Thermal performance of a heat-pipe evacuated-tube solar collector at high inlet temperatures

Abstract

The thermal performance of a heat-pipe evacuated-tube solar collector is investigated in the present paper. Hot water is targeted at a relatively high inlet temperature of 70–90 °C to predict the collector performance that utilized in solar cooling applications such as adsorption and absorption systems. A mathematical model of the system was developed and validated experimentally under weather conditions of Alexandria, Egypt. The theoretical model of the collector is enhanced by considering the effect of the thermal mass of the system, hence the maximum relative error between the experimental and theoretical results was reduced from 12.5% to 4.4%. The effects of the inlet water and ambient temperatures, number of evacuated tubes, water mass flow rate, and solar irradiance on both the exit water temperature and the collector’s efficiency were all considered in comprehensive studies. The results showed that the collector performance was strongly affected by all the above parameters but with different manners. For examples, at certain values of the water mass flow rate and solar irradiance, the exit temperature increases with the increase in the number of evacuated-tubes reaching a specified number above which the increase in the exit water temperature will be eliminated. However, the thermal efficiency is always better at lower numbers of tubes. Consequently, the total number of the evacuated tubes for different series and parallel arrangements can be minimized at specified operating conditions based on the model’s results, leading to minimal capital cost. In addition, an expression for predicting the collector efficiency was derived based on the mathematical model’s results to be used in calculating the exit water temperature at different operating conditions and number of evacuated tubes. This expression can be effectively used for modeling such type of collector employed in a thermally driven cooling cycle.