| Author: | Zheng, Shuyang |
| Title: | Experimental and numerical investigation on direct expansion air conditioning system for improved temperature and humidity control |
| Advisors: | Deng, Shiming (BSE) Yan, Huaxia (BSE) |
| Degree: | M.Eng. |
| Year: | 2020 |
| Subject: | Air conditioning -- Control Humidity -- Control Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Services Engineering |
| Language: | English |
| Abstract: | For many decades, direct expansion (DX) air conditioning (A/C) system has been widely used in most of the small to medium scaled buildings, especially residential buildings, mainly due to the advantages of small size, simple structure and low cost. However, with the increasing concern on the issues of indoor environmental quality and energy consumption, traditional ON/OFF controlled A/C system is becoming far from meeting the needs of better thermal comfort and higher energy efficiency. The main defect for ON/OFF controlled A/C system is the poor controllability of humidity, which is one of the main factors that cause bad thermal comfort in the air-conditioned space. Therefore, the application of variable speed (VS) technology aims at solving the above problems but has also drew a higher request on the controller. Based on the previous experimental investigations, a mathematical model of an experimental VS DX A/C system was established in order to provide a broader insight into the inherent operational characteristics of VS DX A/C system. The simulation results have emphasized that the variation of returned air state, especially the relative humidity, would have a significant impact on system output sensible and latent cooling capacity, as well as compressor coefficient of performance (COP). More importantly, the results have provided a theoretical foundation of VS DX A/C system, which would be useful and helpful to further aid the development of controller. A previous developed high-low control algorithm was experimentally and numerically investigated in this study. Although such a control method could improve the ability of dehumidification without the expense of more energy consumption, the indoor relative humidity level would still be subject to indoor application sensible heat ratio (A SHR) rather than total cooling load according to both the experimental and simulation results. Besides, the main defect of this control method is the over-dehumidification effect under dry condition, resulting in both worse thermal comfort and energy efficiency in some of the cases. While the possible solutions to this issue include optimizing speeds selection at low-speed period and combining ON/OFF and high-low control. Aiming at solving the defect of the high-low control, a more advanced control method was purposed based on the developed model and fuzzy logic control. The developed model was introduced to better predict the required sensible and latent cooling capacity, as well as determine the appropriate compressor and supply fan speed. The purposed control method was tested by simulation and the results show a good controllability of temperature and relative humidity with acceptable control accuracy and sensitivity. Moreover, more simulation tests of the high-low control and the purposed fuzzy logic control were conducted under sudden change and gradual change of indoor loads, respectively. The results illustrate that the fuzzy logic control is able to maintain both temperature and relative humidity in the vicinity of the set-point and avoid unnecessary high-speed operation of the VS DX A/C system to the largest extent, hence higher energy efficiency and better thermal comfort. |
| Rights: | All rights reserved |
| Access: | restricted access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 5349.pdf | For All Users (off-campus access for PolyU Staff & Students only) | 5.16 MB | Adobe PDF | View/Open |
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