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.
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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 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 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 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 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 (AC) in electricity transmission and distribution systems. 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. Most PV systems have modules in a fixed position with the modules facing directly south (in the northern hemisphere—directly north in the southern hemisphere) and at an angle that optimizes the physical and economic performance of the system.
Applications of photovoltaic systems
The smallest photovoltaic systems power 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 (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 impact of PV systems 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. Then, they were widely used for small consumer electronics like calculators and watches. 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. Technological advances, lower costs for PV systems, and various 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.
The U.S. Energy Information Administration (EIA) estimates that nearly 50 billion kilowatthours (kWh) of electricity were generated at utility-scale PV power plants in 2017. Utility-scale power plants have at least 1,000 kilowatts (or one megawatt) of electricity generating capacity. EIA estimates that 24 billion kWh were generated by small-scale grid-connected PV systems in 2017.