These primary energy sources are classified as renewable or nonrenewable. Renewable energy sources are those that
can be replenished quickly or that are nondepletable. Examples include solar, hydropower, wind, geothermal, and
biomass. Nonrenewable energy sources are finite. If we continue to use them, at some point they will run out.
Examples are fossil fuels such as coal, petroleum, and natural gas.
ELECTRICITY: Electricity is a secondary energy source, which means that we must use primary sources to produce
it. About 28 percent of all primary energy consumed in the United States is used to generate electricity. Coal, nuclear,
hydropower, natural gas, and petroleum are the top five primary sources for producing electricity. Unlike the primary
sources, electricity is not classified as renewable or nonrenewable.
TRENDS IN UNITED STATES ENERGY CONSUMPTION: As the economy and population of the United States
have grown, so has energy consumption. However, this increase has been marked by remarkable increases in energy
efficiency. For example, in 1989, the United States used about 9 percent more energy that it did in 1973; however, the
value of the nation’s real gross domestic product GDP (the total value of all the goods and services produced in the
economy in a year) was 46 percent higher! The United States has improved its energy/GDP ratio as fast or faster than
other developed countries. This improvement in energy efficiency was largely a response to the rapid increases in
crude oil prices in the 1970s.
Renewable Energy Sources
RECENT TRENDS: In the 1970s, the federal government’s renewable energy program grew rapidly to include not
only basic and applied research and development (R & D), but also participation in private sector initiatives. In the
1980s, this interest waned as the price of oil fell. In constant dollar (real) terms, government spending for R&D in
renewable energy declined 90 percent from a peak of $875 million in 1979 to a low of $84 million in 1990. In 1990,
this trend was reversed. Constant dollar R&D spending in 1992 was $146 million, and it appears likely there will be
additional funding for additional renewable energy programs. This funding increase reflects fears of environmental
damage from burning fossil fuels, especially acid rain and global warming.
To what extent the United States continues to subsidize the development of renewable energy will be a subject of
much future debate.
RENEWABLE ENERGY SOURCES: The information below identifies basic facts about the different renewable
energy sources, and lists some advantages and disadvantages of each source.
Solar Energy: Solar energy is produced in the core of the sun. In a process called nuclear fusion, the intense heat in
the sun causes hydrogen atoms to break apart and fuse together to form helium atoms. A very small amount of mass is
lost in this process. This lost matter is emitted into space as radiant energy. Less than 1 percent of this energy reaches
the earth, yet it is enough to provide all of the earth’s energy needs. The sun’s energy travels at the speed of light,
186,000 miles per second, and reaches the earth in about eight minutes. Capturing the sun’s energy is not easy, since
solar energy is spread out over such a large area. The energy a specific land area receives depends on factors such as
time of day, season of the year, cloudiness of the sky, and proximity to the equator.
One primary use of solar energy is home heating. There are two basic kinds of solar heating systems: active and
passive. In an active system, special equipment (such as solar collectors) is used to collect and distribute the solar
energy. In a passive system, the home is designed to let in large amounts of sunlight. The heat produced from the
light is trapped inside. A passive system does not rely on special mechanical equipment.
Another primary use of solar energy is producing electricity. The most familiar way is using photovoltaic (PV)
cells, which are used to power toys, calculators, and roadside telephone call boxes. The other primary way to produce
electricity is using solar thermal systems. Large collectors concentrate the sunlight onto a receiver to superheat a
liquid, which is used to make steam to power electrical generators.