Influence of receiver’s downside radiant characteristic on the performance of the heat-pipe evacuated-tube solar collector

The receiver’s downside radiant characteristic of the heat-pipe evacuated-tube solar collector (HESC) has important influence on its performance. This influence is significantly related with the downside emissivity and operation temperature. This paper firstly developed a heat transfer model for the HESC, and a theoretical formula was derived to characterize the relationship between the heat loss and the downside emissivity of the HESC. Then, steady state experiment was performed to measure the heat loss of HESC with different downside emissivity, and the heat loss curves were obtained. Experimental results indicate that the heat loss of the HESC increases with the increase of the downside emissivity, but the increasing level differs for different operation temperatures. When the downside emissivity increases from 0.03 to 0.12, the heat loss of HESC only increases by 31% when the operation temperature is below 100°C, but the heat loss will increase to 96% when the operation temperature is over 200°C. This means that the change of downside emissivity can significantly affect the performance of the HESC at high temperature but affect little at low temperature. Based on this, a novel method by performing roughness treatment on the receiver’s downside is proposed to solve the overheat problem of HESC in summer. Two solar water heaters including 6 HESCs with standard and roughness tubes were tested on real weather condition. Experiment reveals that when the water temperature in tank is below 60°C, the two solar water heaters own similar temperature change. But when the temperature is over 80°C, the solar water heater with rough tube illustrates obviously lower temperature increase than that with standard tube. Therefore, it is very effective to prevent overheat of some solar water heaters used in highland zone in summer by increasing the receiver’s downside roughness.