Researchers in Italy have tested a 10 kW dual-source heat pump using CO₂ as a refrigerant and air or photovoltaic-thermal collectors as heat sources. The results show that solar operation can improve heating performance under favorable irradiance conditions, while an optimized source-switching strategy delivers modest annual electricity savings.
A research team in Italy has investigated the performance of a direct-expansion, dual-source heat pump that uses air and solar energy as heat sources and CO₂ as the refrigerant. The researchers aimed to identify optimal operating conditions and develop advanced control strategies.
“An experimental campaign was carried out under real-field conditions on a 10 kW prototype, considering both Air-Mode and Sun-Mode operation for domestic hot water production and producing a total of 307 tests,” the researchers said. “To overcome the limitations of non-uniform experimental data, artificial neural networks (ANNs) were developed to predict system performance over a wider operating domain.”
The researchers installed the 10 kW heat pump at a research center in Rome, Italy. The prototype operated either with a fan-assisted air evaporator or with CO₂ flowing through copper tubes attached to the rear of 10 photovoltaic-thermal (PVT) collectors. The collectors were south-facing and installed at a tilt angle of 48 degrees.
An additional water circuit, equipped with a 300-liter storage tank, simulated domestic hot water production. The researchers maintained the water inlet temperature at 20 C and adjusted the flow rate to achieve an outlet temperature of approximately 60 C.
During testing, the team varied the compressor frequency from 50 Hz to 70 Hz and monitored refrigerant temperatures, pressures and mass flow rates, along with solar irradiance and total heat pump electricity consumption. Measurements were recorded every 0.5 seconds and averaged over five-second intervals.
The resulting dataset comprised 164 test points in air mode and 143 in solar mode. The researchers used the data to calculate heating capacity and coefficient of performance (COP).
The results showed that solar operation offered a thermodynamic advantage under favorable irradiance conditions. As solar irradiance increased from 322 W/m² to 904 W/m², the refrigerant evaporation temperature rose from 4 C to 12.9 C, reducing the required compression work.
Heating capacity increased by 21.1%, from 8.75 kW to 10.6 kW, while COP improved by 18.7%.
“Conversely, under low irradiance conditions, Air-Mode remained more efficient due to the lower thermal resistance of the air evaporator,” the researchers said. “These results confirm the existence of a minimum irradiance threshold above which Sun-Mode becomes advantageous.”
The team also trained artificial neural networks to predict electricity consumption, COP and compressor frequency in both operating modes. The researchers compared system performance at heating capacities of 8.5 kW and 10 kW and water temperatures ranging from 20 C to 60 C to identify optimal conditions for switching between air and solar operation.
They then conducted annual simulations for seven locations – Rome, Tenerife, San Diego, Toronto, Mexico City, Perth and Beijing – using five domestic hot water demand profiles and a 900-liter storage tank.
“The results show that, although the electrical energy savings associated with the source-switching strategy remain limited (approximately 100–120 kWh/year under the most favorable conditions), the simultaneous electricity generation of the PV/T field significantly affects the overall economic performance of the system,” the researchers said.
The techno-economic assessment also revealed substantial differences between locations. Simple payback periods ranged from approximately six to 10 years in areas with high solar availability to more than 20 years in less favorable climates.
“A complementary thermo-economic assessment highlighted a strong dependence on climatic conditions, with simple payback periods ranging from approximately 6–10 years in locations characterized by high solar availability to over 20 years in less favorable climates,” the researchers added.
The findings are presented in “CO₂ dual-source (air/solar) heat pump on-field testing: results and performance maps development for optimal mode selection,” published in Applied Thermal Engineering. The research team included scientists from Italy’s University of Naples Federico II and the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA).
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