As the name implies the renewable energy gets replenished constantly through natural processes that include solar, wind, ocean, hydropower, biomass, animal waste, geothermal resources, and biofuels and hydrogen derived from renewable resources. It produces no greenhouse gases and is, therefore, seen as an important climate change mitigation tool.
The energy flows in wind, can be used to run wind turbines to generate power. Globally, the long-term technical potential of wind energy could be several times total current global energy production. There are, of course, several factors that would make the realization of this potential economically not feasible including the huge expanses of lands that would have to be put under wind turbines.
Energy in water comes in the form of kinetic energy, temperature differences or salinity gradients and can be harnessed and used. Since water is about 800 times denser than air, a far smaller quantity of water flowing at lower speeds can yield considerable amounts of energy. Usually it is the kinetic energy of the flowing waters that is used to harness energy. Ocean energy includes all technologies to harness energy from the ocean like the marine current power and the tidal stream power using kinetic energy, ocean thermal energy conversion (OTEC) using the temperature difference between the warmer surface of the ocean and the colder lower recesses.
Energy can be collected from sunlight through different means. Two most common methods of collecting solar energy are in the form of heat, as in solar heaters and solar cookers and for generating electricity using photovoltaic solar cells. With increasing use the cost of solar cells is coming down and that day is not far when cost of solar power would be comparable to the power produced from conventional sources.
Plants use CO2 from atmosphere and sunlight to produce biomass by the process of photosynthesis. Biomass 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. Typically biofuel is burned to release its stored chemical energy. Biomass combined with animal manure is also used to produce biogas which can power farm equipment.
This energy is obtained by tapping the heat of the earth itself. Three types of power plants are used to generate power from geothermal energy: dry steam, flash, and binary. Dry steam plants take steam out of fractures in the ground and use it to directly drive a turbine that spins a generator. Flash plants take hot water, usually at temperatures over 200 °C, out of the ground, and allows it to boil as it rises to the surface then separates the steam phase in steam/water separators and then runs the steam through a turbine. In binary plants, the hot water flows through heat exchangers, boiling an organic fluid that spins the turbine and produces power. Source: en.wikipedia.org
Renewables under CDM:
Expectedly, renewable energy projects crowd the CDM registry with more than half of the CDM projects registered so far falling under this category. And of more than 100 baseline and monitoring methodologies approved by the CDM Executive Board so far as many as one third relate to renewable energy sector.