Multivariable control of air temperature and humidity in a space served by a direct expansion (DX) air conditioning (A/C) system

Abstract

Direct expansion (DX) air conditioning (A/C) systems are widely used in small- to medium-scaled buildings, offering many advantages over conventional chilled-water based air conditioning systems. These include a higher energy efficiency and a lower cost to own and maintain a DX A/C system. However, it is difficult to satisfy both indoor air temperature control and humidity control using a DX A/C system with a constant speed compressor and supply fan, which may hinder the wider use of DX A/C systems. Traditionally, on-off cycling a constant speed compressor and supply fan has been the principal method for capacity control for a DX A/C system to maintain indoor dry-bulb temperature only, leaving indoor humidity uncontrolled. Such a control method has led to a poor indoor thermal environment and a low energy efficiency as space overcooling is often seen in order to achieve better dehumidification. The recent advancement of low-cost variable speed drive (VSD) technology offers tremendous opportunities for improving indoor thermal control and energy efficiency for DX A/C systems. Compressor speed can be continuously varied to modulate the output cooling capacity to match the actual thermal load. The supply fan speed can be also continuously altered to affect both sensible heat and latent heat transfer rates across a DX cooling coil. It is therefore possible to simultaneously control indoor air temperature and humidity by simultaneously varying the speeds of both compressor and supply fan in a DX A/C system. However, the related research work on developing a multivariable control strategy for simultaneously controlling both indoor air temperature and humidity in spaces served by DX A/C systems cannot be indentified in published literature. Therefore a study on developing a multivariable control strategy that can simultaneously control indoor air temperature and humidity by varying speeds of both compressor and supply fan in a DX A/C system, through both modeling and experimental approaches, has been carried out and is reported in this Thesis. The Thesis starts with reporting the development of a multivariable control-oriented dynamic model of an experimental DX A/C system. The dynamical model, which consisted of a set of differential equations, was derived based on the principles of energy and mass conservation. The dynamic model was nonlinear in nature, and was therefore linearized and written in a state-space representation which was highly suitable for multivariable control design. The linearized dynamic mathematical model for the experimental DX A/C system was experimentally validated. Experimental tests to obtain the open-loop responses of the experimental DX A/C system after being subjected to step changes in compressor speed and supply fan speed were carried out. The experimental open-loop responses for all major operational parameters obtained were found in good agreement with the simulated open-loop responses from the linearized dynamic model, suggesting that the linearized dynamic model was experimentally validated. Secondly, the Thesis presents the development of a multi-input multi-output (MIMO) controller for simultaneously controlling indoor air temperature and humidity in a space served by the experimental DX A/C system. Based on the linearized dynamic model for the experimental DX A/C system, a multivariable feedback control strategy has been formulated and the MEVIO controller developed using the Linear Quadratic Gaussian technique. Controllability tests for both disturbance rejection capability and command following capability were carried out to evaluate the performance of the MEVIO controller in the experimental DX A/C system. The experimental results demonstrated that the MIMO controller developed can effectively control the indoor air temperature and humidity simultaneously by varying compressor speed and supply fan speed in the experimental DX A/C system. Finally, the development of a new controller for improving the control performance of degree of refrigerant superheat (DS) in a DX A/C system when the fluctuation of operating DS is resulted mainly from varying the speeds of its compressor and supply fan is reported. The new DS controller was developed from a conventional proportional-integral-derivative (PID) DS controller by adding two feed-forward channels so that information of speed changes of compressor and supply fan can be timely passed to the new DS controller to take appropriate control action. Controllability test results showed that an improved DS control performance can be achieved by using the new DS controller instead of a conventional PID DS controller, and resulting in a better operating performance of the DX A/C system in terms of operating efficiency and stability when its compressor speed and fan speed were varied as required by a capacity control algorithm.