Sustainable Energy Sources: What are They?
As the name implies, a sustainable energy source is something that can (in principle) be used indefinitely since the energy that is produced from it can be replaced. The ecology movement has dubbed such a source as "green" energy. In terms of electricity production, fossil fuels and nuclear fuels are regarded as being unsustainable, since they represent a limited resource which will, eventually, be completely consumed. So, just what is a sustainable energy source then? It is a source of energy that can be replaced indefinitely; either because it can be produced by growing a crop (biofuels, for instance) or is practically limitless (e.g. the energy of the sun; tidal; hydro; wind or geothermal energy).
Wind, hydro and tidal energy sources are converted into electrical energy in much the same way as the fossil fuels; their mechanical energy is used to turn a turbine. In the case of solar energy, a different principle is behind the production of power. The materials used to make a solar cell incorporate semi-conductors, which produce electricity by the photo-electric effect, releasing electrons through the conversion of part of the solar radiation spectrum which falls onto the solar collector.
Geothermal energy can be tapped by bringing hot water from underground up to the surface. The electrical energy is produced from a thermocouple in this situation.
These power sources are regarded to be "clean" since no pollution is produced once they are in operation (of course, that can't be said of their actual construction and installation). While the sun will one day die, and the geothermal energy of the planet is not a truly renewable resource, within the context of continued survival of the human race, they can be regarded as infinite resources.