Optimal Cycle and Turbine Design for MW-Scale Waste Heat Recovery ORC With Partial Evaporation

Abstract Efficiently converting waste heat into electricity is crucial for enhancing energy sustainability. Partial Evaporation Organic Rankine Cycle (PE-ORC) technology with wet-to-dry expansion has demonstrated improved conversion efficiency by optimizing heat source utilization over conventional subcritical ORCs. However, PE-ORCs face challenges at the MW scale, such as defining optimal operating conditions and designing turboexpanders for two-phase mixtures. This paper presents a model to determine optimal PE-ORC conditions for specific waste heat sources and outlines a methodology to design a single-stage turbine operating with wet-to-dry expansion and a dry-operated rotor. Two cycle optimizations, for high and low-temperature ranges of the heat source and based on real data, show that PE-ORC is competitive for the low-temperature range, with an increase of power production of about 25 % compared to the best single-phase cycle. A radial inflow turbine design for the low-temperature cycle is presented, focusing on the design, through shape optimization, of the stator cascade, the most critical component due to the supersonic and two-phase flow. The optimum profile is then simulated together with a non-optimized rotor via Computational Fluid Dynamic tool, confirming the possibility of designing a two-phase turbine with an efficiency higher than 85%, as assumed during the cycle design.