Energy, exergy, and economic comparison of ORC with quasi-isothermal expansion with other ORC designs for low-grade waste heat recovery

The present paper investigates different organic Rankine cycle (ORC) configurations, which can convert low-grade industrial waste heat streams (80–100 °C) into electricity. More specifically, the basic ORC, the reheated ORC, the ORC with a quasi-isothermal expander, and the trilateral flash cycle, are analyzed and compared. The quasi-isothermal expansion is achieved through heated oil injection at multiple stages inside the expander. Initially, these cycles are studied parametrically in terms of energy and exergy, considering the same available heat source load. The exergetic evaluation is enhanced through a thorough component-level exergetic analysis. Additionally, the cycles’ performance during a typical winter and a typical summer is examined. The final stage of this analysis includes the techno-economic investigation and comparison of the organic cycle designs. The results indicate that the ORC with quasi-isothermal expansion achieves the best thermodynamic performance compared to the other three designs. The largest calculated values of the net electrical power, the energy efficiency, and the exergy efficiency are 165.6 kW, 9.8 %, and 53.9 %, respectively. In parallel, the same cycle configuration is the most cost-effective, leading to a net present value equal to 2288 k€, and a payback period value of 1.3 years, when the operating hours are equal to 8000 per year. Hence, the ORC with quasi-isothermal expansion is found to be the most proper option for power production at low-temperature heat sources, while the reheated ORC performs marginally poorer from thermodynamic and techno-economic viewpoint.