Clean energies are forms of energy which do not pollute the air, the ground, or the sea.

Clean energies include:

Solar power

Solar power describes a number of methods of harnessing energy from the light of the Sun. It has been present in many traditional building methods for centuries, but has become of increasing interest in developed countries as the environmental costs and limited supply of other power sources such as fossil fuels are realized. It is already in widespread use where other supplies of power are absent such as in remote locations and in space.

As the Earth orbits the Sun, it receives approximately 1,400 W / m² of energy, as measured upon a surface kept normal (at a right angle) to the Sun (this number is referred to as the solar constant). Of the energy received, roughly 19% is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy. The generally accepted standard is 1020 watts per square meter at sea level.

After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and ultraviolet light. Plants use solar energy to create chemical energy through photosynthesis. We use this energy when we burn wood or fossil fuels or when we consume the plants as a source of food.

Wind power

Wind power is the kinetic energy of wind, or the extraction of this energy by wind turbines. This article deals mainly with the intricacies of large-scale deployment of wind turbines to generate electricity.

Wave power

Wave power refers to the capture of ocean surface wave energy to do useful work including electricity generation, desalination, and filling a reservoir with water. Wave power is a form of renewable energy. Though often co-mingled, wave power is physiologically distinct from the diurnal flux of tidal power and the steady gyre of ocean currents which are powered by the earth's rotation. Wave power generation is not a widely employed technology with only a few experimental sites in existence.

Salinity gradient power

Salinity Gradient is a technology that takes advantage of the osmotic pressure differences between salt and fresh water.

If we place a semipermeable membrane (like that in a reverse osmosis filter) between sealed bodies of salt water and fresh water, the fresh water will gradually travel through the filter by osmosis. By exploiting the pressure difference between these two bodies of water we can extract energy commensurate to the difference in pressure.
 
Tidal power

Tidal power is a means of electricity generation achieved by capturing the energy contained in moving water mass due to tides. Two types of tidal energy can be extracted: kinetic energy of currents due the tides and potential energy from the difference in height (or head) between high and low tides.

Geothermal power

Geothermal power is electricity generated by utilizing naturally occurring geological heat sources. It is a form of renewable energy.

Some renewable energies are not clean energies - for example:

Biofuels because they release NOX and particulates into the environment.
Hydroelectric power because it destroys the river basin and has a negative effect on fish migration.