Hybridizing PVT energy with heat pumps, borehole storage

A group of European scientists has sought to combine PV-thermal (PVT) energy coupled with a heat pump (HP) and borehole thermal energy storage (BTES) for powering an Italian swine farm.

“This research is significant and timely, as there is a lack of integrated systems in the agricultural sector combining PVT collectors with BTES and heat pumps,” they explained, adding that the work combines “cutting-edge technologies to address energy challenges in the agricultural sector, examining the synergy between solar energy generation, efficient thermal storage, and the application of heat pumps.”

The novel system was installed to support the nursery barn of a farm located in the province of Modena, northern Italy. The barn consumed 90 MWh of liquefied petroleum gas (LPG) annually before the commissioning of the project, of which 30 MWh was for space heating and 60 MWh for domestic hot water. Heating, the researchers said, is not needed in the summer.

The scientists built a system utilizing 24 PVT collectors based on monocrystalline cells and a thermal absorber. Each had a maximum power of 320 W, for a total project capacity of 25 kW heat and 7.68 kW electricity, with the latter being used to power up the barn's needs and the heat pump. The 35 kW water-to-water heat pump uses a mixture of water and antifreeze glycol with a concentration of 35%.

“The PVT system was designed to use the thermal energy of the collectors to heat the heat transfer fluid (HTF),” they explained. “The heated operating was then directed to the borehole thermal energy storage (BTES) using U-pipes, where the heat would be stored in the shallow sandy aquifer. Another set of U-pipes was present in the BTES to transfer the heat stored to the cold side of the HP.”

In that system, the BETS is used as a seasonal heat storage that can heat the barn in the winter months. The borehole has a storage capacity of 22 MWh, over a volume of 5,405 m3. All system operations are controlled and optimized by a solar central (SC) developed for this project.

“A novel and modular SC was designed to achieve a standardized design that would be used as the basis for other installations within the RES4LIVE project,” the group stated. “This was done to address one of the main barriers to adopting solar thermal and PVT technologies, which is the relative complexity of their installation and configuration compared to conventional systems.”

As the system was only installed recently, the scientists could provide thermal results and BTES analysis for May and June 2023. Due to local regulations, however, they could not conduct electricity metering and, therefore, used software to simulate it.

According to their analysis, the total heat energy produced by the PVT system was 1,807 kWh in May and 2,200 kWh in June. “Analysis of heat production revealed that the thermal energy available for storage during May was lower than that of June, which could be attributed to the reduced amount of insolation,” they highlighted.

As for the BTES system, the scientists found that the whole heat was kept inside the borehole boundaries without dissipation due to groundwater flow. Using numerical simulations, they also concluded that aquifers can handle much more solar thermal energy to support PVT of up to 50 kW.

The electricity production, according to the simulations, was 0.86 MWh for May and 1.11 MWh in June. “The project led to replacing fossil fuel-based energy consumption for the nursery barn with a renewable energy source (RES) and additional efficiency enhancement measures, thus reducing 20,850 kg CO2 per year,” the scientists stated.

Their findings are available in the study “Experimental assessment of a solar photovoltaic-thermal system in a livestock farm in Italy,” published in Solar Energy Advances. The group comprises researchers from Sweeden’s University of Gävle and MG Sustainable Engineering, as well as Italy’s University of Bologna.

The system, which was switched on in April 2023, is part of the renewable energy source for livestock farming (RES4LIVE) project, funded by the EU's Horizon 2020 program