INTRODUCTION

Renewable energy sources are primarily those which are inexhaustible in nature, and which are ultimately derived from the radiant energy of the sun reaching the earth. These include the obvious examples of hydroelectric power, solar energy, and wind power, as well as some not quite so obvious examples, such as combustible renewable wastes and biomass fuels like ethanol made from grain crops. In addition, sources such as geothermal energy and ocean gradient energy, which are derived from the very large quantities of thermal energy stored in the earth's crust and oceans, are often categorized as “renewable,” although clearly in the very long-term they are not completely sustainable. Of course, if we were to take a time-scale of millions (or perhaps billions) of years, even the sun's radiant energy will diminish, and so none of these sources is truly sustainable “for ever.” To a certain extent, then, the definition of “renewable” is somewhat arbitrary, but clearly these are all sources which should still be available to future generations thousands of years from now, and not just in the next few hundred years, as is the case for “non-renewable” sources, such as fossil fuels. Even nuclear power, depending on the technology used to access the energy in nuclear “fuels,” is sometimes considered to be renewable, because potentially it will be available for much longer than fossil fuel-derived energy.