Dual-source CO2 heat pump based on photovoltaic-thermal evaporators

Researchers at the University of Padua in Italy have designed a dual-source heat pump that uses CO2 as the refrigerant and a solar evaporator comprising three PV-T collectors

“The key innovation lies in the combined operation of two evaporators, the finned-coil and the PV-T collectors, enabling selection between air and solar energy or their simultaneous use,” the scientists said. “The novel solution presented here does not require to split the refrigerant flow rate between the two evaporators and at the same time it solves the problem of possible maldistribution at the inlet of the evaporators.”

The proposed 5 kW CO₂ solar-assisted dual-source heat pump (CO₂ SA-DSHP) system can operate with both kinds of evaporators simultaneously or only one of them, with the PVT collector being able to act as an evaporator, as the refrigerant flows directly in the PV-T tubes.

It consists of an inverter-driven hermetic vertical rotary compressor (COMP), an internal heat exchanger (IHE), a gas-cooler (GC), a finned coil heat exchanger used as an air evaporator, and the solar evaporator which consists of three PV-T collectors. The 270 W PV panels were fixed to a plate-and-tube heat exchanger allowing both the evaporation of the refrigerant flow and the cooling of the photovoltaic cells, which in turn improves their performance. It also includes a throttling valve, a circulation pump, and a cylindrical tank acting as a receiver.

The system can operate in three modes: air-mode (AIR-M); solar-mode (SOL-M); and simultaneous-mode (SIM-M). When operating under the AIR-M, the heat pump uses only air as a low-temperature thermal source, while with the SOL-M it utilizes only solar irradiance as the thermal source. Under the SIM-M mode, it uses both solar irradiance and the air.

“In all the operative modes, between the low-pressure receiver (REC) and the suction line of the compressor, the CO₂ flows through the IHE, which is used to superheat the refrigerant before it enters the compressor,” the team explained. “To adjust the degree of superheating, a three-way valve (VB) is used to bypass a portion of the total refrigerant flow rate coming from the REC.”

Through a series of tests carried out in winter environmental conditions, the academics assessed the coefficient of performance (COP) and power consumption of the system. They factored in a compressor and fan speed at 50%, a pump speed of 40%, a high pressure of 80 bar, and water heated from 30 C to 35 C.

“An experimental comparison among the three different operation modes of the heat pump was carried out at the same ambient conditions and operative conditions,” they said, noting that the PV panels were operated to achieve maximum power production.

The analysis showed that the heat pump operating in SIM-M can achieve a higher pressure evaporation and a higher COP compared to those operating in AIR-M and SOL-M. “Simulation results show that the simultaneous-mode operation can be outperformed by the solar-mode only at high irradiance and low air temperature, when the evaporation temperature gets higher than the air temperature,” the scientists further explained.”

Their findings are available in the study “The advantage of running a direct expansion CO₂ heat pump with solar-and-air simultaneous heat sources: experimental and numerical investigation,” published in Applied Energy.

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