Numerical Simulation On Effect Of Connection Modes And Flow Rate On The Energy Efficiency Of PVT System

The photovoltaic thermal（PVT）hot water system combines the original solar hot water system and PVT collectors,which can simultaneously supply electricity and hot water to meet domestic needs,and improve the combined energy output effectively.The connection mode of PVT collectors and flow rate are two critical factors influencing the performance of PVT system.While determining optimum connection mode for PVT collectors has not been previously reported in published literatures,and most of the researches about flow rate are about one particular connection mode of system.In this study,research on the effect of connection mode and flow rate on the energy efficiency of a hot water system under different conditions or radiation zones.The results of this study provide good guidance for determining optimum connection modes for PVT hot water systems in different radiation zones.Based on the law of conversion of energy,considering the ambient temperature,solar radiation,and heating load concurrently,a dynamic heat transfer model representing different connection modes for a PVT hot water system was developed using the FORTRAN programming language.The proposed model was validated through experimentation,and the comparison of the simulation results and experimental data can support the accuracy of the mathematical models utilized in this paper.The PVT hot water system comprised 12 collectors（each collector measured 2 m × 1 m）and 5 kinds of connection modes:N= 1,2,3,4,6,where N is the number of collectors in series;First,the calculation was conducted to optimization index of power output,actual heat gain and actual combined electrical energy respectively,when the flow rate is 0.02 kg·m^-2·s^-1 under different connection modes.For example Tengchong,the results indicate that in two different conditions,the connection mode of maximum power output is N=1,and the connection mode of maximum actual heat gain is different.Based on the annual performance of system,the optimum connection mode are N=1,N=4 and N=3 for the optimization index of power output,actual heat gain and actual combined electrical energy,respectively;Next,five cities are selected in better and available radiation zone respectively.The results indicated that the connection modes corresponding to the maximum actual combined electrical energy were N=2 and N = 3 in better and available radiation zone,respectively.In Shantou,high ambient and main water temperatures contributed to a 40 MJ increase in the actual combined electrical energy when the connection mode changed from N = 3 to N= 2;Finally,the effect of flow rate varies from 0.01 to 0.04 kg·m^-2·s^-1 on the energy efficiency under five different connection modes is analyzed.For example Tengchong,the results indicate that:1)The power output of N=1 decreases when the flow rate is more than 0.025kg·m^-2·s^-1,while power output of other connection modes increases as flow rate increases;2)The actual heat gain of water of N=1 decreases when the flow rate is more than 0.02 kg m^-2·s^-1.The heat gain of water of N=2 increases continuously as flow rate increases,N=3,4,6 increases volatility when flow rate is more than 0.025 kg·m^-2·s^-1,and tendency of volatility is more obvious when the radiation is insufficient;3)According to the the optimization index of annual actual combined electrical energy,the optimum connection mode and the optimum flow rate are N=2 and 0.035kg·m^-2·s^-1 respectively.The optimum flow rate is more than 0.02 kg·m^-2·s^-1 which suggested in relative standard.