Solar Engineering of Thermal Processes, 4th Edition

7.8 Performance of CPC Collectors

The basic equation summarizing the performance of a CPC collector is Equation 7.3.12, where S is calculated by Equations 7.7.1. The remaining question is the calculation of the appropriate thermal loss coefficient Losses from CPCs will include losses from the receivers (which are calculated by methods that are basically the same as for other collectors) plus losses from manifolds, as shown in Figure 7.5.4.

Rabl (1976b) presents a discussion of calculation of loss coefficients for a CPC collector geometry using a flat absorber. Figure 7.8.1 shows his estimations of overall loss coefficients per unit area of absorber for use in Equation 7.3.12. The coefficients are functions of concentration ratio; the method used in their estimation assumed fixed conduction losses independent of concentration ratio, and both radiation and convection from plate to cover are to some degree functions of the concentration ratio. Estimates are shown for two plate emittances and two plate temperatures.

Figure 7.8.1 Estimated loss coefficients for CPC collectors with flat absorbers. From Rabl (1976b).

With estimates of from Figure 7.8.1, from measurements, or from calculations ...