The term ‘photovoltaic’ is derived by combining the Greek word for light, photos, with volt, the name of the unit of electromotive force. The discovery of the photovoltaic effect is generally credited to the French physicist, Edmond Becquerel who in 1939 published a paper describing the experiments with a ‘wet cell’ battery, in the course of which he found that the battery voltage increased when its silver plates were exposed to sunlight.
Photovoltaic cells consist of a junction between two thin layers of dissimilar semi-conducting materials. These two parts are known as positive-type ‘p’ and negative-type ‘n’ semiconductors . These are usually manufactured from silicon although other materials can also be used. n-Typed semiconductors are made of crystalline silicon that has been ‘doped’ with tiny quantities on an impurity (usually phosphorous) in such a way that the doped material possesses a surplus of free electron. On the other hand,
Mirror racks '■
backwards and forwards to track altitude of sun
Fig. 34. Line and point focus by mowing mirrors.
p-type semiconductors are also made from crystalline silicon, but they are doped with very small amounts of a different impurity (usually boron) which causes the material to have a deficit of free electrons. These missing electrons are called holes. By combining these two dissimilar semiconductors, one can produce n-p junction. This sets up an electric field in the region of the junction. Such a set up will cause negatively charged particles to move in one direction, and positively charged particles to move in the opposite direction.
Light is composed of a steam of tiny energy particles called photons. If photons of a suitable wavelength fall within the p-n junction, then they can transfer their energy to some of the electrons in the material so prompting them to a higher energy level. When the p-n junction is formed, some of the electrons in the immediate vicinity of the junction are attracted from the n-side to combine with holes on the nearby p-side. Similarly, holes on the p-side near the junction are attracted to combine with electrons on the nearby n-sides. Hence, the net effect of this is to set up around the junction a layer on the n-side that is more positively charged than it would otherwise be. In effect, this means that a reverse electric field set up around junction such that negative on the p-side and positive on the n-side. Since, the region around the junction is also depleted of charge carriers (electrons and holes), it is therefore, called as the depletion region.
Photovoltaic are much more effective in hazy or partly cloudy conditions and they can be installed even on small residential rooftops.
In recent years, power generation from renewable resources has been counted upon to bridge the gap between global demand and supply of power. The direct conversion technology based on solar photovoltaic has several positive attributes and seems to be most promising. Extensive research activities over the past 25 years have led to significant cost reduction and efficiency amelioration [27,71]. Relatively less attention seems to have been paid to factors related to system reliability [2,17,18,74,77,105]. In this context, stand-alone solar photovoltaic systems consisting of photovoltaic array, electrical interface and the energy storage facilities have often been of interest to researchers. The reliability has not been studies in detail as an important statistical parameter to assess the operational lifetime of the photovoltaic array .
Generation of electricity from sunlight has been a dream of scientists, planners and energy experts since 1950s, when practically the first photovoltaic cell was invented. It is a device that converts the solar radiation directly into electric current via complex photoelectric process. Photovoltaic technology has advanced during the last five decades, making it possible to convert a larger share of sunlight into electricity which has reached as much as 14% in the most advanced prototype systems. They are now widely used, for example, to power electronic calculators, remote telecommunication equipments, electric lights and water pumps.
The 50 MW worth of cells produced in 1990 are only sufficient to power about 15,000 European and Japanese homes . Although the cost of photovoltaic has fallen drastically during the last decades, but still it is four to six times the cost of power generation from fossil fuels. So further reductions are needed for solar power to be competitive with grid electricity. However, photovoltaics are already the most economical way of delivering power to homes far from utility lines. It is expected that this technology will become an economical way of providing supplementary utility power in rural areas, where the distance from plants tends to cause a voltage reduction that is otherwise costly to remedy. As they become more versatile and compact, photovoltaic panels could be used as roofing material on individual homes, bringing about the ultimate decentralization of power generation. In fact, the desert areas are the most attractive and rich regions of the world for the solar irradiation conversion into electric power. Large solar power plants could appear in the world’s deserts, providing centralized power in the same way to today’s coal and nuclear plants.