3D Construction Of Thermodynamic Cycle:Methodology And Application

Low and medium temperature heat is abundant in energy resources,and the amount of available low and medium temperature heat that could be used every year is about 4500 times of the total global energy consumption.As an eternal research direction in thermodynamics,thermal cycle is widely used in the development and utilization of low and medium temperature heat.However,in the existing research,the optimization of traditional cycle cannot break through the limitation of thermosphysical properties of working fluid on cycle efficiency.The construction of novel thermal cycle is complex,and the basic guidance method has not been put forward.In view of many problems existing in the research of thermal cycle at present,based on the idea of working fluid adjustment,a 3D construction method of thermal cycle was proposed,and the research was carried out from aspects of 3D ideal cycle,limiting cycle and actual cycle.In terms of ideal cycle,the ideal cycle of conversion among heat,chemical work and volume work was constructed,and the ideal conversion efficiency was defined.Based on the equation of state of ideal gas,the basic thermodynamic processes involving mass exchange,such as isovolumic isothermal process,isovolumic adiabatic process and isovolumic adiabatic process,were proposed.For the conversion between heat and chemical work,the ideal cycle was constructed by two isotherm isovolumic processes and two isovolumic adiabatic processes.It is found that the ideal conversion efficiency is only related to the heat sink temperature of heat source temperature.For the conversion between chemical work and volume work,the ideal cycle was constructed by two isothermal adiabatic processes and two adiabatic processes,and its ideal conversion efficiency is only related to the chemical potential of the mass source and mass sink.For the conversion between heat,chemical work and volume work,the ideal cycle was constructed by two isothermal adiabatic processes,four isothermal adiabatic processes and two adiabatic processes,and the ideal efficiency is equal to the Carnot cycle efficiency under the same temperature.In terms of limiting cycle,the limiting performance of the cycle was revealed by considering the thermos-physical parameters of the working fluid.For the traditional organic Rankine cycle,in the coordinate of temperature and entropy,the expressions of the limiting thermal efficiency and the limiting thermodynamic perfection of the simple and regenerative organic Rankine cycle using pure and zeotropic working fluids were derived respectively.The results show that the key working fluid properties affecting the limiting performance include the slope of working fluid saturated liquid line,the latent heat of phase change,the specific heat capacity at constant pressure and the glide temperature of zeotropic working fluid.For the 3D thermal cycle,the 3D limiting cycle was defined in the coordinate of temperature,entropy and mass flow rate.The expression of limiting efficiency was derived through the equivalent relationship between the space volume.Through the calculation of commonly using working fluids,it is found that the limiting performance of 3D thermal cycle is 7.3% ～ 16.42% higher than that of traditional organic Rankine cycle.In terms of actual cycle,a novel thermal cycle with multiple working fluids was constructed,which proves the feasibility and the advancement of the 3D construction method.According to the typical characteristics of dual heat sources and large temperature difference of internal combustion engine waste heat,the selection of suitable working fluids for thermal process was studied.Based on the 3D construction method of thermal cycle,the composition adjustable organic Rankine cycle was constructed by matching different working fluids with two heat sources.The thermodynamic performance analysis model was established and the optimal cycle parameters were optimized.Furthermore,the key parameters were analyzed.The results show that R123 / toluene(0.9 / 0.1)is the suitable working fluid in the main loop,and R123 / toluene(0.66 / 0.34)and R123/toluene(0.96/0.04)are the optimal working fluids in the high-pressure branch and low-pressure branch,respectively.The key parameters affecting the cycle performance include primary evaporation temperature,secondary evaporation temperature and separation quality.Under the optimal cycle conditions,the net output work,thermal efficiency and second law efficiency of the composition adjustable organic Rankine cycle are 50.07 kw,10.96%and 45.79%,respectively,which are improved to some extent compared with the simple organic Rankine cycle.In this study,a 3D thermal cycle research framework was initially constructed,and an ideal cycle of "heat-chemical work-volume work" was proposed.The limiting performance of thermal cycle based on the thermos-physical properties of working fluid was defined.The 3D construction method was verified by actual cases.The research could provide guidance for the construction of novel efficient thermal cycle.