By far the largest source of this energy flux is the solar radiation in­tercepted by the earth. Of this flux, the possible channels amenable for use as sources for industrial power are: heat from direct solar radiation, water and wind power, and power derived from the stored energy of pho­tosynthesis.

The total power input from direct solar radiation amounts to 17.7 X 1016 thermal watts. However, this occurs at the low power density of only 0.139 watts/cm2 outside of the earth’s atmosphere, and at greatly reduced density over most of the earth’s surface.

According to Farrington Daniels (1964, Table 1, p. 22), the average solar power reaching the earth’s surface amounts to about 500 cal/cm2/ day. This, when averaged over a full day, amounts to about 2.8 x 10 2 watts/cm2. Large modern power plants have power capacities of about 1,000 megawatts, or 109 watts. Solar cells are capable of converting radi­ant energy to electrical energy with an efficiency of about 10 per cent. Hence, for a 1,000-megawatt solar power plant, it would be necessary to collect 1010 watts of solar power. For the average radiation on the earth’s surface, this would require a collection surface of 36 square kilometers, or a square area 6 kilometers to the side.

Although this is not a large area, the amount of electrical equipment

required to collect this amount of intermittent solar power and to convert it into a steady power output is formidable as compared with that for equivalent power stations using other sources of energy. As long as other sources of energy are available at much lower costs, solar power appears to offer little promise as a source of large-scale power. Solar power for special purposes, such as for rural telephone systems or for spacecraft, is practical, however.