2. THE POTENTIAL OF SOLAR ENERGY The potential of solar energy as a source of power has been recognized and evaluated for more than 100 years. Each square meter of Earth's surface exposed to sunlight at noon receives the equivalent of 1 kW, or a total potential power input 100,000 times larger than the power produced in all the world's electrical generating plants together. Efforts to harness solar energy, which had accelerated during the last half of the 19th and the beginning of the 20th century as industrialization increased the global energy needs, subsided with the successful development of energy economies based on the use of oil and gas. Not until the early 1970's did the development of solar energy technology to produce power — by concentrating solar radiation to generate high temperatures to power heat engines, by direct conversion of solar radiation with photovoltaic processes, by photochemical conversion to produce fuels and by indirect methods such as biomass conversion based on the use of products of photosynthesis, wind energy conversion and ocean thermal energy conversion — again began to be seriously pursued. But these developments are being pursued regionally without — at least so far — consideration of the new, even unique role that solar energy can play as a global energy resource in the future. At times, this role has been repudiated by emphatic dismissal or ridicule or through uninformed underestimates of its global potential. The degree to which solar energy technology will be successfully applied will largely depend on its economics, which, in turn, will depend on the reduced availability of nonrenewable fuels and their future costs. Still, the successful and widespread introduction of solar energy technology will require considerable development to strike the appropriate balance among the conflicting requirements of economics, the environment, and society's needs. The political consequences of increasing solar energy use are likely to be the most far reaching and may require extensive and unprecedented international cooperation, which in conjunction with energy conservation measures could lead to a safer and more stable world than would continued reliance on non-renewable fuels, especially since most of the latter are under the political control of only a few nations. The major challenge to the application of solar energy on a global scale is the fact that it is a distributed resource with a low supply density which must serve societies with an increasing demand density. In the past, population densities were limited by the amount of food that could be produced on a given area of land. Mechanization of agriculture and resulting rural underemployment led to the movement of people to distant cities where they sought an elusive existence. But the existence they sought — and achieved — is now again beginning to elude them as the problems associated with urban population growth continue to increase. The potential of distributed solar technology would appear to be sufficient to meet the energy demands of rural populations. However, urban populations would have great difficulty in meeting their demands with this technology, even if urban centers were located in areas with exceptionally favorable supply densities from renewable resources. In the coming decades, the global population will be less likely to live in villages and single homes. Thus, it would be very optimistic to expect that distributed solar technologies would be able to meet even a low global energy demand unless societies, particularly in industrialized countries, were willing to change their life styles on an unprecedented scale. It is unrealistic to assume that modern societies would deliberately change their life styles to meet changing conditions unless forced to do so. The various solar technologies should be developed so that
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