Renewable energy (sources) or RES capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, wave power , flowing water (hydropower), biological processes such as anaerobic digestion, and geothermal heat flow.

The most common definition is that renewable energy is from an energy resource that is replaced by a natural process at a rate that is equal to or faster than the rate at which that resource is being consumed. Renewable energy is a subset of sustainable energy.

Most renewable forms of energy, other than geothermal and tidal power, ultimately derive from solar energy. Energy from biomass derives from plant material produced by photosynthesis using the power of the sun. Wind energy derives from winds, which are generated by the sun's uneven heating of the atmosphere. Hydropower depends on rain which again depends on sunlight's power to evaporate water.

Even fossil fuels, derive from solar energy, as fossil fuel originates from plant material. However, while theoretically renewable on a very long time-scale, fossil fuels are exploited at rates that may deplete these resources in the near future, and are therefore not considered renewable.

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 biogas from anaerobic digestion or ethanol from biomass.

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.

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