They are forms of energy that are derived from sources that can regenerate or cannot be wiped out completely i.e. heat and power that we can use and that will always be there or that can grow back again. Thus forms of renewable energy are different from forms of non renewable energy such as fossils, and do not produce any forms of green house gases or pollutants that generally arise from fossil fuel combustion. Renewable forms of energy such as wind, water or solar energy have always been in use by mankind, but renewable energy has recently come to the forefront with the coming about of global warming, depletion of global fossil fuel resources and the environmental and political issues associated with nuclear power. Renewable energy makes up for about 14% of global energy consumption, but its potential is large enough to cover much greater levels of consumption. Geothermal and hydropower are seen as the new wave energy forms that cheap on the pocket without need for subsidies. Other technologies such as solar power need much greater amount of capital, although in the future costs may decline with continued R & D.
 Primary Modes of Renewable Energy
Renewable energy forms are derived from rather common phenomena such as sunlight, wind, tides and geothermal heat. Each of these sources are very unique in nature and thus are exploited and used in very different ways.
 Wind Power
Wind can be used to propel wind turbines and some are capable of producing 5 MW of power. Turbines with rated output of 1.5-3 MW are used most commonly for commercial purposes. The power output of a turbine is a function of the cube of the wind speed, so as wind speed increases, power output increases dramatically (European Wind Energy Association). Offshore and high altitude sites where winds are found to be much stronger and constant are ideal locations to set up turbines.
 Water power
Energy in water can be harnessed and used. Since water is about a thousand times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy. There are numerous forms of water energy: -Hydroelectric energy is a term used for large-scale hydroelectric dams and based on rivers. -Micro hydro systems where streams and springs are abundant. They work on similar principal as dams but at a much smaller scale. -Wave power harnesses the power in waves. The waves will usually make large pontoons and go on to rise and subside, leaving an area with reduced wave height in the area known as the "shadow". Wave power has only recently become a commercial venture. -Tidal power captures energy from the tides in a vertical direction. Tides come in, increase water levels within a catchment area, and the tides roll out. Around low tide, the collected seawater is allowed to pass through a turbine.
 Solar Energy
Solar Energy" refers to energy that is collected from sunlight. Solar energy can be applied in many ways, including to: - photovoltaic solar cells. - concentrated solar power. - heating trapped air which turnsturbines within solar updraft towers. -food is cooked with the help of solar ovens. -hot water and central heating can be provided using solar-thermal panels.
 Geothermal energy
Geothermal energy is collected by exploiting the heat of the earth itself, genrally from energy sources hidden deep within the Earth. The initial expenses to build a power station are high but operating costs are low resulting in low energy costs for suitable sites. Ultimately, this energy comes about from the radioactive decay in the core of the Earth, which heats the Earth from within. Biogas Biogas can easily be produced from commonly produced human waste or byproducts, such as: paper production, sugar production, sewage, animal waste and so forth. The waste forms mixed together into a paste and then allowed to naturally ferment, producing methane gas. When a biogas plant has extracted all the methane it can, the remains are used as fertilizer, the remains are sometimes said to be even more fertile than the original biomass.
Renewable energy technologies are essentially developed with the aim of providing world energy security, reducing dependency on fossil fuel resources, and providing opportunities for reducing greenhouse gases in the fight against global warming. A lot of renewable energy forms are past developmental stage and already economically viable while on the other hand other technologies need only limited additional development steps to become ready for the market, and yet other technologies continue to remain expensive and require a few mores years of intense research.
Environment Words- A Dictionary in Plain English, published By Images Asia http://www.edugreen.teri.res.in/EXPLORE/renew/renew.htm suggested reading http://www.earthtrends.wri.org