Environmental Engineering Reference
There are other, less energy-rich sources of energy which are not depletable. These are the
so-called renewable energies, such as those of solar insolation, wind, flowing river currents, tidal
flows, and biomass fuels. In fact, these are the energies that were developed on a small scale in
preindustrial societies, providing for ocean transportation, cooking, sawing of lumber, and milling
of grain. Industrial age technologies have made it possible to develop these sources today on a much
larger scale, yet in aggregate they constitute less than 8% of current world energy consumption.
Renewable energy is currently more costly than fossil energy, but not greatly so, and may yet become
more economical when pollution abatement costs of fossil and nuclear energy are factored in.
How is energy used? It is customary to divide energy usage among four sectors of economic
activity: industrial (manufacturing, material production, agriculture, resource recovery), transporta-
tion (cars, trucks, trains, airplanes, pipelines and ships), commercial (services), and residential
(homes). In the United States in 1996, these categories consumed, respectively, 36%, 27%, 16%,
and 21% of the total energy. Considered all together, energy is consumed in a myriad of individual
ways, each of which is an important contributor to the functioning of these sectors of the national
One prominent use of energy, principally within the industrial and commercial sectors, is
the generation of electric power. This use of energy now constitutes 36% of the total energy use
worldwide, but 44% in the United States. Combined with the transportation sector, these two
uses comprise 70% of the total U.S. energy use. For this reason, electric power production and
transportation form the core energy uses discussed in this text.
How is energy supplied? Except for renewable energy sources (including hydropower), the
main sources of energy are fossil and nuclear fuels, which are depletable minerals that must
be extracted from the earth, refined as necessary, and transported to the end-user in amounts
needed for the particular uses. Given the structure of modern industrialized economies, supplying
energy is a year-round activity in which the energy is consumed within months of being extracted
from its source. 4 While there are reserves of fossil and nuclear fuels that will last decades to
centuries at current consumption rates, these are not extracted until they are needed for current
consumption. 5 Because fossil and nuclear fuel reserves are not uniformly distributed within or
among the continents, some nations are fuel poor and others fuel rich. The quantities of fuel traded
among nations is a significant fraction of overall energy production.
One hallmark of industrialization in the twentieth century has been the growth of the electric power
sector, which today consumes about 36% of the world's energy in the production of an annual
average of 1.4 TW of electric power. In the United States, 44% of total energy is used to generate
an annual average of 0.4 TW of electrical power. Nearly all of this electric power is produced in
large utility plants, each generating in the range of 100 to 1000 megawatts (MW). Fossil and nuclear
fuels supply 63% and 17%, respectively, of the total electric power, the remainder being generated
4 The United States has established a crude oil reserve for emergency use, to replace a sudden cutoff of foreign
oil supplies. The reserve contains only several months' supply of imported oil.
5 In contrast, food crops are produced mostly on an annual basis, requiring storage of food available for
marketing for the better part of a year.