Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 194

concentrator. The collector tracks the sun in one axis (Fig. 5.14). Due to this "one-
dimensional concentration" the geometric concentration factors of 15 to 30 are
considerably lower than those of two-dimensional collectors discussed above.
This is why lower temperatures are achieved when compared to solar tower power
plants. However, this disadvantage is compensated by lower specific costs as well
as a simpler structure and maintenance.

a) b)

Fig. 5.14 Principles of line focussing collectors (a) parabolic troughs, b) Fresnel troughs)

Line focusing solar power plants have a modular structure. Because of this
characteristic and the shape of the solar field, line-focussing solar power plants
have in the past also been referred to as "solar farms".

5.3.1 Technical description

In the following, the technology of parabolic trough power plants and Fresnel
collector and all related components are described.

5.3.1.1

System components

The system elements composing parabolic trough power plants comprise collec-
tor, absorber, heat transfer fluid circuit and power block.

Collectors. The collectors which are typically 100 m, but may nowadays also be
150 m long, are provided with single-axis solar tracking. The annual mean cosine
losses of parabolic troughs vary between 10 and 13 %, whereas those of Fresnel
concepts are double. After deduction of the optical and thermal losses inside the
collector 40 to 70 % of the radiation incident on the mirrors can be used techni-
cally. The percentage depends on the design, the field size and on the geographic
location of the power plant. In the following, the main collector types are dis-
cussed.

Parabolic trough collectors. This collector type is characterised by a parabolic
reflector which concentrates the incident radiation onto a tube positioned in the
focal line (Fig. 5.14 a), Fig. 5.15, Fig. 5.16).
The reflector itself may either consist of one surface provided with a reflecting
layer (metal foil, thin glass mirrors) or of several curved mirror segments arranged
in a truss-type structure; the latter variant is commercially applied. Collectors are