Balancing Efficiency and Economics in Organic Rankine Cycles with Multistage Turbines for Sustainable Waste Heat Utilization

Thermal energy rejected through exhaust gases and cooling systems in marine propulsion units and conventional power plants represents a significant yet underutilized opportunity for improving energy efficiency and reducing carbon emissions. The Organic Rankine Cycle (ORC) has emerged as an effective technology for converting such waste heat into useful power using organic working fluids with favorable thermophysical properties. This study presents a comprehensive thermodynamic, economic, and exergo-economic evaluation of an ORC system incorporating single-stage and multistage turbine arrangements, using R245fa, R123, and R365mfc as working fluids. A validated cycle model is coupled with key economic indicators, including Net Present Value (NPV), Levelized Cost of Electricity (LCOE), and payback period, together with a simplified exergo-economic framework based on exergy destruction costs. The results demonstrate that implementing ORC-based waste heat recovery significantly enhances overall system performance by converting rejected thermal energy into electricity and improving thermal efficiency. Multistage turbine configurations further strengthen performance, increasing net power output and efficiency, with the multistage R245fa system generating more than 530,000 kWh annually. Economically, the single-stage R245fa configuration achieves the lowest LCOE (0.021 USD/kWh) and the shortest payback period, below eight years. Exergo-economic analysis shows that multistage turbines can reduce exergy destruction costs by more than 80%, with benefits becoming pronounced at heat source temperatures above 170 °C.