near future (see: Hubbert peak).

Renewable energy resources may be used directly, or used to create other more convenient forms of energy. Examples of direct use are solar ovens, geothermal heating, and water- and windmills. Examples of indirect use which require energy harvesting are electricity generation through wind turbines or photovoltaic cells (PV cells), or production of fuels such as ethanol from biomass (see alcohol as a fuel). A parameter sometimes used in renewable energy is the tonne of oil equivalent (toe). This is equal to 10,000 Mcal or 41,868 MJ of energy.[1]

In a sense, renewable energy may be categorised as free energy, although most renewable energy sources would not normally be called "free energy". In engineering, free energy means an energy source available directly from the greater environment and which cannot be expected to be depletable by humans. Renewable energy development is concerned with the use of renewable energy sources by humans. For aspects of renewable energy use in modern societies see Renewable energy development. Modern interest in renewable energy development is linked to concerns about exhaustion of fossil fuels and environmental, social and political risks of extensive use of fossil fuels and nuclear energy. For a general discussion, see future energy development.