History of Energy in the United States: 1635-2000
	Renewable Energy
	For all but the most recent fraction of humanity's time on Earth, virtually all energy was renewable energy. Before the widespread use of fossil fuels and nuclear power, which arrived only a relative eyeblink ago, our ancestors warmed themselves directly in the sun, burned brush and fuelwood fashioned by photosynthesis from sunlight and nutrients, harnessed the power of wind and water created mainly by sun-driven atmospheric and hydrologic cycles, and of course used their own musclepower and that of animals.
	We still depend heavily on renewable energy in these basic forms. But various cultures have also found more inventive means of harnessing renewable resources, from mounting sails on wheelbarrows, as did ancient Chinese laborers, to gathering and burning buffalo dung, as did Native Americans and European settlers making their way west. The story of renewable energy is one of the invention and refinement of technologies for extracting both more energy and more useful forms of it from a wider variety of renewable sources. Many energy experts believe that the age of fossil fuels is only an interlude between pre- and post-industrial eras dominated by the use of renewable energy.
	Some renewable energy technologies, such as water- and wind-driven mills, have been in use for centuries. Grain mills powered by waterwheels have existed since at least the first century BCE and became commonplace long ago. In England, for example, the Domesday Book survey of 1086 counted 5,624 mills in the south and east alone. They were to be found throughout Europe and elsewhere and were used for a wide variety of mechanical tasks in addition to milling, from pressing oil to making wire. Some installations were surprisingly large. The Romans built a mill with 16 wheels and an output of over 40 horsepower near Arles in France. A giant 72-foot waterwheel with an output of 572 horsepower, dubbed Lady Isabella, was erected at a mine site on the Isle of Man in 1854. Further development of waterwheels ended with the invention of water turbines. Both types of machines were supplanted by large steam engines, which could be sited nearly anywhere. Turbines, however, found an important niche with the development of hydroelectric power.
	Windmills are a younger but still ancient technology, dating at least to the 10th century in the Middle East, a bit later in Europe. In one form or another, windmills have remained in use ever since, for milling grain, pumping water, working metal, sawing, and crushing chalk or sugar cane. As mentioned in the introduction, American farms of the 19th century erected millions of small windmills to pump water for livestock or household use. In the modern era, technologically advanced windmills have been developed for generating electricity.
	Modern renewable sources in the United States contribute about as much to total energy consumption as does nuclear power (Figure 31). Hydroelectric power generation, which uses dam-impounded water to drive turbine generators, generally accounts for a large share of U.S. renewable energy output. In 2000 (a relatively bad year for U.S. hydropower) that share was 46 percent of total renewable energy consumption. The American hydropower infrastructure includes the great dams of the intermountain West, the Columbia basin, and the Tennessee River valley, as well as hundreds of other smaller installations nationwide.
	Figure 31. Energy Consumption by Source
	Much of U.S. renewable energy comes from wood and waste, a diverse category that accounted for almost half of the total in 2000. It includes not only the obvious candidates (such as wood, methanol, and ethanol) but also peat, wood liquors, wood sludge, railroad ties, pitch, municipal solid waste, agricultural waste, straw, tires, landfill gas, fish oil, and other things. Wood and wood byproducts are the most heavily used form of biomass and are an important source of energy for such industries as paper manufacturing and lumber, which have ready access to them. Geothermal, which began supplying a measurable amount of energy in 1960, accounted for 5 percent of U.S. renewable energy in 2000.
	Despite their cachet, solar energy (photovoltaic and thermal) and wind energy contribute relatively little to the renewable total (about 1 percent each in 2000). The peak year for U.S. manufacturers' shipments of solar thermal collectors was 1981, when 21 million square feet were shipped. During the 1990s, shipments averaged less than half that level, though the 1999 total of 8.6 million square feet was the highest since 1990. Ninety-one percent of the 1999 total went to the residential sector, and 95 percent of the newly shipped collectors were used to heat swimming pools. Four percent were used to heat water. Inflation-adjusted prices for photovoltaic cells have generally declined in recent years and the volume of shipments (expressed as peak kilowatts) has risen steadily since 1985. Over 70 percent of U.S. production went for export in 1999. Wind energy production rose 113 percent between 1989 and 2000 but remains a small factor in the U.S. renewable energy picture.