a b s t r a c tEnergy efficiency is generally used as one of the most important parameters in order to introduce and compare thermal systems including flat plate solar collectors despite of the fact that the first law of thermodynamics is not solely capable of demonstrating quantitative and qualitative performance of such systems. In this paper, a theoretical and comprehensive model for energy and exergy analysis of flat plate solar collectors is presented through which the effect of the entire design parameters on performance can be examined. Upon the verification and confirmation of the model based on the experimental data, effect of parameters such as fluid flow rate and temperature, type of working fluid and thickness of the back insulation on the energy and exergy efficiency of the collector has been examined and based on the analysis and comparison of results, the optimal working condition of the system has been determined. According to the results, designing the system with inlet water temperature approximately 40 more than the ambient temperature as well as a lower flow rate will enhance the overall performance.

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In this paper, a heat pipe evacuated tube solar collector has been investigated both theoretically and experimentally. A detailed theoretical method for energy and exergy analysis of the collector is provided. The method is also evaluated by experiments. The results showed a good agreement between the experiment and theory. Using the theoretical model, the effect of different parameters on the collector's energy and exergy efficiency has been investigated. It is concluded that inlet water temperature, inlet water mass flow rate, the transmittance of tubes, and absorptance of the absorber surface have a direct effect on the energy and exergy efficiency of the heat pipe evacuated tube solar collector. Increasing water inlet temperature in heat pipe evacuated solar collectors leads to a decrease in heat transfer rate between the heat pipe's condenser and water.

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In this paper, an evacuated solar heat pipe collector is investigated theoretically and experimentally. Heat transfer formulas were used for theoretical modeling, and a test method was adopted from ISO 9806-1 to compare the theoretical model with the experimental results. The collector efficiency and useful heat gain were compared between the theoretical and experimental methods. The effect of the working fluid flow rates and collector area were also investigated and discussed. The comparison shows that the theoretical model is in good agreement with the experimental results and is capable of predicting the efficiency, useful heat gain, and working fluid outlet temperature of an evacuated heat pipe collector with good accuracy.