Experimental assessment of material and geometry effects in a medium-temperature latent thermal energy storage for residential heat pumps

This study presents a comprehensive experimental evaluation of medium-temperature latent thermal energy storage systems designed for integration with residential heat pumps powered by renewable energy. Despite significant research on phase change materials, direct comparisons between inorganic and organic PCMs under identical geometries and real-scale operating conditions remain limited, particularly for systems operating in the 45–50 °C range. To address this gap, two finned-coil heat exchanger geometries and two phase change materials—an organic material and a hydrated salt—were tested in a 0.1 m3 storage unit under controlled operating conditions. Charging and discharging cycles were performed across three flow rates and three temperature levels. Results show that the hydrated salt provides 40–50% higher energy density and a 22% higher heat transfer rate, whereas increasing the heat transfer area reduces charging time by around 20%. A scale analysis confirms that conduction within the phase change material dominates the process. An exponential energy-prediction model was developed to estimate stored and released energy as a function of time, inlet temperature difference, and flow rate. The calibrated equation reproduces system dynamics with mean square error values of 0.10–0.39 kWh, enabling fast performance estimation without full experimental characterization. These findings deliver actionable design guidelines for compact and efficient latent thermal energy storage solutions in heat-pump-based residential systems.