Photovoltaics make an important contribution to the energy transition and help to reduce electricity costs. Find out how Photovoltaics works, when a off grid solar system is worthwhile, and which aspects should be considered during Planning.
Climate change, rising electricity prices, and Germany’s ambitious climate targets currently make Photovoltaics one of the most important technologies for climate protection. Solar energy can be converted directly into electricity using Photovoltaics – an almost climate-neutral way of producing electricity. Photovoltaics are now not only easily accessible for private individuals but also a worthwhile investment. This guide gives you an overview of how Photovoltaics works, its advantages, and what steps are necessary to plan your system.
What is photovoltaic?
The word “photovoltaics” (short: PV) is made up of the Greek word “photos” (light) and the word “volt” (unit of measurement for electrical voltage). Photovoltaics is a technical process in which light energy is converted into electrical energy. It uses the so-called photoelectric effect, through which direct current is obtained with the help of solar cells. This, in turn, is converted by employing a solar inverter into the alternating current that can be used in the household.
Where is photovoltaic used?
Photovoltaics are used in many everyday devices, such as pocket calculators, watches, solar lights, or parking ticket machines. The electricity required for operation is small and can be generated with just a few solar cells. This is also why many of these applications have existed since the beginning of the technical use of Photovoltaics when the cells were nowhere near as efficient as they are today. In the meantime, many cool boxes, boats, mobile homes, and electric garden tools are also equipped with solar cells.
However, today’s most important application area for Photovoltaics is high-performance, grid-connected photovoltaic systems (PV systems) on private homes, commercial properties, and public buildings. In addition, it is used in large solar parks, in which electricity is generated for numerous households and industries.
In the private sector, PV systems are mainly used on house roofs. These systems usually consist of 10 or more solar modules, but it is also possible to use individual solar modules on the balcony or terrace. The higher efficiency of solar cells makes the use of Photovoltaics more and more attractive, even on a small scale. Although the small plants known as balcony power plants cannot cover the electricity demand, they help to lower electricity costs and also reduce the CO2 emissions of the household.
Solar thermal energy, in which solar energy is converted into heat, offers an alternative to Photovoltaics. This can be used for heating or water heating. However, solar thermal competes with Photovoltaics regarding roof area, and the question arises as to whether solar collectors or PV modules should be installed. What speaks in favor of Photovoltaics is that it fulfills many functions because the electricity generated can also be used for heating (e.g., with a heat pump), for heating water (e.g., via a heating rod), and last but not least for electromobility (sector coupling).
Other types of use of Photovoltaics
While photovoltaic systems are most commonly installed on roofs, other options exist. For example, solar modules can be used as facade photovoltaics or a parking lot roof. Further applications are PV fences, floating photovoltaic systems, or in the agricultural sector as Agri-PV.
Photovoltaic has these advantages
Photovoltaics contributes to the energy transition: around 2.5 million photovoltaic systems are currently installed in Germany (as of autumn 2022). In 2021, Photovoltaics covered around 9.1% of gross electricity consumption. For private individuals, Photovoltaics primarily produces cheap electricity themselves, thus reducing electricity costs. Thanks to high-performance solar cells, PV systems on the roofs of detached or semi-detached houses also work economically.
Here is an overview of the advantages of Photovoltaics:
- Solar energy is freely available and costs nothing.
- Photovoltaic works are emission-free and noiseless.
- Photovoltaics enable an independent energy supply.
- The electricity generated can also be used using an electricity storage system at night.
- Those who use Photovoltaics are already meeting future legal requirements (keyword: solar roof obligation).
- Solar power can be used in many ways, such as heating the house and water.
- If there is enough space, the size of a photovoltaic system can be tailored to individual power requirements. Subsequent expansions of the PV system are usually possible.
- Photovoltaic systems have a long service life of 20 years and more.
- Photovoltaic systems are low-maintenance.
- Subsidies can be claimed for the installation of a photovoltaic system.
The function of Photovoltaics and the structure of a PV system
Photovoltaic systems consist of several solar modules, which in turn consist of photocells or solar cells. In the solar cells, the incident sunlight is converted into electricity. Light consists of photons, the carriers of electromagnetic radiation.
Solar cells use the so-called “inner photoelectric effect,” discovered as early as 1839. The French physicist Alexandre Edmond Becquerel noticed that certain materials exhibit higher electrical conductivity when exposed to sunlight. Becquerel researched selenium compounds.
Today, silicon contaminated with foreign atoms (technically: “doped”) is often used for solar cells, but also material combinations such as gallium arsenide (GaAs), cadmium telluride, or CIGS (copper indium gallium diselenide). Solar cells can also be made from organic compounds.
What all the materials used have in common is that they are semiconductors. This refers to solids whose electrical conductivity lies between the conductivity of electrical conductors (e.g., iron) and non-conductors (e.g., glass and many plastics). In addition, the conductivity of the semiconductors also increases as the temperature rises.
How does photovoltaic work?
In Photovoltaics, different semiconductor layers are combined in the solar cells. How this generates electricity will be explained using the example of a silicon semiconductor cell:
To create photoelectrically active silicon layers, the silicon is doped. This means that different foreign atoms are introduced into the regular silicon lattice. This can, e.g., B. phosphorus (P) or boron (B).
Phosphorus has five outer electrons, i.e., one more than silicon, while boron only has three and, therefore, one less than silicon. This means that there is a semiconductor layer with an excess of electrons (“n-conducting,” where n = negative) and one with a lack of electrons (“p-conducting,” p = positive).
Suppose the p- and n-conductive layers are brought together. In that case, an equalization occurs in that the electrons and the “holes” (places in the lattice where electrons are missing) migrate toward one another and “recombine.”
A narrow, neutral boundary layer forms, bordered by two charged layers, a positive layer doped with boron and a negative layer doped with phosphorus. This creates an electric field across the boundary layer.
If light falls on the boundary layer, the negative electron and the positive hole are separated and guided to the other side by the electric field if you connect the two charged outer layers of the solar cell with an electrical conductor, current flows.
The voltage and current produced by a single solar cell are very low. Only by interconnecting many cells can the function of Photovoltaics be used sensibly to generate energy. For a long time, a standard solar module consisted of 60 cells; today, 144 half-cells are mostly in use. Depending on the output, 25 to 30 modules are required for a ten kWp system.
How high the yield of a photovoltaic system depends not only on size and orientation but also on the technology of the solar cells used. Solar modules with monocrystalline cells, which have an efficiency of up to 22%, are usually installed on house roofs. This allows high performance to be achieved on the limited roof area. The less expensive thin-film modules with lower efficiencies are mainly used where the area is not the limiting factor, such as in solar parks.
From the solar cell to the solar module
By connecting several solar modules, the output and, thus, the amount of solar power that can be generated can be proportionally increased. Rule of thumb: To generate an output of 1kWp, solar modules with an area of around 6 square meters are required – more or less depending on the output of the solar cells. The Solar radiation in Germany is around 1,000 kilowatt hours (kWh) per square meter per year and is around five times higher in summer than in winter.
Calculation example for dimensioning the system: A family of four’s average annual electricity requirement is 4,500 kWh. The PV system should be dimensioned so that it at least mathematically covers the demand, so in the present case, a nominal output of at least 4 to 6 kWp makes sense. This means an approximate roof area of 24 to 36 m² is required.
Since the exact calculation depends on many factors, planning by a trained specialist is necessary. This determines the optimal structure so the system can yield the highest possible.
From direct current to alternating current
Photovoltaic components always generate direct currents; most electrical devices in the home use alternating currents. The conversion of direct current into alternating current is the task of the power inverter. In addition to the solar modules and the inverter, the photovoltaic system includes a substructure, cabling, electrical connections, and a 2-way meter (supply and purchased electricity). The PV system can also include an energy management system that controls consumption and a power storage unit.