Modeling and optimization of a network of evacuated solar collectors attached to an absorption refrigeration system

The vapor absorption refrigeration system is an alternative cooling technology to vapor compression that can operate with low-grade energy sources, such as solar thermal energy. This study provides an analytical model, developed using heat and mass transfer equations in Microsoft EXCEL, to estimate the fluid outlet temperature of the collector (water), and thermal efficiency of the evacuated solar collectors as a function of inlet water temperature, solar intensity, and flow rate. To evaluate the optimum operating values to obtain the target hot water temperature required for operating the absorption system, a three-level Box-Behnken design matrix was generated using Design Expert-10 software selecting an appropriate input range, i.e., flowrate: 3 to12 L min^-1, inlet water temperature: 25°C to 35°C, and solar intensity: 550 to 1150 W m^-2. Performance values (response) were evaluated using the developed thermal model, and values obtained were inserted for each generated input combination in the design matrix followed by an analysis of variance (ANOVA) and regression analysis. Equations were generated to predict responses after testing the significance of the response surface quadratic model. Results showed an increase in the collector fluid outlet temperature with a lower flow rate of collector fluid, higher ambient temperature and solar intensity. Optimization showed the optimal range of collector fluid outlet temperature between 83°C and 90°C and thermal efficiency of 37.45% to 39%. This study can lead to the development of more efficient system design including solar collectors by maximizing the utilization of solar energy.