The increasing penetration of photovoltaic generation in residential buildings has intensified the temporal mismatch between electricity availability and thermal demand, particularly in heat pump-based heating systems. Thermal energy storage represents a key enabling technology to increase on-site renewable energy self-consumption and system flexibility. Among available options, latent thermal energy storage (LTES) based on phase change materials (PCMs) offers high energy density and compact system layouts, which are especially attractive for space-constrained residential applications. Organic PCMs are of particular interest due to their chemical stability, predictable phase change behavior, and absence of supercooling-related issues. Nevertheless, the practical integration of PCM-based storage into real heat pump systems remains challenging due to heat transfer limitations and internal temperature non-uniformities. As a result, latent storage solutions cannot be considered plug-and-play components and require careful experimental validation under realistic operating conditions. Within this context, the LIFE ITS4ZEB project adopts an application-driven approach, focusing on industrially feasible configurations rather than idealized laboratory concepts.
This work presents the experimental characterization of a medium-temperature latent thermal storage unit based on an organic phase change material and integrated with a hydraulic loop representative of residential heat pump systems. The storage module consists of an aluminum tank with an internal volume of approximately 0.1 m³, filled with a commercial organic PCM with a phase change temperature suitable for space heating applications. The resulting nominal storage capacity is approximately 8 kWh. Heat exchange between the water loop and the PCM is provided by a commercial finned-tube heat exchanger adapted for latent storage use, representative of compact and cost-oriented industrial solutions. Charging and discharging tests were carried out by varying inlet water temperature differences (10–20 K) and volumetric flow rates (4–8 L/min), covering a range of operating conditions relevant to residential heat pump operation. Thermal power, cumulative stored and released energy, and internal temperature distribution were monitored using calibrated instrumentation. A distributed network of thermocouples enabled the analysis of thermal stratification and spatial temperature non-uniformities within the PCM volume.
The experimental results show that the performance of the organic PCM-based storage is strongly influenced by the imposed boundary conditions. Increasing the inlet temperature difference leads to higher peak thermal power and shorter charging and discharging times, while enabling a more effective utilization of the available storage capacity. At constant temperature difference, increasing the flow rate significantly boosts peak power but may reduce the total usable energy when an application-oriented power threshold is adopted to define the end of the process. Charging and discharging behaviors are largely symmetric, reflecting the stable and repeatable phase change characteristics of the organic PCM. Internal temperature measurements reveal the development of thermal stratification during both operating phases, with temperature gradients increasing under more aggressive conditions. Although such non-uniformities do not prevent correct operation, they influence charging and discharging dynamics and limit the uniform participation of the PCM volume in the phase change process.
The experimental investigation confirms that organic PCM-based latent thermal storage can deliver energy and power levels compatible with residential heat pump applications, albeit with a lower energy density compared to inorganic solutions. The absence of supercooling contributes to stable and predictable operation, but heat transfer limitations and thermal stratification remain key design challenges. System performance is highly sensitive to operating conditions, and excessive emphasis on peak power may compromise temperature uniformity and effective energy utilization. These results demonstrate that, even for organic PCMs, latent thermal energy storage cannot be considered a plug-and-play solution. Careful system design, appropriate selection of operating conditions, and experimental validation under realistic boundary conditions are essential to achieve a balanced compromise between performance, robustness, and industrial feasibility. Within the ITS4ZEB framework, the investigated configuration represents a reliable and scalable solution for enhancing renewable energy self-consumption in residential heat pump systems.
