Modelling Analysis Of Partial Evaporation Organic Rankine Cycle With Volumetric Expander

Efficient waste heat conversion into electricity is essential for enhancing energy sustainability. Partially Evaporated Organic Rankine Cycles (PE-ORCs) can offer higher conversion efficiencies than conventional subcritical Organic Rankine Cycles (ORCs) by optimizing heat source utilization and minimizing exergy losses during heat transfer. In PE-ORCs, the working fluid undergoes only pre heating and partial evaporation before entering the expander, allowing for a better thermal match with a sensible heat source and reducing isothermal heat transfer inefficiencies. However, their implementation faces challenges, including the identification of optimal working fluids and operating conditions, and the design of expanders capable of handling two-phase mixtures. Volumetric expanders, compared to turboexpanders, are recognized as being better able to withstand the expansion that occurs, partially or fully, in the two-phase state. This work presents a numerical analysis of a PE-ORC for low temperature heat recovery. The ORC model consists of sub-models for the system's main components. The expander sub-model simulates two-phase expansion by means of a lumped-parameter semi-empirical approach for volumetric expanders, in which a vapor flashing model is integrated. The heat exchangers are modelled using a moving boundary approach and the ε-NTU method. The PE-ORC model has been validated using experimental data from a kW-scale test bench equipped with a piston expander and using HFC-134a as the working fluid. The results show good agreement, with Relative Root Mean Squared Errors below 10%, for key output variables such as working fluid mass flow rate and expander power, under heat source temperatures between 40°C and 75°C and vapour qualities at the expander inlet between 0.2 and 0.9.