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of thesis entitled: Diagnosis and Robust Control of Complex Building Central Chilling Systems for Enhanced Energy Performance Submitted by : GAO Diance For the degree of : Doctor of Philosophy at The Hong Kong Polytechnic University in April, 2012 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 iii 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