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

periods and extend the system operation beyond the solar hours. Fuel hybridization prevents
direct normal irradiation (DNI) variations from adversely affecting the engine efficiency;
therefore, hybridized solar systems typically have higher solar-to-electricity conversion
efficiency and hence produce more solar power than similar nonhybrid systems. Various storage
options can also be added to most solar thermal systems.
The three main components of a concentrated solar thermal system are (a) the
reflector/concentrator that reflects concentrated light onto the aperture of a receiver positioned at
its focus, (b) the receiver that converts the radiation to heat (or chemical potential), and (c) the
engine that converts heat to electricity. The power conversion efficiency ηPC is the product of the
receiver and engine efficiencies:
ηPC = ηrec ηeng . (1)
Figure 73 demonstrates the characteristics of solar thermal conversion efficiency by showing the
variation of ideal system efficiency with temperature and sunlight concentration. The ideal
receiver efficiency, assuming no losses in the optical concentration component and negligible
conduction and convection losses, is
where
σ = Stefan-Boltzmann constant
TH = effective receiver reradiation temperature
TL = ambient temperature
I = flux (or intensity) of the DNI at design conditions
C = sunlight concentration ratio.
ηrec = 1 - σ(TH 4 -TL 4 )/IC, (2)
The limiting efficiency of a heat engine is given by the Carnot expression for the ideal engine:
ηeng = 1- TL/TH . (3)
In this simplified ideal case, the upper engine temperature is assumed to be equal to the effective
receiver reradiating temperature.
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