Alternative energy is an umbrella term that refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels. Typically, official uses of the term, such as qualification for governmental incentives, exclude fossil fuels and nuclear energy whose undesired consequences are high carbon dioxide emissions, the major contributing factor of global warming according to the Intergovernmental Panel on Climate Change, and difficulties of radioactive waste disposal. Over the years, the nature of what was regarded alternative energy sources has changed considerably, and today because of the variety of energy choices and differing goals of their advocates, defining some energy types as "alternative" is highly controversial.
The term "alternative" presupposes a set of undesirable energy technologies against which "alternative energies" are opposed. As such, the list of energy technologies excluded is an indicator of what problems that the alternative technologies are intended to address. Controversies regarding dominant forms of energy and their alternatives have a long history.
Historians of economies have studied the key transitions to alternative energies and regard the transitions as pivotal in bringing about significant economic change. Prior to shift to an alternative energy, supplies of the dominant energy type became erratic, accompanied by rapid increases in energy prices.
For example: Coal as an alternative to wood; Petroleum as an alternative to whale oil; Alcohol as alternative to fossil fuels; Coal gasification as alternative to expensive petroleum.
Renewable energy sources such as biomass are sometimes regarded as an alternative to ecologically harmful fossil fuels. Renewables are not inherently alternative energies for this purpose. For example, the Netherlands, once leader in use of palm oil as a biofuel, has suspended all subsidies for palm oil due to the scientific evidence that their use "may sometimes create more environmental harm than fossil fuels". The Netherlands government and environmental groups are trying to trace the origins of imported palm oil, to certify which operations produce the oil in a responsible manner. Regarding biofuels from foodstuffs, the realization that converting the entire grain harvest of the US would only produce 16% of its auto fuel needs, and the decimation of Brazil's CO2 absorbing tropical rain forests to make way for biofuel production has made it clear that placing energy markets in competition with food markets results in higher food prices and insignificant or negative impact on energy issues such as global warming or dependence on foreign energy. Recently, alternatives to such undesirable sustainable fuels are being sought, such as commercially viable sources of cellulosic ethanol.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For More Information: Renewable Energy & Alternative Energy