Introduction to Solar Photovoltaics

Solar energy is usually divided into two categories, although they can be employed together in solar installations.

Solar thermal energy is generated from heat and employs heat directly to heat water, the ambient temperature in buildings, or steam to power electricity generators.

Solar photovoltaic electricity is generated from light, employing photovoltaic modules or cells, which convert sunlight into electricity using cells with semi conductors.

Solar thermal technologies use the sun’s heat. They include non-grid solar thermal technologies; water heating systems, solar cookers, solar drying applications and solar thermal building designs. These technologies help to conserve energy in heating and cooling applications. Solar thermal devices use direct heat from the sun, concentrating it to produce heat at useful temperatures. As with many other advances in the energy sector, modern solar thermal industry began with the oil embargo of 1973-1974 and was strengthened with the second embargo in 1979. In the early 1980’s, a 354 MW solar power plant was built in the Mojave Desert, in California. The heat contained in solar rays, concentrated by reflecting troughs and raised to 400^oC, produces steam that runs a conventional power generator. When the sun is not shining, the plant switches to natural gas. The latest generation of this type of plant incorporates new engineering solutions and new scientific principles such as non-imaging optics, which makes it possible to build much more efficient concentrators at lower costs. Solar thermal technology has many applications both for grid-connected power generation, in isolated locations where grid connected electricity is not viable, and in domestic and commercial situations.

Solar photovoltaics or solar PV are solid-state semiconductor devices that convert light directly into electricity. Solar PV’s are mostly made of silicon with traces of other elements and are closely related to transistors, LEDs and other electronic devices. The electricity is direct current but can be converted to alternating current or stored for later use.

Bell Telephone researchers discovered the PV cell in 1954 when examining the sensitivity of a properly prepared silicon wafer to sunlight. From the late 1950s, PVs were used to power US space satellites, which generated commercial applications for PV technology. The simplest PV systems power many of the small calculators and watches in everyday use. More complex systems provide electricity in off-grid applications and generate electricity for the grids.

Advanced technology is required to manufacture PV cells and modules, but the cells themselves are simple to use. PV modules are usually low-voltage DC devices with no moving or wearing parts, although arrays of PV modules can be wired for higher voltages. Once installed, a PV array does not require much maintenance except for an occasional cleaning, and even that is not imperative. Most PV systems contain storage batteries, which require some water and maintenance similar to that required by the battery in a car.

A solar cell consists of layers of semiconductor materials with different electronic properties. In a typical solar crystalline silicon cell, most of the material is silicon. The silicon is doped with a small quantity of boron to give it a positive or p-type character. A thin layer on the front of the cell is doped with phosphorous to give it a negative or n-type character. The interface between these two layers contains an electric field and is called a junction. Light consists of particles called photons and when the light hits the solar cell, some of the photons are absorbed in the region of the junction, freeing electrons in the silicon crystal. If the photons have enough energy, the electrons are able to overcome the electric field at the junction and are free to move through the silicon and into an external circuit. As they flow through the external circuit they give up their energy as useful work (turning motors, lighting lamps, etc.) and return to the solar cell. The photovoltaic process is entirely solid-state and self-contained; there are no moving parts and no materials are consumed or emitted.