A study of unidirectional spiral tube for air evacuation in a solar heater with phase-change material

Abstract

Solar air heaters combined with a phase-change material (SAH-PCM) have become a focus of research on the use of solar thermal energy. However, the specific heat of the air is small and the heat exchange efficiency is slow. In this study, we carried out three designs to improve the thermal efficiency of evacuated tube solar air heater including unidirectional flow, PCM-rod, and spiral tube. An experimental test platform was built to evaluate the thermal performance of the SAH-PCM, the temperature field and flow field in the vacuum tube were analyzed using ANSYS software, and the environmental assessment was also carried out. The numerical analysis obtained the optimal size of the PCM-rod, and also calculated the heat enhancement coefficient of the spiral tube to be 1.1. The experimental tests found that the outlet temperature fluctuation of the SAH-PCM could be effectively avoided, but the average outlet temperature is reduced by 31.5 °C; the heat loss rate was reduced by 58.5%; the thermal efficiency of PCM can reach up to 844.9%; the operating hours at night have been extended by more than 3 h. In addition, the correlation equation between the outlet temperature and the air flow rate is also established. Concerning the environmental aspect, the 8 m2 SAH-PCM can reduce CO2 emissions by more than 226.4 kg per hour. The results show that the structural design of unidirectional flow and spiral tube and the use of phase-change materials can improve the thermal performance of the air heater.