Quick Facts
- Arizona’s large desert areas offer some of the highest solar power potential in the country.
- California leads the Nation in electricity generation from nonhydroelectric renewable energy sources, including geothermal power, wind power, fuel wood, landfill gas, and solar power. California is also a leading generator of hydroelectric power.
- Two solar power plants are proposed for central California, covering 12.5 square miles and generating as much as 800 megawatts of power.
- In 2008, Connecticut ranked in the top-ten States for solar power capacity within the United States.
- Two renewable energy technology manufacturers plan to transform an abandoned automobile factory in Wixom to the Nation’s largest renewable energy park, producing solar panels and large-scale batteries to store power for the electric grid.
- Nevada has become a substantial producer of solar energy.
Present & Future
There are two main types of technologies for converting solar energy to electricity: photovoltaic (PV) and solar-thermal electric. PV conversion produces electricity directly from sunlight in a photovoltaic (solar) cell. Solar-thermal electric generators concentrate solar energy to heat a fluid and produce steam to drive turbines. In 2008, less than 1% of the Nation’s electricity was from solar power with a capacity of approximately 8,775 MW according to the Solar Energy Industries Association. In addition, the US market for PV is expected to be around $27 billion dollars by 2020 according to the Department of Energy, and directly and indirectly create 150,000 new jobs.
Sources: Department of Energy, Solar Energy Industries Association
Background
Solar energy can be used for generating electricity, heating and lighting homes and other buildings, hot water heating and solar cooling, as well as for numerous commercial and industrial uses. Various methods of harnessing solar energy are used in order to perform the above functions. These methods include concentrating solar power, photovoltaic systems, passive solar energy and solar thermal energy.
Production
Concentrating solar power uses the sun’s heat to produce electricity. Three main concentrating solar power systems are currently available: parabolic-trough, dish/engine and power tower. Parabolic-troughs use long rectangular, curved (U-shaped) mirrors tilted toward the sun to concentrate the sun’s heat on pipes running through the center of the trough. Oil in the pipes is then heated and boils water located in a conventional steam generator to create electricity.
Similar to the parabolic-trough, a dish/engine system uses a set of mirrors. The mirrors are shaped like dishes and resemble large satellite dishes. The surface gathers and focuses the sun’s heat onto a receiver. The heat is absorbed and transferred to fluid within the engine. The heated fluid expands against a turbine or piston and produces mechanical power. In turn, the mechanical power runs a generator or alternator to create electricity.
Like the parabolic-trough and dish/engine methods, a power tower system also uses a set of mirrors to capture the sun’s heat. The heat is concentrated on the top of a tower, which houses a receiver and through which molten salt flows. The heated salt generates electricity through a conventional steam generator. Due to molten salt’s ability to retain heat efficiently, it can be stored for a period of days before being converted into electricity. Thus, electricity can still be created on days with little sun or after sunset.
Electricity can also be created using the photovoltaic system, more commonly known as the solar cell. Solar cells can convert sunlight directly into electricity and are often used in calculators and watches. Semiconducting materials, much like computer chips, absorb sunlight. The energy from the sunlight knocks electrons loose from their atoms. Electricity is then produced as the electrons flow through the semiconducting material. Solar cells can be combined into larger units in order to provide electricity on a larger scale. Thin film solar cells can be used as shingles or roof tiles to power households and other buildings. However, larger solar cells are much more expensive to produce. Scientists continue to search for ways to make solar cells more cost efficient, despite significant advances in solar cell technology made during the past fifty years.
In much the same way that solar energy is captured to produce electricity, solar energy can be captured to heat and light buildings. The method most commonly used to perform this function is called passive solar energy heating and daylighting. Since the south sides of buildings generally receive the most sunlight, buildings intended to use passive solar energy have large, south-facing windows. Built into the walls and floors, sunlight absorbing materials store the sun’s heat during the day and slowly release it at night in a solar design feature called direct gain. Passive solar heating also incorporates other design features known as sunspaces and trombe walls. A sunspace acts like a greenhouse and stores heat, which is then circulated through the building via special ventilation systems. Trombe walls face south and are painted black in order to absorb heat and slowly release it as the day cools into night.
Passive solar heating designs are also used to naturally lighten buildings. Systems of windows along rooftops allow light to pass through rooms, even those that face north. Of course, too much heating and daylighting can occur during warmer seasons. However, overhangs can be used to cover windows, and sunspaces can be closed off from reaching the rest of the building.
Finally, solar thermal energy heats water in buildings. Solar water heating systems for buildings generally require two parts: a solar collector and a storage tank. The most commonly used collector is the flat-plate collector. It is a thin, flat box with a transparent cover which faces the sun and is placed on a rooftop. Inside the box, small tubes carry water or another type of fluid, such as antifreeze, and are attached to an absorber plate. Heat from the sun builds in the collector and heats the fluid passing through the tubes. The heated water is contained in the storage tank, which is usually large and heavily insulated. If a fluid other than water is used, it passes through coils that heat the water in the tank. Active or passive systems can be used for solar water heating. Active systems are more common and use pumps to move fluids, while passive systems rely on gravity.
Sources: National Renewable Energy Laboratory (NREL), U.S. Department of Energy (Energy Efficiency and Renewable Energy) and The American Solar Energy Society