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

作为卡诺电池的关键部件，填料床蓄热系统是提高效率和降低成本的科学方法。初步研究表明，与传统流体相比，以液态金属作为传热流体的填充床储热能够实现更高的能源效率，特别是在填充床的低孔隙度（即高固体分数）下。该概念结合了低成本的填充床，并通过使用高导热性和低粘度的传热流体（低普朗特数流体，通常是液态金属），有效地将热量传递到存储介质。因此，它可以实现功能分离：一方面，可以根据储热介质的存储参数进行选择和优化，如比热容和密度大，成本低；另一方面，传热流体提供高传热率，只在少量使用。液态金属的导热系数比传统流体大一到两个数量级，因此其传热速率明显高于传统流体。此外，使用液态金属作为传热流体，可以在灵活的温度范围内（例如，在100°C至500°C的温度范围内使用液态钠）将热量储存在卡诺电池中，而不会发生相变，并且在热交换器等其他系统组件中也具有高效的传热。这使得卡诺电池中的热存储和能量转换子系统紧凑且经济高效。然而，关于填料床中低普朗特数流体的传热相关性，文献中存在空白。这就是为什么本研究的重点是基于高质量的实验数据，对填料床中低普朗特数流体强制对流的传热相关性进行适应。为此目的，设计、建造了一个项目特定的测试部分，并将其集成到现有的液态金属试验台中。然后将发现的传热相关性包含在现有的2D-1D多尺度模型中，以改进传热模型参数，这是本项目的重点。因此，温度场的模拟以及在此基础上的充电、待机和放电条件下的动态行为，构成了存储系统在容量、几何形状、材料等方面的效率评估和针对性设计的基础，可以在优先计划内与合作伙伴进行优化和交流。为了支持反向方法并促进与主题领域B的合作伙伴的交流，2D-1D多尺度模型将在项目早期使用，用于快速评估概念、参数、交换数据规范等。

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.