Carnot batteries (CBs) store electrical energy as thermal exergy. Along discharging it is aimed to restore the electrical energy. For high efficiencies each part of the system must approach thermodynamic reversibility. Beside efficiency, also costs and further criteria will be important in future energy markets. In this project an inverse design of CBs using multi-component mixtures will be investigated and mathematically optimized, using detailed models for machines, heat exchangers, storages, and fluids. Selected points will be investigated experimentally, aiming to prove the validity of the models and their predictions. The project heavily relies on co-operations within the priority program 2403. Criteria for the inverse design will come from projects from energy systems analysis, and detailed features and correlations of the parts will be obtained from thermodynamics, heat transfer and termo-fluid machine projects.

The main hypothesis is that relatively simple cycles with co-optimized working fluid mixtures, and working conditions lead to a high enough degree in flexibility for an inverse design approach and meet the criteria formulated from energy systems analysis, like round trip efficiency per investment costs and given storage to power ratio for different temporal application-profiles, to meet the market needs and to evaluate the limits of Carnot batteries. The second main hypothesis is that actual thermodynamic models of CBs neglect too many details, like local properties, fluid dependent efficiencies, time dependencies, and the coupling effects so that their predictions suffer from large uncertainties, especially due to a lacking comparison with experiments. Here, it is expected that the results of this project will allow to assess the level of detail needed in models for good accuracy predictions and reliable inverse design.

At the end of this funding period, it is expected to have a CB model, which can be used for inverse engineering, and is experimentally validated at selected operation points.

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

In Carnot batteries, mainly Rankine or Brayton cycles have been used so far to realize heat supply and regeneration. In the proposed project, an Ericsson process will be targeted for the first time for cases where only the Rankine process has been considered so far. Therefore, this concept is called Ericsson battery. The background is that the Rankine cycle differs from the Carnot cycle, which leads to exergy losses. In heat pump mode, expansion losses, and in power generation mode, preheating losses dominate. In Carnot batteries, these have so far been countered by the use of an additional sensible heat storage. As an alternative, a cycle is now to be used instead, in which these types of losses do not occur. In principle, the Ericsson cycle is suitable for this purpose, but the technical implementations have not yet been successful so far, since the volumetric power density is low due to the gas used as the working fluid and the necessary heat transfer is low due to the lack of a phase change. For this reason, a new approach is being used, whose basic features have been developed at the Bitzer Chair of Refrigeration, Cryogenics and Compressor Technology in recent years. It is a Recuperative Two-Phase Cycle (RTPC), in which complete recuperation is enabled by the use of a zeotropic mixture with a highly nonlinear profile in the two-phase area. This cycle can adopt the shape of the Ericsson cycle, while still having the desired phase change. The goal is to use it in thermal electricity storage systems, resulting in a storage concept that is based only on latent heat storage and does not require additional sensible storage. In the proposed project, this concept will be investigated for the first time. For this purpose, the optimal cycle process parameters with the required working fluid mixtures have to be identified first. This requires a parallel optimization of process parameters, mixture components and mixture composition. As a result, the most important thermodynamic characteristics of the Ericsson battery are obtained, such as the ideal storage temperature and the specific heat flow. In the next step, the functionality of the mixture will be verified on a demonstrator, although the experiment is intended to represent only part of the desired overall process. The project is being carried out in cooperation with other partners. Thereby, energy-economical requirements (A) are received, methodical approaches (B) are integrated into the own calculation tools and requirements for necessary components (C) are passed on. In a final part, all theoretical and experimental findings will be used to quantify the potential of a technical implementation of the Ericsson battery. 

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