Energy sources that regenerate and can be sustained indefinitely, sometimes called “green” renewables, contribute much less to global warming and climate change than exhaustible fossil fuels like petroleum and coal, which pollute when they are burned. Currently, renewable-energy sources provide about 6 percent of the nation's total energy output.
Hydrogen — Unlike fossil fuels or renewable-energy sources like solar or wind energy, hydrogen is not converted directly into electricity. Rather, it must be produced from hydrogen-containing materials — like water, natural gas or methane — using electricity generated from another fuel. The hydrogen is then stored in a tank located adjacent to a fuel cell — which uses the hydrogen to run vehicles, machinery or appliances. The tank has to be refilled periodically at a hydrogen fuel station.
Hydrogen fuel cells are non-polluting: Their only byproducts are water and heat. And because hydrogen is abundantly available, fuel-cell technology could potentially free countries from dependence on foreign energy sources.
Hydrogen molecules are typically separated from other materials by electrolysis or steam reforming. Electrolysis uses electric current to split molecules of water (H2O) into hydrogen and oxygen. As the gases bubble up through the water tank, the hydrogen can be captured and stored. Steam reforming combines steam and a hydrocarbon — such as methanol (CH3OH) — at high pressure and temperature to produce hydrogen and carbon dioxide.
Wind — Windmills have been used to generate electricity in remote locations in the United States since the early 1900s. But after the 1973 OPEC oil embargo caused oil and gas prices to spike, the federal government began funding research into new wind-power technology. NASA developed the first modern wind turbines in the mid 1970s. By 1979, after a second round of oil-price shocks, federal funding for wind-power research and development exceeded $50 million a year. However, when oil prices fell during the Reagan administration, federal funding for wind-power research plummeted, dropping to $17 million by 1982.
Wind power continued to develop, however, in several states. California began installing wind farms in 1983, and by 1990 the state was producing more than half the world's wind power. Since then, European countries, led by Germany, have taken the lead in wind generation by heavily subsidizing the construction of wind farms.
Solar — Solar thermal energy results when solar radiation is converted into heat. This can be done with a simple, passive solar heating system, such as south-facing windows opening onto a dark-colored stone floor, which stores and slowly releases the heat into the surrounding room. In more complex rooftop systems, solar panels containing a fluid — such as water or oil — are heated by sunlight, and the fluid is then pumped through a continuous circuit of pipes to heat interior rooms.
Solar photovoltaic technology converts solar radiation into electricity, which can be used for any purpose. Developed by Bell Labs in the early 1950s, photovoltaics got a boost from federal funding after the energy crises of the 1970s. By 1981, Boeing and Kodak had improved the technology's efficiency with the first thin-film photovoltaic cells. But, as with wind energy, leadership in solar technology has shifted to Europe and Japan, especially after Germany's Siemens AG bought California-based ARCO Solar in 1990, then the world's largest photovoltaic company.
Geothermal — Technology converting underground steam to electricity originated in Italy in the early 1900s, but the first commercial-scale geothermal facility in the United States did not appear until 1960, when Pacific Gas & Electric installed a 10-megawatt unit in California, called The Geysers. Federal support to develop geothermal facilities peaked in the late 1970s at about $100 million a year and now stands at around $30 million a year. Nevertheless, the United States maintains a leading position in the global geothermal industry; since 1985, U.S. developers have added nearly 1,000 megawatts of geothermal electric generating capacity outside The Geysers, and California Energy is the world's largest geothermal company.
Biomass — Half the world's population burns wood for heating and cooking, which contributes to deforestation, causes air pollution and exacerbates global warming by releasing carbon dioxide. But organic matter can be an important source of renewable energy. Moreover, unlike other renewable-energy sources, biomass can be converted directly into transportation fuels. Ethanol, a form of alcohol, is made by fermenting carbohydrate-rich plants such as corn to produce a gasoline additive that reduces harmful emissions. Biodiesel, which can be made from recycled cooking oils and animal fats, can be used alone or added to regular diesel fuel to reduce emissions. Biomass can also be converted into a fuel oil to generate electricity or be burned directly to produce steam for electrical generation or other industrial uses. Critics oppose ethanol because they say it takes more net energy to produce ethanol than is saved by burning it.
Wave Energy — Traditional hydropower, which converts the energy of moving river water into electricity, is a major source of power in the West. But a vast, largely untapped reserve of energy is held in the waves, tides and even the heat of the world's oceans. Tidal energy can be captured by erecting a dam across the opening of a tidal basin and using traditional hydropower technologies to turn turbines to generate electricity from the elevated water in the basin as the tide recedes. Britain, whose western coastline has some of the most powerful wave action in the world, is launching a program to capture wave energy to generate electricity. Wave energy is still in the research stage in the United States.