Operational characteristics and controller developments of a variable speed direct expansion air conditioning system

Abstract

Compared to chilled water-based large-scale central air conditioning (AlC) systems, direct expansion (DX) A/C systems are simpler in configuration, more energy efficient and generally cost less to own and maintain. Therefore, for the last few decades, DX A/C systems have found increasingly applications in buildings, particularly in small to medium-scale buildings. However, currently most DX A/C systems are equipped with single-speed compressors and fans, relying on on-off cycling to maintain indoor dry-bulb temperatures only. This results in an uncontrolled equilibrium indoor humidity and possible space over-cooling if indoor humidity level is to be maintained at an acceptable level. These will lead to a reduced level of thermal comfort for occupants, poor indoor air quality (IAQ) and low energy efficiency. The development of variable-speed (VS) technology has made the continuous control of compressor speed and supply fan speed in a DX A/C system more practical, paving the way for achieving simultaneous control of indoor air temperature and relative humidity using DX A/C systems. For a VS DX A/C system, varying its compressor speed and supply fan speed influences on its output sensible and latent cooling capacities, which is part of the operational characteristics of the VS DX A/C system. Previous extensive experimental studies demonstrated that inlet indoor air temperature and humidity would, however, have also influence on the operational characteristics of a VS DX A/C system, with the extent of influences to be further studied. On the other hand, different attempts have been made to develop control strategies for the simultaneous control of indoor air temperature and humidity using a VS DX A/C system. Each controller developed has however its inadequacy because of both the difficulties of accurately modeling the complex heat and mass transfer between the refrigerant and air in a DX evaporator, and the coupling effects for the control loops for air temperature and humidity. Recently, artificial intelligent control strategies have been widely applied to building heating, ventilation and air conditioning (HVAC) systems. Among various artificial intelligent control strategies, fuzzy logic (FL) is promising for achieving improved control of HVAC systems, because FL is very useful when the processes under consideration are too complex to be analyzed by conventional quantitative techniques or when the available sources of information are interpreted qualitatively, inexactly or uncertainly. Furthermore, artificial neural network (ANN) has been proven to be powerful in modeling the dynamic performance of a nonlinear multivariable system without requiring a physical model. Therefore, the application of fuzzy logic or ANN, individually or jointly, helps address the issue of simultaneously controlling indoor air temperature and humidity using a VS DX A/C system. Therefore, in this Thesis, a study to further investigate the operational characteristics of a VS DX A/C system at different inlet air states, and to develop novel controllers that can simultaneously control indoor air temperature and humidity using a VS DX A/C system following fuzzy logic and ANN approaches is reported. The Thesis, first of all, begins with presenting an experimental study on the operational characteristics of a VS DX A/C system at six different inlet air states to the system, which is a follow-up investigation to a previous related experimental study but at only two inlet air states. The study results suggested that different inlet air states to a DX A/C system influenced the operational characteristic of the system, in terms of the Inherent Correlations (IC) between its output total cooling capacity (TCC) and equipment sensible heat ratio (E SHR). Therefore, a further data processing method was developed using regression, by which the ICs of the VS DX A/C system at non-test inlet air states can be predicted with adequate accuracy. This study has therefore, on one hand, provided a further detailed understanding of the operational characteristics of the DX A/C system, and on the other hand, paved way to developing advanced control strategies for indoor thermal environment (i.e., air temperature and humidity) based on the known or predicted ICs within the possible operating ranges of inlet air temperature and humidity to the DX A/C system. Secondly, this Thesis reports on the developments of two novel controllers as two different approaches to solve the same problem of simultaneously controlling indoor air temperature and humidity using a VS DX A/C system. For the first one, based on the obtained ICs of the VS DX A/C system, a novel ANN aided fuzzy logic controller, which combined the complementary merits of fuzzy logic controller and ANN modeling, was developed and experimentally validated. A novel control principle was proposed to decouple the temperature and humidity control loops by introducing two interim variables of sensible output cooling capacity and latent output cooling capacity of the VS DX A/C system. A fuzzy logic system was redesigned to simplify both its calculation and structure by using weights assigned to linguistic variables. To enable ANN models developed to be functional over the normal operational range of indoor air parameters, the obtained ICs of the VS DX A/C system were used for training and testing the ANN models. The novel controller developed was tested using the experimental VS DX A/C system. Both the command following tests and disturbance rejection tests showed that air dry-bulb and wet-bulb temperatures were properly controlled by the controller developed with satisfactory control performances in terms of control accuracy and sensitivity. However, on the other hand, the other controller developed was a novel PD-law based fuzzy logic controller (PFC). To weaken the coupling effect between two control loops, fuzzy logic was deployed. A PD law was used instead of a PID law in the PFC, which could help simplify not only calculation but also the structure of the PFC. The controller developed was also validated by carrying out the controllability tests, including the command following test, disturbance rejection test and the command following with disturbance test, with the experimental conditions covering the normal operational range of a VS DX A/C system. The experimental results of the controllability tests suggested that the novel PFC developed was also capable of realizing the simultaneous control of indoor air temperature and humidity satisfactorily, in terms of control accuracy and sensitivity. Thirdly, this Thesis presents the performance comparisons between the two developed novel controllers, using that of an ON / OFF controller as a benchmark for the VS DX A/C system, in terms of control performance and energy efficiency. Comparison experiments for the three controllers were carried out under different operating conditions using the VS DX A/C system. Both the merits and shortcomings of the developed novel controllers are reported. With this study reported in this Thesis, a better understanding of operational characteristics of a VS DX A/C system has been obtained. Furthermore, novel controllers based on fuzzy logic and ANN have been developed to address the simultaneous control of indoor air temperature and humidity using a VS DX A/C system. The outputs from this study can help improve occupants' thermal comfort level and indoor air quality and enhance the energy efficiency of VS DX A/C systems.