Basic Research Needs for Solar Energy Utilization - Office of ...

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Temperature [K]
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Carnot
Figure 73 Variation of ideal power conversion efficiency of a solar thermal system with
temperature and sunlight concentration ratio; assumed DNI = 1000 W/m 2 [after Fletcher and
Moen (1977)]
As shown in Figure 73, ideally, the power conversion efficiency of solar thermal systems
increases with temperature and concentration. However, the overall efficiency of a real system
depends also on the optical efficiency; that is:
ηsys = ηopt ηrec ηeng . (4)
Here the optical efficiency ηopt is the ratio between the solar radiation reaching the receiver
aperture (i.e., the “target”) and the direct normal solar radiation approaching the
reflector/concentrator collection area.
After some simplification, the optical efficiency becomes
where
ηopt = ρ n (1 - Sp)(1 - Sh)(1 - Bl) , (5)
ρ = reflectivity of optical components
n = number of reflections
Sp, Sh, Bl = spillage (including back reflection), shadowed, and blocked portions
of the incoming sunlight, respectively.