Preliminary experimental study on dynamic characteristics of a CO₂-based multi-mode system

The dynamic characteristics of Heating, Ventilation and Air Conditioning (HVAC) systems play a pivotal role in improving occupant comfort and promoting sustainable building development. In this paper, the temporal characteristics of a CO₂-based multi-mode integrated system are experimentally investigated under different control strategies and operating conditions. Two control strategies are used for the integrated system, including valve opening regulation and compressor frequency regulation. An experimental program is developed and the corresponding experimental control procedures are delineated. The reliability of the results is quantified through uncertainty analysis to improve the accuracy of the conclusions drawn from the empirical data. Results show that, the system exhibits distinct characteristics across different modes. In cooling and heating modes, compressor inlet and outlet pressures and temperatures fluctuate before stabilizing. In hot water and combined cooling/hot water modes, outlet pressure and temperature increase gradually, while inlet conditions remain stable. Compressor power consumption rises rapidly initially, then fluctuates. The electronic expansion valve (EEV) significantly affects compressor inlet pressure and outlet temperature, complicating the adjustment process. The system achieves a maximum coefficient of performance of 8.1 at 100 Hz, and increasing the frequency from 100 Hz to 140 Hz reduces the time to reach the target tank temperature by 33.4 %.