Experimental Comparison of Organic and Inorganic PCMs in a Full-Scale Latent Thermal Energy Storage for Residential Heat Pump Integration

Latent thermal energy storage (LTES) systems are increasingly recognized as a key enabler for enhancing the flexibility and efficiency of residential heat pump applications, especially when integrated with renewable energy sources. By decoupling heat generation from heat demand, LTES allows heat pumps to operate when renewable electricity is available, storing thermal energy for later use. The performance of such systems heavily depends on the choice of phase change material (PCM), whose selection is far from trivial due to the trade-offs among thermal properties, stability, costs and compatibility. This study experimentally investigates and compares two commercially available PCMs — a bio-based organic and a hydrated salt having a phase change temperature between 46 and 48 °C— within an LTES prototype designed in the framework of a European LIFE project (LIFE22-CCM-IT-LIFE ITS4ZEB GA n° 101113714) aimed at market deployment of optimized heat pump-LTES integration. Organic PCMs are chemically stable, widely available, and exhibit excellent long-term reliability, but suffer from low thermal conductivity (around 0.2 W/m·K) and moderate energy density. Hydrated salts, on the other hand, offer higher thermal conductivity (around 0.6 W/m·K) and volumetric energy density, but are prone to supercooling, phase separation, and material compatibility issues such as corrosion. The tested LTES prototype has an internal volume of approximately 0.1 m³ (430×520×440 mm) and integrates a finned-coil heat exchanger composed of 7 rows of aluminum fins and copper tubes. A preliminary design optimization showed that only 25% of the original copper tube coverage was necessary, as the target application prioritizes thermal capacity and economic sustainability over rapid charge/discharge cycles. Water circulates inside the copper tubes, while the PCM is distributed in the shell side to facilitate phase change. Both PCMs were tested under identical boundary conditions, maintaining constant volume and varying mass due to density differences. Three water flow rates and three temperature gradients between inlet water and PCM were used to evaluate thermal performance during both charging (melting) and discharging (solidification) phases. Temperature was monitored at 22 locations throughout the LTES to assess thermal distribution and transient behavior. Experimental results show that the hydrated salt stores significantly more energy (approximately 12 kWh vs. 7 kWh for the organic) and achieves a more homogeneous temperature field due to its higher latent heat and thermal conductivity, despite a slightly longer absolute charging and discharging time (around 1.50 h vs. around 1.30 h). When normalized to the energy stored, the salt exhibited faster relative charging/discharging dynamics. However, considerable temperature gradients were observed even with the hydrated salt, highlighting the need for further geometric optimization. This comparative case study demonstrates the superior thermal performance of hydrated salts for large-scale LTES applications, while also emphasizing the necessity of full-scale testing due to material behavior dependence on system size. The findings support the selection of the hydrated salt for the next project phase and lay the groundwork for further improvements in heat exchanger design to enhance performance and reduce material-related limitations.