ABSTRACT
Only two methods are used to generate significant electric power. The first, discovered by Michael Faraday in 1821 and in commercial production by 1885, requires the relative movement of a magnetic field and a conductor and hence an external engine or turbine. The second is photovoltaic generation using solar cells (more technically called photovoltaic cells). These devices produce electricity directly from electromagnetic radiation, especially light, without any moving parts. The photovoltaic effect was discovered by Becquerel in 1839 but not developed as a power source until 1954 by Chapin, Fuller and Pearson using doped semiconductor silicon. Photovoltaic power has been one of the fastest growing renewable energy technologies: annual production of cells grew tenfold from about 50MW in 1990 to more than 500MW by 2003 (see Figure 7.31), with this growth continuing since. Demand has been driven by the modular character, standalone and grid-linked opportunities, reliability, ease of use, lack of noise and emissions, and reducing cost per unit energy produced. Photovoltaic generation of power is caused by electromagnetic radia-
tion separating positive and negative charge carriers in absorbing material. If an electric field is present, these charges can produce a current for use in an external circuit. Such fields exist permanently at junctions or inhomogeneities in photovoltaic (PV) cells as ‘built-in’ electrostatic fields and provide the electromotive force (EMF) for useful power production. Power generation is obtained from cells matched to radiation with wavelengths from the infrared ∼10m to the ultraviolet ∼03m, however, unless otherwise stated, we consider cells matched to solar short wave radiation ∼05m. The built-in fields of most semiconductor/semiconductor and metal/semiconductor cells produce potential differences of about 0.5V and current densities of about 400Am−2 in clear sky solar radiation of 10kWm−2. Commercial photovoltaic cells, dependent on price, have efficiencies of 10-22% in ordinary sunshine, although laboratory specimens and arrangements reach greater efficiency. The cells are usually linked in series and fixedwithinweather-proofmodules,withmostmodules producing
about 15V. The current from the cell or module is inherently direct current, DC. For a given module in an optimum fixed position, daily output depends on the climate, but can be expected to be about 0.5-1.0 kWh /m2 day−1. Output can be increased using tracking devices and solar concentrators. Junction devices are usually named ‘photovoltaic cells’ although the
solar radiation photons produce the current, since the voltage is already present across the junction. The cell itself provides the source of EMF. It is important to appreciate that photovoltaic devices are electrical current sources driven by a flux of radiation. Efficient power utilization depends not only on efficient generation in the cell, but also on dynamic load matching in the external circuit. In this respect, photovoltaic devices are similar to other renewable sources of power, although the precise methods may vary (e.g. by using DC-to-DC converters as ‘maximum power tracking’ interfaces, Section 7.9.2). On a reasonably sunny site of insolation 20MJ/ m2 day−1, power can
be produced that is significantly cheaper over extended use than that from diesel generators, especially in remote areas where fuel supply and maintenance costs may be large. The eventual aim is to be competitive with some daytime peak grid-electricity prices, which is most likely if the polluting forms of generation are charged for external costs. Sections 7.2-7.5 outline the basic science and technology of photovoltaic
cells. Preliminary analysis will always refer to the silicon p-n junction single crystal solar cell (Figure 7.1) since this is the most common and wellestablished type. Section 7.6 considers how cells are constructed, and how this has been improved over the years to overcome many of the limitations of such cells. Variations, including the development of cells of materials other than Si, are discussed in Section 7.7. Sections 7.8 and 7.9 examine and illustrate the circuits and systems in which photovoltaics are actually used. Readers whose main interest is in applications may wish to read these sections first, along with Section 7.10, which examines economic, social
and environmental aspects of the use and production of photovoltaics. Before 2000, most photovoltaics were in stand-alone systems, progressing from space satellites to lighting, water pumping, refrigeration, telecommunications, solar homes, proprietary goods and mobile or remotely isolated equipment (e.g. small boats, warning lights, parking meters). Grid connected PV power, e.g. incorporated with buildings, has become a major activity for the 21st century.
