Optimal cycle and turbine design for MW-scale waste heat recovery organic Rankine cycle with partial evaporation

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 organic Rankine cycles (ORCs). However, PE-ORCs face challenges at the MW scale, such as defining optimal operating conditions and designing turbo-expanders 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 nonoptimized 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.