Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 432

Figure 14.3.10 Schematic of heat collection element adopted in LFR.

14.3.8 Heat collection element
The heat collection element transfers concentrated solar energy to the fluid. Compared
to trough technology, there are several heat collection element concepts available, all
of which can be merged into three different categories:

- Single absorber tube;

- Secondary reflector/concentrator;

- Cavity receiver.

A schematic of each category is shown in Figure 14.3.10. The single absorber tube
concept is similar to that of parabolic trough. Solar radiation is concentrated by the
primary mirrors on the heat collection element, which has a glass tube to limit thermal
losses. Compared to PT, the HCE is much simpler since it does not move with the
structure; connections and differential thermal expansions are much simpler. In this
case, the size of each mirror is similar to the heat collection element. Mirrors are usually
flat or have a small curvature.
The second category is based on the adoption of a secondary reflector or con-
centrator which further concentrates solar radiation coming from the primary mirrors
on the absorber tube. Considering that the mirror's width is about the same as the
secondary reflector's (i.e. larger than HCE), this configuration can increase the con-
centration ratio of the single absorber tube configuration. Novatec (Novatec Biosol,
2012) and ENEA concepts are based on the secondary receiver (Grena & Tarquini,
2011).
Besides the higher concentration ratio, the advantages of the secondary reflector
concepts are lower thermal losses: the upper part of the absorber tube does not see the
sky temperature, but the temperature of the reflectors which is higher. Moreover, the
secondary reflector can be insulated and a glass plate can be used to close the secondary
reflector cavity, reducing convective heat loss. A disadvantage of this configuration is
the additional reflection of solar radiation, with optical penalties.
Optimization of the secondary reflector shape has been rigorously investigated
and optimized in the literature, since the optical analysis has much higher degrees
of freedom than the single absorber tube. A picture showing the optical and thermal