2.2 Renewable Energies: Photovoltaic systems

Photovoltaic systems

Solar cells are the basis for generating electrical energy in photovoltaic (PV) systems and are able to convert the radiation energy of the sun directly into electrical energy. The basic principle for this conversion is the so-called photoelectric effect. In the photoelectric effect, the energy of photons, which can be imagined as the smallest units of light, is transferred to electrons. To ensure that this energy transfer can take place and finally lead to the generation of electrical energy, the material of the solar cells is crucial.

Solar cells mainly consist of two thin layers of the element silicon. Silicon is a semiconductor and reacts to the photons of solar radiation by releasing negatively charged electrons from their positively charged counterparts. As now there is an electron missing, these counterparts are afterwards referred to as “holes”. Thus, these electrons now can move freely. By adding some boron to the lower silicon layers (“p-doped”) and some phosphorus to the upper layer (“n-doped”), an electric field is created in the solar cell – the semiconductor is “doped”. The doping ensures that the electric field is created in the boundary layer between the two silicon layers and the released electrons migrate to the n-doped layer. A conductive metal contact on the top of the solar cell collects the electrons. An electric current is generated. The electrons are guided via a metallic conductor to the consumer and further to the other side of the solar cell, where there is also a metal contact. In order to generate larger, usable amounts of electricity, individual solar cells are connected together to form modules (PV modules).

Structur of a solar cell by Name (CC BY-SA)

A central parameter for describing the technological performance of solar cells is the electrical efficiency. The electrical efficiency describes how much of the radiant energy arriving at the solar cell is converted into electrical energy. Currently, commercially available solar cells achieve an efficiency of 17 to 20 %. Most of the losses occur because only part of the incoming photon energy is transferred to the electrons due to physical limitations. Since solar cells generate direct current, but the electricity grid in Germany, as well as most consumers, need alternating current, PV systems are usually equipped with an inverter. The conversion in the inverter causes further losses, but these are small compared to the internal losses of the solar cell.

In order for PV systems to deliver maximum electricity yield, the modules must be oriented in such a way that solar energy irradiated onto the module surface is maximised. In the northern hemisphere of the earth, PV systems deliver maximum yields during the course of the day respectively the year if they are oriented towards the south. The optimal inclination of the modules depends strongly on the latitude - i.e. the distance to the equator - of the respective location. In Central European latitudes, the highest annual yields can usually be achieved with angles of inclination between 25 and 45°. If the focus is not on maximising the annual yield, but rather on providing electricity as constantly as possible throughout the entire year, a much steeper angle of inclination of about 60° is advantageous in Central Europe. The steeper angle of inclination leads to an optimisation of the (relatively low) electricity yield in winter, while the (higher) electricity yield in summer is reduced. Such a smoothing of the annual generation profile can for example be useful for private PV rooftop systems in order to maximise self-consumption of the generated electricity.