Experimental and simulation on night heat dissipation characteristics and freezing process of evacuated tube solar water heaters

Abstract

The winter operation of all-glass evacuated tube solar water heaters (ETSWH) often encounters the problem of ice damage. Studying their nighttime heat dissipation and freezing characteristics can further reduce the risk of frost damage during winter use. A numerical model for the nighttime heat dissipation of all-glass evacuated tube collectors is proposed in this study. An enthalpy equation is also introduced to analyze the heat dissipation process and freezing conditions of water inside the ETSWH under lower temperature conditions. Based on this, a nighttime heat loss model for solar vacuum tube collectors, including the effects of radiation and phase change factors, is established and validated through nighttime temperature drop experiments, which can effectively predict the temperature distribution and antifreeze performance of ETSWH during the nighttime heat dissipation process. In addition, the independent effects of the initial temperature of the water inside ETSWH, the thickness of the water tank insulation layer, and the absorptive coating emissivity on the heat dissipation process of ETSWH are studied, and the heat dissipation of ETSWH during winter nights in five different climatic zones in China is simulated. Experimental and simulation results show that under cold and clear winter weather conditions in Luoyang, ice forms in ETSWH during nighttime. The icing first occurs at the bottom of the vacuum tube, and the icing length on the wall and the icing thickness at the bottom can reach 1145 mm and 15 mm, respectively. In severely cold areas and cold regions such as Harbin and Beijing, there is a high risk of frost damage to ETSWH during winter. Keeping the initial working fluid temperature in the heater above 50 °C, ensuring a minimum insulation thickness of 50 mm, and reducing the emissivity of the absorptive coating in the heater to below 0.07 can effectively solve the freezing problem and have significant implications for the promotion and application of all-glass evacuated tube collectors.