First Experimental Results of the Power Cycle of a 50 kWe Carnot Battery Integrated into a Flooded Mine

The ongoing electrification, combined with the increasing penetration of intermittent renewable energy sources, is profoundly transforming generation, transmission and distribution systems and creating new challenges on the grid. In this context, energy storage solutions covering multiple time scales are becoming essential, and the Rankine–Carnot Battery (CB) emerges as a particularly promising long-duration option. The technology integrates two high-TRL subsystems: a Heat Pump (HP, power-to-heat) and an Organic Rankine Cycle (ORC, heat-to-power), coupled through a thermal storage medium. Despite its reliance on mature components, the overall TRL of CBs is still constrained by the scarcity of large-scale demonstrators and real industrial integrations. This paper presents the first experimental results of the ORC power cycle of a semi-reversible CB with a nominal electrical output of 50 kWe (peak 77 kWe) and a thermal capacity of 500 kWth in HP mode, integrated into a decommissioned flooded slate mine in Martelange (Belgium) that acts simultaneously as seasonal thermal storage. The system architecture, the on-site instrumentation and the specific challenges associated with deploying and operating the technology are described. The ORC uses R1233zd(E) as working fluid and is equipped with a semi-hermetic screw expander (535 m³/h at 50 Hz). Three commissioning steps are reported: by-pass operation, expander operation under no magnetic load, and successful grid synchronization. Experimental results are analysed in terms of cycle behaviour, expander and refrigerant-pump performance, energy-balance closure (residual ≈ 4 %) and refrigerant-charge distribution. A peak electrical output of 37.18 kWe were achieved, currently limited by the available inverter capacity (2 of 4 modules), with a measured ORC
cycle efficiency of 5–7.2%. The two main limitations identified, refrigerant-pump cavitation at low charge level, are discussed. Overall, the results represent a significant step forward in the technological maturity of CB systems, validating their operation at real industrial scale within a standardized industrial environment.