A solar system is a technical system for converting solar energy into another form of energy. If in the energy conversion chain kinetic energy occurs; that is, when the solar system for converting solar energy into electrical energy moving parts (such as a turbine, a generator or a motor ) contains, as one speaks of a solar power plant.
Solar systems can be divided into three basic types according to the working principle and the form of energy obtained:
- Thermal solar systems in smaller scale supply heat energy in the low temperature range, mainly for direct use in households (for example, solar panels, solar cookers ).
- Solar thermal power plants also deliver heat but in the larger, industrial scale and typically at much higher temperatures. The heat is mainly converted into electricity (eg solar tower power plant ). A direct thermal use, for example, for chemical processes is possible.
- Photovoltaic systems supply electrical energy (direct current), which is usually through an inverter to the power grid (AC) is fed.
Other renewable power generation facilities are in principle also - albeit indirectly - users of solar radiation energy. By absorbing the radiation is converted into other forms of energy, such as heat. As an example, thermal power plants are mentioned. So the sun is the primary driving force also in the energetic use of wind, biomass and natural water circulation.
- Photochemical solar systems: photochemical reactions can be used for industrial purposes using solar technology.
Especially in the treatment of wastewater and photosynthesis, various techniques have been developed, inter alia, of the solar research at DLR. This lightly or heavily focusing solar systems are used, which are also used in the field of solar thermal energy.
Thermal solar systems
Thermal solar systems - such as flat solar collectors and evacuated tube collectors - can be used for heating of potable water ( shower and bath water) and to generate heat for space heating and for example for cooking ( process heat).
Here, specially coated surface area within a so-called thermal absorber " collector " ( = collector ) is heated by the electromagnetic solar radiation in the visible and infrared regions of the spectrum. Through the tubes of the absorber, a liquid flows, and a rare gas (for example air), which receives that heat ( heat transfer medium). By means of a pump or a fan - sometimes even by the buoyancy of the heating - is directed to a storage medium that, where it is cooled and fed back to the input of the absorber ( circuit).
In Central Europe can with solar thermal systems - depending on the region - usually 50 to 60 percent of the energy used in the heating of drinking water. It also higher contributions or the use in building heating systems are possible. The solar thermal system can support the heating, however, the contribution margin on various boundary conditions (demand, storage, media, etc.). What is possible, showed the Swiss engineer Josef Jenni in the project of Obernburger solar house in 1989: With enough time and effort one hundred percent coverage of the heat demand of a family home with solar energy is available, this is also possible with a multi-family house.
Furthermore, solar thermal systems can be used for process heat in industry and commerce. Particularly in the food industry, there are many applications for which the necessary temperature can be generated from 60 to 100 ° C with collectors. The household use ( solar cookers ) is uncommon in Europe, Africa and India but has been realized in some projects.
The supply of district heating networks using solar energy is widely used now in Denmark, Sweden and Austria. With special collectors for large systems the conventional energy supply is solar assisted in about 100 villages. This solar heat replaces the summer in many small networks a biomass boiler. But even in urban areas there are notable approaches, such as in Graz.
Another field of application is the provision of cold ( Solar air conditioning ). Chillers, which are driven by heat from collectors, solar energy use particularly efficient because of the high cooling requirement coincides with the strongest sunlight. There are now over a hundred model systems for research and demonstration, in recent years, large commercial projects were realized. The most prominent installations are located in Qingdao / China in the Olympic Sailing Village in 2008, the Renewable Energy House in Brussels, in Lisbon, on the main building of Caixa Geral de Depósitos as currently the largest solar cooling in offices worldwide and in Pristina / Kosovo to the building of the European Agency for the reconstruction of Kosovo.
See also Green Building
Solar thermal power plants
Solar thermal power plants are delimited by solar systems not only by their capacity. While mostly a flat absorber exploits also diffuse light at the smaller solar systems, direct sunlight collimating mirror come with solar power plants usually used. Diffuse light can not be used here. Accordingly, the technique has also impact on the choice of location ( dry deserts, etc.). In the light collection there are at least three different concepts:
- Parabolic trough are one or two axes track the sun and focus the radiation onto an absorber pipe in the focal line. In that there is a thermal oil, which drives a turbine and a power generator after the heating of a conventional steam cycle. Since 1985, parabolic trough power plants in the California desert for commercial purposes in operation ( total capacity about 354 MW).
- Parade are large biaxial tracking the sun, parabolic mirror with a focal point in the Stirling engine, to which a current- generating generator is mounted directly. Experimentally, a solar Stirling were achieved with an attached generator efficiency by 20 percent with very large plants using.
- Heliostats are usually large mirror. They are used to focus the incident sunlight. The heliostats of a solar tower power plant reflect sunlight onto a central receiver located at the top of a high tower. The levels of heliostats are aligned so that they all reflect accurately on the absorber. This results in very high temperatures are reached. The heat thus obtained is converted into a downstream "conventional" thermal power plant into electricity. Solar tower power plant example in Germany: Solar tower power plant Juelich ( STJ )
Photovoltaic systems convert the electromagnetic spectrum our sun in some cases only very thin semiconducting layers " directly " into electricity. Core element of the solar cells ( assembled in modules ) which produce by the photon bombardment of radiation, a separation of positive and negative charges. Now, when an electrically conductive connection is produced between the charge zones, a current flows. The power thus obtained can either be used directly, stored in solar batteries or with the aid of inverters in one or the public grid are fed. Meanwhile, plants will be built and operated with a peak power of several megawatts.