Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 196

trough collectors. Such losses may at least partially be compensated by alternating
mirror positions.
Each reflecting segment rotates around its centre of gravity. Segments are actu-
ated either individually or as a group. Fresnel collectors thus require a more so-
phisticated control system than parabolic trough collectors since a specifically
higher number of drives must be used. This is why Fresnel collectors have to date
only been tested on a minor scale and have, also on a minor scale, first been
commercially applied since 2004 in Liddell, Australia /5-17/.

Absorber / Heat Collecting Element (HCE). Individual horizontal tubes are
used as absorbers in the focal line of the collectors; for Fresnel collectors also tube
groups may have to be used due to their wider focal line. Today's stainless steel
absorber tubes of parabolic trough collectors (i.e. Heat Collecting Element (HCE))
are enclosed in an evacuated glass tube to minimise heat losses (Fig. 5.16). In case
of parabolic trough collectors the vacuum also serves to protect the sensitive
highly selective coating. Nowadays, such selective coatings remain stable up to
temperatures of 450 to 500 °C; solar absorption is above 95 %, and at a tempera-
ture of 400 °C emissivity is below 14 % /5-18/.

Evacuation opening Evacuation opening Absorber tube Absorber tube Hydrogen getter Hydrogen getter

Glass/metal joint Glass/metal joint Glass tube Glass tube Extension bellows Extension bellows

Fig. 5.16 Absorber tube of a parabolic trough collector

In addition to the proven HCE design (Fig. 5.17 a)), Fig. 5.17 b) and c) show
two additional variants using a secondary concentrator and a tube bundle receiver.
Both options have been proposed to suit the optics of Fresnel collectors.

Evacuated glass tube Evacuated glass tube Secondary concentrator Secondary concentrator Isulation Isulation

a) a) b) b) c) c)

A A A A A A

Fig. 5.17 Design principle of receivers of parabolic trough collectors (a); see also
Fig. 5.16) and Fresnel collectors b) and c)) ( A width of the focal line)