The performance of thermodynamic cycles based on the properties of working fluids

Abstract

With the increase of global energy demand, themodynamic cycles, such as Rankine cycle, refrigeration or heat pump, have been widely employed to generate work or transfer heat. Since these cycles generally consist of a linked sequence of state points with physical properties variables, the physical properties of working fluid are essential to the cycle analysis. They directly determine the design of components, the cycle performance, the cycle stability and safety. However, in the thermodynamic analysis, the determination of cycle performance and the calculation of required physical properties are usually seperated. The thermodynamic peroperties of working fluids are often obtained from the complex experimental equations, so that the relationship between the cycle performance and the working fluids has not been established yet. What′s more, due to the fact that Carnot efficiency doesn′t contain detailed information on the properties of working fluids, a nature idea emerges how to derive the efficiency limit under the constraint of working fluids. Thus, in this work, the influences of thermodynamic properties on power cycles are investigated and the limiting efficiencys are proposed. In order to derive the cycle performances from the physical properties of working fluids, a general cubic equation of state is employed to obtain the residual properties, such as residual enthalpy, residual entropy and residual internal energy. Thereafter, on the basis of the properties of ideal gas, thermodynamic parameters of working fluids at any state are determined. According to the thermodynamic general relationships, the required heat is obtained for four classical processes, namely isochoric, isobaric, isothermal and isentropic processes. Based on the derived expressions, the output work and cycle efficiency are obtained for power cycles including Carnot cycle, Rankine cycle, Brayton cycle and Sterling cycle. The relationship between the cycle performance and the temperature, the properties of working fluids is developed. As a theoretical upper bound of cycle efficiency, Carnot efficiency is only determined by temperatures of heat source and sink. While the output work of Carnot cycle is a function of temperatures and the properties of working fluids. For other power cycles, it can be concluded that the thermodynamic performances are related with temperatures and working fluids, based on the derived expressions from cubic equation of state. Furthermore, compared with the Carnot efficiency under the same heat source and sink, the thermodynamic perfections of cycles are very low in practical engineering. Therefore, performance limits of thermodynamic cycles are investigated on the basis of the characteristics of working fluids in this paper. Limiting performance is derived from the equation of state and the temperature-entropy diagram of working fluids. For Rankine cycle and Brayton cycle, a maximum isobaric slope in the temperature-entropy diagram is employed to cut out the unexploited area from the enclosed area of Carnot cycle, so that the limiting work and efficiency can be obtained from the cycle areas. For Sterling cycle, when the used working fluid is ideal gas, the cycle efficiency is equal to Carnot efficiency. Although the proposed limiting performance can not be achieved by practical cycles, it can provide some theoretical guidance for the operating condition and the optimal design of thermodynamic cycles.