A team of scientists led by Italy’s University of Padova has numerically simulated a novel heat pump that can reportedly combine three renewable energy sources with high efficiency.
The system uses photovoltaic-thermal (PVT) collectors as the solar source, a U-tube borehole heat exchanger (BHE) for the ground source, and a finned coil heat exchanger (FCHE) for the air source.
“Two configurations of the multisource heat pump are investigated: solar-air mode (SA-mode) with finned coil and PV-T collectors running simultaneously and ground-air mode (GA-mode) with finned coil and BHE running simultaneously,” the team explained. “The multisource system operates as a dual-source heat pump (DSHP) in each mode. It is important to note that this configuration is different from all existing parallel and series setups found in the literature.”
The numerical simulation was constructed using MATLAB software. Aside from the PVT units, BHEs, and the FCHE, the system also included a compressor, a gas cooler (GC), an internal heat exchanger (IHE), an electronic expansion valve (EEV), and a low-pressure receiver (REC). The finned coil evaporator is fed in dry expansion, while the solar and ground evaporators operate in flooded mode. It is working with CO2 as the refrigerant.
“The superheated refrigerant is compressed and sent to the GC to be cooled down. It then passes through the IHE, where it undergoes further subcooling. After expansion in the EEV, the refrigerant enters the two-phase region and evaporates inside the FCHE,” the group explained. “With an increased vapor quality, the CO2 enters the REC, where the two-phase fluid separates into vapor and liquid due to density differences. Liquid CO2 is taken from the bottom of the REC and directed either to the BHE in GA mode or to the PV-T collectors in SA-mode. After the evaporation process, the refrigerant returns to the REC. Finally, the saturated vapor is drawn back into the compressor.”
The system was simulated to work in the northern Italian city of Padova, with its average monthly solar irradiance of 300 W/m², an annual average soil temperature of 13 C and an average air temperature of 7 C. It was tested with different amounts of BHE, each with a depth of 30 m, and a changing number of PVT units. Those with a power output of 270 W were placed at a 45° tilt angle.
The simulation showed that, for both systems, the solar or ground evaporators operate simultaneously with the air evaporator, and 1 K of air temperature increase results in a 4.8% coefficient of performance (COP) rise in SA-mode and a 4.3% COP rise in GA-mode. Furthermore, it demonstrated that the SA-mode is influenced by solar irradiance, where each 100 W/m² global tilted irradiance (GTI) increase improves the COP by 2.8%, while GA-mode is affected by soil temperature and 1 K of soil temperature increase produces a rise in COP equal to 0.9%.
Moreover, the academics found that adding one to three BHEs results in an 8%–21 % increase in the soil temperature range from 7 C to 13 C. Still, adding 1 to 4 PVT units achieved a 4%–22% COP increase within a GTI range of 300 W/m² to 1,100 W/m².
“More importantly, the COP of the multisource heat pump is always higher than that of the air-source heat pump,” the researchers emphasized, noting that they are now planning to the effect of varying the number of PV-T modules and borehole heat exchangers on heat pump performance.
The system was described in “Investigation on a direct expansion multisource carbon dioxide heat pump to maximize the use of renewable energy sources,” published in Applied Thermal Engineering. The group also included scientists from the Netherlands’ Delft University of Technology.
Another research group at the University of Padova recently designed a 5 kW direct-expansion solar-assisted heat pump that uses alternatively two different evaporator technologies. The cooling of the PV module unit by CO2 evaporation increases power production by 8%.
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