Modeling and control of a direct expansion (DX) variable-air-volume (VAV) air conditioning (A/C) system

Abstract

A direct expansion (DX) variable-air-volume (VAV) air conditioning (A/C) system, a typical type of air-cooled packaged A/C systems found in commercial buildings, consists of a VAV air-distribution sub-system and a DX refrigeration plant. Such a system offers more advantages than a conventional chilled-water based VAV A/C system, such as higher energy efficiency and generally costing less to own and operate. Currently, the major deterrence to its wider application is to continuously match the output cooling capacity from its DX refrigeration plant with the varying cooling load in its VAV air-distribution sub-system. However, the development of both variable-speed compressor and electronic expansion valve (EEV) technologies offer a good opportunity to address the problem of capacity-load matching. This thesis firstly reports on the development of an experimental rig for a DX VAV A/C system. The rig was operated by a computerized logging and control supervisory system. All major operating parameters can be real-time measured, monitored and recorded, using high-precision measuring devices/sensors. The setting up of the rig would help facilitate developing both a dynamic model and its experimental validation, and new control strategies for a DX VAV A/C system. Secondly, a complete dynamic mathematical model for the experimental DX VAV A/C system has been developed based on the principle of mass and energy conservation, and using the correlations describing the operational performance of various components in the experimental DX VAV A/C system, which were either field-tested or available from manufacturers. Mathematical correlations representing the control logics of various controllers were also included. The model was component-based and of partial-lumped-parameter type. It consisted of separate constituent sub-models for both the DX refrigeration plant and the VAV sub-system in the DX VAV A/C system. Both the steady-state and dynamic behaviors from both the DX refrigeration plant and the VAV sub-system can be simultaneously simulated by the model. Thirdly, the experimental validation for the dynamic model developed using the experimental rig is presented. The open-loop responses from both the DX refrigeration plant and the dynamic sub-model representing the DX plant were compared and found in good agreement. Fourthly, the thesis reports on the simulated closed-loop responses from the DX VAV A/C system with all its conventional proportional-integral (PI) control loops being enabled, using the validated model. Simulation results demonstrated that the dynamic model developed can behave as expected in a similar manner to a real DX VAV A/C system with all its control loops enabled. This further confirmed that the model developed was correct and could be useful for studying control issues for a real DX VAV A/C system. Finally, the development of a novel DDC-based feedforward capacity controller, for matching the output cooling capacity from the DX refrigeration plant with the varying cooling load in the VAV sub-system, is reported. The feedforward controller consisted of both a numerical calculation algorithm, which was fundamentally based on the principle of energy balance using a number of real-time measured system's operating parameters, and a dead-band for avoiding unstable compressor operation.Controllability tests for the feedforward controller have been carried out in the experimental rig. Test results suggested that the novel feedforward controller developed was able to provide satisfactory control sensitivity and accuracy, resulting in a higher operating efficiency of, and better environmental control by, a DX VAV A/C system.