Experimental investigation on low temperature heat source with a pumpless gravity-driven closed loop thermosyphon organic Rankine cycle

In this study, a pumpless gravity-driven Organic Rankine Cycle (PORC) system was developed and experimentally tested for low-temperature heat recovery applications. The system utilized a modified scroll expander, originally designed for automotive air conditioning, and operated entirely via natural circulation, without a mechanical pump. The effects of refrigerant charge, heat source temperature, and resistive electrical load were investigated to evaluate system behavior and power generation efficiency. Results revealed that stable operation was achieved with a refrigerant charge between 2.1–2.3 kg, heat-source temperatures of 40–60 °C, and a system height of 2.8–3.1 m. The maximum work output of the expander reached 18.44 W, while electrical power output peaked at 0.80 W under optimal conditions. The scroll expander isentropic efficiency ranged from 30-86%. A positive, approximately linear correlation was observed between system height and work output within the tested range. Although theoretical and experimental efficiencies diverged significantly—highlighting mechanical and electrical losses—the study confirmed the technical feasibility of pumpless ORC systems. Although the output is modest, it is comparable to other small-scale ORC systems operating at similar source temperatures, demonstrating comparable efficiency without a mechanical pump. These findings support the application of gravity-driven ORC systems for power production in space-constrained and off-grid environments using low-grade thermal energy sources. • A closed-loop, pumpless ORC system was experimentally evaluated for low-grade heat recovery. • Natural circulation was achieved using hydrostatic pressure without a mechanical pump. • Maximum electrical output was 0.90 W; isentropic efficiency reached 86%. • Increasing system height showed potential to enhance work and energy output. • The system achieved a maximum pressure differential of 0.38 bar and theoretical work output of 19.68 W.