Carnot batteries (CBs) store electrical energy in the form of thermal exergy and convert it back into electrical energy during discharge. In addition to high efficiencies, costs, flexibility, and operation under transient and part-load conditions are important for market relevance in future energy systems. In this project, the inverse design of Carnot batteries is investigated within a multi-objective optimisation framework based on comparatively simple thermodynamic cycle concepts. Multicomponent working fluid mixtures, in particular zeotropic mixtures with a large temperature glide for sensible energy storage, are treated as key design variables and co-optimised together with process parameters and operating conditions. Building on the previous funding period, detailed and modular models of machines, heat exchangers, storage units, and fluids are extended, while effects already considered—such as local variations in thermophysical properties and fluid-dependent component efficiencies—are systematically refined and augmented. A particular focus is placed on representing transient charging and discharging with variable cycle lengths, as well as on quantifying part-load, start-up, and standby losses. The resulting model predictions are validated in laboratory experiments in order to reduce uncertainties and to assess model fidelity in a systematic manner. The project closely cooperates with other projects within the Priority Programme: design criteria and time-dependent load profiles are adopted from energy system analysis, while thermophysical properties, correlations, and component models are drawn from projects in thermodynamics, heat transfer, storages, and thermofluid machinery. The central hypothesis is that jointly optimised working fluid mixtures, process parameters, and operating strategies can provide sufficient flexibility for inverse design even in simple cycle configurations and can meet market-based quality criteria—such as efficiency–cost trade-offs at required energy-to-power ratios—under realistic temporal operating profiles. At the end of the funding period, an experimentally validated, transient-capable Carnot battery model will be available as a basis for inverse engineering of Carnot battery systems.

Professor Dr. Burak Atakan
Universität Duisburg-Essen
Institut für Energie- und Material-Prozesse (EMPI)
Lehrstuhl für Thermodynamik

Transcritical Battery (TraCBat) – Assessment of the potential and operability of Carnot Battery Concepts based on the Recuperative Two-phase Cycle – is a sub-project of the second project period of the DFG Priority Programme ‘Carnot Batteries: Inverse Design from Markets to Molecules’ (SPP2403). The subject of the investigation are Carnot batteries that use a Recuperative Two-Phase Cycle (RTPC) as charging and discharging process. This concept, developed at the Schaufler Chair, enables the reduction of exergy losses in thermodynamic cycles caused by the temperature lift in the cycle by using an asymmetric working fluid mixture and a liquid/two-phase recuperation. In heat pump cycles, these are primarily throttling losses, while in power cycles these are preheating losses. In addition, there are exergy losses during heat transfer in the recuperator. A compromise must be found between capital expenditure and the round trip efficiency (RTE). For this reason, a transcritical cycle variant of the RTPC was identified as a promising approach for the application in a Carnot battery in the first project period. The concept only requires one high-temperature storage unit and uses the environment as a heat source or a heat sink. This concept will be investigated in the second project period using sub-models from area C. In addition to models for the working fluid mixtures, a storage model and a turbomachinery model will be integrated. In addition to models for the working fluid mixtures, a storage model and a turbomachinery model are to be integrated, thereby the inverse optimisation can be extended to the component level. Furthermore, the focus is also on the flexibility of the cycle concept. The cycle must be adapted to variable ambient temperatures and different temperature spreads in the heat storage unit. This capability is being investigated experimentally, with an adjustable heat sink being added to the existing test rig. Specifically, an operating strategy for the expansion valve needs to be developed for variable boundary conditions and the inlet conditions in the compression process need to be investigated. The inlet conditions shift with the variable temperature levels, resulting in a two-phase state at the compressor inlet even in a transcritical RTPC. For this reason, the focus is on the compressor and the composition shift caused by the two-phase inlet state. Based on the theoretical potential investigations and the experimental work on cycle flexibility, the feasibility of using the Recuperative Two-Phase Cycle in Carnot batteries and its potential benefits will then be evaluated.

Professorin Dr.-Ing. Christiane Thomas
Technische Universität Dresden
Institut für Energietechnik