Environmental Engineering Reference
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Figure 14.2.1 Ideal conversion efficiency from solar energy to mechanical work assuming different
concentration ratios (C) and absorber temperatures.
From this equation it can be noted that the thermal efficiency and, consequently,
the heat recovered from the collecting system increases with the concentration ratio;
Q EMITTED is constant since it depends only on absorber temperature, but, being divided
by the concentration factor, its relative contribution decreases.
In order to better explain these concepts, the resulting ideal conversion efficiency
(see Equation 14.2.1) as a function of maximum temperature and concentration ratio
is summarized in Figure 14.2.1 (for this example α and ε are assumed equal to 0.94
and G to 800 W/m 2 ).
In addition to the thermodynamic advantages previously discussed and shown in
Figure 14.2.1, the adoption of a concentration system substitutes expensive compo-
nents working at high temperature, like the absorber, with cheaper mirrors at ambient
temperature. Moreover, since the performance of the absorber is fundamental and the
amount required is reduced, research activity can focus on its improvement, pushing
its performances to higher values.
An example of the material absorptivity/emissivity impact on system efficiency
is shown in Figure 14.2.2. For simplicity a constant absorptivity is assumed, while
three different values of emissivity are considered (0.94, 0.5 and 0.1). Reducing the
emissivity while keeping a high absorptivity reproduces the properties of an advanced
material with high performance values, though probably at higher cost. However, a
very high concentration ratio can significantly reduce the economic impact because of
the overall limited influence on total plant costs. Moreover it can be noted that low
emissivity is fundamental for a low-medium concentration ratio (in the range of 100)
and high absorber temperature ( > 800 K).
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