Heat Transfer Mechanism And Experimental Research Of Trough Solar Collector With Heat-pipe Evacuated Tube

In the solar thermal utilization technology,the non-uniform distribution of wall temperature and working medium temperature in the receiver have an impact on the heat transfer performance of the trough collector system with linear evacuated tube.Heat pipe has good isothermal property and high thermal conductivity.In this paper,the solar collector which combines heat-pipe evacuated tube receiver with parabolic trough concentrator was studied as follows.First,the structure of parabolic trough collector system with heat-pipe evacuated tube,was introduced.The system includes trough concentrator and heat-pipe evacuated tube receiver.Heat pipe evacuated tube includes two-phase closed thermosyphon and singlelayer glass tube.The working principle of of heat-pipe evacuated tube was analyzed.According to the working conditions of the system,the working fluid of heat pipe is water and the shell material is stainless steel.According to the working characteristics of heat pipe evacuated tube,the azimuth-altitude axis tracking mode was chosen in this paper.Secondly,according to the characteristics of the tracking mode of the azimuth-altitude axis of the collector system,the optical performance of the concentrator under four different manual adjustment modes was simulated with SolTrace software.The simulation results show that the optimum tilt angle of the concentrator is determined by the solar altitude angle at the noon of the same day,and the adjusting time of the azimuth axis is not more than 3 minutes.Furthermore,taking optical efficiency and the uniformity of heat flux distribution as the measurement standard,and taking into account tracking error.The out diameter of absorbing tube and the annular space gap between the metal pipe and the glass envelop were optimized.The diameter of the metal pipe of the receiver is selected as 30 mm,and the annular space gap between the metal pipe and the glass sleeve is selected as 10 mm in the paper.Thirdly,a azimuth-altitude tracking trough concentrator experimental platform was built.The optical performance of the concentrator was tested with different test positions along the main optical axis of the concentrator,solar altitude angle and concentrator inclination angle.The results show that the test position of the most concentrated spot distribution on the receiving target surface is the focal length position of the trough concentrator designed theoretically.It can be judged that the linearity error of the fabricated trough concentrator surface is small.When the trough concentrator is tilted,and azimuth single-axis tracking,the solar altitude angle and the tilt angle of the concentrator mainly affect the end loss of the spot.By comparing the experimental results with the simulation results,there are 2° errors when the tilt angle of the manual tracking test platform loaded with the concentrator is adjusted.Finally,according to the heat transfer characteristics of parabolic trough collector system with heat-pipe evacuated tube,a one-dimensional heat transfer resistance mathematical model was established.Under the conditions of different solar direct irradiation intensity,ambient temperature,inlet temperature of heat transfer fluid and wind speed,the heat transfer resistance of the system was calculated.The results show that,it is only related to the structure and material characteristics of heat-pipe evacuated tube which have the thermal resistance of the wall of heat-pipe evacuated tube,radiant and heat transfer resistance between metal tube and glass envelope and radiant resistance between glass envelope and sky.The remaining thermal resistance is greatly affected by the solar direct radiation intensity and the inlet temperature of the heat transfer fluid.The influence of ambient temperature and wind speed on the thermal resistance between the glass sleeve and the outside environment is greater.In all thermal resistance of heat-pipe evacuated tube,the heat transfer resistance between the metal pipe and the glass envelope is the maximum,and the conduction resistance of the tube wall in the evaporator section is the minimum.