Page 140 - A Comprehensive Guide to Solar Energy Systems

Chapter 7 • Concentrating Solar Thermal Power 139

FIGURE 7.7 Front view of a typical heliostat. The 32 facets composing its reflecting surface are clearly shown.

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water at 249°C into superheated steam at 565°C and 160 × 10 Pa. Because of the high ther-
modynamic quality of this superheated steam the annual solar-to-electricity efficiency of
this plant is 28.7%, which is significantly higher than the 14.4% efficiency of the plant PS-10.
The size of the heliostats used in a CSTP plant with central receiver does not depend
on the nominal power of the plant. As there is no consensus about the most cost-effective
strategy to achieve the required total reflective surface there are companies using helio-
stats with a large reflective surface per unit (>150 m ), while other companies are using
2
small heliostats (<20 m ). PS-10 and IVAnPAH plants are good examples of these opposite
2
2
approaches: the PS-10 plant uses 120 m heliostats for a total nominal power of 10 mW e
2
(net), while IVAnPAH uses 15 m heliostats for a total nominal power of 377 mW e (net). A
common approach among all the manufacturers of heliostats is the use of silvered glass
mirrors for the facets, with two variants: back-silvered thick-glass mirrors or sandwich-
type mirrors (a silver layer deposited between two glass pieces). Borosilicate glass is pre-
ferred whenever possible due to its high solar transmittance.
Central receivers for saturated or superheated steam are composed of walls made of
many parallel steel tubes inside which the feed water is converted into steam as it circu-
lates from the receiver inlet to the outlet. The outer wall of the steel tubes composing the
receiver are painted with a black thermal paint providing a high solar absorptance and
suitable for high-temperatures. Because of the high solar flux onto the receiver it appears
as bright as a fluorescent lamp when the receiver is in operation, despite the external black
color provided by the thermal paint (Fig. 7.6).