Multi-objective thermo-economic optimization of a combined organic Rankine cycle and vapour compression refrigeration cycle

Abstract

This study focuses on the multi-objective optimization of a combined power (organic Rankine cycle) and vapour compression refrigeration cycle based on heat source temperatures ranging from 120 °C to 150 °C. The primary purpose is to achieve an optimal system through the use of efficiency and cost functions. The size of the heat exchangers and turbines (indicated by the total heat transfer coefficient UAtotal and size parameter SPtotal, respectively) used in the system affect the cost function of the system, whereas the efficiency function of the system is affected by the thermal efficiency (ηth) and the irreversibility ratio (IR value, representing the total exergy destruction) of the system. The sensitivity analysis conducted herein revealed that the ηth value increased by 107.9% when the evaporator pressure was varied from the minimum to the maximum value within the bound constraints, whereas the UAtotal value increased by 15.5%. The system was optimized based on six input parameters. The optimization results indicated that the system optimized for R236ea provided the lowest cost and the lowest system efficiency, system optimized for R245ca provided the highest cost and the highest system efficiency, and the system optimized for R245fa was between the two above-mentioned extremes in terms of cost and efficiency. At a heat source temperature of 150 °C and with R245fa as the ORC fluid, the optimum UAtotal, ηth, IR, and SPtotal values were 99.6 kW/K, 35.8%, 54.2%, and 0.386 dm, respectively.