An international research team has proposed integrating photovoltaic-thermal (PVT) collectors, borehole thermal energy storage (BTES), and a dual-source heat pump (DSHP) into a single hybrid energy system designed to supply space heating for livestock buildings. The concept combines three complementary technologies to improve overall efficiency and reduce reliance on conventional fossil-fuel heating.

“The integrated system allows to provide heating to livestock buildings, by almost nullifying the electric consumption,” the research's lead author, Francesco Tinti, told pv magazine. “The PVT provides electric energy, but also thermal energy to store in the BTES; the BTES allows recovery thermal energy from underground; and the DSHP use the underground energy, but also, in order not to deplete the BTES, the air source when needed.”

“Our analysis showed that the electrical energy consumed by the DSHP is almost entirely covered by the electricity generated by the PVT unit,” he continued. “The capital cost of the system, however, remains significant compared to conventional fossil fuel systems or air-source heat pumps. On the other hand, it is comparable to standard geothermal heat pumps, as the additional cost of the PVT and solar field helps reduce the required borehole length. Moreover, a key advantage of the PVT–BTES–DSHP system is that it significantly reduces primary energy consumption.”

A prototype of the system was tested at the Golinelli swine farm in Mirandola, in the Italian northern province of Modena, which is housing 500 sows and 2,500 weaners across multiple barns. Previously, the nursery barn was heated by a 34 kW LPG boiler supported by thermal lamps in each room.

The PVT–BTES–DSHP system combines a 35 kW dual-source heat pump capable of using air or ground as heat sources, an underground borehole thermal energy storage (BTES) field with eight 30 m deep boreholes, a rooftop photovoltaic-thermal (PVT) array of 24 collectors providing both thermal and electrical energy, and a centralized solar control station that manages energy flows between components.

Image: University of Bologna, Geothermics, CC BY 4.0

The PVT array is hydraulically linked to the BTES, and a rule-based control system ensures efficient operation by activating circulation only when solar radiation is sufficient or when temperature gradients favor heat transfer, thereby avoiding unnecessary energy losses. The DSHP is centrally controlled via PLC automation, maintaining a constant supply temperature of 50–55 C. It uses advanced compressor modulation and dual-evaporator control to optimize performance, prioritizing the more efficient ground source whenever conditions allow.

The operational switching between ground and air sources depends on glycol temperature thresholds, ensuring efficiency, freeze protection, and reliable winter operation, while additional systems manage defrosting under extreme cold conditions. All system components are continuously monitored via a cloud-based platform, enabling real-time tracking of temperatures, pressures, and performance indicators.

System design was based on detailed monthly heating demand and solar injection load calculations, balancing seasonal energy needs with long-term ground storage capacity and regulatory limits on subsurface temperatures. Overall, the integrated system is designed to reduce fossil fuel dependence, enhance seasonal energy storage, and maximize renewable energy use for livestock heating applications.

During the first monitoring year, the system demonstrated stable thermal behaviour in both the BTES and building supply circuits. Minimum BTES outlet temperatures remained above operational limits even in winter, confirming that solar-assisted heat injection effectively prevented ground overcooling and avoided thermal depletion of the storage field.

Maximum BTES temperatures gradually increased toward spring, indicating seasonal thermal recovery driven by reduced heating demand and intermittent solar recharge. On the building side, supply temperatures consistently met nursery heating requirements without critical drops, while maximum values reflected peak demand periods. Temperature stability contributed to steady indoor conditions, supporting animal welfare by avoiding thermal fluctuations.

The DSHP operated mainly between November and April, with a defrosting rate of only 3.6% during this period, confirming robust winter performance. Over the year, the system delivered 147,133 MJ of heat to the building while consuming 38,917 MJ of electricity, resulting in an overall seasonal performance factor (SPF) of 3.78.

The system also extracted 109,425 MJ of ambient energy, of which 38.6% came from ground-only operation, 6.0% from air-only operation, and 55.4% from hybrid operation. This confirms that the air source mainly supports peak loads, while the ground remains the primary energy provider. Seasonally, winter months were dominated by hybrid operation, while spring favored ground-only operation, demonstrating adaptive source switching based on conditions.

The analysis also showed that the system achieved an average coefficient of performance (COP) of about 4.07, with values exceeding 4 during favorable periods. In addition, the combination of PVT, BTES, and DSHP significantly reduces borehole length requirements while achieving comparable output to conventional systems with substantially lower geothermal infrastructure demand.

“The double-circuit BTES reduces the required length of the borehole heat exchanger (BHE) by around one third, since, in the specific location and hydrogeological conditions, the natural subsoil temperature is low, while the BTES allows increasing it by approximately 5 °C,” Tinti stressed. “However, not all hydrogeological conditions allow the storage of solar heat, which can be dissipated by natural groundwater movement.”

“The proposed configuration is especially suitable for farms with sufficient available land and favourable shallow geothermal conditions, providing a practical pathway to reduce greenhouse gas emissions while enhancing energy resilience,” he concluded.

The system was introduced in the study “Solar-assisted borehole thermal energy storage coupled with heat pump for livestock buildings: results from a full-scale installation,” published in Geothermics. The research team included scientists from Italy's University of Bologna, Greece-based heating and cooling specialist Psyctotherm G, and Swedish engineering firm MG Sustainable Engineering AB. 

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