Proposal and experimental study on a CO₂-based multi-mode integrated HVAC system

Heating, ventilation, and air conditioning (HVAC) systems are major contributors to energy consumption in buildings, making performance improvement crucial for addressing energy challenges. In this paper, a novel CO₂-based multi-mode integrated HVAC system is proposed. The operational modes and feasibility of the integrated system are analyzed. Experiments conducted on a self-built platform reveal variations in compressor suction and discharge pressure, temperature, ambient temperature, heating loads, and refrigerant charge under hot water mode and combined refrigeration/hot water mode. Energy, exergy, environmental, and economic analyses are performed. Results indicate that suction and discharge pressure and temperature increase with ambient temperature. However, as refrigerant charge increases, both suction and discharge pressures rise, while temperature decrease. The optimal refrigerant charge for the combined refrigeration/hot water mode is 1.80 kg, surpassing the 1.65 kg for the hot water mode. The electronic expansion valve exhibits the highest energy consumption. Compressor costs account for 51.8 % of lifecycle expenses. When ambient temperature increases from −5℃ to 35℃, pollutant emissions decrease by 20.56 %. At the same water tank outlet temperature (40℃), the CO₂ system achieves a COP of 8.7 in combined refrigeration/hot water mode, 5.45 % higher than the R410A system, confirming CO₂ as an effective alternative.