Simulation Study On Performance Of A New Medium Temperature Parabolic Trough Collector

A new type of medium-temperature parabolic trough collector is developed,which can absorb solar radiation with high efficiency and low cost.It generates medium-temperature hot water at 120°C~200°C,and drives solar double-effect air conditioners.The design,optical properties,one-dimensional heat transfer characteristics and three-dimensional thermal simulations of the collector are studied.The main contents and conclusions are as follows.1)The optical and tracking systems of the collector are theoretically studied.Parameters such as concentrating ratio,edge angle and focal length are quantitative analyzed.From the perspective of saving material,easy processing and reducing errors,the new medium-temperature collector equationx~2(28)3.2z(7)-0.894m?x?0.894m(8)is designed.At the same time,a single-axis tracking system is designed for the parabolic trough collector(PTC),and a programmable logic controller is used to initiatively track the the position of the sun.2)Based on the Monte Carlo ray tracing method,a concentrator tracking model of the collector is set up,which aims at the simulation of the optical characteristics of the collector during operation in Nanjing.The distribution of energy flow density in the occlusion area and the decreasing area has the largest gradient,followed by the increasing area.The truncation factor is used to analyze the errors of collector.The shape error,tracking error and installation error have the largest influence on the truncation factor.The error during the operation is to ensure that the optical efficiency of the collector is not less than 75%and the distribution of the energy flow density on the surface of the metal abso rber tube is relatively uniform.The shape error of collector must be satisfied k?2.5mrad.The accuracy of PLC control system must be satisfied(35)p?0.3?.The deviation of the receiver in the X-axis direction must satisfy(35)x?0.6cm.The deviation in the Z direction must satisfy-0.4cm?(35)z?0.8cm.3)According to the energy flow,a one-dimensional heat transfer model of the collector is established.The result of the simulation is lower than the experimental result of National Renewable Energy Laboratory(NREL),and the relative error is within 5%.The factors which influence the heat transfer of metal heat pipe,annular space and glass tube are calculated and analyzed.The mathematical models of heat loss,instantaneous thermal efficiency and temperature difference at the inlet and outlet of the collector are established.The heat loss of the collector increases with the increase of the temperature of the heat pipe.But the heat efficiency of the collector decreases with the increase of the temperature of the heat pipe.The temperature difference between the inlet and outlet of the collector is greatly affected by the inlet temperature of the Heat transfer fluid(HTF),the flow rate of the HTF and the direct radiation intensity(DNI).Under the condition of good vacuum degree,the ambient temperature and wind speed have little effect on thermal performance parameters.The heat loss increases 2.1W/m,the instantaneous thermal efficiency decreases 0.12%,and the temperature difference decreases 0.14°C,when the ambient temperature decreases 10°C and the wind speed increases 2m/s.4)Considering the limitation of the one-dimensional heat transfer model,the circumferential energy flow density obtained by optical simulation is used as a boundary condition.ANSYS software is used to simulate the temperature field and velocity field during the operation of the collector.The thermal simulation results of the temperature difference between the inlet and outlet of the collector are lower than the results of one-dimensional heat transfer simulation,and the relative error is within 7%.The temperature of the metal absorption tube during the operation of the collector is unevenly distributed in the axial and radial directions,and the area with the largest temperature gradient is between 0.01m and 0.03m in arc length.Inlet flow velocity and direct radiation intensity have the greatest influence on the temperature field of the collector.The temperature gradient of the collector decreases with the increase of the flow rate and increases with the increase of the direct radiation intensity.