The world’s requirements and resources of industrial energy are large and complex subjects which, owing to limitations of space in the present vol­ume, can only be summarized. However, this summary is based upon two more extensive reviews (Hubbert, 1962; 1969) to which reference may be made for more detailed information and documentation.

Flux of Energy on the Earth

The use of energy for nonbiological or industrial purposes can best be appreciated in the context of the earth’s total matter-and-energy econ­omy. In this context, the earth may be regarded as a material system whose gain or loss of matter during the last billion years has been negligi­ble. Into and out of this surface environment, however, there occurs a con­tinuous flux of energy, in consequence of which the earth’s material con­stituents undergo continuous or intermittent circulation.

The principal sources of this energy are: solar radiation, geothermal energy conducted and convected to the earth’s surface from the earth’s hotter interior, and tidal energy derived from the combined gravitational and kinetic energy of the earth-moon-sun system (see accompanying tab­ulation). Of these three sources of energy, that from solar radiation is overwhelmingly the largest. Heat from solar radiation is received at a rate of 2 gram-calories per square centimeter per minute. Converted to power units, this amounts to a radiation of thermal power at a rate of 0.139

Source

Solar radiation. Geothermal heat Tidal energy…

watts/cm2, and the total power intercepted by the earth’s diametral plane amounts to 17.7 X 1016 watts. This is about a hundred thousand times the world’s present installed electric-power capacity. By comparison, energy inputs from geothermal and tidal sources amount only to about 32 x 1012 and З x 1012 watts, respectively.

Geothermal energy occurs initially as heat, which eventually assumes the lowest temperature of the earth’s ambient surface environment. Tidal energy is dissipated into heat by the friction of tidal currents in the oceans and in the shallow seas, coastal bays, and estuaries around the world. Of the solar energy input, about 35 per cent is directly reflected into outer space (see accompanying tabulation). Of the remaining energy, about 42 per cent is absorbed and converted directly into heat. Another part is ab­sorbed by the atmosphere and the oceans, causing thermal expansion and providing the energy for atmospheric and oceanic circulation, and about 23 per cent becomes the latent heat of evaporation of water. This, to­gether with atmospheric circulation, is responsible for the hydrologic cy­cle, including the precipitation and runoff of water on all of the land areas of the earth. Finally, a small fraction (less than 1 per cent) of the total in­put of solar energy is captured by the leaves of plants and is stored as chemical energy in the process of photosynthesis whereby inorganic ma­terials such as 02, C02, and H20 are converted into organic compounds and provide the energy base for the entire plant and animal kingdoms. Upon decay, the organic matter of plants and animals oxidizes, and the stored chemical energy is released as heat.

The rate of decay of organic material is almost exactly equal to its rate of production. However, a minute fraction of this material may be­come deposited in sedimentary muds or in peat bogs in an oxygen-free en­vironment and thus be preserved. The accumulation of this small fraction of preserved organic matter over the last 600 million years of geological history has resulted in the world’s present supply of fossil fuels — coal, pe­troleum and natural gas, and related materials.

The end product of all of the terrestrial energy transformations, ex­cept for the fraction of solar energy directly reflected and the minute frac­tion preserved and stored by organisms, is degradation ultimately into heat at the lowest ambient temperature. This heat then leaves the earth by spent, long-wavelength, thermal radiation.

One additional form of terrestrial energy is that stored in atomic nu­clei, particularly in the heavy elements uranium and thorium, and in the light element hydrogen. Uranium and thorium have an abundance in the surface rocks of the earth of about 16 parts per million, and are slightly radioactive. By this process, nuclear energy is being spontaneously con­verted to heat, which appears to be a major source of the earth’s geo­thermal energy. The extraction of the stored nuclear energy from both the heavy and light elements by artificial means is the basis for the recently acquired ability to produce nuclear power.

From this brief review, it is seen that the energy sources appropriate for large-scale industrial uses must be either the earth’s supplies of stored energy: the fossil fuels, nuclear energy, and to some extent geothermal energy; or else the various channels of the energy flux: solar power, water and wind power, plants and animals, and geothermal and tidal power. With regard to the stored energy, the problem of present interest is prin­cipally the magnitudes of the supplies, and about how long they can be depended upon to provide a major fraction of the world’s potential re­quirements. For the contemporary energy fluxes, the problem is the mag­nitude of the industrial power that can be derived from each.