Thermodynamic Performance Analysis And Working Fluid Selection Of Trilateral Cycle Power Generation System

With the depletion of fossil energy such as coal,oil and natural gas and more and more serious environmental problems,countries around the world attach great importance to the efficient use of energy.In recent years,waste heat recovery has become a new field,which has been widely studied by scholars all over the world.Organic Rankine cycle(ORC)is a technology for efficient utilization of medium and low temperature resources.Trilateral cycle system is proposed to optimize the problem of large loss caused by poor match of heat source and working fluid temperature in organic Rankine cycle evaporator.Based on the trilateral cycle system with isentropic expansion process,the thermodynamic optimization is carried out in this paper.In addition to the system thermal efficiency,system exergy efficiency and net output power,the evaluation indicators also focus on the expander flow ratio and heat source utilization ratio.The mathematical model of trilateral cycle is established and the effectiveness-NTU method is used to calculate the heat transfer of heater.The key factors affecting the thermodynamic performance of the system,such as the inlet temperature of the expander,the condensing temperature,the ratio of the average heat capacity flow rate,and the inlet temperature of the heat source,are optimized and analyzed using the control variable method.It is concluded that increasing the inlet temperature of the expander(not exceeding the critical temperature of the working fluid)and decreasing the condensing temperature can improve the system performance,however,the system performance is limited by the expander flow ratio and isentropic efficiency of the expander;when the inlet temperature of the expander approaches the critical temperature of the working fluid,the system has the best performance.The system has the best performance when the ratio of heat capacity flow rate between working fluid and heat source is 1.There are two main lines of working fluid selection: one is to pay attention to the expander flow ratio;the other is to match the heat source.The effects of thermal physics properties,such as critical temperature,saturated vapor pressure at condensing temperature and density of working fluid,are analyzed emphatically.The results show that the system performance is optimal when the inlet temperature of heat source is close to the critical temperature of working fluid;n-butane is the best working fluid with low expander flow ratio and high output power;two fitting formulas of expander flow ratio and saturated vapor pressure at critical temperature of working fluid and condensation temperature of working fluid are obtained respectively.To improve the utilization rate of heat source in single trilateral cycle power generation system,a combined cycle system is proposed to realize cascade utilization of heat source.The trilateral cycle is regarded as the top cycle(high temperature section)and the organic Rankine cycle as the bottom cycle(low temperature section).The heat source of top cycle is high temperature exhaust heat(diesel engine exhaust and gas turbine exhaust),or high temperature hot water with a certain pressure;the heat source of bottom cycle is top condenser heat release and low-temperature waste heat.The results show that the utilization ratio of heat source,net output power,thermal efficiency and exergy efficiency of the combined cycle system are better than those of the single trilateral system.The value of secondary recovery is significant.The study of the important parameters of the combined cycle system shows that there are optimum values for the top condensing temperature,the bottom evaporation temperature and the ratio of heat capacity flow rate between top cycle working fluid and heat carrier.