Design and modelling of a reversible HP/ORC Carnot battery tailored for waste heat integration in flooded mines

Carnot Batteries (CBs) are a promising option for energy storage, acting as a buffer for the variability from renewables and enabling multi-energy integration and dispatch, converting electricity to heat and back to electricity. Although techno-economic studies report promising costs and high feasibility, especially when components from both cycles are shared in long-term storage, there are few prototypes, and the technology readiness level remains near 4. This paper presents a reversible Rankine-based CB designed for integration with an abandoned flooded mine. The system is under construction, being the largest machine of its type. A physics-based model was developed and validated against manufacturer data to assess performance under realistic constraints. The key focus is the role of auxiliaries and temperature-glide control. By actively modulating secondary-loop pump rotational speed, the Organic Rankine Cycle (ORC) achieves up to a 36 % increase in efficiency and the Heat Pump (HP) mode up to 20 % increase in relative efficiency to a constant-glide strategy. Highlighting that no single pair of glide settings is optimal across the full operating envelope, underscoring the need for adaptive control. Neglecting auxiliaries leads to substantial errors: a relative difference of 24 % in round-trip efficiency (RTE) can be achieved when auxiliaries are omitted, resulting in unrealistic performance values and, consequently, an unrealistic feasibility. With auxiliaries and constraints included, the modelled charge–discharge RTE ranges from 22.8 % to 34.7 %, lower than conventional storage but consistent with reported limits for CB technology. However, CBs can also supply industrial heat, reject heat to district heating networks, and/or deliver cooling, making RTE efficiency an incomplete metric for this technology. The analysis indicates that efficiency depends more on operating conditions than on component selection. This highlights that, for CBs connected to low-temperature storage, auxiliary components are decisive for performance. Achieving high efficiency requires water pumps with high part-load efficiency (including both pump and motor), refrigerant pumps capable of high efficiency at low net positive suction head, and the deployment of active control laws governing charge management and pump operation.