Experimental and numerical study of a thermal storage with phase-changing material and two heat transfer circuits

It is estimated that 50% of the final form of energy consumption in Europe is accounted for by heat, with the majority of this (75%) still produced by fossil sources. Thermal storage plays an important role in the decarbonisation of heat production. It enables the decoupling of production from consumption, thereby facilitating the integration of renewable sources such as biomass, thermal solar panels and waste heat recovery. Latent heat thermal energy storage (LHTES) technology is particularly suited to situations where there are space limitations and where the temperature differential between the cold and hot sources is relatively small. Recent developments have demonstrated the relevance of LHTES with two distinct heat transfer fluid circuits: the first for heat storage and the second for heat restitution. This component is capable of simultaneously charging and discharging the storage. These characteristics are beneficial for systems such as Carnot batteries, solar heat for industrial processes (SHIP) and district heating networks. This thesis work discusses the use of LHTES with two circuits and shell-and-tube technology components, with monophasic or diphasic heat transfer fluids. The numerical system-scale modelling of this kind of storage is decomposed into three steps and validated against experimental data. The objective is to develop a numerical tool for the design and piloting of these systems.The software DYMOLA was employed for the development of this model. This is discussed in the first section of the document, with particular attention paid to the depiction of thermal enhancement structures surrounded by phase-change materials (PCM), monophasic heat transfer models and diphasic heat transfer models.Then, the initial validation of the model is presented and validated against experimental data obtained from a demonstrator in which the heat transfer fluid is monophasic and flows through an annular tube surrounded by helicoidal aluminum fins. The results demonstrate the significance of the thermal resistance between the fluid and the wall in the macroscopic thermal behavior of the system, the modelling of the collectors and the PCM fusion temperature.A second validation is presented, based on experimental data from a second storage system. In this prototype, the heat transfer fluid is diphasic, and the finned tube is surrounded by an aluminum vertical insert. The results and sensitivity test demonstrated the significance of the slip effect between the two phases and the methodology for determining the equivalent thermal conductivity of PCM surrounded by aluminum thermal enhancement structures.Ultimately, this study demonstrates that the numerical model accurately simulates the thermal behavior of shell-and-tubes LHTES, irrespective of the fluid state (monophasic or diphasic). Additionally, it highlights the pivotal parameters of such models.