Comparative Analysis of PCM and Water-Based Thermal Energy Storage Units in a Photovoltaic and Thermal System for Residential Applications

Photovoltaic thermal (PVT) systems, which may be incorporated into buildings, can generate both electricity and heat. When utilized in residential structures with limited space, the miniaturization of thermal energy storage (TES) units becomes crucial. The phase change material (PCM)-based thermal energy storage unit developed in this study enables the installation of TES systems in residential environments where securing sufficient installation space is challenging. The study was conducted on a PVT system installed on the Korea Institute of Energy Research's KePSH-2 building. Under actual PVT operating conditions with real-time fluctuations in solar irradiance, the performance of an 11.51 L PCM 1-based TES unit and an improved 7.71 L PCM 2-based TES unit was comparatively analyzed against that of a 500 L water-based TES unit. We observed the instantaneous changes in the charging rate from April 18 to June 30, 2025, over three months. The operational performance of the storage units, including the energy storage density, was evaluated by comparing the heat collected by the PVT modules and stored in the water heat transfer fluid versus the PCM-based units. The average solar collection by the PVT system was 15.2 kWh. The average energy storage densities were 14.1 kWh/m3 (24.6% lower than water) for PCM 1, 47.4 kWh/m3 (153.4% higher than water) for PCM 2, and 18.7 kWh/m3 for the water-based TES. The average charging rates were 2.6 kW for the water-based TES, and only 0.1 kW for both PCM units. This indicates that while PCM-based units can be miniaturized due to their high energy storage density, their charging performance remains constrained by internal heat-transfer design. The PCM 1-based unit showed low storage density and slow charging behavior, suggesting that its internal configuration limited heat transfer efficiency, whereas the PCM 2-based unit—equipped with an improved internal structure—achieved significantly higher storage density than the water-based TES. Further research is needed to develop an integrated system that utilizes the high charging rate of the TES tank and the high energy storage density of the PCM, creating an optimal thermal storage solution for residential buildings.