A research team in Italy developed a PVT-driven heat pump system for domestic hot water production that combines packed-bed latent heat storage with sensible thermal storage to better balance supply and demand. Their simulations showed that hybrid storage coupled with temperature-based routing significantly improves system efficiency, raising heat pump coefficient of performance and increasing renewable energy utilization.
Researchers from the Polytechnic University of Bari and the University of Padua in Italy have developed a photovoltaic-thermal (PVT)-driven residential heat pump system for domestic hot water (DHW) production based on a hybrid thermal energy storage concept, in which a packed-bed latent heat thermal energy storage (PB-LHTES) unit complements a sensible thermal energy storage (STES) buffer.
This system configuration is intended to enhance flexibility by combining fast-response latent heat storage with the buffering capacity of sensible storage, smoothing thermal supply and demand mismatches. Packed-bed latent storage is used to increase energy density, reducing storage volume requirements while maintaining high thermal performance.
“The aim of this system design is to capture surplus PVT heat and shift it to demand peaks while limiting auxiliary electricity use,” corresponding author Aminhossein Jahanbin told pv magazine. “More specifically, the study quantifies how temperature-based routing at the storage–heat pump interface affects 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, which enabled 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 charging, or heat pump operation depending on system control. The heat pump acts as a backup heat source, ensuring DHW supply when solar input and stored energy are insufficient.
In addition, the system utilizes a downstream STES tank supplying instantaneous DHW demand, while a mixing stage ensures stable delivery temperatures at the user set-point. The PB-LHTES model is based on a thermal non-equilibrium formulation using a concentric dispersion approach, capturing axial heat transfer and phase change dynamics. Governing 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 incorporating occupancy patterns, seasonal variability, and typical daily usage peaks.
Using MATLAB and TRNSYS, the scientists developed a co-simulation framework to evaluate performance of the hybrid system. Annual simulations were performed with a 1-minute time step using realistic weather data and detailed component representations, including PVT collectors, stratified storage tanks, piping 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 required. Hysteresis-based control logic was implemented to prevent short cycling and improve operational stability.
For their simulations, the scientists considered a five-storey residential building in Bari characterized by high solar availability and moderate heating demand. Four configurations were analyzed. Case 1 introduces a PB-LHTES unit downstream of the PVT field to store surplus thermal energy and support DHW charging, thereby reducing reliance on the heat pump. Cases 2 and 3 replace the air-to-water heat pump with a water-to-water unit and integrate PB-LHTES on the load side to improve heat pump operating conditions. The fourth configuration serves as a reference case, representing a baseline PVT-assisted heat pump system for domestic hot water production.
The analysis showed that, across the four configurations, adding PB-LHTES and switching to water-to-water heat pumps improves system performance by enhancing energy matching and reducing heat pump electricity use. Case 3 was found to deliver the best results due to conditional thermal routing, which prioritizes direct storage use and optimizes heat pump operating conditions.
Heat pump coefficient of performance (COP) was also found to increase consistently across configurations, rising from about 2.5 in the reference case to around 2.9–3.1 in Case 1 and reaching around 4.3 in Case 3, reflecting reduced temperature lift and improved system integration.
The renewable energy factor also improved significantly, especially in PB-LHTES cases, increasing from roughly 14–37% in the reference configuration to summer peaks of 75–80% in Case 1, and stabilizing at about 40–60% year-round in Cases 2 and 3, indicating higher PV self-consumption and more balanced seasonal performance.
“Overall, the results indicate that in PVT-driven DHW systems, major performance gains are not achieved through storage integration alone, but through the synergistic coupling of hybrid latent–sensible storage with temperature-aware routing strategies,” said Jahanbin. “This combination maintains the quality of recovered thermal energy, enhances the stability of DHW supply conditions, and consistently lowers reliance on grid-driven heat pump operation over the course of the year.”
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