Page 18 - Design of Solar Thermal Power Plants

1.2 BRIEF INTRODUCTION TO SOLAR THERMAL POWER GENERATION 11

FIGURE 1.3 Relationship of solar thermal power generation system efficiency, thermal-
collecting Temperature, and concentration ratio.

CSP generation system design. The greater of the concentration ratio, the
more possible it is to achieve a higher maximum temperature (Fig. 1.3).
The concentration ratio is the ratio of mean radiation flux density that
gathers on the surface of the receiver’s aperture to the solar direct normal
irradiance that enters the aperture of the concentration field. Annual po-
wer generation is a key factor that determines the benefits of an CSP plant.
The annual power generation of an CSP plant is the product of the CSP
plant’s annual efficiency and that solar direct normal irradiance that
has been cast on the aperture area of the concentration field. Thus the
CSP plant’s annual efficiency and solar direct normal irradiance at the
construction siteof the CSP plant are two extremely critical factors.The CSP
plant’s annual efficiency (which can also be deemed the system efficiency)
is determined by the thermal collection efficiency and the efficiency of the
thermal engine. As shown in Fig. 1.3, based on a certain concentration
ratio along with increases in the thermal collection temperature, the
system efficiency curve will demonstrate a “saddle point,” which is
mainly caused by the increased efficiency of the thermal engine along
with the increment of the thermal collection temperature. However, due to
increased heat losses by the receiver, thermal collection efficiency de-
creases after reaching a certain level. Therefore, in the CSP generation
system, simply increasing the working temperature of the system is not
advised; instead, the concentration ratio and thermal collection temper-
ature should be comprehensively considered by applying the high-ratio
daylight concentration and high-performance absorber techniques.
Based on the concentration mode, CSP generation technologies can be
divided into two systems, point focusing and line focusing, with the point