| Although countries around the world have formulated various policies and conducted extensive researches to reduce building energy consumption,the effect is not very satisfactory.HVAC energy consumption accounts for a large proportion of building energy consumption,and a large part of this is caused by performance degradation,poor operating conditions or failures.Although fault detection and diagnosis technology is widely used in various scientific research and industrial fields,its application in HVAC systems is not very mature.Based on Modelica,a multi-domain modeling language,this paper selects appropriate model libraries to test the critical components of the chillers and connect them to form a chiller system model.In steady-state simulations,the system shows good steady-state working conditions.Then according to the principle of automatic control,four different control strategies were designed,including non-control strategy,independent control of expansion valve,independent control of compressor and coordinated control.By refering to the survey of common chiller faults,four different typical faults were selected,including Reduced Condenser Water Flow,Reduced Chilled Water Flow,Cooling Tower Malfunction,and Air Handling Unit(AHU)Fan Malfunction.The dynamic simulation of different fault levels was performed under different control strategies.The results were analyzed and studied,and some of the results were compared to the experimental data in the ASHRAE 1043-RP research project.The results show that different control strategies have a significant effect on the performance of the chillers,and there are huge differences in the parameter trends of the same fault.The design of the control strategy plays a crucial role in optimizing the chiller,HVAC system and reducing building energy consumption.Under the non-control strategy,it is helpful to analyze the causes of system parameter changes in different faults,but the system is not stable.Under the independent control of the expansion valve,it is possible to optimize the efficiency in a small range by adjusting the flow of the refrigerant and avoid major compressor failures,but it is impossible to greatly adjust the power and increase the efficiency.Under the independent control of the compressor,by adjusting the speed of compressor revolution,the temperature of the chilled water outlet is stabilized at the set point,and the working state is effectively changed under certain conditions.However,the gas-liquid separator in the system affects the stability and increases the cost.Under cooperative control,the advantages of the expansion valve and the compressor control strategy are combined to adjust the opening of the expansion valve to control the evaporator outlet overheat and to adjust the number of compressor revolutions to maintain the chilled water outlet temperature.The gas-liquid separator can be cancelled.It makes the system more stable,the efficiency is improved obviously,and there are good performances in all kinds of faults,which should be widely used in practice.A comparison with the ASHRAE 1043-RP experimental results shows that the model and experimental results have a high degree of consistency in the trend and extent of changes in most of the important parameters for Reduced Condenser Water Flow.For the Reduced Evaporator Water Flow,the control system of this model has superior performance compared to the experimental results,and can effectively avoid unnecessary energy consumption.The comparison results prove the reliability and high efficiency of the chiler and control system.The model can be further applied in research and industrial fields.The above studies set up a simulation platform for the performance analysis of chillers,control system design and testing of fault detection and diagnosis technologies,which helps to optimize the design and operation of chillers,reduce the energy consumption of HVAC systems,and then reduce the energy consumption. |
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