Photovoltaic Technologies

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A French physicist, Alexandre Edmond Becquerel, was the first to record his observation of the photovoltaic effect (photo denotes light and voltaic denotes the generation of electricity) in the 19th century. Since then, many scientists have worked to develop energy technologies based on this effect. It is a process in which electricity is generated in the boundary layers of certain semiconductor materials when they are illuminated. Today's photovoltaic semiconductor materials include silicon, gallium arsenide, copper indium dise-lenide, cadmium sulfide, and cadmium telluride.

Photovoltaic materials are classified as either crystalline, polycrystalline, or thin-film in form. These classifications represent the three major PV technologies. These are the building blocks for today's commercial PV products, which include consumer electronics (such as a solar-powered calculator or watch), remote electric power systems, utility-connected power systems, and building-integrated systems.

One of these PV materials, silicon, is highly abundant; it constitutes more than 25% of the Earth's crust. Silicon is used in more than 90% of all PV applications, including building-integrated photovoltaics or BIPV. Silicon solar technologies can be grouped in these three basic areas: single-crystal silicon, polycrystalline silicon, and thin-film amorphous silicon. The primary distinctions among the three technologies are their sunlight-to-electricity conversion efficiency rates, the methods by which they are manufactured, and their associated manufacturing costs.

The efficiency of each BIPV product is specified by the manufacturer. Efficiencies range from as low as 5% to as high as 15%-16%. A technology's conversion efficiency rate determines the amount of electricity that a commercial PV product can produce. For example, although thin-film amorphous silicon PV modules require less semiconductor material and can be less expensive to manufacture than crystalline silicon modules, they also have lower conversion efficiency rates. Until these conversion efficiencies increase,

The Photovoltaic Effect

Sunlight is composed of photons—discrete units of light energy. When photons strike a PV cell, some are absorbed by the semiconductor material and the energy is transferred to electrons. With their new-found energy, the electrons can escape from their associated atoms and flow as current in an electrical circuit.

PV arrays require no care other than occasional cleaning of the surfaces if they become soiled or are used in dusty locations. However, they must be kept clear of snow, weeds, and other sources of shading to operate properly. PV cells are connected in series, so shading even one cell in a module will appreciably decrease the output of the entire module.

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PV Cells

PV cells are the basic building blocks of PV modules. They are made of semiconducting materials, typically silicon, doped with special additives. Approximately 1/2 volt is generated by each silicon PV cell. The amount of current produced is directly proportional to the cell's size, conversion efficiency, and the intensity of light. As shown in the figure below, groups of 36 series-connected PV cells are packaged together into standard modules that provide a nominal 12 volt (0r 18 volts @ peak power). PV modules were originally configured in this manner to charge 12-volt batteries. Desired power, voltage, and current can be obtained by connecting individual PV modules in series and parallel combinations in much the same way as batteries. When modules are fixed together in a single mount they are called a panel and when two or more panels are used together, they are called an array. (Single panels are also called arrays.)

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