MODELLING ANALYSIS OF PARTIAL EVAPORATION ORGANIC RANKINE CYCLE WITH VOLUMETRIC EXPANDER

Efficient conversion of low-grade waste heat to electricity remains a key challenge in industrial energy systems. Partially Evaporated Organic Rankine Cycles (PE-ORCs) offer improved thermal matching with sensible heat sources compared to conventional subcritical ORCs by limiting the isothermal evaporation zone, thereby reducing exergy losses. However, the two-phase conditions at the expander inlet – which necessitate the use of volumetric rather than turbomachinery expanders – require dedicated modelling frameworks that go beyond black-box isentropic efficiency assumptions.,This paper presents a semi-empirical, lumped-parameter model of a micro-scale PE-ORC equipped with a reciprocating piston expander. The expander sub-model integrates an Interface Exchange Model (IEM) to account for non-equilibrium flash evaporation during two-phase expansion. Heat exchangers are modelled using the moving-boundary method coupled with the ε-NTU approach, and the feed pump is characterised by experimentally derived characteristic curves. All sub-model parameters are calibrated against 18 steady-state operating points from a kW-scale test bench using R134a as the working fluid. Validation against an independent experimental dataset covering heat source temperatures from 40 °C to 75 °C and expander inlet vapour qualities from 0.2 to 1 yields Relative Root Mean Squared Errors (RRMSE) below 10% for all primary output variables – including working fluid mass flow rate (RRMSE = 2.6%), expander power output (6.5%), and evaporator thermal power (5.1%). A parametric analysis reveals that net power output peaks at an expander inlet vapour quality of approximately 0.75, confirming partial evaporation as a viable and controllable operating strategy rather than a merely degraded condition. The validated model constitutes a reliable tool for the design optimisation and performance assessment of PE-ORC systems in low-temperature waste heat recovery applications.