Although most of man's activities release heat to the environment, the vast preponderance of the heat released comes from the production or consumption of energy. The human body gives off waste heat at the rate of approximately 100 W (thermal), but this is two orders of magnitude smaller than the per capita energy use of the United States (10,000 W). About 20% of U.S. energy use goes to the production of electricity; other major energy uses include transportation (20%), space heating (15%), and industrial use of heat (20%).l The per capita energy consumption for the entire world is currently one-sixth of that in the United States, and the distribution of energy uses in the U.S. is also substantially different from that in less-developed nations. World energy use has increased substantially over the past century. Figure 2.1 displays an estimate of the growth of the world's consumption of energy between 1925 and 1971. Between 1925 and 1968, world energy consumption increased at a rate of about 3.5% per year. However, the growth rate itself increased from 2% per year between 1925 and 1938 to 5.5% per year after 1960.2 This increase is a function of both an increasing world population and an increasing per capita energy demand, although the rise in per capita consumption is probably the dominant of the two factors.^ In the United States alone, increased population has accounted for only about 20% of the increased electric power consumption. The other 80% of the increase is a result of increases in per capita demand.^ The growth rate of per capita energy use in the United States is estimated at 2-3% per year.5 As less-developed countries become more modernized, substantial increases in energy use can be expected. Furthermore, as supplies of natural resources become depleted, it is likely that energy-intensive substitutes will have to be developed, which will further increase worldwide per capita energy use. Weinberg and Hammond^ project future energy needs in an industrial society to be as much as 20 kWt* per capita, which is double the current U.S. consumption. It is obvious that energy use and the resulting heat rejection to the environment will continue to increase. How large these increases will be and what the ultimate heat rejection of the earth might be are the important issues. Table 2.1 contains some estimates of 1970-71 energy use and projections for the future.1»6“10 There is some disagreement on exactly what the present world energy use is, but most estimates predict increases by a factor of five over current levels by the year 2000. *See p. iv, "Definitions of Unit Symbols."
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