Environmental Engineering Reference
In-Depth Information
The individual collectors rotate on their longitudinal axis and in this way follow the
course of the sun. The mirrors concentrate the sunlight more than 80-fold at the
focal point onto an absorber pipe. This is embedded in an evacuated glass casing
in order to reduce heat loss. A special selective coating on the absorber pipe reduces
the heat radiated from the pipe surface. With conventional systems, a special thermal
oil fl ows through the pipe, which heats up to temperatures of about 400 °C as a
result of the solar radiation. The heat is transmitted over heat exchangers to a water
steam cycle, vaporized and overheated again. The steam drives a turbine and a
generator, which produces electric power. Behind the turbine it condenses again
into water and through a pump again enters into the cycle (Figure 7.5). The principle
of producing electricity using steam turbines is called the Clausius-Rankine process,
named after its inventors. This process is also used in classic steam power plants,
such as coal-fi red plants.
Figure 7.5
Parabolic trough power plant with thermal storage.
During periods of bad weather or at night a parallel burner can also be used to
operate the water steam cycle. In contrast to photovoltaics, this guarantees a daily
output of power. It also increases the attractiveness of and planning security in the
public electricity supply. For totally carbon-free plant operation, either biomass or
renewably produced hydrogen can be used as a supplementary fuel, or the burner
can be eliminated entirely. Instead a thermal storage tank can be integrated. The
solar fi eld heats up the storage during the day using excessive heat. At night and
during periods of bad weather the storage feeds the water steam cycle (Figure 7.6).
The storage must be designed to handle temperatures above 300 °C. Molten salt is
suitable as a storage medium for this temperature range.
The development of solar thermal parabolic trough power plants dates back to
1906. In the USA and near the Egyptian city of Cairo - at the time still under British
rule - research facilities were set up and the fi rst tests were successful. Based on
