Heat Removal Factor Analysis Of Solar Collector And Its Application In Performance Optimization

The thermal performance of two types of solar collectors,namely,double-glazing flat plate collector and linear Fresnel reflector collector,is studied in this paper using heat removal analysis method.The mathematical analytic expressions of some important parameters such as instantaneous efficiency,total heat loss coefficient and heat removal factor are deduced.The accuracy and rationality of the mathematical model is verified by experiments.The main research contents are summarized as follows.Firstly,the structure of double-glazing flat plate collector is analyzed as well as its thermal performance.The thermal network diagram is drawn,and the energy loss of each part is analyzed.It is found that the heat loss at the top is the main proportion,and the mathematical analytic expression of the total heat loss coefficient UL is given.Finally,the instantaneous efficiency equation and the mathematical analytic expression of the heat removal factor FR are deduced.Secondly,the experimental system of double-glazing flat plate collector is built.According to the national standard GB/T4271-2007 Test methods for the thermal performance of solar collectors,the thermal performance of double-glazing flat plate collector is tested.Under the working condition of standard flow rate 0.02kg/m2·s,the instantaneous efficiency equationη=0.83-2.90T*is obtained by fitting the test results with the least square method,and the instantaneous efficiency curve is drawn.Based on the instantaneous efficiency equation,the total heat loss coefficient UL=3.12W/m2·°Cand the heat removal factor FR=0.93are calculated.The theoretical and experimental values of heat removal factor under different working flow rates are compared.The error is between 1.27%～1.82%,which verifies the rationality of the mathematical model.Specific measures to optimize the collector performance of plate collector are put forward.Thirdly,the basic structure of linear Fresnel reflector collector is introduced.The structure curve of the CPC cavity absorber is introduced.The choice of the optimum initial angles of the mirrors in the linear Fresnel reflector system are analyzed,and the calculating formula under different sun directions are given.The heat removal performance of the CPC cavity absorber is studied and its thermal network diagram is drawn.The energy balance equation and the calculation formula of the heat removal are given.The mathematical analytic formula of the total heat loss coefficientULand the instantaneous efficiency equation of the linear Finel reflector are derived.The heat removal factor FR is introduced into the instantaneous efficiency equation and its mathematical expression is derived.Finally,the experimental system of linear Fresnel reflector collector is built.The instantaneous efficiency equation η=0.65-1.97T*of the linear Fresnel reflector collector is obtained and the instantaneous efficiency curve is drawn under the standard flow rate 0.02kg/m2·s.According to the instantaneous efficiency equation,the heat removal factor FR=0.88of the linear Fresnel reflector collector and the total heat loss coefficient UL=9.75W/m2·°Care calculated under the standard flow rate.