Multi-objective optimization and fluid selection of organic Rankine cycle (ORC) system based on economic-environmental-sustainable analysis

Global issues such as the energy crisis and environmental pollution impulse the development of waste heat recovery technologies. Organic Rankine cycle (ORC) systems are a promising solution to utilize renewable energies and recover waste heat. However, the different heat source temperatures often force the ORC to use different working fluids. Matching the heat source temperatures with suitable fluids is important to enhance the system performance and promote the marketability of this technology. In this work, a double-layer multi-objective optimization framework is proposed for subcritical ORC systems applied in the geothermal field. Four kinds of objective functions are selected in the optimization model, including net power output, total product unit cost, greenhouse gas emissions, and ecological life cycle cost to characterize system thermodynamic, exergoeconomic, environmental, and sustainable performances, respectively. The feature of the established model allows simultaneous system comprehensive design and fluid screening under specific geothermal temperatures. Results showed that the impacts of decision variables, including evaporation pressure and condensation temperature, on system performances were different under the different performance indicators. According to the balanced weighting factor case study, it is found that R134a had excellent thermodynamic and sustainable performances while R600a performed better from economic and environmental aspects at 393.15 K geothermal temperature. In addition, it was observed that the obtained optimal point shifted regularly on the Pareto curve with the change of geothermal temperature and weighting factor. Finally, under different weighting factor schemes, the optimal plans of fluid selection were obtained at certain geothermal temperatures. HCs fluids showed superior overall performance in the double-layer optimization framework considering ecological impacts, while R152a exhibited excellent comprehensive performance among the selected safe fluids under the premise of strict consideration of system security.

Automated summary

This paper proposes a double-layer multi-objective optimization framework for subcritical ORC systems in the geothermal field. Matching suitable fluids with specific geothermal temperatures is crucial for enhancing system performance and promoting marketability. Results show that the impacts of decision variables on system performance differ under different performance indicators.