Environmental Engineering Reference
In-Depth Information
Heat transfer medium. To date, high-boiling, synthetic thermal oil has been
applied as heat transfer medium in the absorber tubes. Due to the limited thermal
stability of the oil, the maximum working temperature is limited to scarcely
400 °C. This temperature requires to keep the oil pressurised (approximately 12 to
16 bar). This is why collector tubes as well as expansion reservoirs and heat ex-
changers must be of pressure-resistant design. Relatively high investments are
thus required.
Hence, as an alternative, molten salt has been proposed as heat transfer me-
dium. Molten salt is characterised by the advantages of lower specific costs, a
higher heat capacity and thus potentially higher working temperature on the one
hand, and by the higher viscosity of the medium and a higher melting temperature,
requiring trace heating, on the other hand. Due to the higher heat capacity, pump-
ing power requirements are still expected to be lower when compared to thermal
oil. To date, only prototypes of this variant have been built.
For this reason, investigations of direct steam generation inside the collector
tubes are promoted since great cost saving and efficiency potentials are expected.
The advantages are the higher possible working temperature of steam as a work-
ing medium and that there is no secondary heat transfer fluid loop required includ-
ing the necessary heat exchangers. The expected problems related to evaporation
of water in horizontal tubes (including two-phase flow and thus different heat
transmission) can be solved by available technology (forced-circulation boiler
with a relatively high recirculation rate and water/steam separator). It is thus pos-
sible to directly generate saturated steam by line focussing collectors. However,
the high steam pressure (usually between 50 and 100 bar) requires a relatively
high tube wall thickness, so that for very wide collectors tube bundles might be
more suitable than the well proven individual tube /5-40/.
Collector fields. Nowadays, collector fields are composed of a certain number of
loops of an approximate length of 600 m each. These loops are connected to one
feed ("cold header") and one discharge line ("hot header") each. Collectors are
north-south oriented to allow for high and constant energy yields.
With regard to the collector field design special emphasis must be laid on the
distance between the individual collector rows. The distance determines shading
during morning and evening hours and thus the corresponding efficiency reduc-
tion of the whole field. Furthermore, land and piping costs as well as thermal and
pump losses must be taken into account. Since the effect of shading also depends
on the latitude, each field design must be optimised with regard to the site-specific
conditions. As a rule of thumb, the distance between lines of parabolic troughs
typically amounts to three times the aperture width.
Collectors are positioned horizontally; a slope of several percent may also be
admissible. However, more severe unevenness of the site must be compensated or
terraced.
The achievable thermal output of the entire field is limited by pressure losses of
the heat transfer medium and the piping costs. Currently, the maximum economi-
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