Due to the ongoing expansion of renewable energy, the need for large-scale electrical energy storage is growing. In particular, providing longer storage periods of several days to a few weeks is a key challenge for bridging so-called Dunkelflauten. Carnot batteries offer a promising solution by storing electrical energy as thermal energy via thermodynamic processes. Carnot batteries based on high-temperature heat pumps and organic Rankine cycles (HP-ORC systems), in particular, combine cost-effective, scalable thermal storage with readily available components. Since both processes require similar components, a reversible integration of both processes within a single system enables further savings. In addition, the thermal integration of (industrial) waste heat as a heat source for the heat pump can significantly increase the power-to-power efficiency of a Carnot battery. However, many of these concepts are still at the research or pilot stage. The present work investigates the use of zeotropic fluid mixtures to improve the efficiency and flexibility of these systems. Zeotropic fluid mixtures are characterized by a non-isothermal phase change, known as the temperature glide. Adapting the mixture composition, so-called adaptive fluid mixtures, to the process conditions promises higher efficiencies across a wide operating range. Two research questions were addressed: (1) What are the benefits of (adaptive) fluid mixtures for reversible HP-ORC systems, and (2) what needs to be considered when implementing such a system in practice? To answer these questions, thermodynamic simulations and experimental investigations were conducted on a reversible HP-ORC pilot plant. Both pure substances and zeotropic mixtures were tested in the pilot plant. Furthermore, concepts for composition-adjustable mixtures were discussed and evaluated techno-economically in scenario-based simulations. The results indicate a multi-objective design problem involving power-to-power efficiency, storage density, and heat-source utilization in the design of reversible HP-ORC systems. A systematic evaluation using Pareto frontiers allows the selection of favorable design points. Experiments with the pure fluid R1233zd(E) achieved a maximum COP of 11.2 in heat pump operation (at a 9 K temperature lift), and a maximum ORC efficiency of 4.40% (at a 40 K temperature gradient). The resulting power-to-power efficiency was 39.1% under 100/80°C hot-water conditions. ORC performance increased to 5.36% through fluid charge optimization, highlighting the importance of charge management. Mixtures proved to be advantageous. Zeotropic mixtures better match sensible source and sink temperature profiles and reduce heat exchanger exergy losses. In tests with a low-glide mixture, R1336mzz(E)/R1336mzz(Z), and a high-glide mixture, CO2/R1233zd(E), mixtures outperformed their pure components, typically by 5–20% in both modes. Notably, the optimal composition varies substantially depending on the operating conditions. Scenario-based simulations indicate that (adaptive) mixtures can raise mean annual power-to-power efficiency from about 50% to 60-70% under the investigated conditions. A passive concept for composition adjustment in reversible HP-ORC systems was developed and offers low complexity and fast adjustment. Medium- to high-glide mixtures are particularly suitable because only small shifts in the circulating composition are required to adjust the temperature glide. Economic feasibility remains limited under current German day-ahead electricity prices. Net margins per stored kilowatt-hour do not cover the levelized cost of storage. Feasibility improves with wider price spreads, multi-day storage needs, and supportive policy (e.g., capacity remuneration, balancing-market revenues, CO2 pricing). Beyond electricity arbitrage, reversible HP-ORC systems can flexibly provide power-to-heat and heat-to-power services, expanding revenue opportunities. For large-scale deployment, the results motivate the use of fixed-direction fluid flow with continuous liquid receiver integration, zeotropic mixtures based on low-GWP refrigerants with sufficiently different boiling points, accumulator-based composition control, and, at scale, a separate expander and compressor while sharing heat exchangers and piping. Overall, this work underscored that fluid mixtures, in particular composition-adjustable mixtures, offer considerable potential to increase the efficiency and flexibility of Carnot batteries.
Maximilian Weitzer (Thu,) studied this question.