Concentrating solar power (CSP) technology uses reflective materials to concentrate the sun’s heat energy, which drives a generator to produce electricity. CSP technologies include trough systems, power towers, and dishengine systems, which feature a large reflective dish that collects energy from the sun, concentrating it on a small area. A thermal receiver absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the generator.

Active and passive solar collectors also absorb the sun’s heat energy, but the heat is used directly for space heating or water heating in homes and industrial facilities. Passive solar collectors take advantage of warmth from the sun through design features such as south-facing windows, and wall and floor materials that absorb warmth during the day, and release it at night. Active systems collect and absorb solar radiation, and are combined with fans and pumps to transfer the collected heat. They usually incorporate an energy- storage system that provides heat when the sun is not shining.

This grid-tied, concentrator photovoltaic (CPV) system supplies electricity to the Arizona Public Service grid. Dual-axis tracking modules use small mirrors to focus sunlight on multijunction cells. Individual fins extend outward to cool each cell. (Arizona Public Service/DOE/NREL)

Geothermal Energy

While solar energy harnesses heat from the sun, geothermal energy captures heat from the Earth. Clean and sustainable, geothermal energy sources range from shallow ground to hot rock and water found miles beneath the Earth’s surface. Wells, similar to oil wells, are drilled into underground reservoirs to tap steam and extremely hot water that is brought to the surface and used to heat buildings, grow plants in greenhouses, and drive turbines for electricity generation. Currently, most geothermal reservoirs are located in the western United States, Alaska, and Hawaii.

In most locations, the upper ten feet of the Earth’s surface maintains a nearly constant temperature between 50 and 60°F. A geothermal heat pump with pipes buried in the shallow ground near a building can heat and cool the building all year. In winter, heat from the warmer ground goes through a heat exchanger into the building; in the summer, hot air from the house is pulled through the heat exchanger into the cooler ground. That removed heat can be used as a no-cost energy to heat water. This technology is also used by municipalities to melt snow and ice on sidewalks.


Hydropower is the fourth-largest source of U.S. electricity generation, producing nearly 10 percent of the country’s electricity, according to the DOE. In areas such as the Northwest and New York, hydropower makes an even larger contribution to electricity generation. Hydropower (or hydroelectric power) facilities generate enough power to supply 28 million households with electricity — the equivalent of nearly 500 million barrels of oil.