A research team in Italy developed a PVT-powered heat pump system for domestic hot water production, which combines latent heat storage in a packed bed with sensible thermal storage to better match supply and demand. Their simulations showed that hybrid storage combined with temperature-based routing significantly improves system efficiency, increases the heat pump coefficient of performance and increases the use of renewable energy.
Researchers from the Polytechnic University of Bari and the University of Padua in Italy have developed a photovoltaic-thermal (PVT) powered residential heat pump system for hot water (DHW) production based on a hybrid thermal energy storage concept, where a packed bed latent heat storage (PB-LHTES) complements a meaningful thermal energy storage (STES) buffer.
This system configuration aims to increase flexibility by combining fast-response latent heat storage with the buffer capacity of sensible storage, mitigating the mismatch between thermal energy supply and demand. Latent storage in a packed bed is used to increase energy density, reducing storage volume requirements and maintaining high thermal performance.
“The goal of this system design is to capture excess PVT heat and shift it to demand peaks, while limiting the consumption of additional electricity.” corresponding author Aminhossein Jahanbin narrated pv magazine. “More specifically, the study quantifies how temperature-based routing at the storage-heat pump interface influences usable heat quality, heat pump operation and seasonal performance. To support this analysis, we also developed a computationally efficient PB-LHTES model for minute-resolution dynamic simulations and validated it against experimental data, allowing its integration into building-scale DHW analyses.”
The system uses PVT collectors to provide both electrical and thermal energy, with electricity offsetting auxiliary consumption and heat supporting direct use, storage charge or heat pump operation, depending on system control. The heat pump acts as a backup heat source and provides the DHW supply when solar energy and stored energy are insufficient.
In addition, the system uses a downstream STES tank that provides immediate hot water demand, while a mixing stage ensures stable supply temperatures at the user-set point. The PB-LHTES model is based on a thermal nonequilibrium formulation using a concentric dispersion approach, capturing axial heat transfer and phase change dynamics. Applicable equations describe coupled heat transfer between the heat transfer fluid and encapsulated phase change material (PCM). Realistic DHW demand profiles are generated using a stochastic Gaussian-based model that includes occupancy patterns, seasonal variability and typical daily consumption peaks.
Using MATLAB and TRNSYS, the scientists developed a co-simulation framework to evaluate the performance of the hybrid system. Annual simulations were performed at a 1-minute time step, using realistic weather data and detailed representation of components including PVT collectors, tiered storage tanks, pipe networks and variable speed heat pumps. A hierarchical control strategy prioritized direct solar thermal use, followed by storage charging and discharging, and finally heat pump activation when needed. Hysteresis-based control logic was implemented to prevent short cycles and improve operational stability.
For their simulations, the scientists looked at a five-story residential building in Bari, which is characterized by high solar energy availability and moderate heating demand. Four configurations were analyzed. Case 1 introduces a PB-LHTES unit downstream of the PVT field to store excess thermal energy and support hot water charging, reducing dependence on the heat pump. Cases 2 and 3 replace the air-water heat pump with a water-water unit and integrate PB-LHTES on the load side to improve the heat pump operating conditions. The fourth configuration serves as a reference scenario and represents a basic PVT-assisted heat pump system for domestic hot water production.
The analysis found that, in the four configurations, adding PB-LHTES and switching to water-to-water heat pumps improves system performance by improving energy matching and reducing heat pump electricity consumption. Case 3 was found to deliver the best results thanks to conditional thermal routing, which prioritizes direct storage use and optimizes heat pump operating conditions.
The heat pump coefficient of performance (COP) was also found to increase consistently in all configurations, from about 2.5 in the reference case to about 2.9-3.1 in case 1 and about 4.3 in case 3, reflecting reduced temperature rise and improved system integration.
The renewable energy factor also improved significantly, especially in PB-LHTES cases, from around 14-37% in the reference configuration to summer peaks of 75-80% in case 1, and stabilized throughout the year at around 40-60% in cases 2 and 3, indicating higher self-PV consumption and more balanced seasonal performance.
“Overall, the results indicate that in PVT-powered DHW systems, large performance gains are achieved not only through storage integration, but through the synergistic coupling of hybrid latent-sensitive storage with temperature-aware routing strategies,” said Jahanbin. “This combination maintains the quality of recovered thermal energy, improves the stability of DHW supply conditions and consistently reduces dependence on grid-powered heat pump operation over the course of the year.”
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