Effect Of Non-condensable Gas On Organic Rankle Cycle Performance

Organic Rankine Cycle(ORC),as one kind of low-grade waste heat utilization technology,can not only improve the utilization rate of primary energy,but also has broad market prospects in the utilization of renewable energy.However,during the operation of the ORC system,the non-condensable gas(NCG)in the system has a great influence on the condensation heat transfer process of the working fluid,which significantly reduces the output power and thermal efficiency of the system.At present,the research on NCG mainly focuses on the fields of heat pipe,refrigeration,steam condensation,etc.,while there are few researches on the influence of NCG.To bridge this gap,the thermodynamic model of the ORC system with NCG and the condensation heat transfer model of the condenser are constructed in this paper.The influence of different NCG on the thermodynamic performance of the ORC system and the heat transfer performance of the condenser were analyzed.Firstly,the condenser liquid storage tank model was established,and the gas-liquid phase equilibrium data with NCG was calculated based on the PR equation.The thermodynamic model of ORC containing NCG was further established,and the types of NCG that may be generated in the system were analyzed: In subcritical ORC,the effects of air and hydrogen fluoride on the performance of eight different working fluids systems were analyzed.In the transcritical ORC,the effects of air,methane,ethane and propylene on the performance of three different working fluids systems were analyzed.The results show that the NCG causes the condensing pressure of the system to increase,while the inlet and outlet pressure ratio of the expander,the net power output of the system,and the thermal efficiency to decrease.When the air mass fraction was 5%,the net output power of the ORC system using R1234 yf working fluid decreased by 27.09%,and the thermal efficiency decreased by 1.47%.When the mass fraction of methane was5%,the net power output of the ORC system using n-pentane working fluid decreased by 6.63%,and the thermal efficiency decreased by 0.72%.Based on the analysis of the thermodynamic performance of ORC,a plate condenser in a transcritical ORC system with three working fluids is designed.Based on the diffusion layer theory,the condensation heat transfer model of the working fluid containing NCG on the vertical wall is established.The results show that NCG will cause a significant decrease in the heat transfer coefficient and heat transfer coefficient.Among them,methane in the n-pentane system has the greatest influence.When the mass fraction of methane is 5%,the condensation heat transfer coefficient decreases by54%.The NCG will cause incomplete condensation of the working fluid at the outlet of the condenser,which will bring the risk of cavitation to the operation of the working fluid pump.Using the ORC system of n-pentane,when the mass fraction of methane is5%,the vapor quality of n-pentane at the condenser outlet becomes 0.25.To eliminate the influence of NCG on the vapor quality of the working fluids at the outlet of the condenser,the maximum heat exchange area of the condenser needs to be increased by about 46.2%.