Wenjie Gang

Robust optimal design of building cooling systems concerning uncertainties using mini-max regret theory

Various uncertainties exist in cooling systems at the plan and design stage. The conventional design method usually selects the cooling system or configurations without considering uncertainties, which may be risky because the performance of the cooling system may deviate from the expected, together with increased cost and reduced benefit due to uncertainties and variations in actual working conditions. A new, simple, and effective method is proposed to optimize the design of cooling systems and get the robust optimal cooling system by considering uncertainties in the information used at the design stage. The mini-max regret theory is used to realize the method. Two important problems in the design of cooling systems are studied: chiller combinations and chilled water pump configurations. By considering the uncertainties in the cooling load, the robust optimal chiller combination can be obtained. By considering the uncertainties in the resistance of chilled water pipelines, the robust optimal chilled water pump configuration can be determined.

District cooling systems and individual cooling systems: Comparative analysis and impacts of key factors

District cooling systems are used in many areas, especially where the building density is high. However, its efficiency is still quite controversial compared with conventional individual cooling systems, especially in China. It is highly necessary to conduct a detailed study and give clear answers when a decision is made between the district cooling system and individual cooling system. Key factors that affect the decision need to be studied. This article therefore conducts a comprehensive performance assessment of district cooling systems by comparing with individual cooling systems. The comparative performance of both systems is analyzed when the combination of buildings with different functions varies. The optimal combinations for each system are obtained using genetic algorithms, and recommendations are summarized for better application of both systems. Impacts of the efficiency of chillers, the resistance of chilled water networks, and the cooling loads on the comparative performance are quantified considering uncertainties. Economic performance of both systems is analyzed, including the capital costs and operational costs. Suggestions are summarized for the future application of district cooling systems and individual cooling systems.

A model-based adaptive method for evaluating the energy impact of low delta-T syndrome in complex HVAC systems using support vector regression

Low delta-T syndrome refers to the situation where the measured differential temperature of the overall terminal air-handling units is much lower than the normal value expected. It widely exists in the existing heating, ventilating, and air-conditioning systems and results in increased energy consumption. This paper presents a model-based method to evaluate the energy impact on the chilled water pumps due to the low delta-T syndrome in a complex chilled water system. When the low delta-T syndrome occurs, the chilled water pumps would deviate from their normal working conditions with increased power consumption. Models are developed to predict the reference benchmarks of the chilled water pump power based on the current cooling load, control rules, and preset set-points. The energy impact on the chilled water pumps can be determined by comparing the measured current pump power with the predicted benchmark. Support vector regression method is introduced for predicting the chilled water flow rate of the overall terminal units. Adaptive concept is employed to enhance the prediction accuracy of the overall pressure drop of the hydraulic water network under various working conditions. The proposed method is tested and validated in a dynamic simulation platform built based on a real complex heating, ventilating, and air-conditioning system. Results show that the proposed method can accurately evaluate the impact of the low delta-T syndrome on energy consumption of the chilled water pumps. Practical application: Low delta-T syndrome widely exists in existing HVAC systems and results in increased energy consumption. This paper presents a model-based method for practical applications in assessing the energy impact on the chilled water pumps due to the low delta-T syndrome in a complex chilled water system. When the low delta-T syndrome occurs in a system, this method can be used to predict the reference benchmark of energy use of chilled water pumps based on the measured cooling load profiles, the control rules used, and the preset set-points. The energy impact can be determined by comparing the measured actual energy consumption with the predicted benchmark. The evaluation results could help the operators to conveniently monitor the energy performance of the chilled water distribution system as well as to judge whether or not taking measures to identify and correct the related faults that result in the low delta-T syndrome.