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.
Source: National Energy Education Development Project (public domain)
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.