Assessing the heat transfer performance of an LTES tank with PCM encapsulated in tubes: Scale-up insights

Climate change and the urgent need for a sustainable energy transition demand to increase the integration of renewable energy sources and to improve techniques for energy management. Latent thermal energy storage (LTES) is a promising solution to decouple energy supply and demand, offering higher energy density compared to sensible heat storage. In residential applications, LTES can address both heating and cooling requirements depending on the phase change material (PCM) selection and related melting temperature range. This work evaluates the thermal performance of a lab-scale shell-and-tube LTES prototype with an unconventional configuration, where the PCM is encapsulated inside the tubes while the heat transfer fluid (HTF) circulates on the shell side. Two configurations are investigated: a five-tube arrangement and a thirteen-tube arrangement. Moreover, two distinct PCMs are tested for different applications: a Na2CO3-water eutectic solution (melting temperature of -2.1 °C) for cold storage utilization, and a commercial paraffin-based PCM (melting temperature of 32 °C) for heating applications. The experimental setup consists of the LTES tank connected to a thermostatic bath, whose integrated pump circulates an ethylene glycol-water mixture through the shell side. Three complete charging-discharging tests are performed for each configuration, by setting adequate temperature extremes to ensure full phase change transition. Data analysis allows the quantification of the exchanged latent heat, calculated through the energy balance on the HTF side. The results are compared with differential scanning calorimetry (DSC) measurements, delving into the differences between small-scale laboratory measurements and larger-scale experimental systems. Furthermore, the correlation between the number of tubes and the thermal energy exchanged is investigated and discussed with a view to the design of these systems. The results directly contribute to the design and scale-up of the LTES demonstration systems for the Horizon EU project ECHO (Efficient Compact Modular Thermal Energy Storage), which aims to develop an innovative TES system integrating both PCMs and thermochemical materials. Beyond this application, the study offers useful insights into the challenges and opportunities encountered when evaluating the thermophysical properties of PCMs, transitioning from small-scale characterization to real-world prototype testing, an essential step for practical implementation of LTES technologies in residential buildings.