Exergy Analysis for the Performance of Solar Collectors The optimum control and performance evaluation of solar collectors are analyzed from the standpoint of exergy. The pressure drop inside the collector is introduced to the analysis using the Hottel-Whillier model. By treating the friction process as exergy loss, the optimum operating conditions are presented in a simple statement. The maximum capability of collectors is determined and expressed by a relationship among the collector parameters and the environment in which they operates. Introduction The optimum operating conditions of solar collectors have so far been investigated on the basis of collected thermal energy. The criterion generally adopted is to maximize the difference between the collected thermal energy and the required pumping power First, exergy analysis using the Hottel-Whillier model Exergy Analysis Using the Hottel-Whillier Model The properties of flat-plate solar collectors are, in general, given by the relationship among the outlet and inlet temperatures T 0 , Tj, the mass flow rate m, and insolation /, as From equation (1), instantaneous thermal efficiency, i},, defined by o'm /kg. s" 1 ) For the investigation of the optimum control and performance evaluation of solar collectors, it is sufficient to assume a constant environment for the first step. Therefore, only the instantaneous efficiency is considerd in the following discussion, and the daily efficiency is not treated. As one example, the insolation, I, and ambient temperature, T a , are fixed as 7=650 W/m 2 , r a = 300 K The stagnation temperature, T sl , is obtained by setting ij, = 0 in equation (3), as With respect to this case, r/, is depicted in Assuming that the specific heat, C P , at constant pressure is constant, A/z and As in equation Ae Eliminating T 0 by equation Next, the maximum exergy efficiency ijf ax over all of 7", and rh is considered. Expanding equations Setting equations = collector area = specific heat at constant pressure = inner diameter of absorber tube = friction factor = collector efficiency factor = heat removal factor = volumetric flow rate = solar radiation incident on the collector = absorber tube length = mass flow rate = ambient temperatur
