Simultaneous use of Air and Solar Evaporators in a CO2 Heat Pump to Maximize the use of Renewable Sources

ABSTRACT

This study explores a dual-source heat pump design utilizing CO₂ as the refrigerant. The innovation lies in the possibility to operate the heat pump in three distinct evaporation modes: air-mode, solar-mode, and a simultaneous-mode using both thermal sources. The experimental data reported in this work confirms that the operation in simultaneous-mode can increase the evaporation pressure and improve the coefficient of performance as compared to the air-mode or solar-mode. The simultaneous-mode offers greater flexibility and design advantages because even with limited solar panel area, the combined utilization of both sources enhances the performance compared to a traditional air-source heat pump.

INTRODUCTION

Air source heat pumps (ASHP) are the most common technology for replacing gas boilers and decarbonizing the building sector, but their performance is reduced in cold temperatures. One alternative can be represented by solar-assisted heat pumps (SAHP), which use solar radiation as a heat source but depend strongly on solar irradiance. The coupling of the two technologies in one single system is realized in solar-air dual-source heat pumps (SA-DSHP). However, managing the switch between the two heat sources, to maximize the system performance, is a challenge that can be overcome by trying to simultaneously utilize both the two heat sources. There can be two different configurations of direct SA-DSHPs: parallel configuration, where the refrigerant flow rate is split between the solar and the air evaporators, and series configuration, where the refrigerant flows in the two evaporators sequentially. In addition to the enhancement of the performance, another key aspect is the use of low global warming potential refrigerants and natural refrigerants. Following the phase-down of the HFC refrigerants promoted by the Kigali Amendment (2016), this work introduces the use of CO₂ in a SA-DSHP. Furthermore, a numerical model of the heat pump is employed to assess the performance when varying the solar irradiance.

DESCRIPTION OF THE EXPERIMENTAL APPARATUS AND OPERATION MODES

The experimental prototype is a 5 kW heating capacity SA-DSHP and it is installed at the Department of Industrial Engineering at the University of Padova. The heat pump layout (see Figure 1) includes two evaporators: a conventional finned coil heat exchanger and three photovoltaic-thermal (PV-T) solar collectors.

An inverter-driven rotary compressor (COMP) sends high-pressure superheated refrigerant to a gas-cooler (GC) which is a brazed plate heat exchanger. The refrigerant then passes through an internal heat exchanger (IHE) and an electronic expansion valve (EEV). The heat pump can operate in three evaporation modes:

1. Air-mode: Valve V1 directs the flow to the finned coil and valve V2 is closed. The finned coil heat exchanger is used as evaporator and the thermal source is air.
2. Solar-mode: Valve V1 directs the flow to the receiver and valve V2 is open. Refrigerant flows to the low-pressure receiver, then is pumped to the PV-T collectors, using solar irradiance as the thermal source.
3. Simultaneous-mode: Valve V1 directs the flow to the finned coil and valve V2 is open. Refrigerant goes through the finned coil evaporator and then to the PV-T collectors, using both solar irradiance and air as the thermal source.

In both solar and simultaneous modes, the PV-T evaporator is fed with liquid CO₂ with forced circulation avoiding possible maldistribution issues and the presence of superheated vapor at the outlet of the collectors. Vapor-phase refrigerant from the low-pressure receiver is superheated in the internal heat exchanger before entering the compressor.

EXPERIMENTAL RESULTS IN THE THREE OPERATION MODES

Defining the COP as the ratio between the heating capacity produced by the heat pump and the total power consumption (including the consumption of the compressor, the fan of the finned-cooled evaporator and the pump for the liquid CO₂) it is possible to compare the three different operation modes (air, solar, and simultaneous). Figure 2a shows some experimental data, when the heat pump worked with 50% of compressor speed, 80 bar of high-pressure and a water heater from 30 °C to 35 °C. The heat pump in simultaneous-mode achieved the highest COP values (COP=4.65), about 25% higher compared to air-mode (COP=3.71) and solar-mode (COP=3.83). The higher COP in simultaneous-mode can be explained considering at the dual-source operation reduces the compressor power consumption and consequently the total consumption of the heat pumps (P_tot) by approximately 8% and increases the heating capacity (Q_GC) by approximately 18%, as shown in Figure 2b. The compressor power reduction is due to an increase of about 6 K in the evaporation temperature (T_evap) compared to single-source modes (Figure 2a).

RESULTS OBTAINED WITH THE NUMERICAL MODEL OF THE HEAT PUMP

A numerical model of the heat pump (Conte et al., 2024, Applied Energy, vol 369) has been used to investigate the effect of solar irradiance on the heat pump performance. Figure 3 illustrates the relation between COP and the solar irradiance when considering 5°C air temperature, 50% of compressor speed, 80 bar of high-pressure and water heated from 30 °C to 35 °C. The data indicates that in simultaneous-mode, the COP increases almost linearly with solar irradiance and the increase is lower than that obtained for the solar-mode. However, the simultaneous-mode provides higher COP values compared to the solar-mode as long as the solar irradiance remains below 1050 W∙m^-2. Exceeding this value reduces the heat pump’s performance in simultaneous-mode because the evaporation temperature surpasses the air temperature, causing the finned coil to no longer aid in the evaporation process. On the other hand, the simultaneous-mode always offers a higher COP compared to air-mode even with an irradiance equal to 500 W∙m^-2 and this advantage increases with the solar irradiance.

CONCLUSIONS

When operating a dual source heat pump using a specific thermal source (in this case solar or air), it necessitates a comprehensive control algorithm to choose the most efficient evaporator. The algorithm must continuously monitor and forecast the heat pump’s performance to decide how to switch between the thermal sources. Interestingly, the operation in simultaneous mode reduces the requirement for constant algorithm adjustments, maintaining optimal performance across various environmental conditions.

A solar-air dual source heat pump in simultaneous-mode provides a compact, high-performance solution, particularly interesting even with a limited useful area for the PV-T. The simultaneous use of thermal sources enables the achievement of improved performance as compared to a mere air source installation and maximizes the utilization of renewable sources.

Acknowledgements

This study was developed in the framework of the Project “Network 4 Energy Sustainable Transition—NEST”, Spoke 1, Project code PE0000021, funded under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.3— Call for tender No. 1561 of 11.10.2022 of MUR; funded by the European Union—NextGenerationEU