Before a solar module can convert sunlight into electricity it has to undergo a multi-stage production process. First of all, silicon has to be extracted from sand, which is then melted down and shaped into blocks, the so-called ingots which in turn are cut into slices of a few millimeters thickness, the wafers. In the next step these are coated to produce solar cells which are then put together to form solar modules.
As the second most frequent element in the earth’s crust the availability of silicon is almost unlimited. In nature it occurs exclusively as an oxide, either in the form of silicon dioxide, or as silicon-containing minerals. Thus, sand and quartz mainly consist of silicon dioxide. On an industrial scale elementary silicon is produced by the reduction of silicon dioxide with carbon in a furnace at temperatures of about 2,000 °C. For the production of solar cells the raw silicon must be further purified to become solar-grade silicon.
In a second working step solar wafers are made from the solar-grade silicon. To this end the silicon is melted at a temperature of more than 1,400 °C and cast into blocks or ingots. In the melting process either the mono-crystalline or the poly-crystalline process can be used. When making mono-crystalline wafers only one single crystal is drawn from the silicon melt. In the poly-crystalline process the liquid silicon melt solidifies and thousands of small crystals are formed into one block. The blocks are then broken up into columns with a square cross section from which the very thin wafers are cut with wire cutters or by laser.
The machining and coating of their surfaces turns the wafers into solar cells. Now, the cells already possess all the technical properties needed to convert sunlight into electric power. They constitute the basic element of a solar module. A solar cell consists of two layers of silicon. An electrical field is formed at the interfaces of the two layers. Physical processes triggered by incident light cause electrical energy to flow between the metal contacts that have been fixed to these silicon layers. Today, the average degree of efficiency of the solar cells, i.e. their ability to convert solar energy into electrical power, amounts to some 18 percent.
In a last working step the solar cells are combined into solar modules. Solar power modules are the solar end product and ready for solar power generation. They are framed and encapsulated to be weather-proof. In the modules the sunlight is converted into electrical energy. A distinction is made between mono-crystalline and poly-crystalline modules. Photovoltaic modules made of mono-crystalline solar cells are more efficient, hence they are particularly suitable for small roof areas.
Photovoltaic systems convert the electromagnetic spectrum of sunlight into electrical current. Core elements are the solar cells which in turn are combined into modules. The photon bombardment by the incident light causes a separation of positive and negative charges. If an electrically conductive connection is established between the charging zones electricity will flow. Depending on the size and type of system the individual solar modules are interconnected in line into so-called “strings”. As a result, the voltages of the individual modules are additive. The solar modules are as a rule mounted on a sub-structure that ideally aligns the modules with the sun in such a way that the highest possible or a consistent energy yield is achieved in the course of the year. The sub-structure can also be designed to track the sun in order to optimize the energy yield. By way of an inverter the direct current generated is converted into alternating current and then fed into the national grid, or directly consumed right on the spot.
The most frequent form of PV system is the rooftop system on a private property where the existing building carries the sub-structure for the photovoltaic system. At the same time the inclination of the roof can optimize the alignment of the PV system which would otherwise have to be achieved by additional design measures. The operator of the system can sell the power to and feed it into the national grid or alternatively consume it himself. In general with a 5 kWp photovoltaic system, equal to more or less 40-50 square meters of the roof area, you are able to produce electricity needed for an average household in the EU.
A fully integrated system is one where the photovoltaic system replaces parts of the outer shell of the building, i.e. of the façade cladding and the roof cover. The advantage is that the place of the elements of the roof and the façade needed anyway is now taken by the elements of the PV system. In addition, esthetic arguments are also quoted in favor of this building approach because often the elements adjusted in color to conventional roof covers are optically less obtrusive than the usual systems mounted on the roof. Here again, the power is either fed into the national grid or consumed right on the spot.
Industrial or commercial rooftop systems are usually larger photovoltaic systems that are placed on industrial or commercial buildings like warehouses or refrigeration halls. As most industrial rooves are flat these systems require system and frame technology with which they can be optimally aligned with the sun. Often the power generated in this way is consumed by the operator of the industrial building on the spot. But it is not unusual for this power to be fed into the national grid either.
By free-field system we mean a photovoltaic system that is not fitted on a building but on the ground in a free-field site. A free-field system may be a fixed installation in which a sub-structure is used to align the photovoltaic modules at a certain angle to the sun. In addition to the fixed installation free-field systems there are also the so-called tracker systems that follow (i.e. track) the position of the sun. Frequently, free-field systems are large installations whose output is in the multi-digit megawatt range. Their operators in many cases act as professional utilities. It can also happen that free-field systems are established to supply the power to an industrial enterprise. In this case, the latter’s owner and the operator of the free-field system conclude an individual contract on the purchase of the power generated.