Experimental and numerical analysis of a phase change material-based shell-and-tube heat exchanger for cold thermal energy storage

This work experimentally and numerically investigates the thermal performance of a vertical shell-and-tube heat exchanger, filled with a biological phase change material (PCM), linked to a water-chiller system for cold thermal energy storage. The system provides the cooling service to a 150 m2 single-family house. An experimental apparatus has been designed to collect the PCM temperature data through the employment of multiple thermocouples located at different heights of the heat exchanger. Starting from the experiment, a comprehensive 3D numerical model of the system has been developed based on the enthalpy-porosity method using COMSOL Multiphysics®. The PCM temperature profiles and the evolution of the solid-liquid interface location at various time instants are used as performance indicators of the model reliability and accuracy. The numerical agreement with the experimental findings is proved using statistical indices, e.g., maximum values of mean absolute error (MAE) and root mean square error (RMSE) equal to 1.18 °C and 1.33 °C, respectively, with regards to central thermocouples. The results reveal that natural convection highly affects the PCM charging/discharging processes, as proved by the PCM liquid fraction, which moves from full to 51 %, between two levels spaced by 0.78 m. The developed 3D model, based on the enthalpy-porosity method, allows to consider boundary conditions variability with both angle and height, which are not usually covered by simplified 2D models. Hence, this model confirms that only 39 % of the PCM mass experiences the complete phase change process, proving that the current design does not allow the PCM to totally exploit the thermal storage potential, thereby making optimization pivotal and challenging.