Nonrenewable energy sources are materials of geologic origin, i.e., petroleum, natural gas, coal, shale oil, and uranium, which cannot be replaced once their supply is exhausted. Renewable sources, on the other hand, are those that can be replenished on a predictable time basis and are known collectively as biomass; these include such cellulosic products as wood, bagasse, agricultural waste, and residue from the forest products industries (shavings, bark, sawdust, etc.), which yield the same heating value per dry ton as one barrel of crude oil. Several of these sources have been in use for some years as on-site fuels in the sugarcane, plywood, and paper pulp industries. Another instance is methane obtained from animal manures (biogas), which is being developed on a large scale in the west and southwest. Interest in the mass cultivation of algae and hydrocarbon-producing plants such as guayule and copaiba for fuel has been reported. There are also a number of mechanical energy sources whose development involves engineering rather than chemistry, namely, solar radiation, wind, water flow, tides, and thermal gradients in ocean water. Several of these are already in limited operation, but in most cases their development will be slow and costly. Hydroelectric power has long been an accomplished fact, though it accounts for only about 1% of electrical needs. Solar energy is being actively researched, but is unlikely to be a factor of consequence for at least another decade. The following entries discuss the chemical sources of energy more specifically:
