In this project proposal, scientific methods for efficiency improvement and cost reduction of a packed-bed heat storage system, as a key component of the Carnot Battery, are addressed. As preliminary work showed, a packedbed heat storage with liquid metals as heat transport fluid is able to perform more energy efficient than with conventional fluids, especially at low porosities in the packed bed (i.e. high solid fraction). The concept combines a low-cost packed bed and an efficient heat transport to the storage medium by using a heat transport fluid with high heat conductivity and low viscosity (low-Prandtl-number fluid, typically liquid metal). Therefore, it allows a separation of functions: The heat storage medium, one the one hand, can be chosen and optimized according to its storage parameters, e.g. large specific heat capacity and density and low cost; the heat transport fluid, on the other hand, offers high heat transfer rates and is only used in low quantities. Liquid metals offer significantly larger heat transfer rates compared with conventional fluids due to their thermal conductivities being one to two orders of magnitude larger. Furthermore, the use of liquid metal as the heat transport fluid allows the heat storage to be used in a Carnot battery over a flexible temperature range (e.g. with liquid sodium in the temperature range from 100°C to >500°C) without phase change and with highly efficient heat transfer in other system components like heat exchangers too. This enables compact and cost-efficient heat storage and energy conversion sub-systems in a Carnot battery. However, there is a gap in the literature regarding heat transfer correlations of low-Prandtl number fluids in packed beds. This is why this proposal focusses on the adaptation of a heat transfer correlation for forced convection of low-Prandtl number fluids in packed-beds based upon high quality experimental data. For this purpose, a project-specific test section is designed, constructed and integrated into an existing liquid metal test rig. The found correlation for the heat transfer is then included in an existing 2D1D multi-scale model to improve the heat transfer model parameter being in the focus of this project. As a result, the simulation of the temperature field and, based on that, the dynamic behaviour during charge, standby and discharge conditions, which forms the basis for efficiency evaluation and targeted design of storage systems in terms of capacity, geometry, material etc., can be optimized and exchanged with the partners within the Priority Programme. To support the inverse approach and foster exchange with the partners in Subject Area B, the 2D-1D multi-scale model will be used already early in the project for fast evaluation of concepts, parameters, specification of exchange data etc. 

Dr.-Ing. Klarissa Niedermeier
Karlsruher Institut für Technologie (KIT)
Institut für Thermische Energietechnik und Sicherheit (ITES)

The development of Carnot-Batteries with high round-trip efficiency and low cost requires sophisticated thermal energy storage (TES) systems and a comprehensive understanding of their transient behavior. The conspicuous lack of validated and computationally efficient TES models for latent heat storage represents an important barrier to successful inverse design of Rankine based Carnot-Batteries. The present project intends to bridge this gap by developing an accurate, computationally efficient and experimentally validated latent-heat TES multi-scale simulation model and providing the result to the members of the priority program. Moreover, the project aims at the formulation of a framework for describing Carnot-Battery concepts consistently across the priority program using uniform high-quality metadata.

Thermal energy storage is a key component in a Carnot-Battery. For Rankine-based Carnot Batteries, latent heat storage systems promise a high roundtrip efficiency because of the excellent temperature matching between the isothermal melting/solidification in the storage and the evaporation/ condensation of the working fluid during charging/discharging of the storage.

In state-of-the-art latent heat storage systems, a heat exchanger is embedded into the phase change material (PCM) to enable heat transfer between the working fluid and the storage medium. As typical storage materials have a low thermal conductivity, extended heat transfer respectively fins structures are required to ensure sufficiently high power densities. Complex transient temperature and phase distribution fields arise in between these structures during charging and discharging. Knowledge of the local and temporal progression of the temperature and melting phase distribution is a prerequisite for the design of the storage and the identification of an optimal design that ensures high roundtrip efficiency of the Carnot battery. An integration of this model complexity in higher-level design optimizations for an entire Carnot battery is however not yet possible. Current design and simulation tools for latent heat storage are furthermore rarely validated.

The current project consists of a main part (MP), formulated by the first principal investigator (Vandersickel) and devoted to the development of a consistent multiscale model for latent TES and a transfer module (TM) formulated by the second PI (Thess) serving the distribution of the developed methodology across the priority program. As a result of the MP, this project will provide a well validated latent heat storage model and a model reduction/parametrization suited for the simultaneous optimization of storage and Carnot Battery design and operation. As a result of the TM, the project will supply a methodology for a uniform description of Carnot-Battery systems using metadata as well selected transient data of the DLR Carnot Battery pilot plant to all interested members of the priority program.

Professor Dr. André Thess
Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)
Institut für Technische Thermodynamik (ITT)

Prof. Dr.-Ing. Annelies Vandersickel
Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)
Institut für Technische Thermodynamik (ITT)