Diagnosis and robust control of complex building central chilling systems for enhanced energy performance

Pao Yue-kong Library Electronic Theses Database

Diagnosis and robust control of complex building central chilling systems for enhanced energy performance

 

Author: Gao, Diance
Title: Diagnosis and robust control of complex building central chilling systems for enhanced energy performance
Degree: Ph.D.
Year: 2012
Subject: Air conditioning -- Energy consumption.
Air conditioning -- Control.
Air conditioning -- Equipment and supplies.
Hong Kong Polytechnic University -- Dissertations
Buildings -- Cooling.
Department: Dept. of Building Services Engineering
Pages: xxv, 224 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2615859
URI: http://theses.lib.polyu.edu.hk/handle/200/6964
Abstract: Low delta-T syndrome (i.e., low chilled water temperature difference disease) and deficit flow problem (i.e., the required flow rate of secondary loop exceeds that of the primary loop) widely exist in many building chilled water systems, which make the system fail to operate as efficient as anticipated, degrading the overall building energy performance. This thesis focuses on developing diagnosis and robust control strategies to avoid or improve the low delta-T syndrome and enhance the operation and energy performance of chilled water systems. The main contributions of this thesis include an in-situ case study to diagnose the low delta-T syndrome in a complex building chilled water system, an online optimal control strategy for complex chilled water systems involving heat exchangers, a fault-tolerant control strategy for secondary chilled water systems, and a fault diagnosis strategy integrated with energy impact evaluation method for low delta-T syndrome. These proposed strategies are developed based on an actual chilling system in a super high-rise building, and are tested and validated on a dynamic simulation platform developed in this study prior to site implementation. Firstly, an in-situ case study to diagnose the low delta-t syndrome and deficit flow problem that frequently occurred in the studied super high-rise building is conducted. An in-situ diagnosis method of low delta-t syndrome for practical applications is proposed, which involves history operation data analysis and experimental validation. The reasons for low delta-T syndrome in this studied system are detected and identified, and corresponding suggestions are provided as well. Secondly, based on the causes that resulted in low delta-T syndrome found in the actual super high-rise building under study, an adaptive optimal control strategy for online control of complex chilled water systems involving intermediate heat exchangers is developed. This optimal control strategy searches for the optimal settings of the outlet water temperature after heat exchangers and the required operating number of heat exchangers and pumps in order to minimize the total energy consumption of pumps under various working conditions. The strategy has enhanced control robustness and reliability including avoiding deficit flow problem when compared with the conventional strategies. An adaptive method is utilized to update the key parameters of the proposed models online. The proposed strategy is validated to be robust and reliable to eliminate deficit flow problem when facing disturbances. The test results also demonstrate that the proposed strategy can accurately identify the optimal control settings. Significant energy of pumps therefore can be saved when compared with other conventional methods for online practical applications.
Thirdly, a fault-tolerant control strategy for secondary chilled water pumps is developed not only for eliminating the low delta-T syndrome and deficit flow problem but also for enhancing the energy efficiency of the chilled water distribution systems. This fault-tolerant strategy employs the developed flow-limiting technique that is activated when deficit flow tends to occur and eliminates it by resetting the differential pressure set-point for pumps control. This strategy also integrates optimal differential pressure set-point that can minimize flow resistance of chilled water loop while still satisfying cooling energy demand. The performance of this proposed strategy is evaluated in a simulated environment representing a chilled water system in a super high-rise commercial building by comparing it with two conventional control strategies. Results show that the proposed strategy can effectively eliminate the deficit flow in the bypass at both starting and normal operation periods. Fourthly, a fault detection and diagnosis (FDD) strategy integrated with an energy impact evaluation method for low delta-t syndrome is developed to identify the low delta-T problem caused by cooling coils under various working conditions, and predict its energy impact on the pumps using a model-based method. Fifthly, an in-situ approach is presented for experimental validation of the possibility by using a check valve (i.e., putting a one-direction check valve in the chilled water by-pass line) in the studied super high-rise building to solve the deficit flow problem and thus to enhance the overall operational performance. The experimental tests were carried out on the complex central chilled water system in a super high-rise building by using a 'simulated' check valve through fully closing one of the butterfly valves in the by-pass line when the system operated with significant excess flow demand and experienced with low delta-T problems. The results showed that the system operational performance can be improved greatly when the 'simulated' check valve was used as compared to that without using the check valve. Lastly, the software tools and implementation guidelines for applying these online supervisory and optimal control strategies in practice are presented.

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