Basic Research Needs for Solar Energy Utilization

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0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

Temperature [K]

C = 60

C = 100

C = 500

C = 1,000

C = 1,500

C = 2,000

C = 5,000

C = 10,000

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

ηopt = ρn (1 - Sp)(1 - Sh)(1 - Bl) , (5)

where

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.