Page 131 - A Comprehensive Guide to Solar Energy Systems

130 A COmPREHEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

has shown that most of the investment required to implement CSTP plants is spent at re-
gional or national level, thus empowering the local economy and reducing currency flight
to foreign economies.
At present, four different technologies are available for CSTP systems:

1. Parabolic-trough collectors
2. Central receiver systems
3. Compact linear Fresnel concentrators
4. Stirling dishes

The main differences among these four technologies are: the way they concentrate the
direct solar radiation onto the receiver; the geometry of the receiver, and the concentra-
tion ratio (i.e., the quotient between the solar flux density at the surface of the receiver and
the flux density of the primary direct solar radiation). The higher the concentration ratio,
the higher the optimum temperature that can be achieved by the working fluid in the re-
ceiver. The optimum temperature is that achieving the highest efficiency of the conversion
from thermal energy to electricity in the power block.
If a simplified model of a CSTP plant is used to assess the overall plant efficiency versus
the concentration ratio, the curves given in Fig. 7.2 are obtained. These curves show that
the optimum working temperature and the associated maximum efficiency increases with
the concentration ratio, C. Hence the importance of achieving high concentration ratios to
increase the temperatures and efficiencies.

FIGURE 7.2 Theoretical efficiency and associated working temperature versus the concentration ratio, C. [8].