Performance Research Of Non-Concentrating Medium-temperature Solar Evacuated Collectors

For non-concentrating solar collectors,vacuum insulation is the main means to increase their working temperature.To meet the needs of medium-temperature heat collection at 100?200?,the following research was carried out on evacuated tube collector(ETC)with additional thermal resistance and evacuated flat plate collector(EFPC):Using the method of exergy analysis,the heat loss and exergy analysis models of ETC with additional thermal resistance were established,and the numerical model was verified by experimental results.The influence of additional thermal resistance on the performance of ETC was analyzed,and the effects of irradiance,operating temperature and working fluid flow rate on the exergy efficiency of ETC with additional thermal resistance were explored.The results show that additional thermal resistance can effectively increase exergy efficiency of ETC.When the solar irradiance is 800 W/m2,the exergy efficiency relative value of ETC can be increased by 7.1%at the operating temperature of 80? and 38.2%at the operating temperature of 180?.In addition,additional thermal resistance can increase the maximum exergy efficiency from 14.6%to 17.5%,and increase the maximum exergy efficiency temperature from 135? to 156? at the solar irradiance of 800 W/m2.Additional thermal resistance can greatly reduce the radiation heat transfer irreversible losses of the glass tube and the absorber plate,which is the main reason of the exergy efficiency improvement.An outdoor test rig for EFPC was developed.The experimental results show that when the solar irradiance is 910 W/m2,the normalized efficiency intercept of the EFPC is 76.3%.In the whole experimental working temperature range,the thermal efficiency of EFPC is more than 50%.Compared with ETC with additional thermal resistance,EFPC has higher thermal efficiency,and the thermal efficiency of EFPC can be increased by 10.2%,10.0%and 9.8%at the operating temperature of 100?,120? and 140?,respectively.The model of EFPC was established,and the numerical model was verified by experimental results.The performance of EFPC and flat plate collector was compared,and parameter analysis of EFPC was conducted.The simulation results show that when the solar irradiance is 800 W/m2,the maximum exergy efficietcy of EFPC is 17.0%at the operating temperature of 151?.Compared with flat plate collectors,EFPC has significant advantages.The absolute value of the thermal efficiency of EFPC has increased by 37.1%,47.5%and 57.3%at operating temperatures of 80?,100?,and 120?,respectively.