Heat transfer in packed bed thermal energy storage with low Prandtl number fluid

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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 packed-bed 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 2D-1D 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 behavior 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 Program. 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.