Environmental Engineering Reference
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Figure 5.3 Plot of system efficiency g L (mechanical power output divided by sun power input)
versus collector temperature (referred to as T c in text), assuming sun at 6000 K and ambient exhaust
at 300 K. The peak efficiency is 0.854 at 2544 K ([53], p. 52, Figure 3.1.
A plot of the ef ciency as a function of the collector temperature T c is shown in
Figure 5.3.
5.3
Solar Thermal Electric Power
This category is also known as concentrated solar power, CSP, with twomain forms,
the large central receiver and the parabolic trough system. Large central receiver
solar thermal electric power plants are shown in Figure 5.4. Design of the mirrors
and their tracking is a feat of engineering. Only in a large systemdoes it seem likely
that the theoretical ef ciency 84.5%could be approached because of the dif culty in
maintaining components and heat-carrying fluids at temperatures approaching
2544 K. In such high temperatures, it seems that molten salts, or a molten metal
such as lithium, with boiling point 1615 K, are good candidates, because oils
will decompose and steam would require an unattainably large pressure. From
Figure 5.3, Equation 5.3a, the efficiency at 1615 K is 0.81, much higher than found
in photovoltaic systems, as we will see. In fact, the PS10 system, left in Figure 5.4,
has 11MW capacity, with 624 metal glass mirrors of 120m 2
area, and uses
pressurized steam at 250 C and 40 bar
(http://www.solarpaces.org/Tasks/
Task1/ps10.htm.).
So, this system, working at 250 C, must be far short of the available ef
ciency. As a
benchmark, tungsten has a melting point 3683 K, has been used in light bulbs for
decades, and would allow a much higher temperature receiver assembly. Other high-
temperature metals are molybdenum, tantalum, and titanium. (Aircraft manufac-
turers and architects have learned to use titanium to considerable advantage.)
At 250 C, the theoretical system ef ciency is in the vicinity of 43%, by looking
at Figure 5.3 and using Equation 5.3a. An empirical ef ciency estimate for PS10
can be made easily: the receiving area is 624
120m 2 , with input 1000W/m 2 ,
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