solar energy index: solar energy and photovoltaic roofs types of photovoltaic systems

The photovoltaic effect


Fig. 1: Monocrystalline silicon photovoltaic. The silver strips are the ohmic contacts which allow for the connection in series of more cells.

Photovoltaic technology allows to directly convert energy from solar radiation directly into electric energy, with an overall efficiencyGlossary between 16% and 18% for a single monocrystalline photovoltaic cell Glossary.

This technology makes use of the photovoltaic effect which is based on the properties of certain semiconducting materials which can convert solar radiation energy into electric energy without the use of moving mechanical parts and without the use of fuel (Fig. 1).

These devices are made from semiconductorGlossary, materials, such as siliconGlossary (Si), gallium arsenide (GaAs) and copper sulphate (Cu2S). In a photovoltaic cellGlossary, the photonsGlossary of the incidental solar radiation break the ties of the semiconductor's electrons, thus allowing the electrons to move freely in the semiconductor. The positions left free by the electrons act as positive charges and take the name of "holes". The photovoltaic cells generally consists of two thin regions, one above the other, each with specially added impurities called dopants. The result is that one region is of "type n"Glossary, with an excess of electrons (negative), while the other is of "type p", with an excess of positive holes. This 2-region structure, called a p-n junction, produces an internal electric fieldGlossary. When the photons create free electrons and holes in proximity to the p-n junction, the internal electric field makes them move in opposite directions: the electrons move towards the side n and the holes move towards the side p. So a tension (electromotive force, e.m.f.) is generated between the p and n regions, with p positive and n negative. Using wires, the side p and n are connected to a "load", e.g. a light bulb, and an electric current runs through the load.

Fig. 2: Single photovoltaic cells (1) connected in series form a photovoltaic module (2). Several modules assembled together create a photovoltaic system (3)..

Silicon in crystalline form is the material most commonly used to make photovoltaic cells, which typically measure 12cm x 12cm. The cells are assembled together to obtain photovoltaic modulesGlossary with a surface of approximately half a metre squared (Fig. 2).
Other types of cells are those in polycrystalline and amorphous silicon, which are not as efficient, and those with more than two junctions, which are more efficient but also more expensive. At the moment considerable efforts are being made to develop plastic cells with polymers, which should have lower costs but also reduced efficiency.

Photovoltaic systems are made up of many panels connected in series and in parallel; this modularity permits the systems to be highly flexible. A photovoltaic system can be a stand aloneGlossary system or a grid connectedGlossary) system. In both cases it is necessary to convert the continuous electric current supplied by the cells into alternate current through the use of an inverter Glossary .

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