Photovoltaics is the direct generation of electricity from light, where the system exploits not only direct solar irradiation but also diffuses light on overcast days. A solar power plant consists of solar modules that generate direct current from solar energy, the inverter that converts this direct current into alternating current, the counters that measure the electricity fed into the grid, the wiring and the assembly system.
In order to exploit the plant´s efficiency to the full, all components have to be perfectly synchronized and optimally adapted to the structural conditions of the respective building. The price of a solar power plant is determined by the installed output in kWp.
Nowadays solar power plants make use of the photovoltaic (=photoelectric) effect. This involves light absorbed in a "semi-conducting material" generating charge carriers that are discharged via electrodes affixed to the front and back of the absorber material whereupon it is supplied to the consumer.
At present, the dominant technology involves crystalline silicon. The high-purity silicon that is produced in blocks is cut into thin wafers. In further processing steps, the properties of these wafers are adjusted to the photovoltaic requirements by means of intentional contamination (doping). In order to increase light absorption, the front of the wafers are given an antireflection coating. As the tapable currents generated by an individual wafer are too low for technical application, the wafers are connected to one another in series. In order to protect the wafers against mechanical and environmental influences, these wafer systems are then sandwiched between a front disk and a rear film to make a module.
The technique has been tried and tested and developed over decades and with a market share of approx. 90 percent makes up the lion´s share of all photovoltaic products.
With the newer thin-film technology, the functional layers are for the most part applied to a glass substrate by means of a vacuum method rather than using raw silicon wafers. This involves processes similar to those used in coating architectural glass or in the manufacture of flat screens. The basic design of front contact, absorber, and rear contact is the same as that used in the crystalline technology, but the material and energy requirements are considerably lower. Whereas in the crystalline method, wafers measuring approx 250 µm are used, the absorbers in the thin-film technology are only a few micrometers thick.
Common to both techniques is the fact that the individual cells are turned into technically usable systems by connecting them in series. While this is done by soldering the individual wafers to one another in the case of crystalline technology, this process is directly integrated into the production process with the thin-film modules. By separating the individual layers in strips between the individual coating stages, generally by laser, it is possible to concatenate the individual elements. As in the conventional crystalline technology, the modules are then assembled into a complete module by adding a cover or rear glass plate.
Grid-connected solar power plants per se are then made up of a large number of modules that are connected in series in so-called strings. Several strings are then connected in parallel to a so-called inverter that transforms the direct current generated by the PV facility into in-phase alternating current and then feeds it into the electricity grid of the power supplier.
