Numerical simulation and validation of commercial hot water tanks integrated with phase change material-based storage units

Abstract

Thermal energy storage (TES) allows to extensively exploit solar thermal technologies by effectively handling the mismatch between energy production and demand thus possibly causing a downsizing of generation units. Among thermal energy storage technologies, those based on phase change materials (PCM) are particularly interesting because relatively large latent heat values may guarantee more compact systems (as compared to sensible TES). In this work, we report a numerical and experimental investigation on a hybrid latent-sensible heat storage characterized by a commercial hot water tank integrated with macro-encapsulated phase change materials. Those hybrid systems are interesting as they can possibly increase the overall thermal capacity of a sensible water tank. Despite all this, we demonstrate that increasing the effective storage capacity is a non trivial task with standard conditions and materials. To this end, three different numerical models have been developed and experimentally validated. The first model is based on the enthalpy porosity method and simulates the charge and discharge of a PCM storage unit in a climatic chamber. The second model is a one-dimensional description of the water storage tank without PCM. Finally, the third model is obtained by coupling the previous two and simulates the whole PCM-water thermal storage. This final model was validated through an experimental test, which consisted in inserting 94 modules of PCM in the water tank and observing the resulting thermal behaviour for three days, applying the load curve of a detached house of 200m2. The model proved to be very accurate, with determination coefficients between 92.10% and 99.80% for the considered physical quantities (temperatures and thermal powers). As a main contribution of this work, we proved that the hybrid thermal storage system did not exploit the full latent heat potential of the PCM, since only 40% of it actually changes its phase, due to is thermal transport properties that negatively affect heat transfer.