Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 433

Figure 14.3.11 Optical assessment (left side), thermal analysis (center and right) for the Novatech
secondary reflector configuration (Binotti et al., 2011).
assessment for Novatec's secondary receiver is shown in Figure 14.3.11. Similar assess-
ments were performed by Barale et al. (2010), Veynandt et al. (2010) and Morin &
Dersch (2009).
The third category involves a cavity receiver, which places this concept in between
the single absorber and the secondary concentrator. The configuration is based on
multiple absorber tubes being placed in a cavity, which provides thermal insulation,
thus reducing heat loss.
In general, the single absorber tube has higher heat losses but is much simpler
and cheaper than the secondary reflector and cavity concepts. Adoption of a cavity or
secondary reflector system increases complexity, but also cost. Moreover, both cavity
receiver and secondary reflector systems shadow part of the primary reflector, reducing
optical efficiencies.
The absorber tubes in LFR must have the same physical properties as PT applica-
tions. High absorptivity and low emissivity are required in order to reduce as much
as possible the heat losses. For this reason the same technology adopted in parabolic
trough systems can be used in Fresnel (i.e. Schott PTR 70® (SCHOTT PTR® 70
Brochure).
14.3.9 Reflectors
Reflectors in LFR are also called primary to distinguish them from the secondary
reflector. Their function is to reflect and concentrate solar direct beam radiation onto
the HCE. Compared to PT, where just one parabolic reflector is assumed, LFR is based
on several reflectors, which can be flat or have a very small curvature. The reflector
width is generally close to the absorber diameter for the single tube configuration; the
width of secondary reflectors is equal to the aperture width of the secondary receiver.
Reflectors must have a high reflectivity when new and operating in clean condi-
tions, as well as over their entire lifetime. Since the reflectors are ground-based, i.e.
below 1-2 m altitude, wind speed over the system is lower than in parabolic trough
installations, thus reducing wear, especially by sand in desert locations.
Since the function of mirrors is the same in both LFR and PT applications, their
reflectors share the same manufacturers. Therefore mirrors can be made from: (i) a
glass layer with low iron concentration and a reflective silvered film (Flabeg); (ii)