Energy, Exergy, and Economic Performance Comparison and Parametric Optimization of Organic Rankine Cycles Using Isobutane, Isopentane, and Their Mixtures for Waste Heat Recovery

The possibility of applying the organic Rankine cycle (ORC) to further recycle the low-grade waste heat efficiently is studied in the present work. The energy, exergy, and economic models of the ORC system are established, and the isobutane, isopentane, and their mixtures are selected as the organic working mediums (OWMs). Due to the slip characteristics of mixed OWM, four operational conditions of the ORC system are proposed, and then the contrastive analysis of energy, exergy, and economic performances under the four operational conditions are conducted. Finally, the optimal mixture mass fraction and crucial parameters of the ORC system are separately determined through the bi-objective optimization. The results show that the ORC system using the mixed OWM can achieve a larger net power output and exergy efficiency by comparing the pure OWM when the condensing temperature is set as the saturated vapor temperature during the condensation process. The electricity production cost first rises and then decreases with the rising mass fraction of isobutane in mixed OWM, and the ORC system using the isopentane can achieve the smallest electricity production cost. By taking the low-grade flue gas of 433.15 K as the ORC heat source, four operational conditions have the same optimal ORC crucial parameters, namely the evaporating temperature of 393.15 K, condensing temperature of 308.15 K, and superheat degree of 0 K. The pure OWM of isobutane can achieve better overall performance by setting the condensing temperature as the saturated liquid temperature.