Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 195

mounted on a mounting structure and track the sun's diurnal course by a single-
axis system following the longitudinal axis.

Evacuated glass tube

Direct
solar radiation

Absorber tube

Supporting stand
with single-axis
tracking

Parabolic trough
concentrator with a
reflecting surface

Fig. 5.15 Functional principle of a parabolic trough

The mirror segments typically consist of back-silvered white low iron glass to
achieve high reflectivity values in the solar spectrum. When the mirror segments
are clean the mean solar reflectivity amounts to approximately 94 %. Since the
glass is weatherproof the reflectivity of the cleaned reflector remains practically
unchanged.
One collector (SCA = Solar Collector Assembly) is composed of a number of
collector elements (SCE = Solar Collector Elements) with a typical length of 12 m
each. The largest collector built to date (Skal-ET) consists of 12 SCEs (6 on each
side of the central drive pylon). It has an overall length of 150 m and an aperture
width of 5.77 m.
Each collector unit is equipped with an angular position sensor to track the po-
sition of the collector and optionally additionally with a sun sensor. First electric
motors provided with gearboxes or cable winches were used as drives. For the
more recent LS-3 and EuroTrough collectors, cost-efficient hydraulic drives have
been applied /5-16/.

Fresnel collectors. For this collector type the parabola profile is approximated
with individual segments (Fig. 5.14 b)). Individual long rectangular mirror seg-
ments of an approximate width of up to 2 m are tracking the sun similar to helio-
stat fields, so that they reflect the incident radiation to a common focal line. All
segments are mounted at the same level (either near-ground or higher on mount-
ing structures). Due to their lower width they are exposed to lower wind loads
compared to parabolic trough collectors. However, the different segments shadow
each other. Due to their specific geometry, Fresnel collectors are characterised by
lower concentrations and a lower optical efficiency when compared to parabolic