Study Of Irreversibility Of Heat Transfer And Performance Of Heat Work Conversion On Thermodynamic Cycles

Studies on irreversibility of heat transfer(matching degree of heat transfer and irreversible degree of heat transfer)and heat work conversion(thermodynamic perfectibility and ecological function)of five thermodynamic cycles are conducted based on thermodynamic mean temperature(TMT)of the fluid thermal process.The five cycles are organic Rankine cycle(ORC),Carnot cycle,trilateral cycle(TLC),Brayton cycle and Lorenz cycle.Moreover,the relevant evaluation indicators are redefined.Thermal efficiency can be expressed as a universal function of TMT of heating process and cooling process;and the maximum thermal efficiency of heat source/cold source using TMT is established.Matching degree of heat transfer and irreversible degree of heat transfer that characterize the irreversibility of heat transfer are defined based on TMT.The mathematical models and relationships of irreversibility of heat transfer and performance of heat work conversion are established.For ORC,subcritical and transcritical conditions,pure working fluids and mixed working fluids are analyzed to select the optimal working fluid and the optimal conditon.Results show that: thermodynamic perfectibility should be higher than 0.5 that ecological function is greater than zero,so the thermodynamic cycles or operation conditions can be beneficial and significant.The maximum ecological function is a condition balancing the maximum net power output and the maximum thermal efficiency.Ecological function provides a new method or criterion to the optimization and selection of the thermodynamic cycles,working fluids and suitable operation conditions.In addition,Lorenz cycle is an ideal cycle for matching heat source/cool source with superior performance than other thermodynamic cycles.For real cycles,the performance of TLC(or transcritical ORC)is the best,and ORC with monotonical increase is the second one,but Brayton cycle is poor.For subcritical ORC,the cycle performance has shift characteristics and the corresponding shift-temperature of heat source.When the temperature of heat source is lower than the shift-temperature of heat source,the performance parameters have an extreme value with evaporation temperature;otherwise,the performance parameters changes with the evaporation temperature monotonically.Moreover,the performance without extreme value is better than that with extreme value.