Photovoltaics

Photovoltaics is the direct conversion of light energy, mostly from sunlight, into electrical energy by means of solar cells. It has been used in space travel since 1958, and later also to supply energy to individual electrical devices such as calculators or parking ticket machines. Today, grid-connected power generation on roofs and as ground-mounted systems is by far the most important area of application for replacing conventional power plants.

The term is derived from the Greek word for "light" (φῶς, phos, in the genitive case: φωτός, photos) and from the unit for electrical voltage, the volt (after Alessandro Volta). Photovoltaics is a sub-area of solar technology, which includes other technical uses of solar energy.

At the end of 2018, photovoltaic systems with a capacity of more than 500 GW were installed worldwide. Between 1998 and 2015, global installed photovoltaic capacity increased at a growth rate of 38% per year on average. According to a 2019 paper in Science, installed capacity is expected to reach about 10,000 GW by 2030 and could reach 30,000 to 70,000 GW by 2050. In 2014, crystalline silicon cells accounted for about 90% of the global market share. Forecasts assume that silicon cells will remain the dominant photovoltaic technology in the long term and, together with wind turbines, will be the "workhorses" of the energy transition.

Photovoltaics was long considered the most expensive form of electricity generation using renewable energy; however, this view is now outdated due to the sharp cost reductions in system components. From 2011 to 2017, the cost of generating electricity from photovoltaics fell by almost 75%. In the US, tariffs of less than 5 US cents/kWh (4.2 euro cents/kWh) are common for solar farms (as of 2017); similar levels were possible in other states at the time under favorable circumstances. In several states, record values of 3 US cents/kWh (2.5 euro cents/kWh) were achieved in tenders. In 2020, several solar farms were awarded with tariffs well below 2 US cents/kWh. As of April 2020, the cheapest awarded bid is 1.35 US cents/kWh (1.13 ct/kWh) for a solar farm in Abu Dhabi. In Germany, too, the LCOE of newly constructed large-scale photovoltaic plants has been lower than for all other fossil or renewable energies since 2018.

In 2014, the electricity production costs of photovoltaics in certain regions of the world were already at the same level or even lower than those of fossil competitors. Including storage facilities, which become necessary when photovoltaics account for a high share of the electricity mix, the costs at this time were still higher than for fossil power plants. However, solar power would have been competitive even at this point if the external costs of fossil power generation (i.e. environmental, climate and health damage­) had been taken into account; in fact, however, they were only partially internalised.

History of photovoltaics

Photovoltaics is based on the ability of certain materials to convert light directly into electricity. The photoelectric effect was discovered as early as 1839 by the French physicist Alexandre Edmond Becquerel. This was subsequently researched further, with Albert Einstein in particular playing a major role in this research with his 1905 work on light quantum theory, for which he was awarded the Nobel Prize in Physics in 1921. In 1954, it was possible to produce the first silicon solar cells ­with efficiencies of up to 6 %. The first technical application was found in 1955 in the power supply of telephone amplifiers. Photovoltaics became widespread in exposure meters for photography.

Photovoltaic cells have been used in satellite technology since the late 1950s; the first satellite to use solar cells, Vanguard 1 was launched into Earth orbit on March 17, 1958, and remained in operation until 1964. In the 1960s and 1970s, demand from space led to advances in the development of photovoltaic cells, while photovoltaic systems on Earth were only used for certain stand-alone systems.

Triggered by the oil crisis of 1973/74 and later by the nuclear accidents in Harrisburg and Chernobyl, a rethinking of energy supply began. Since the end of the 1980s, photovoltaics has been intensively researched in the USA, Japan and Germany; later, financial subsidies were added in many countries of the world in order to stimulate the market and to make the technology cheaper by means of economies of scale. As a result of these efforts, global installed capacity increased from 700 MWp in 2000 to 177 GWp in 2014 and continues to grow.

Notation

Usually the spelling photovoltaik and the abbreviation PV are used. Since the German spelling reform, the spelling Fotovoltaik is the new main form and Photovoltaik is still a permissible alternative spelling. In German-speaking countries, the alternative spelling photovoltaik is the common variant. The spelling PV is also common in international usage. For technical fields, the spelling in standardisation (here also photovoltaics) is an essential criterion for the spelling to be used.

Photovoltaic system tracking the position of the sun in Berlin-Adlershof


Sale of solar plants in Ouagadougou, Burkina Faso

Technical basics

For energy conversion, the photoelectric effect of solar cells is used, which in turn are connected to form so-called solar modules. The electricity generated can be used directly, fed into electricity grids or stored in accumulators. Before being fed into AC power grids, the generated DC voltage is converted by an inverter. The system consisting of solar modules and the other components (inverter, power line) is called a photovoltaic system.

Functional principle

Photovoltaic functional principle using the example of a silicon solar cell. Silicon is a semiconductor. The special feature of semiconductors is that free charge carriers can be generated in them by supplied energy (e.g. in the form of light or electromagnetic radiation).

1. The upper silicon layer is interspersed with electron donors (e.g. phosphorus atoms) - negatively doped. There are too many electrons here (n-layer).
2. The lower silicon layer is interspersed with electron acceptors (electron receivers - e.g. boron atoms) - positively doped. Here there are too few electrons, i.e. too many defects or holes (p-layer).
3. In the boundary region of the two layers, the excess electrons of the electron donors bind loosely to the vacancies of the electron acceptors (they occupy the vacancies in the valence band) and form a neutral zone (p-n junction).
4. Since there is a lack of electrons at the top and a lack of defects at the bottom, a constantly present electric field is formed between the upper and lower contact surfaces.
5. Photons (light quanta, "sun rays") enter the transition layer.
6. Photons with sufficient energy transfer their energy in the neutral zone to the loosely bound electrons in the valence band of the electron acceptors. This releases these electrons from their bond and lifts them into the conduction band. Many of these free charge carriers (electron-hole pairs) disappear after a short time by recombination. Some charge carriers drift - moved by the electric field - to the contacts in the similarly doped zones (see above); i.e. the electrons are separated from the holes, the electrons drift upwards, the holes downwards. A voltage and a usable current are generated as long as further photons continuously generate free charge carriers.
7. The "electron" current flows through the "outer circuit" to the lower contact surface of the cell and recombines there with the holes left behind.

Rated power and yield

The nominal power of photovoltaic systems is often specified in the notation Wp (Watt Peak) or kWp and refers to the power at test conditions, which roughly correspond to the maximum solar radiation in Germany. The test conditions are used to standardize and compare different solar modules. The electrical values of the components are specified in data sheets. Measurements are taken at 25 °C module temperature, 1000 W/m² irradiance and an air mass (abbreviated AM) of 1.5. These standard test conditions (usually abbreviated STC) have been defined as an international standard. If these conditions cannot be met during testing, the nominal power must be calculated from the given test conditions.

For comparison: The radiation intensity of the sun in space near the earth (solar constant) is 1367 W/m² on average. (Approximately 75% of this energy reaches the ground in clear weather).

The decisive factor for the dimensioning and amortisation of a photovoltaic system is, in addition to the peak power, above all the annual yield, i.e. the amount of electrical energy generated. The radiation energy fluctuates depending on the day, season and weather. For example, a solar system in Germany can have up to ten times the yield in July compared to December. Up-to-date daily feed-in data with a high temporal resolution are freely available on the Internet for the years from 2011 onwards.

The yield per year is measured in watt hours (Wh) or kilowatt hours (kWh). The location and orientation of the modules as well as shading have a significant influence on the yield, whereby in Central Europe roof pitches of 30 - 40° and orientation towards the south provide the highest yield. Oriented to the maximum height of the sun (midday sun), the optimum inclination in Germany for a fixed installation (without tracking) should be approx. 32° in the south of the country and approx. 37° in the north. In practice, a somewhat higher angle of inclination is recommended, as the system is then optimally aligned both twice a day (in the morning and in the afternoon) and twice a year (in May and in July). This is why such orientations are generally chosen for ground-mounted systems. Although the average solar altitude distributed over the year and thus the theoretically optimal inclination can be calculated exactly for each latitude, the actual irradiation along a latitude varies due to various, mostly terrain-dependent factors (e.g. shading or special local weather conditions). Since the system-dependent effectiveness also varies with respect to the angle of irradiation, the optimum alignment must be determined in each individual case for each site and system. In these energetic investigations, the site-related global radiation is determined, which includes not only the direct solar radiation but also the diffuse radiation incident via scattering (e.g. clouds) or reflection (e.g. nearby house walls or the ground).

The specific yield is defined as watt hours per installed nominal power (Wh/Wp or kWh/kWp) per time period and allows the simple comparison of systems of different sizes. In Germany, with a reasonably optimally aligned permanently installed system, an annual yield of approx. 1,000 kWh can be expected per module area with 1 kWp, whereby the values vary between approx. 900 kWh in northern Germany and 1150 kWh in southern Germany.

Mounting systems

On-roof / in-roof mounting

A distinction is made between on-roof systems and in-roof systems. In an on-roof system for sloping house roofs, the photovoltaic system is attached to the roof with the help of a mounting frame. This type of mounting is chosen most often, as it is the easiest to implement for existing roofs.

In an in-roof system, a photovoltaic system is integrated into the roof cladding and takes over its functions such as roof tightness and weather protection. The advantages of such systems are the visually attractive appearance and the fact that no roof covering is required, which means that the higher installation costs can often be compensated for.

In addition to tiled roofs, on-roof mounting is also suitable for sheet metal roofs, slate roofs or corrugated sheets. If the roof pitch is too shallow, special hooks can compensate for this to a certain extent. The installation of an on-roof system is usually easier and less expensive than that of an in-roof system. An on-roof system also provides adequate rear ventilation for the solar modules. The mounting materials must be weather-resistant.

Another form is flat roof mounting. Since flat roofs are not inclined at all or only slightly, the mounting system angles the modules between 6 and 13°. An east-west inclination is also frequently used in order to achieve a higher utilisation of space. In order not to damage the roof cladding, the mounting system is secured by ballasting if the load-bearing capacity is sufficient.

The in-roof system is suitable for roof renovations and new buildings, but is not possible on all roofs. Tiled roofs allow in-roof installation, sheet metal roofs or bitumen roofs do ­not. The shape of the roof is also decisive. In-roof mounting is only suitable ­for sufficiently large pitched roofs with a favourable orientation to the sun's path. In general, in-roof systems require greater slope angles than on-roof systems to allow adequate rainwater runoff. In-roof systems form a closed surface with the rest of the roof covering and are therefore more attractive from an aesthetic point of view. In addition, an in-roof system has a higher mechanical stability against snow and wind loads. However, the cooling of the modules is less efficient than with the on-roof system, which reduces the output and yield somewhat. A 1 °C higher temperature reduces the module output by approx. 0.5 %.

Free-space assembly

In the case of mounting systems for ground-mounted systems, a distinction is made between fixed mounting and tracking systems. In the case of fixed mounting, a steel or aluminium frame is anchored in the ground by driving or screwed to concrete blocks, depending on the subsoil; the angle of the modules is not changed after mounting.

Tracking systems follow the course of the sun to ensure that the modules are always optimally aligned. This increases the yield, but also increases the investment costs as well as the operating costs for maintenance and the energy required for tracking. A distinction is made between single-axis tracking - either horizontal only (the panel follows the position of the sun from east to west from sunrise to sunset.) or vertical only (the south-facing panel rotates depending on the height of the sun above the horizon.) and dual-axis tracking - horizontal and vertical. This increases the yields compared to fixed mounting: in Central European latitudes by about 20 % with only single-axis tracking and by over 30 % with dual-axis tracking.

Another form of free-space mounting is floating mounting on bodies of water, where the modules are mounted on plastic floats. However, the yield increases due to the cooling effect of the water. The investment costs are 20-25 % higher than with conventional mounting. The Fraunhofer Institute estimates the potential for floating PV systems alone on 25 % of the land destroyed by lignite mining to be 55 GWp if flooded.

In Baden-Württemberg, a plant with modules installed vertically was commissioned in 2020.

Developments

Up to now, the majority of photovoltaic systems worldwide have been based on silicon technology. In addition, various thin-film technologies have been able to gain market share. Other semiconductors such as cadmium telluride or gallium arsenide are also used. Layers of different semiconductors are used in so-called tandem solar cells.

The development of solar modules based on perovskite is considered to be very promising due to the low-cost production. The cells can be built significantly thinner than silicon cells. However, the low durability is still a problem.

Another research goal is the development of organic solar cells. Together with partners, the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg has succeeded in producing a low-cost organic solar cell on flexible film.

House roof with photovoltaic system for electricity and solar collector for hot water production


Solar radiation potential in Europe


Photovoltaic functional principle using the example of a silicon solar cell (see text for explanations of the figures)


Radiation atlas based on satellite data from 1991-1993