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Title: Modeling and control of a three-evaporator air conditioning (TEAC) system with an emphasis on improved indoor humidity control
Authors: Yan, Huaxia
Degree: Ph.D.
Issue Date: 2016
Abstract: Multi-evaporator air conditioning (MEAC) systems featuring variable refrigerant flow, with a three-evaporator air conditioning (TEAC) system being a typical example, have increasingly become attractive. This is because MEAC systems have a number of advantages such as higher energy efficiency and system design and installation flexibility. Although both a single-evaporator air conditioning (SEAC) system and an MEAC system are operated based on the same vapor compression cycle, there exist a number of noticeable differences between the two. Usually in an MEAC system, there are two or more indoor units (IU) installed in parallel without any pressure regulators, which results in strongly coupled operational parameters in each IU. Although there have been studies on the relationships among the coupled operating parameters in an MEAC system, most of them were simulation based. No previous experimental based studies on the operating characteristics of MEAC systems having more than two evaporators may be identified in the open literature. On the other hand, for buildings located in hot and humid climates such as Hong Kong, controlling indoor air humidity at an appropriate level is critically important since this directly affects building occupants' thermal comfort, indoor air quality and the operating efficiency of the building air conditioning installations. Although MEAC systems worth many billions of dollars are sold worldwide, only a very limited number of capacity control algorithms for MEAC systems can be identified in the open literature, with most of them focusing on the control of indoor air temperature only. Furthermore, there is a lack of experimentally validated dynamic mathematical MEAC models which can better understand the complex coupled operating characteristics and develop novel capacity controller for MEAC systems for improved control accuracy and higher energy efficiency. Therefore, a programmed research work on the modeling and control of a TEAC system, with an emphasis on improved indoor humidity control, has been carried out and the study results are presented in this thesis. This thesis begins with reporting the development of an experimental TEAC system, consisting of three indoor spaces (IS) and one outdoor space. The experimental TEAC system was operated by a computerized logging and control supervisory system. All its major operating parameters can be real-time measured and recorded using high precision measuring devices. The availability of the experimental TEAC system would help facilitate studying the operating characteristics of a TEAC system, experimentally validating a dynamic TEAC mathematical model to be developed and developing novel capacity controllers for a TEAC system with an emphasis on indoor humidity control.
Secondly, the thesis presents the results of two Sets of specifically designed experiments on the operating characteristics of the experimental TEAC system. In the first Set, changes in the operating parameters in one of the three IUs/ ISs were introduced, but that in two of the three IUs/ ISs in the second. Similar coupling effects of mutual influences reported in previous dual-evaporator air conditioning (DEAC) systems may also be observed in the first Set. However, when the operating parameters in two of the three IUs/ ISs were varied, the coupling effects became more complicated, with mutual influences being more remarkable than that in the first Set with even less variation magnitude. Furthermore, the operating characteristics on both the refrigerant side and air side of each IU/ IS in the experimental TEAC system were also studied, which is the first of its kind in the open literature. Thirdly, the development of a dynamic mathematical model for the experimental TEAC system is presented. The model was built based on the sub-models of major system components in the TEAC system, including a variable speed compressor, an air-cooled condenser, three electronic expansion valves, three IUs and ISs. Unlike all other reported TEAC models, both sensible and latent heat balances on the air side of all IUs were specifically taken into account in the TEAC model developed. The model was experimentally validated using the experimental TEAC system. Model predictions were found to be within {171}6% of the measured values, suggesting that the model developed was capable of simulating both the steady state and dynamic operation of a TEAC system with an acceptable accuracy. Finally, the development of a novel capacity controller for the experimental TEAC system for improved indoor humidity control is reported. The novel controller was developed by integrating two previously developed control algorithms, one for a DEAC system for temperature control and the other for an SEAC system for improved indoor humidity control. Both simulative and experimental controllability tests were carried out, using the validated TEAC model and the experimental TEAC system, respectively. The controllability test results showed that, with the novel controller, improved control over indoor air humidity and better energy efficiency for the TEAC system could be obtained, as compared to the use of traditional On-Off controllers extensively used in MEAC systems.
Subjects: Air conditioning.
Humidity -- Control.
Hong Kong Polytechnic University -- Dissertations
Pages: xxi, 202 pages : color illustrations
Appears in Collections:Thesis

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