A simulation study on the capacity control of a direct-expansion variable-air-volume air conditioning system

A direct-expansion (DX) variable-air-volume (VAV) air conditioning (AJC) system which integrates a VAV air-distribution sub-system and a DX refrigeration plant, is believed to be highly energy-efficient, and can help reduce peak demand. It can also provide flexible independent zoning control, with easy future space or zone reconfiguration. However, it is understood that the main technical deterrence to the wider application of a DX VAV AJC system lies in matching the output cooling capacity from its DX refrigeration plant with the varying cooling load in its VAV air-distribution sub-system. The capacity control methods presently adopted by DX AJC systems, such as on-off cycling refrigeration plant, hot-gas bypass, staging-control are not totally satisfactory because of either poor control accuracy or poor energy efficiency. The use of variable-refrigerant-volume (VRV) technology featured with a variable-speed compressor and an EEV in a DX VAV A/C system is expected to help solve the matching problem. Using a complete dynamic mathematical model for DX VAV A/C systems which is well validated by experimental data, the simulation study for the capacity control for a DX VAV A/C system equipped with variable-speed compressor and EEV was carried out. The closed-loop responses of the DX VAV A/C system was obtained after being subjected to: (1) a step-change in space internal thermal load; (2) a step-change in the set-point of space air temperature. The total simulated operation period is 5400 s, i.e., 90 min. Ten minutes after starting time, i.e., t=600 s, the internal sensible load in Space A which is one of two air conditioned spaces in the DX VAV A/C system was step changed down to 1080 W, or a 50% reduction. At t=3000 s, the set-point of air temperature in Space B was step-changed from 25°C down to 24°C. From the analysis of simulated results, it can be concluded that the application of variable-speed compressor together with EEV could achieve the continuous, quick and precise capacity control for the DX VAV A/C system and therefore space air temperatures can be satisfactorily controlled at their set-points during the whole operation period.