The parabolic trough technology represents the most important technology of the line focus collectors. It is the only solar thermal technology for electricity generation that can look back on 30 years of commercial operation. The first parabolic trough power plants were installed already in 1984 in California. Since 2008, the technology has undergone a renaissance, first in Spain but now also worldwide, as part of the promotion of renewable energy.
In addition to parabolic trough technology, also linear Fresnel collectors are counted among the line focus collectors. Line focus collectors are suitable for implementation at temperature levels up to 500°C. They generally consist of concentrating mirrors with uniaxial tracking of the sun, as well as an absorber tube in the focal line. A heat transfer fluid flowing through the absorber transfers the energy tothe consumer (e.g. power plant processes, process heat consumers or thermal energy storage).
Parabolic trough collectors are the commercially most successful technology for solar thermal power plants. The state of the art is represented by the plants of ANDASOL type, installed in Spain with a nominal electrical outputof 50 MWe and integrated thermal energy storage for about 8 full-load hours. Parabolically bent glass mirrors mounted on steel frameworks constitute concentrator modules with an aperture width of 5.8 meters and a length of 12 meters, which are put together to 150 meters long collectors. The absorber tubes have a selective coating and are surrounded by a glass envelop with vacuum isolation in between. While thermal oil is used as heat transfer medium in the solar field, the thermal energy storage is filled with molten salt due to the lower costs.
In linear Fresnel collectors, the concentrator consists of slightly bent, thin facets that concentrate the sunlight on a stationary absorber. Due to the comparatively low wind load of the concentrator, significant savings can be realised on materials and investment costs. The drawback of the technology is the reduced optical efficiency as well as the geometrical disadvantages in comparison to the parabolic trough. So far, the installed plants generate hot water or saturated steam.
Thermal oil, which at present is commonly used, is limiting the efficiency of the steam turbine circuit with its inlet temperature of about 400°C. In order to fully utilise the temperature potential of the collectors, the usage of alternative heat transfer media is investigated. Direct steam generation in the collector field reaches live steam conditions up to 120bar/550°C. Similar values can be achieved with molten salt. Further alternative heat transfer media the corresponding processes are investigated as well.
At the present state of technology, the potential for further efficiency improvements of key components is limited. Therefore, objectives for the development of components shall rather concern the reduction of costs for materials, manufacturing, transport and erection, without retrenching the performance. New collector concepts rely on different construction methods, for instance with sandwich structures, alternative materials like concrete or innovative collector geometries.
In addition to the use in solar thermal power plants, line focus collectors are also installed for industrial use in order to provide process heat or cooling power. On this market, system integration is the most important challenge.
Fields of activity
The department for line focus systems is situated in Almería, Cologne and Stuttgart. We work on relevant issues concerning line focus systems in the entire spectrum from basic research and theory, over experimental and numerical methods up to demonstration under real operation conditions including economic evaluation. Our main research topics are at the moment:
► Molten Salt Systems
► Direct Steam Generation
► Collector Development
► Industrial Process Heat
► Process Optimisation