A New Thermal Performance Factor for Solar Air Collectors

It has been shown by different authors that thermal performances of flat plate solar air heaters are strong functions of the number and nature of the transparent covers and selectivity of the absorber of the collector. In this paper we develop a mathematical model to analyze the heat exchanges in four different types of solar air collectors. For each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first order non-linear differential equation that is able to predict the thermal behaviour of the collector. Our heat transfer model clearly demonstrates the existence of a new dimensionless parameter, referred to as the thermal performance factor of the collector, which compares the useful thermal energy which can be extracted from the heater to the overall thermal losses of that collector for a given set of input parameters. This new thermal performance factor is therefore a useful tool for assessing the ability of different collectors to convert solar radiation for the same set of input working conditions. A sensitivity analysis of our thermal model has been performed for the most significant input parameters such as the incident solar irradiation, the inlet fluid temperature, the air mass flow rate, the depth of the fluid channel, the number and nature of the transparent covers in order to measure the impact of each of these parameters on our model. We succeeded to show in this study that the thermal behaviour of an air collector can be presented in the form of a chart where the values of the fluid temperature rise between the entrance and the exit of the heater per unit incident irradiation and of the thermal efficiency η are easily read when the air mass flow rate per unit collector area G, the fluid channel depth ef and the incident solar irradiation I are known for the collector under consideration. This is a very useful tool for designers to answer the what if questions.