Experimental characterization of a reversible heat pump – Organic Rankine cycle pilot plant as a thermally integrated Carnot battery
Maximilian Weitzer, Sabine Reiß, Daniel Steger, Sebastian Kolb, Jürgen Karl
Abstract
• First detailed experimental characterization of a reversible heat pump – ORC pilot plant. • 150 stationary operating points investigated in heat pump mode and ORC mode. • Reversible screw machine analyzed in depth as the critical component for efficiency. • Verification of results by means of energy balances and reproducibility tests. • Results show successful proof of concept with further optimization potential. This paper presents a first detailed experimental characterization of a reversible heat pump – Organic Rankine Cycle pilot plant, highlighting its potential as a thermally integrated Carnot battery for efficient energy storage. A total of 150 stationary operating points were investigated in both heat pump and ORC modes. The study analyzes and discusses the main trends and interactions at both the system and component levels. As expected, heat pump and ORC operations exhibit an opposing behavior. While heat pump operation yields a maximum COP of 9.5 at a 15 K temperature lift, the peak net ORC efficiency reaches 4.4 % with a 40 K temperature gradient. Neglecting additional storage losses, this leads to a power-to-power (round-trip) efficiency of up to 41.8 % for the Carnot battery. The absence of a liquid receiver in the ORC mode leads to significant subcooling and increased condensation pressures, which affect the expander’s power generation due to excess refrigerant accumulation. Moreover, the experimental results reveal that the reversible screw machine is the critical factor influencing efficiency. The screw machine’s maximum isentropic efficiencies are 78 % and 60 % in compressor and expander operation, respectively. The reliability of the results was verified by means of an energy balance and reference operating points, underlining high reproducibility. Additionally, potential system improvements were derived from the experimental results. Altogether, this paper successfully demonstrates the concept of reversible heat pump – ORC systems and provides experimental results for further advancements in this field.