The emergence of solar power generation is part of the overall movement toward renewable energy production. Interest in this type of energy production grew in the early 1970s with an increased public awareness of the negative impact of technological developments on the environment. The use of solar power, of course, was not new. Heat produced by the sun was used for all sorts of purposes from the early history of humankind. In the search for renewable energy sources, the direct use of the sun’s heat has continued in the use of solar panels. In these panels, heat from the sun is absorbed by water flowing in pipes, and the hot water can then be used for heating purposes. In the twentieth century, two types of thermal solar energy systems developed: (1) active systems that used pumps or fans to transport the heat; and (2) passive systems that use natural heat transfer processes. In 1948 a school in Tucson, Arizona, with a passive solar energy system was built by Arthur Brown. In 1976 the Aspen-Pitkin County airport was opened as the first large commercial building in the U.S. that used a passive solar energy system for heating. However, the original idea of using passive solar energy goes back to ancient times. Archeologists have found houses with passive solar energy systems dating back to the fifth century AD (see Buildings, Designs for Energy Conservation).
What was new in the renewable energy trend of the twentieth century was the conversion of solar energy into electricity in order to replace the energy that was produced from fossil, or nonrenewable sources. The device that was developed to realize this conversion is the photovoltaic (PV) cell. This cell is based on the PV effect of a number of semiconducting materials, first discovered in selenium by Willoughby Smith in 1873. In 1877 William G. Adams and Richard E. Day discovered this effect in a selenium–platinum junction, and went on to build the first selenium solar cell. The effect was subsequently seen in a variety of other semiconducting materials such as germanium and silicon. In 1954 Bell Laboratories researchers demonstrated their first solar cells, primarily for space applications. The following year Western Electric sold the first licenses for producing silicon PV cells. Commercial production of PV cells started in the same year by Hoffman Electronics Semiconductor Division. In 1958 the satellite Vanguard I was the first to be powered by PV solar cells.
Silicon is still often used for producing PV cells. The most important types of silicon cells are monocrystalline (based on single crystals), polycrystalline (based on numerous grains of monocrystals), and amorphous silicon (no crystals but thin homogeneous layers). The monocrystalline cells have the highest efficiency, but the polycrystalline cells are cheaper. Amorphous cells are the cheapest and also the thinnest type, which has advantages when the cell is to be integrated into a device. Apart from silicon, cadmium telluride and copper indium diselenide are used for making thin film cells. For thick films, gallium arsenide is an alternative material for silicon. Production processes are different for different types. Crystalline cells are produced in wafers, and amorphous cells are made by depositing the silicon on a substrate (a steel or a glass sheet covered with a layer of tin oxide).
The earliest PV cells had efficiencies of just a few percent. For example, Hoffman Electronics first cells in commercial production had an efficiency of only 2 percent, and by 1957 this had increased to 8 percent. In the course of the second half of the twentieth century, considerable research and development were done to improve the efficiency of the PV cells and to reduce the price of PV electricity. This was not without success. The average efficiency of the monocrystalline and polycrystalline silicon cells increased from 11 percent in 1985 to 16 percent in 1995. The efficiency of the amorphous silicon cells increased from 5 to 10 percent in the same period. The price of all types of silicon cells dropped to less than half the 1985 price in these years. As a result, PV electricity can now be produced for $0.25 to 0.40 per kilowatt-hour (kWh), but this is still five times as much as electricity produced by burning coal and gas. The structure of a PV cell is shown in Figure 12. There are several layers in the cell, and in the middle there are two semiconducting layers, one ntype (negative) and one p-type (positive). When sunlight hits the cell, electrons in the semiconducting layers receive energy that makes them free to move. An electric field in the semiconducting layers forces them to move; and when a load is connected to the cell, an electric current can flow. This current is the PV electricity that is produced by the cell. There are two conducting layers on both sides of the pair of semiconducting layers for connecting the load. To protect the cell, there is a covering glass layer. An antireflection layer prevents  incoming sunlight from being reflected away from the semiconducting layers. The size of such cells varies from 1 to 10 centimeters in diameter. Individual PV cells can only serve as an energy source for low-power applications. Several cells, however, can be connected and used together to form a module, also referred to as a ‘‘solar panel,’’ not to be confused with the water-based solar panels mentioned above. The PV cells produce direct current (DC). As most electric appliances are based on the use of the electricity network with its alternating current (AC), PV systems usually have a converter that transforms DC into AC. The output of an average module is 12 volts, which is converted into the 110 or 230 volts that most electric appliances need. The power that is generated by an average module is around 50 to 80 watts. The average electricity demand of a household is 1.5 to 2 kilowatts, so it is common to have around 20 to 30 modules in a PV system for supplying electricity in a house. This requires an area of around 15 by 15 square meters. The efficiency of a complete PV energy system not only depends on the quality of the cells in the panels but also on the extent to which the changing position of the sun can be taken into account. In a number of applications the position of the panels is fixed; for example, when they are integrated into the roof of a house; in which case the house can be oriented for the optimal use of sunlight. In other cases, when a tracking system is used, the position of the panels can change, as when solar panels are used in a satellite. Tracking systems can have one or more axes in order to capture the optimum amount of sunlight. Concentrators (Fresnel lenses with concentration ratios of 10 to 500 times, or mirrors) for focusing the sunlight allow the panel to use the available sunlight more effectively. Such concentrating systems started to be used in the late 1970s.
Two types of PV cell applications can be distinguished: stand-alone and grid-connected. In stand-alone applications, the PV system functions independently of the electricity network. This type of application is usually found where connection with the network is problematic because of large distances. As there is no backup energy source in this case, a battery has to be part of the PV system. Some examples of practical applications are: energy supply for villages in developing countries; energy supply for water pumping systems; lighting of beacons in the sea; and energy supply in satellites and electrically driven boats and cars. In the case of a grid-connected system, there is an exchange of energy between the PV system and the grid. In case there is a lack of energy in the PV system (at night or dark sky days), energy can be retrieved from the electricity network. When there is a surplus of energy in the PV system, however, this surplus can be sold to the energy company by feeding it into the grid. Figure 13 is a schematic drawing of a gridconnected PV panel application in a house.
The total amount of solar energy as a contribution to the total energy production in most countries at the end of the twentieth century is still relatively small, even though it has been calculated that the sales of PV systems have increased from $2 million in 1975 to more than $750 million in 1993. For the year 1996 it was estimated that of the off-grid residential PV systems operational worldwide, about 10,000 were in remote vacation homes in Scandinavia. Among the reasons for the relatively low numbers is that the price of solar cells and the often-needed batteries is still too high to make a PV system economically competitive with nonrenewable (fosil fuel) energy production. A second, but less important, reason is that it is not yet clear if some types of solar cells really have better environmental properties over the whole life-cycle of the systems