Energy from the sun
The sun has produced energy for billions of years. The energy in the sun's rays that reaches the earth (solar radiation) can be converted into heat and electricity.
In the 1830s, the British astronomer John Herschel famously used a solar thermal collector box (a device that absorbs sunlight to collect heat) to cook food during an expedition to Africa. Today, people use the sun's energy for a variety of purposes.
Solar energy can be used for heat and electricity
When converted to thermal energy, solar energy can be used to heat water for use in homes, buildings, or swimming pools; to heat spaces inside homes, greenhouses, and other buildings; and to heat fluids to high temperatures to operate turbines that generate electricity.
Solar energy can be converted into electricity in two ways:
- Photovoltaic (PV devices) or solar cells change sunlight directly into electricity. Individual PV cells are grouped into panels and arrays of panels that can be used in a variety of applications ranging from single small cells that charge calculator and watch batteries, to systems that power single homes, to large power plants covering many acres.
- Solar thermal/electric power plants generate electricity by concentrating solar energy to heat a fluid and produce steam that is then used to power a generator.
There are two main benefits of solar energy:
- Solar energy systems do not produce air pollutants or carbon dioxide.
- When located on buildings, solar energy systems have minimal impact on the environment.
There are two main limitations of solar energy:
- 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, time of year, and weather conditions.
- Because the sun doesn't deliver that much energy to any one place at any one time, a large surface area is required to collect the energy at a useful rate.
Where Solar is Found
Solar energy is sunshine
The amount of solar energy that the earth receives each day is many times greater than the total amount of energy consumed around the world. However, on the surface of the earth, solar energy is a variable and intermittent energy source. The amount of sunlight and the intensity of sunlight varies by location. Weather and climate conditions affect the availability of sunlight on a daily and seasonal basis. The type and size of a solar energy collection and conversion system determines how much of available solar energy can be converted into useful energy.
Solar thermal collectors
Low-temperature solar thermal collectors absorb the sun's heat energy to heat water or to heat air for heating in homes, offices, and other buildings.
Concentrating solar energy technologies use mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat. This thermal energy can then be used to produce heat or electricity with a steam turbine or a heat engine driving a generator.
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 used by hundreds of homes.
Hundreds of thousands of houses and buildings around the world have PV systems on their roofs. Many multi-megawatt PV power plants have also been built. Covering 4% of the world's desert areas with photovoltaics could supply the equivalent of all of the world's electricity. The Gobi Desert alone could supply almost all of the world's total electricity demand.
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
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 a semiconductor material. When photons strike a PV cell, they may be reflected, pass right through, or be absorbed by the semiconductor material. Only the absorbed photons provide energy to generate electricity. When enough sunlight (solar energy) is absorbed by the material, 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 the electrons naturally migrate to the surface of the cell.
The flow of electricity
When the electrons leave their position, holes are formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of electrical charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. Electrical conductors are placed on the cell to absorb the electrons. When the conductors are connected in an electrical circuit to an external load, such as an appliance, 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 semiconductor material and PV cell technology. The efficiency of most commercially available PV modules ranges from 5% to 15%. Researchers around the world are trying to achieve higher efficiencies.
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.
PV cells are electrically connected together in a packaged, weather-tight PV module or panel. PV modules vary in size and vary in the amount of electricity they can produce. PV module electricity generation 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 several thousand PV modules. The number of PV modules connected together in a PV array determines the total amount of electricity that 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 in electricity transmission and distribution systems. Devices called inverters are used on PV modules or in arrays to convert the DC electricity to alternating current (AC) electricity.
PV cells and modules will produce the largest amount of electricity when they are directly facing the sun. Tracking systems can be used to move PV modules to constantly face the sun, but these systems are expensive. Most PV systems have modules in fixed positions with the modules facing directly south and at an angle that optimizes the physical and economic performance of the system at the location where it is installed.
Applications of photovoltaic systems
The simplest photovoltaic systems are solar-powered calculators and wrist watches. 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 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 impact of PV systems is minimal.
History of photovoltaic
The first practical PV cell was developed in 1954 by Bell Telephone researchers who were examining the sensitivity of a properly prepared silicon wafer to sunlight. Beginning in the late 1950s, PV cells were used to power U.S. space satellites. PV cells were next widely used for small consumer electronics like calculators and watches and were used to provide electricity in remote or off-grid locations where there were no 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. Technology advances, lower costs for PV systems, and various financial incentives and government policies helped to greatly expand PV use since the mid-1990s. There are now hundreds of thousands of grid-connected PV systems installed in the United States.
Solar Thermal Power Plants
Solar thermal power systems use concentrated solar energy
Solar thermal power 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 throughout 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, which include parabolic troughs and linear Fresnel reflectors
- Solar power towers
- Solar dish/engine 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 positioned 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 that are typically aligned in a north-south orientation to maximize solar energy collection. This configuration enables the mirrors to track the sun from east to west during the day and concentrate sunlight continuously onto the receiver tubes.
A parabolic trough collector has a long parabolic-shaped reflector that focuses the sun's rays on a receiver pipe located at the focus of the parabola. The collector tilts with the sun to keep sunlight focused on the receiver as the sun moves from east to west 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 the longest operating solar thermal power facility in the world, the Solar Energy Generating System (SEGS), which has nine separate plants and is located in the Mojave Desert in California. The first plant, SEGS 1, has operated since 1984, and the last SEGS plant that was built, SEGS IX, began operation in 1990. With a combined electricity generation capacity of 354 megawatts (MW), the SEGS facility is one of the largest solar thermal electric power plants in the world.
In addition to the SEGS, many other parabolic trough solar power projects operate in the United States and around the world. The three largest projects in the United States after SEGS are
- Mojave Solar Project: a 280 MW project in Barstow, California
- Solana Generating Station: a 280 MW project in Gila Bend, Arizona
- Genesis Solar Energy Project: a 250 MW project in Blythe, California
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 approximately 30 times its normal intensity. The only operating linear Fresnel reflector system in the United States is a compact linear Fresnel reflector (CLFR)—also referred to as a concentrating linear Fresnel reflector—a type of LFR technology that has multiple absorbers within the vicinity of the mirrors. Multiple receivers allow the mirrors to change their inclination to minimize how much they block adjacent reflectors' access to sunlight. This positioning improves system efficiency and reduces material requirements and costs.
Solar power towers
A solar power tower system uses a large field of flat, sun-tracking mirrors called heliostats to reflect and concentrate 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 capability allows the system to produce electricity during cloudy weather or at night.
The U.S. Department of Energy, along with several electric utilities, built and operated the first demonstration solar power tower near Barstow, California, during the 1980s and 1990s. Three solar power tower projects now operate in the United States:Learn more about the history of solar power in the Solar Timeline.
- Ivanpah Solar Power Facility: a 392 MW project located in Ivanpah Dry Lake, California
- Crescent Dunes Solar Energy Project: a 110 MW project located in Nevada
- Sierra Sun Tower: a 5 MW two-tower project located in the Mojave Desert in southern California
Solar dish/engine systems use a mirrored dish similar to a very large satellite dish. To reduce costs, the mirrored dish is usually composed of many smaller flat mirrors formed into a dish shape. The dish-shaped surface directs and concentrates 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 the 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 used with a solar dish can be mounted at the focal point of the dish, making it well suited for remote locations, or the energy may be collected from a number of installations and converted into electricity at a central point.
The U.S. Army is developing a 1.5 MW system at the Tooele Army Depot in Utah with 429 Stirling engine solar dishes. The system is scheduled to be fully operational in 2017.
Solar Thermal Collectors
Heating with the sun's energy
Solar thermal energy can be used to heat water or air. It is most often used for heating water in buildings and in swimming pools. Solar thermal energy is also used to heat the insides of buildings. Solar heating systems can be classified as passive or active.
Passive solar space heating happens in a car on a hot summer day. The sun's rays pass through the windows and heat up the inside of the car. In passive solar heated buildings, air is circulated past a solar heat-absorbing surface and through the building by natural convection. No mechanical equipment is used for passive solar heating.
Active solar heating systems use a collector and a fluid to collect and absorb 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 water heating systems usually include a tank for storing water heated by the system.
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). Flat-plate collectors are the most common type of nonconcentrating collectors and are used when temperatures lower than 200°F are sufficient. Nonconcentrating collectors are often used for heating water or air for space heating in buildings and in swimming pools.
There are many flat-plate collector designs, but they generally have four specific components:
- A flat-plate absorber that intercepts and absorbs the solar energy
- A transparent cover that allows solar energy to pass through but reduces heat loss from the absorber
- A heat-transport fluid (air or liquid) flowing through tubes to remove heat from the absorber
- A layer of insulation on the back of the absorber
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 maintains a high degree of concentration on the absorber.
Solar Energy & the Environment
Solar energy does not produce air or water pollution or greenhouse gases. However, using solar energy may have some indirect negative impacts on the environment. For example, 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. 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 affects on habitat areas for 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 sunlight a solar power tower creates can kill birds and insects that fly into the beam.