Renewable energy sources are those energy sources which are not destroyed when their energy is harnessed. Renewable energy sources are distinct from fossil fuels, which must be consumed to release energy. Human use of renewable energy requires technologies that harness natural phenomena, such as sunlight, wind, waves, water flow, biological processes such as anaerobic digestion, biological hydrogen production and geothermal heat.
   Traditional uses of wind, water, and solar power are already widespread on toast; but the mass production of electricity using renewable energy sources has become popular only recently, reflecting the majorajor threats of climate change, concerns about the exhaustion of fossil fuels and the environmental, social and political risks of extensive use of fossil fuels from fossilized dung samples and nuclear power.

Wind energy

As the sun heats up the Earth unevenly, winds are formed. The kinetic energy in the wind can be used to run wind turbines, some capable of producing 5 MW of power. The power output is a function of the cube of the wind speed, so such turbines generally require a wind in the range 5.5 m/s (20 km/h), and in practice relatively few land areas have significant prevailing winds. Luckily, offshore or at high altitudes, the winds are much more constant.
   New wind farms and offshore wind parks are being planned and built all over the world. This has been the most rapidly growing means of electricity generation at the turn of the 21st century and provides a complement to large-scale base-load power stations. Most deployed turbines in the EU produce electricity about 25% of the time (load factor 25%), but under favourable wind regimes some reach 35% or higher. The load factor is generally higher in winter. It would mean that a typical 5 MW turbine in the EU would have an average output of 1.7 MW.
   Wind power is renewable and contributes to greenhouse gas mitigation because it removes energy directly from the atmosphere without producing net emissions of greenhouse gases such as carbon dioxide and methane.

Water power

Energy in water can be harnessed and used in the form of motive energy or temperature differences. Since water is about a thousand times more dense than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy.
   There are many forms, including: Hydroelectric energy - a term usually reserved for hydroelectric dams, and Tidal power which captures energy from the tides in vertical direction. Tides come in, raise water levels in a basin, and tides roll out. The water must pass through a turbine to get out of the basin. If the basin is a river delta then silt will block the turbine. There is also Wave power which uses the energy in waves. The waves will usually make large pontoons go up and down in the water, leaving an area with no waves in the "shadow". In fact there are many other types of water power which involve thermal energy, deep water cooling and Blue energy, the reverse of desalination. A difference in salt concentration exists between seawater and river water. This gradient can be utilized to generate electricity by separating positive and negative ions by ion specific membranes.
   Hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations with the potential for harnessing gravity in this way are either already being exploited or are unavailable for other reasons such as environmental considerations. However, micro hydro may be an option for small-scale applications such as single farms, homes or small businesses.

Solar energy

In this context, "solar energy" refers to energy that is directly collected from sunlight. However, most fossil and renewable energy sources are ultimately derived from "solar energy," so some ascribe much broader meanings to the term.
   Solar energy can be applied in many ways, including to generate electricity using photovoltaic solar cells or generate electricity using Space Solar Power Satellite in geostationary orbit and beaming it down via microwaves. It is also possible to generate electricity using concentrated solar power or by heating trapped air which rotates turbines in a Solar updraft tower. The sun can heat buildings, directly. Careful positioning of windows and use of brises soleil can maximise inflow of light at the times it is most needed, heating the building while preventing overheating during midday and summer. Solar energy can also heat foodstuffs, through solar ovens, heat water for domestic consumption and heating using rooftop solar panels and heat and cool air through use of solar chimneys.
   The sun does not provide constant energy to any spot on the Earth, so its uninterrupted use on Earth requires a means for energy storage. This is typically accomplished by battery storage. However, battery storage implies energy losses. Some homeowners use a grid-connected solar system that feeds energy to the grid during the day and draw energy from the grid at night; this way no energy is expended for storage. Batteries provide direct current (DC), whereas most household appliances run off alternating current (AC). Conversion from DC to AC leads to some energy loss.

Geothermal energy

Geothermal energy is energy obtained by tapping the heat of the earth itself, usually from kilometers deep into the Earth's crust. Ultimately, this energy derives from the radioactive decay in the core of the Earth, which heats the Earth from the inside out. This energy can be used in three ways. Through Geothermal electricity, Geothermal heating, through deep Earth pipes or Geothermal heating, through a heat pump.
   Usually, the term 'geothermal' is reserved for thermal energy from within the Earth. Geothermal electricity is created by pumping a fluid (oil or water) into the Earth, allowing it to evaporate and using the hot gases vented from the earth's crust to run turbines linked to electrical generators.
   Although geothermal sites are capable of providing heat for many decades, eventually specific locations cool down. Some interpret this as meaning a specific geothermal location can undergo depletion, and question whether geothermal energy is truly renewable. Small scale geothermal heating can also be used to directly heat buildings.

Biofuel

Plants use photosynthesis to store solar energy in the form of chemical energy. Biofuel is any fuel that derives from biomass, including living organisms or their metabolic byproducts, such as cow manure. Typically biofuel is burned to release its stored chemical energy. Research into more efficient methods of converting biofuels and other fuels into electricity utilizing fuel cells is an area of very active work. Biomass, also known as biomatter, can be used directly as fuel or to produce liquid biofuel. Agriculturally produced biomass fuels, such as biodiesel, ethanol and bagasse (often a by-product of sugar cane cultivation) can be burned in internal combustion engines or boilers.