How it works

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Last updated: 23 November 2005

A photovoltaic (PV) system consists of one or more photovoltaic (PV) modules, the basic building block of PV-systems. One PV-module consists of about 40 photovoltaic solar cells; the cells convert the light into electricity. The PV modules are connected either in series or in parallel and they are called the PV field. Because PV-fields are built with PV modules of 50 or 100 watts each, photovoltaic systems are exceptionally modular, which provides for easy transportation and rapid installation, and enables easy expansion if power requirements increase.

To be able to use the generated electricity, more components need to be added to the system. PV systems for stand-alone applications (also called off-grid applications) may comprise also a control, storage (e.g. battery), cables and a load (e.g. lights, radio, television). PV systems for grid-connected applications need an inverter to convert the direct current (d.c.), generated by the PV-modules, into alternating current (a.c.).

Photovoltaic cell converts light into electricity

The photovoltaic cell is the component responsible for converting light to electricity. When sunlight strikes a photovoltaic cell, part of the light particles (photons), which contain energy, is absorbed by the cell. By the absorption of a photon a (negative) electron is knocked loose from a silicon atom, and a positive "hole" remains. The freed electron and the positive hole together are neutral. Therefore, in order to be able to generate electricity, the electron and the hole need to be separated from eachother. Therefore a photovoltaics cell has an artificial junction layer, also called the p/n-layer. Now, the freed electronics cannot return to the positive charged holes. When the electric contacts on the front and rear are being connected through an external circuit, the freed electrons can only return to the positively charged holes by flowing through this external circuit, thus generating current.

As shown in the image, a photovoltaic cell has more layers. The upper layer of multicrystalline silicon cells looks blue, due to a anti-reflection coating, which is used to optimize the absorption of light by the cell. Both the upper layer and rear layer have contacts, to enable the freed electrons to flow through the external circuit from the negative layer to the positive layer. The silicium part, the actual photovoltaic cell, also has a negative (N-layer) and positive (P-layer), which both are chemically polluted: the upper layer consists of material of which the atoms have one electron extra than silicum, whereas the layer on the rear side is made of material of which the atoms have one electron less than silicium.

The photovoltaic effect will continue as long as light strikes the photovoltaic cell: every time the photons will create new electron/hole-pairs. This implies that no materials are being wasted; it is a renewable process.

The electrical power that can be extracted from a photovoltaic cell is proportional to its area and to the intensity of the sunlight that hits the area, and is measured in watt (W).

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