Photovoltaics for beginners
Photovoltaics and solar thermal energy. These are catchwords which we read almost daily in the newspapers. Festo supports the manufacturing processes which are part of these technologies in a number of different ways. But what are the actual basic principles of these technologies?
Solar power can basically be subdivided into two important technologies for energy production: solar thermal energy and photovoltaics.
Solar thermal energy
Solar thermal energy is the conversion of the sun's energy into useable thermal energy. For domestic use, a heat transfer medium (e.g. a water-propylene glycol mixture) is generally heated by flat or vacuum solar collectors and used for heating water or for supporting heating systems.
Commercial solar thermal power stations, such as the Andasol power stations in Andalusia, concentrate the sun's rays onto receiver tubes through parabolic mirrors. These receivers convert the solar energy into heat, and steam is generated via heat exchangers which drive turbines that generate electricity.
In contrast to solar thermal energy, photovoltaics produce electricity directly in a solar cell. This occurs without the intermediate step of generating steam and without using chemical and mechanical processes. The function of a solar cell is based on the fact that sunlight generates electric charges in photoactive semiconductors such as silicon. It is this photoelectric effect that is made use of. Massless light particles, so-called photons, have enough energy to push individual electrons out of their orbits around the atoms in semiconductor materials. Negatively charged "free" electrons are produced. A positively charged "electron hole" remains. By doping the semiconductor, for example by using phosphorus, the charge carriers are separated, so that a voltage difference of approx. ±0.5 V can be tapped from the electrical connections of the solar cell.
This can be used by many semiconductor materials. Quartz sand (silicon) is the second most common element on earth after oxygen.
Crystalline or thin-film technology for solar cells?
We primarily distinguish between two types of solar cells. Crystalline cells made of silicon (c-Si) are square plates with a thickness of approx. 0.2 mm, an edge length of typically 156 mm and a weight of approx. 13 g. The degree of efficiency of these cells can be up to 19%. A c-Si solar module generally consists of 60 or 72 cells switched in series. This results in a voltage of up to 36 V and approx 200 W power across an area of 1 x 0.6 metres.
With thin-film technology, a coating of semiconducting material of approx. 5 μm is placed on solar glass with a thickness of a few mm. The coating is either chemically sprayed on or vapour-deposited under high vacuum. Thin layer cells are cheaper and are also able to process diffuse light, meaning without direct sunshine.
However, they only achieve a lower degree of efficiency (up to 11%) than crystalline material.
How does Festo support the photovoltaics industry?
The Festo team for the solar industry sector has thoroughly analysed the production processes in the manufacture of c-Si and thin-film solar modules. Together with our customers, industry-specific products and solutions are designed to provide significant advantages and create differentiation.