All naturally occurring products, substances, or sources of energy that are useful to human beings are termed resources. Any resource, including any energy resource, is either renewable or non-renewable. The major non-renewable energy resources are fossil fuels such as coal, petroleum, and natural gas or elemental fuels such as uranium (U). Although these materials might be renewed or replaced by natural processes over millions of years, they are not renewed on the time-scale of human history. For all realistic purposes, once they are used, they are gone forever.
Some renewables, like solar energy, tidal and wave energy, hydropower (electricity produced by rivers running downhill through dams), and wind energy, are perpetual. That is, no matter how many windmills are raised or solar panels are exposed to the light, the same amount of wind will blow and the same amount of sun will shine. However, other renewable energy resources are renewable only so long as human beings do not exploit them too greedily or change the environmental conditions that allow their renewal. Among these contingent or fragile renewables are those like fuel wood, bio-diesel, or ethanol (C2H5OH) that depend on agriculture and forestry. Farmlands and forests can be permanently ruined by practices that destroy the fertility of the soil, ending the cycle of renewal.
The main sources of renewable energy in the world today--also termed alternative energy--are biofuels (fuel wood or liquid fuels manufactured from crops), hydropower, geothermal energy (energy from heat sources below Earth's surface), solar energy, and wind energy.
Biofuel energy--firewood--was the first concentrated energy source besides muscle power to be exploited by humans. Fire was first controlled between 1 and 1.5 million years ago. Water and wind power came into use several thousand years ago, while non-renewable fuels such as coal and petroleum only began to be widely used starting with the Industrial Revolution in the late eighteenth century. Some renewables, such as hydropower and wood, remained in use even after the Industrial Revolution, though their share of energy supply dwindled.
Renewable energy sources received new attention in the United States and other energy-importing countries after the OPEC (Organization of Petroleum Exporting Countries) oil embargo of 1973. For decades, coal and oil had easily supplied nearly 90 percent of the United States' energy, but the sudden oil shortage prompted the U.S. government and others to invest in alternative and renewable energy sources as well as in more efficient ways to use energy. The U.S. commitment to efficiency and renewables, symbolized by the installation of solar hot-water panels on the roof of the White House by President Jimmy Carter (1924-) in 1979, was greatly scaled back in the 1980s, a change signaled by the removal of the Carter panels by President Ronald Reagan (1911-2004) in 1986.
The 1980s and 1990s were decades of cheap and abundant fossil fuel, and U.S. development of efficiency and renewable energy stagnated. The share of U.S. primary energy met by renewables--mostly large-scale hydropower from mega-dams like the 6.8 gigawatt (GW) Grand Coulee Dam power station--peaked in the mid 1990s at about 7.5 percent, then began to decline. By 2005 only about 6 percent of U.S. energy came from renewables, and about 45 percent of that came from large hydropower plants--about the same figures as in 1979. However, in the late 1990s and early 2000s renewable energy again became prominent thanks to increasing concerns over rapidly rising oil prices, pollution, coal mining, and climate change. Climate change is driven largely by carbon dioxide (CO2) released from the burning of fossil fuels; most forms of renewable energy do not contribute to climate change. Hopeful attention was also turned to renewable energy by political issues, such as growing U.S. dependence on Middle Eastern oil reserves and the protection of wildlife and offshore areas that contain U.S. oil reserves.
It should be noted that energy is not the same as electricity. Electricity is only one form of energy. Energy is also consumed as automotive power in vehicles and as heat in buildings, cooking, and industrial processes.
According to the Energy Information Administration (EIA) of the U.S. Department of Energy (DOE), renewable energy sources account for about 12 to 15 percent of all electricity generated. Conventional hydroelectric projects and biomass conversions contribute the majority of renewable energy. Biomass includes wood used to heat homes, the burning of solid municipal waste (garbage), and the addition of ethanol from fermented plant material (mostly corn in the United States) to gasoline for use in internal combustion engines. Another biomass energy source is landfill gas, methane (CH4) that is emitted by decaying garbage in landfills.
Hydropower uses the force of dammed water running downhill to turn turbines, which produce electricity. Future U.S. hydropower supplies will not increase greatly, as most of the most productive sites have already been developed. Several Third World countries have viewed hydropower as a cheap source of future energy and are developing new projects, but environmentalists and native peoples have protested the damming of more rivers and the destruction of natural habitats and human communities associated with large hydropower facilities. Also, research since the 1990s has shown that large hydropower projects in the tropics may contribute as much to climate change as making an equal amount of electricity by burning fossil fuels. This is possible because of the seasonal nature of tropical rainfall. In the dry season, water levels in reservoirs fall, exposing large flat, muddy areas where plants grow quickly. These plants, like all others, build their tissues by extracting carbon from carbon dioxide in the atmosphere. Carbon dioxide is the major greenhouse gas. When the rainy season raises the water level again, the plants are submerged. They then die and decay through an oxygen-free (anaerobic) process that releases the plants' carbon as methane, another greenhouse gas. Molecule for molecule, methane is about twenty times as effective as carbon dioxide at causing climate change. Tropical dams therefore can act as factories that transform carbon dioxide into methane, contributing to global warming. Dams in the U.S. and other countries far from the equator do not suffer from this drawback, but do disrupt river ecosystems and drown many square miles of territory.
The United States is the world's largest producer of geothermal energy, accounting for about 29 percent of the world total. Other nations with large geothermal facilities include the Philippines, Indonesia, Mexico, and Italy. Geothermal sources produced about 5 percent of United States renewable energy (0.30 percent of U.S. electricity output) in 2011. Geothermal energy is produced by using steam-powered turbines to make electricity or by using Earth's heat directly. High-intensity heat is not the only source of geothermal energy: the technology known as geothermal heat pumping heats and cools buildings by exploiting the steady temperature of soil and rock only a few feet below the surface. U.S. geothermal power continues to grow, with the number of installed geothermal heat pumps increasing by about seven percent per year with a number of large, high-intensity geothermal electric generation projects under construction in California and other western states. Geothermal plants are also found in the Philippines, New Zealand, and Iceland. Iceland derives about 20 percent of its electricity and almost 90 percent of its space heat (interior building heat) from geothermal sources.
Solar energy accounted for only 2 percent of the electricity generated in the United States in 2012 but by 2020 had tripled to 6 percent. Solar energy systems include passive systems and active systems. Passive solar systems are buildings that are designed to use the sun's heat and light directly, while active systems use mechanical methods to capture the sun's energy as heat or electricity.
Photovoltaics are active solar systems that use flat electronic devices termed solar cells to convert a fraction of the energy in sunlight directly to electricity. Photovoltaics' contribution to the U.S. and world energy supply was growing rapidly in the early 2000s as improving technology lowered costs. The world's largest producer of photovoltaic power in 2018 was China, with a capacity of 176,100 MW, followed by the United States (62,600 MW), Japan (56,000 MW), and Germany (45,400 MW).
By 2018, wind energy accounted for 6.5 percent of U.S. electricity production. Wind energy is produced when wind turns windmills that convert the wind's kinetic energy (energy of motion) into electricity. Technology improvements helped wind energy to become the world's fastest growing alternative energy, lowering its price and making it competitive with other energy sources, including coal. During the 1990s, countries such as Germany, Denmark, Spain, and Japan invested heavily in wind production. In the United States wind energy capacity continues to increase (generating 100,125 megawatts in 2019), and is now second only to China in the world.
The U.S. Energy Information Administration (EIA) estimates that about 24 percent of global electricity generation is from renewable energy sources (biofuels, biomass, geothermal, hydropower, solar, and wind sources). The EIA expects that percentage of production to increase to 25 percent before 2040.
The EIA also estimates that 15 percent of the total global energy consumption relies on renewable energy sources (biofuels, biomass, geothermal, hydropower, solar, and wind sources). The EIA estimates that reliance on renewables will grow to 25 percent by 2040.
Engineers and scientists are researching other renewable energy sources besides wind, solar, hydropower, and biofuels. For example, some countries are developing technologies to capture the energy in the ocean's waves (wave power) or tides (tidal power) or in warm ocean water (ocean thermal energy conversion). Algae in glass tubes may prove an efficient way to harvest solar energy as biofuel. Improved technologies for producing, storing, and using hydrogen (which can be made from water and which produces only water when burned) may, according to many experts, extend to the usefulness of various renewables.
However, renewable energy sources also face several obstacles. (1) Because they harvest energy from natural flows, most renewables have low power density. That is, the area of land needed to produce a given amount of power from solar cells, biofuel crops, windmills, or other renewable methods tends to be fairly large. Without reducing energy demand through high-efficiency usage, it will be difficult to meet growing global demand for energy services through renewables or indeed any other means, including coal or nuclear power. (2) The two major non-hydro renewable sources of electricity, namely solar and wind, tend to be intermittent--that is, a windmill only produces power when the wind is blowing above a certain speed, while a solar cell only produces power during the daytime. Fuel-fired plants such as those running on coal or uranium do not have this particular disadvantage, though they do experience outages due to mechanical failures and refueling. For example, the average U.S. nuclear power plant must shut down for refueling for thirty-seven days once every seventeen months. However, the intermittency problem has sometimes been exaggerated: a quilt or patchwork of complementary renewables could produce output that was much more steady than that from any single windmill or solar panel. This is because renewable generators can be complementary, that is, make up for each other's shortcomings. Stormy weather tends to increase wind output while decreasing solar output, demand for electricity is higher during the daytime when the sun is shining, and countries such as the U.S. are so large that the wind is almost certain to be blowing across at least a certain fraction of wind-turbine locations at all times.
The production of fuel ethanol has been criticized both as a net energy consumer (or poor net-energy producer) because of the fossil fuels needed to grow corn and other ethanol crops. It has also been criticized as injurious to the world's poor. Biofuel manufacturers compete directly with food buyers in the market for corn, and high demand for biofuel crops caused growers to switch acreage away from food production. The result was higher food prices. Expert estimates of how much of the rise in global food prices was due to biofuel programs ranged from 10 percent to well over 50 percent.
Despite such difficulties, many renewables--especially solar, wind, and biogas--are increasingly affordable and are being deployed around the world at an increasing pace. The Energy Policy Act of 2005 included numerous provisions regarding alternative energy sources, including increasing the amount of biofuels mixed with gasoline, loan guarantees for development of clean energy technologies, funding for geothermal energy technology, and subsidies for wind and other renewable energy producers. In 2009, the American Recovery and Reinvestment Act was passed and included loan guarantees for the development of renewable energy source technologies such as biofuels.
As of 2020, the U.S. Energy Information Administration claims that wind and solar energy sources account for about 75 percent of new U.S. electricity capacity (not actual electricity generated) anticipated to come online over the next two years.