The stagnation parameter was obtained by carrying out testing under stagnation conditions; that is, energy collected by concentrator was used to raise the temperature of working fluid to its stagnation temperature. The ratio of the temperature of the stagnation temperature and ambient temperature to the incidence solar radiation is defined as the stagnation temperature. In addition, the optimal operating temperature can be determined which could in turn effect minimum entropy generation (Kalogirou, 2004).