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Solar basics

Energy from the sun

The sun has produced energy for billions of years and is the ultimate source for all of the energy sources and fuels that we use today. People have used the sun's rays (solar radiation) for thousands of years for warmth and to dry meat, fruit, and grains. Over time, people developed technologies to collect solar energy for heat and to convert it into electricity.

Radiant energy from the sun has powered life on earth for many millions of years.
The sun

Source: NASA

Collecting and using solar thermal (heat) energy

An example of an early solar energy collection device is the solar oven (a box for collecting and absorbing sunlight). In the 1830s, British astronomer John Herschel used a solar oven to cook food during an expedition to Africa. People now use many different technologies for collecting and converting solar radiation into heat energy for a many uses.

We use solar thermal energy systems to heat

  • Water for use in homes, buildings, or swimming pools
  • The inside of homes, greenhouses, and other buildings
  • Fluids to high temperatures in solar thermal power plants

Solar photovoltaic systems convert sunlight into electricity

Solar photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes.

Solar energy has benefits and some limitations

Using solar energy has two main benefits:

  • Solar energy systems do not produce air pollutants or carbon dioxide.
  • Solar energy systems on buildings have minimal effects on the environment.

Solar energy also has some limitations:

  • The amount of sunlight that arrives at the earth's surface is not constant. The amount of sunlight varies depending on location, time of day, season of the year, and weather conditions.
  • The amount of sunlight reaching a square foot of the earth's surface is relatively small, so a large surface area is necessary to absorb or collect a useful amount of energy.

Where solar is found and used

Solar energy is sunshine

 Map of direct normal solar irradiance (DNI) in the United States showing greatest concentration in the southwest.

Source: National Renewable Energy Laboratory, U.S. Department of Energy

Click to enlarge

Map of direct normal solar irradiance (DNI) in the United States showing greatest concentration in the southwest.

Source: National Renewable Energy Laboratory, U.S. Department of Energy

Click to enlarge

World map of solar resources
World Map of Solar Resources showing greatest concentration in the southern portion of the Northern Hemisphere, South America, Africa, the Middle East, southern Eurasia, the South Pacific, and Australia

Source: United Nations Environment Programme (UNEP), NASA Surface meteorology and Solar Energy (SSE), 2008.

Click to enlarge

Sunshine is radiant energy from the sun. The amount of solar radiation, or solar energy, that the earth receives each day is many times greater than the total amount of all energy that people consume each day. Use of solar energy, especially for electricity generation, has increased a lot in the United States and around the world in the past 30 years.

Latitude, climate, and weather patterns are major factors that affect insolation—the amount of solar radiation received on a given surface area during a specific amount of time. Locations nearer the equator and in arid climates generally receive higher amounts of insolation than other locations. Clouds, dust, volcanic ash, and pollution in the atmosphere affect insolation levels at the surface. Buildings, trees, and mountains may shade a location during different times of the day in different months of the year. Seasonal (monthly) variations in solar resources increase with increasing distance from the earth’s equator.

The type of solar collector also determines the type of solar radiation and level of insolation that a solar collector receives. Concentrating solar collector systems, such as those used in solar thermal-electric power plants, require direct solar radiation, which is generally greater in arid regions with few cloudy days. Flat-plate solar thermal and photovoltaic (PV) collectors are able to use global solar radiation, which includes diffuse (scattered) and direct solar radiation. Learn more about solar radiation.

Solar thermal collectors

Low-temperature solar thermal collectors absorb the sun's heat energy to heat water for washing and bathing or for swimming pools, or to heat air inside buildings.

Concentrating collectors

Concentrating solar energy technologies use mirrors to reflect and concentrate sunlight onto receivers that absorb solar energy and convert it to heat. We can use this thermal energy for heating buildings or to produce electricity with a steam turbine or a heat engine that drives a generator.

Photovoltaic systems

Photovoltaic (PV) cells convert sunlight directly into electricity. PV systems can range from systems that provide tiny amounts of electricity for watches and calculators to systems that provide the amount of electricity that hundreds or thousands of homes use. Millions of houses and buildings around the world have PV systems on their roofs and there are many large PV power plants.

Use of solar energy

The U.S. Energy Information Administration (EIA) estimates that total solar energy use in the United States increased from about 0.06 trillion British thermal units (Btu) in 1984 to about 1,870 trillion Btu in 2022. Solar electricity generation accounted for about 97% of total solar energy use in 2022 and direct use of solar energy for space and water heating accounted for about 3%.

Total U.S. solar electricity generation increased from about 5 million kWh in 1984 (nearly all from utility-scale, solar thermal-electric power plants) to about 204 billion kWh in 2022. In 2022, utility-scale PV power plants accounted for 70% of total solar electricity generation, small-scale PV systems accounted for 29%, and utility-scale solar thermal-electric power plants accounted for 1%. Utility-scale power plants have at least 1,000 kilowatts (kW) (or one megawatt [MW]) of electricity generation capacity. Small-scale PV systems have less than one MW generation capacity.

According to EIA’s International Energy Statistics, total world solar electricity generation grew from 0.4 billion kWh in 1990 to about 1.306 billion (about 1.3 trillion) kWh in 2021. The top five producers of solar electricity and their percentage shares of world total solar electricity generation in 2021 were:

  • China–33%
  • United States–16%
  • Japan–9%
  • Indian–6%
  • Germany–%

Solar photovoltaic

Photovoltaic cells convert sunlight into electricity

A photovoltaic (PV) cell, commonly called a solar cell, is a nonmechanical device that converts sunlight directly into electricity. Some PV cells can convert artificial light into electricity.

Photons carry solar energy

Image of how a solar photovoltaic water pumping system for livestock in a remote location.

Source: National Renewable Energy Laboratory (copyrighted)

Image of a solar photovoltaic system on the roof of a house.

Source: National Renewable Energy Laboratory (copyrighted)

Image of a utility-scale solar photovoltaic system.

Source: National Renewable Energy Laboratory (copyrighted)

Sunlight is composed of photons, or particles of solar energy. These photons contain varying amounts of energy that correspond to the different wavelengths of the solar spectrum.

A PV cell is made of semiconductor material. When photons strike a PV cell, they may reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the absorbed photons provide energy to generate electricity. When the semiconductor material absorbs enough sunlight (solar energy), electrons are dislodged from the material's atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to the dislodged, or free, electrons so that the electrons naturally migrate to the surface of the cell.

The flow of electricity

The movement of electrons, each carrying a negative charge, toward the front surface of the cell creates an imbalance of electrical charge between the cell's front and back surfaces. This imbalance, in turn, creates a voltage potential like the negative and positive terminals of a battery. Electrical conductors on the cell absorb the electrons. When the conductors are connected in an electrical circuit to an external load, such as a battery, electricity flows in the circuit.

The efficiency of photovoltaic systems varies by the type of photovoltaic technology

The efficiency at which PV cells convert sunlight to electricity varies by the type of cell material and technology. The efficiency of PV modules averaged less than 10% in the mid-1980s, increased to around 15% by 2015, and is now approaching 20% for state-of-the art modules. Experimental PV cells and PV cells for space satellites are nearly 50% efficient.

How photovoltaic systems operate

The PV cell is the basic building block of a PV system. Individual cells can vary in size from about 0.5 inches to about 4 inches across. However, one cell only produces 1 or 2 Watts, which is only enough electricity for small uses, such as for powering calculators or wristwatches.

PV cells are electrically connected in a packaged, weather-tight PV module or panel. PV modules vary in size and in the amount of electricity they can produce. PV module electricity generating capacity increases with the number of cells in the module or in the surface area of the module. PV modules can be connected in groups to form a PV array. A PV array can be composed of two or hundreds of PV modules. The number of PV modules connected in a PV array determines the total amount of electricity the array can generate.

Photovoltaic cells generate direct current (DC) electricity. This DC electricity can be used to charge batteries that, in turn, power devices that use direct current electricity. Nearly all electricity is supplied as alternating current (AC) in electric power lines. Devices called inverters are used on PV modules or in arrays to convert the DC electricity to AC electricity.

PV cells and modules will produce the largest amount of electricity when they are directly facing the sun. PV modules and arrays can use tracking systems that move the modules to constantly face the sun, but these systems are expensive and are mostly used in large PV power plants. Most PV systems on buildings have modules in a fixed position with the modules facing directly south (in the northern hemisphere—directly north in the southern hemisphere). Learn more about PV collector tilt angles and PV collector tracking systems.

Did you know?

Solar photovoltaic cells are grouped in panels (modules), and panels can be grouped into arrays of different sizes to produce small to large amounts of electricity, such as for powering water pumps for livestock water, for providing electricity for homes, or for utility-scale electricity generation.

Applications of photovoltaic systems

The smallest photovoltaic systems power calculators and wristwatches. Larger systems can provide electricity to pump water, to power communications equipment, to supply electricity for a single home or business, or to form large arrays that supply electricity to thousands of electricity consumers.

Some advantages of PV systems are

  • PV systems can supply electricity in locations where electricity distribution systems (power lines) do not exist, and they can also supply electricity to an electric power grid.
  • PV arrays can be installed quickly and can be any size.
  • The environmental effects of PV systems located on buildings is minimal.

History of photovoltaics

The first practical PV cell was developed in 1954 by Bell Telephone researchers. Beginning in the late 1950s, PV cells were used to power U.S. space satellites. By the late 1970s, PV panels were providing electricity in remote, or off-grid, locations that did not have electric power lines. Since 2004, most of the PV panels installed in the United States have been in grid-connected systems on homes, buildings, and central-station power facilities. Technical advances, lower costs for PV systems, and financial incentives and government policies have helped to greatly expand PV use since the mid-1990s. Hundreds of thousands of grid-connected PV systems are now installed in the United States.

EIA estimates that electricity generated at utility-scale PV power plants increased from 6 million kilowatthours (kWh) in 2004 to 143 billion kWh in 2022. Utility-scale power plants have at least 1,000 kilowatts (or one megawatt) of electricity generating capacity. EIA estimates that 59 billion kWh were generated by small-scale grid-connected PV systems in 2022, up from 11 billion kWh in 2014. Small-scale PV systems are systems that have less than 1,000 kilowatts of electricity generation capacity. Most small-scale PV systems are located on buildings and are sometimes called rooftop PV systems.

Solar thermal power plants

Solar thermal power systems use concentrated solar energy

Solar thermal power (electricity) generation systems collect and concentrate sunlight to produce the high temperature heat needed to generate electricity. All solar thermal power systems have solar energy collectors with two main components: reflectors (mirrors) that capture and focus sunlight onto a receiver. In most types of systems, a heat-transfer fluid is heated and circulated in the receiver and used to produce steam. The steam is converted into mechanical energy in a turbine, which powers a generator to produce electricity. Solar thermal power systems have tracking systems that keep sunlight focused onto the receiver during the day as the sun changes position in the sky.

Solar thermal power systems may also have a thermal energy storage system component that allows the solar collector system to heat an energy storage system during the day, and the heat from the storage system is used to produce electricity in the evening or during cloudy weather. Solar thermal power plants may also be hybrid systems that use other fuels (usually natural gas) to supplement energy from the sun during periods of low solar radiation.

Types of concentrating solar thermal power plants

There are three main types of concentrating solar thermal power systems:

Linear concentrating systems

Linear concentrating systems collect the sun's energy using long, rectangular, curved (U-shaped) mirrors. The mirrors focus sunlight onto receivers (tubes) that run the length of the mirrors. The concentrated sunlight heats a fluid flowing through the tubes. The fluid is sent to a heat exchanger to boil water in a conventional steam-turbine generator to produce electricity. There are two major types of linear concentrator systems: parabolic trough systems, where receiver tubes are positioned along the focal line of each parabolic mirror, and linear Fresnel reflector systems, where one receiver tube is above several mirrors to allow the mirrors greater mobility in tracking the sun.

A linear concentrating collector power plant has a large number, or field, of collectors in parallel rows with a north-south orientation so that the mirrors track the sun from east to west and concentrate sunlight continuously onto the receiver tubes during the day.

Parabolic troughs

Parabolic trough power plant
Picture of a parabolic trough power plant.

Source: Stock photography (copyrighted)

A parabolic trough collector has a long parabolic (curved-shaped) reflector that focuses the sun's rays on a receiver pipe located at the focus of the parabola. The collector moves to focus sunlight on the receiver as the sun moves through the sky during the day.

Because of its parabolic shape, a trough can focus the sunlight from 30 times to 100 times its normal intensity (concentration ratio) on the receiver pipe, located along the focal line of the trough, achieving operating temperatures higher than 750°F.

Parabolic trough linear concentrating systems are used in one of the longest operating solar thermal power facilities in the world, the Solar Energy Generating System (SEGS) located in the Mojave Desert in California. The facility has had nine separate plants over time, with the first plant in the system, SEGS I, operating from 1984 to 2015, and the second, SEGS II, operating from 1985 to 2015. SEGS III–VII (3–7), each with net summer electric generation capacities of 36 megawatts (MW), came online in 1986, 1987, and 1988. SEGS VIII (8) and IX (9), each with a net summer electric generation capacity of 88 MW, began operation in 1989 and 1990, respectively. SEGS 3, 4, 5, 6, 7, and 8 all ceased operation in 2021, leaving only SEGS 9 in operation as of December 31, 2021.

In addition to the SEGS, many other parabolic trough solar power projects operate in the United States and around the world. The four largest projects in the United States after SEGS are:

  • Solana Generating Station: a 296 MW project in Gila Bend, Arizona
  • Mojave Solar Project: a 275 MW project in Barstow, California
  • Genesis Solar Energy Project: a 250 MW project in Blythe, California
  • Nevada Solar One: a 69 MW plant near Boulder City, Nevada

Linear Fresnel reflectors

Linear Fresnel reflector (LFR) systems are similar to parabolic trough systems in that mirrors (reflectors) concentrate sunlight onto a receiver located above the mirrors. These reflectors use the Fresnel lens effect, which allows for a concentrating mirror with a large aperture and short focal length. These systems are capable of concentrating the sun's energy to about 30 times its normal intensity. Compact linear Fresnel reflector (CLFR)—also referred to as a concentrating linear Fresnel reflector—are a type of LFR technology with multiple absorbers within the vicinity of the mirrors. Multiple receivers allow the mirrors to change their position so they do not shade other reflectors. This improves system efficiency and reduces costs. A demonstration CLFR solar power plant was built near Bakersfield, California, in 2008, but it is currently not operating.

Solar power towers

A solar power tower system uses a large field of flat, sun-tracking mirrors called heliostats to reflect and focus sunlight onto a receiver on the top of a tower. Sunlight can be concentrated as much as 1,500 times. Some power towers use water as the heat-transfer fluid. Advanced designs are experimenting with molten nitrate salt because of its superior heat transfer and energy storage capabilities. The thermal energy-storage allows the system to produce electricity during cloudy weather or at night.

The U.S. Department of Energy and several electric utilities built and operated the worlds's first demonstration solar power tower near Barstow, California, during the 1980s and 1990s. Two solar power tower projects now operate in the United States:

  • Ivanpah Solar Power Facility: a facility with three separate collector fields and towers with a combined net summer electric generation capacity of 393 MW in Ivanpah Dry Lake, California
  • Crescent Dunes Solar Energy Project: Crescent Dunes Solar Energy Project: a 110 MW one-tower facility with an energy storage component in Tonapah, Nevada

Solar dish/engines

Solar dish/engines
Image of a solar dish collector.

Source: Stock photography (copyrighted)

Solar dish/engine systems use a mirrored dish similar to a very large satellite dish. To reduce costs, the mirrored dish usually has many small flat mirrors formed into a dish shape. The dish-shaped surface directs and focuses sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to an engine generator. The most common type of heat engine used in dish/engine systems is the Stirling engine. This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power runs a generator or alternator to produce electricity.

Solar dish/engine systems always point straight at the sun and concentrate solar energy at the focal point of the dish. A solar dish's concentration ratio is much higher than linear concentrating systems, and it has a working fluid temperature higher than 1,380°F. The power-generating equipment of a solar dish can be placed at the focal point of the dish, making it well suited for remote locations, or the energy may be collected from many dishes and converted into electricity at a central point.

There are no utility-scale solar dish/engine projects in commercial operation in the United States.

Solar thermal collectors

Heating with the sun's energy

People use solar thermal energy to heat water and air. The two general types of solar heating systems are passive systems and active systems.

Passive solar space heating happens when the sun shines through the windows of a building and warms the interior. Building designs that use passive solar heating usually have south-facing windows that allow the sun to shine on solar heat-absorbing walls or floors during the day in the winter. The absorbed solar energy heats the building by natural radiation and convection at night. Window overhangs or shades block the sun from entering the windows during the summer to keep the building cool.

Active solar heating systems use a collector and a fluid that absorbs solar radiation. Fans or pumps circulate air or heat-absorbing liquids through collectors and then transfer the heated fluid directly to a room or to a heat storage system. Active solar water heating systems usually have a tank for storing solar heated water.

Solar collectors are either nonconcentrating or concentrating

Nonconcentrating collectors—The collector area (the area that intercepts the solar radiation) is the same as the absorber area (the area absorbing the radiation). Solar systems for heating water or air usually have nonconcentrating collectors. Flat-plate collectors are the most common type of nonconcentrating collectors for water and space heating in buildings and are used when temperatures lower than 200°F are sufficient.

Flat-plate solar collectors usually have three main components:

  • A flat metal plate that intercepts and absorbs solar energy
  • A transparent cover that allows solar energy to pass through the cover and reduces heat loss from the absorber
  • A layer of insulation on the back of the absorber to reduce heat loss

Solar water heating collectors have metal tubes attached to the absorber. A heat-transfer fluid is pumped through the absorber tubes to remove heat from the absorber and transfer the heat to water in a storage tank. Solar systems for heating swimming pool water in warm climates usually do not have covers or insulation for the absorber, and pool water is circulated from the pool through the collectors and back to the pool.

Solar air heating systems use fans to move air through flat-plate collectors and into the interior of buildings.

Concentrating collectors—The area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. The collector focuses, or concentrates, solar energy onto an absorber. The collector usually moves so that it keeps sunlight focused on the absorber. Solar thermal power plants use concentrating solar collector systems because they can produce high temperature heat.

Solar energy & the environment

An array of solar photovoltaic panels supplies electricity for use at Marine Corps Air Ground Combat Center in Twentynine Palms, California
An array of solar panels supplies energy for necessities at Marine Corps Air Ground Combat Center in Twentynine Palms, Calif.

Source: U.S. Marine Corps photo by Pfc. Jeremiah Handeland/Released (public domain)

Solar energy systems/power plants do not produce air pollution, water pollution, or greenhouse gases. Using solar energy can have a positive, indirect effect on the environment when solar energy replaces or reduces the use of other energy sources that have larger effects on the environment.

Some toxic materials and chemicals are used to make the photovoltaic (PV) cells that convert sunlight into electricity. Some solar thermal systems use potentially hazardous fluids to transfer heat. Leaks of these materials could be harmful to the environment. U.S. environmental laws regulate the use and disposal of these types of materials.

As with any type of power plant, large solar power plants can affect the environment near their locations. Clearing land for construction and the placement of the power plant may have long-term effects on the habitats of native plants and animals. Some solar power plants may require water for cleaning solar collectors and concentrators or for cooling turbine generators. Using large volumes of ground water or surface water in some arid locations may affect the ecosystems that depend on these water resources. In addition, the beam of concentrated sunlight a solar power tower creates can kill birds and insects that fly into the beam.