The study presents a latent heat storage system consisting of a phase change material (PCM) and a heat transfer fluid (HTF), placed in direct contact, for storing thermal energy derived from renewable energy sources or industrial waste heat. It is important that the two media are immiscible for the function of such a storage system. In this study the PCM was an eutectic mixture of the two salt hydrates, magnesium nitrate hexahydrate and magnesium chloride hexahydrate and the HTF was a mineral oil. The direct contact leads to intensified heat transfer compared to that in the indirect contact heat storage systems. This intensified heat transfer results in short loading and unloading periods of the storage tank with a high heat exchange performance. Furthermore, a higher storage density is observed in comparison to the sensible storage systems, particularly when combined with low temperature differences between the charged and discharged storage systems. In our study, the heat transfer between liquid PCM and liquid HTF (rises drop-shaped in the PCM) is investigated. For this purpose, a storage tank has been built with only one HTF inlet opening in the bottom. For the time and space resolved data, the axial temperature profile, and the surface temperature of the rising droplet, as well as the PCM temperature, are measured with a specially developed, fast, near infrared sensitive, and fiber-coupled technique. These measurements made it possible to determine a local heat transfer coefficient between the PCM and HTF. With the chosen parameters and materials in this study a heat transfer coefficient of 1845 W/m² K was calculated.

中文翻译：

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液-液直接接触潜热储存系统传热系数研究

该研究提出了一种潜热存储系统，由直接接触的相变材料（PCM）和传热流体（HTF）组成，用于存储来自可再生能源或工业废热的热能。对于这种存储系统的功能来说，两种介质不能混溶，这一点很重要。在本研究中，PCM 是两种盐水合物（六水合硝酸镁和六水合氯化镁）的低共熔混合物，HTF 是矿物油。与间接接触储热系统相比，直接接触导致强化的传热。这种强化的传热导致储罐装卸时间短，换热性能高。此外，与显热存储系统相比，观察到更高的存储密度，特别是当与充电和放电存储系统之间的低温差相结合时。在我们的研究中，研究了液体 PCM 和液体 HTF（在 PCM 中呈滴状上升）之间的传热。为此，我们建造了一个储罐，其底部只有一个导热油入口。对于时间和空间分辨数据，轴向温度分布、上升液滴的表面温度以及 PCM 温度，均采用专门开发的快速、近红外敏感光纤耦合技术进行测量。这些测量使得确定 PCM 和 HTF 之间的局部传热系数成为可能。根据本研究中选定的参数和材料，计算出传热系数为 1845 W/m² K。