| In the overall social energy structure,a large proportion of energy will be attributed to buildings,in which air conditioning facilities consume considerable energy.Up to now,most primary systems are designed with constant flowrate,which fails to adjust in accordance with the actual load,thus pumps always run with full power.Since systems are often run under part-load condition for approximate 95%of their operation time,the capacity of pumps always exceeds the actual requirements.High operation expenses have made air conditioning systems a heavy burden in building management.Therefore,enhancing the efficiency of water transport systems possesses great significance in reducing the energy consumption of air conditioning system.Seeing the problems of constant flow system during the design and operation stage,this research aims to offer pertinent insight into optimal operation of variable primary flow(VPF) systems,by incorporating the chilled and chilling water systems into a single model.Firstly,feasibility and efficiency of VPF are analyzed.It is shown that only minor decrease of the chiller's performance takes place if the chilled and chilling water change is restricted within certain limits.Comparably,the chilling water change induces greater fluctuation of COP than chilled water.The analysis of correlation of chillers and pumps shows an increasing energy saving rate along with the decreasing load.The larger ratio of pump power against chiller power,the more energy will be saved.At the meantime,design key points, problems and solutions of VPF are analyzed.In addition,the non-uniform change of water flow and temperature differences between the consumer side and cooling source side are analyzed, which is determined by the heat transfer characteristics of surface coolers and chillers. Therefore,the main pipe bypass approach is introduced to balance the foregoing non-uniformity.Then,based on optimization theory and operation principles of VPF,this research establishes the mathematical model of VPF optimal operation,aiming to ensure both comfort and energy saving requirements.Identification method of model expressions and related parameters is given and then validated.By selecting chilled and chilling water flow rate as control parameters,and listing constraints with security concerns,this research applies genetic algorithm to compile and solve optimization program,with MATLAB tool box.With the proposed optimization method,parameters in VPF can be properly ascertained,which contributes to improve the efficiency of air conditioning systems,especially under part-time load.Finally,a real case is investigated by employing the optimization approach into the transformation of VPF.Comparison of energy consumption of a project before and after optimization shows great amount of energy consumed by pumps can be saved in spite of minor energy increase by chillers.Therefore the system COP_s is greatly enhanced.The lower cooling load brings much less energy consumption and more efficiency.For most of the time,the water temperature difference both in the chilled and chilling side of optimized system is maintained at 5～6℃,with the lowest exceeding 3.5℃.A single load ratio will respond to a optimum flow rate, which ensures the maximum COP_s.After transformation,63.1%of total electricity consumption by pumps,or 12.8%by the whole water system are saved.In the design of VPF systems,special attention should be paid to power ratio between chiller and pump,chiller performance under variable water flow,as well as cooling load,et al.Targeted at efficient operation of air conditioning networks,based on optimization theory, this research incorporates the genetic algorithm into the design of VPF systems,and proposes a set of optimal control strategies,which are applicable to energy saving design,reconstruction, as well as operation management of air conditioning systems.
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