Analysis of the thermodynamic performance limits of the organic Rankine cycle in low and medium temperature heat source applications

In this paper, an exploration of the practical thermodynamic performance limits of the organic Rankine cycle (ORC) under working fluid and cycle parameter restrictions is presented. These performance limits are more realistic benchmarks for the thermodynamic cycle than the efficiency of the Carnot cycle. Subcritical ORC configuration with four typical case studies that are related to temperature ranging from 373.15 to 673.15 K is taken into account. The ORC is defined by its cycle parameters and working fluid characteristic properties. The cycle parameters involve evaporation temperature (T_eva), condensation temperature (T_con) and superheat degree (ΔT_sup), while the working fluids are represented by the characteristic properties including critical temperature (T_c), critical pressure (p_c), acentric factor (ω), and molar ideal gas isobaric heat capacity based on the principle of corresponding states. Subsequently, Pareto optimum solutions for obtained hypothetical working fluids and cycle parameters are achieved using multi-objective optimization method with the consideration of both thermal efficiency (η_th) and volumetric power output (VPO). Finally, sensitivity analysis of the working fluid characteristic properties is conducted, and the second law of thermodynamics analysis, especially the applicability of entropy generation minimization, is performed. The results show that the current commonly used working fluids are widely scattered below the Pareto front that represents the tradeoff between η_th and VPO for obtained hypothetical fluids. T_eva and T_con are the most dominant cycle parameters, while T_c and ω tend to be the most dominant characteristic property parameters. The entropy generation minimization does not give the same optimal results.