Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 440

14.3.14 Collector field
The collector field is based on a large number of heliostats with a tracking control
system to continuously focus direct solar radiation onto the receiver aperture area.
Heliostats are usually supported by a metallic structure with the tracking systems.
They can be flat or also a parabolic shape with small curvature (Buck and Teufel,
2009).
The optical efficiency of the solar field, which depends on the heliostat field per-
formances, is equal to the ratio of the net power intercepted by the receiver and the
product of the direct insolation and the total mirror area. This parameter includes the
cosine effect, mirror properties as reflectivity, shadowing, blocking, aberration and
atmospheric attenuation, and receiver spillage.
Since optical efficiency is fundamental for achieving a high solar-to-electricity con-
version, and the solar field complexity is significant (the solar field counts hundreds of
heliostats), several modelling codes have been developed in order to determine plant
performances and to find the optimal configuration (Belhomme et al., 2009; Delsol
Modelling Tool; Noone et al., 2012; Pitz-Paal et al., 2011; Wei et al., 2010).
These models can predict optical efficiency as well as the thermal flux density with
a good degree of accuracy. For example, in the following Figure 14.3.16 there is a com-
parison of experimental measurement and the simulation results for the PS-10 power
plant at noon on 21 March. The simulation required detailed information about the
heliostat number, positions and optical properties, as well as receiver characteristics.
All these simulation tools are based on optical and geometrical correlations which
aim at maximizing the yearly optical efficiency. In the literature, there are several
solar field configurations which belong to three different groups: surround fields (the
heliostats surround the receiver), north fields (the heliostats are just on the north side
of the receiver) and a configuration in between these two. In general, north fields have
a higher efficiency for high latitude or high incidence angles, while surround fields are
typical of locations close to the equator. Examples of two configurations are shown in
Figure 14.3.17.
As anticipated, the optical efficiency of central tower systems is higher than
single-axis tracking systems since the solar incidence angle on the heliostats is lim-
ited. An example of the incidence angle modifier for a central tower system is shown
Figure 14.3.16 Measured flux on PS-10 tower (top) and simulation results with Delsol for the same
plant on the 21st of March at midday (Colzi et al., 2010).