Thermodynamic Analysis And Optimization Of Supercritical CO₂ Cycle

Since the 21st century,supercritical CO2 cycle has received extensive attention from researchers.S-CO2 cycle is characterized by high efficiency,simple system structure and compact components.It is widely used in nuclear power,solar energy,low temperature waste heat and coal-fired power plants.S-CO2 cycle is mainly divided into S-CO2 Rankine cycle and S-CO2 brayton cycle.Due to the low critical temperature of CO2 and the difficulty of cooling,researches focus on S-CO2 brayton cycle in recent years.S-CO2 brayton cycle is divided into many different circulation modes according to the number of compressor,intercooler,regenerator and reheater.Among them,the first stage recompression cycle,that is S-CO2 recompression brayton cycle is the most widely studied,and its structure is simple with good performance,which is widely recognized by researchers.The traditional methods are based on the first and second laws of thermodynamics,but the classical thermodynamics has some limitations to deal with the multi-process coupled system.Under the framework of modern thermodynamics,this paper introduces thermodynamic flow and thermodynamic force to establish the expression of actual engine efficiency.From the perspective of T-Q curve,the enclosed area of T-Q curve of the heat transfer process in the heat exchanger is defined as heat energy conversion dissipation and represents available energy loss.The quantitative relationship between heat load and heat energy conversion dissipation during heating and cooling is established.In consideration of the thermal coupling between heating and cooling,the expression of actual engine efficiency is obtained.The actual heat engine efficiency expression tells us that no matter how complicated a heat engine is,the heat resistance ratio should be reduced as much as possible in order to improve the heat engine efficiency.This theory provides guidance for the design and operation of the generalized heat engine.Based on the modern thermodynamic theory,it is further applied to the analysis of S-CO2 cycle to guide the construction of S-CO2 cycle and further verify the correctness and universality of the actual efficiency theory of the heat engine.For the S-CO2 recompression brayton cycle,the effect of the part flow ratio is analyzed.The new method points out that the thermal efficiency of the heat engine is only related to the heat exchange coupling between the heat engine and the external heat source.From this point of view,this paper deeply analyzes the cause of the influence of the part flow ratio on the cycle performance,and further analyzes the cause of the phenomenon from the change of the internal regenerator.The results show that the coupling performance of the external and internal of the engine mainly depends on the heat transfer path of the two fluids,which is finally reflected in the area of T-Q curve.The heat transfer dissipation of the internal regenerator will further affect the heat transfer process between the external heat source and the working fluid,thereby affecting the external coupling performance of the engine and ultimately affecting the efficiency of the engine.When the part flow ratio is 0.3,the heat energy transfer dissipation of both the high temperature regenerator and the low temperature regenerator is relatively small.At this time,the energy dissipation of the corresponding heat exchanger and the cold source heat exchanger is also relatively small,and the heat engine efficiency is relatively high at 50.81%.In order to further improve the performance of CO2 cycle,the introduction of ORC as the bottom cycle is considered to absorb the waste heat of CO2 cycle,and the actual thermal efficiency theory is adopted to guide the coupled heat transfer of CO2 cycle and ORC.The results show that the combined cycle can improve the efficiency by at least 2 percentage points on the basis of a single cycle,and when the pinch point of ORC is between the preheating section of the coupled heat exchanger,that is,the organic working fluid is better matched with CO2 and the closed area shown in the T-Q curve is smaller,so that the combined cycle can improve the system performance more.