The enormous potential power in solar energy is revealed by measurements which show that solar heat reaches the earth in the Temperate Zone at the average rate of about 4,000,000 calories per square yard per day. During the three months of greatest sunshine, an acre of land receives directly from the sun an amount of heat equivalent to burning approximately 250 tons of high-grade coal. This measurement indicates that unobstructed solar radiation, transformed completely into useful energy, would produce approximately one horsepower per square yard.
This energy determines our climates, causes winds, ocean currents, and rainfall, and produces photochemical reactions whereby a portion of the energy is stored in plants. Thus, solar energy is the ultimate source of our fuels—wood, coal, oil, and gas—as well as of power derived from wind or falling water. However, the stores of fuel energy in coal, oil, and gas, while great, are not inexhaustible. It is therefore of ultimate importance to investigate and develop alternative sources of heating and power.
Because of the enormous amount of solar energy freely available, the practical problem is not to find means of using it with a high percentage of efficiency but rather to find methods that will be cheap enough to make solar energy economically useful.
There have been many attempts in which mirrors were used to concentrate sunshine as a source of energy for heat engines but, while some of them have been technically successful, they were economically unsound. The problem of economically collecting the sun’s heat is a baffling one.
Research on electrical apparatus for utilizing solar energy suggests three approaches. These are vacuum or gas-filled photoelectric cells, thermopiles, and boundary-layer apparatus, such as the copper–copper oxide cell. None of these have been successfully adapted to convert sun energy for power purposes. Although all are notoriously inefficient, one cannot assume that further knowledge and its application may not entirely change the situation. If it should, the possibilities are enormous.
It is evident today, however, that much more information is needed before specific applications can be successfully planned. On the other hand, the ignorance is so vast at present that there remains the possibility that developments in this direction may supersede many other means of power conversion and provide an inexhaustible source of cheap power.