Optimal Control Of Two-stage Pump Chilled Water System Under Variable Floe Operation

With the developing of the socio-economic, air-conditioning system in theapplication of modern architecture is more and more widely used. Accompanied bythe increasing in energy consumption, in the modern society requirements andguidelines of the energy saving policies, the energy saving of air-conditioningsystem is imperative.Nowadays in the air-conditioning system, chillers are running in accordancewith the constant flow but when the air-conditioning system is running in the partialload or the large temperature difference circumstances, it will greatly increase thepumps’ energy consumption. When chillers are running in accordance with thevariable flow, on the one hand it will lead to increased energy consumption ofchillers, but on the other hand pumps will reduce energy consumption. Because ofchillers have a certain flow range and with the development and application ofinverter technology, variable flow system will be more and more popular.This article will through the opitimal control of two-stage pump chilled watersystem under variable flow operation and the altered evaporator to be used in thealteration of the current civil air-conditioning systems and related industrialrefrigeration systems, in order to reduce the chilled water pump and chillers’ totalenergy consumption, so as to achieve the purpose of energy-saving.In order to reserch the chilled water pump and chillers’ minimum total energyconsumption under the two-stage pump chiller chilled water system under variableflow of the optimum chilled water flow ratio and the optimum chilled waterswitching flow ratio after the altered evaporator, so various mathematical models ofthe two-stage pump chiller chilled water system be established, including themathematical models of the chiller, the mathematical models of the chilled waterpumps, the mathematical models of the mixing process of chilled water and themathematical models of the before and after the altered evaporator, these modelsprovide the convenient for the next research.For the research of the optimum chilled water flow ratio, objective function isdetermined and the range of the nine reference parameters and the two variables aredetermined. The study found that the main factors of the optimum chilled water flowratio are the ratio factor of the actual load and the design load, the design differencetemperature of the users, the design temperature of the inlet chilled water of theusers, the first loop design pressure drop, the second loop design pressure drops andthe design pressure drop of the evaporator, so as to found that the optimum chilled water flow ratio can be fitting to a piecewise function about the ratio factor of theactual load and the design load.In oder to combine the advantages the before and after the altered evaporator,soa modified valve control of the evaporator is proposed and the optimum chilledwater switching flow ratio is raised. As the optimum chilled water switching flowratio is closely related to the switching load ratio and correspondence between thetwo, so to study the optimum chilled water switching flow ratio is the same toreserch the switching load ratio. Through the research found that the main factorof the optimum chilled water switching flow ratio is the design pressure drop of theevaporator and found that the optimum chilled water switching flow ratio is due tothe design pressure drop of the evaporator.Through the comparative analysis of the energy consumption between thebefore and after the altered evaporator of the pump chilled water system running atthe optimum chilled water flow ratio and the constant flow chillers running in twodifferent working conditions found that the variable flow chillers operation isfeasible. If the optimum chilled water switching flow ratio isn’t exit, the energyconsumption of after the altered evaporator is less than before. While the optimumchilled water switching flow ratio is exit, if the actural load ratio is lower than theswitching load ratio, then the energy consumption of before the altered evaporator isless than after, otherwise the energy consumption of after the altered evaporator isless than before the altered evaporator.Because of the optimum chilled water flow ratio and the optimum chilled waterswitching flow ratio are both energy-saving and closely related to the load ratio, sothe control-based load method is proposed. If the optimum chilled water switchingflow ratio isn’t exit, the entire system is running under the optimum chilled waterflow ratio of after the altered evaporator.While the optimum chilled water switchingflow ratio is exit, if the actural load ratio is lower than the switching load ratio, thenthe entire system is running under the optimum chilled water flow ratio of before thealtered evaporator, otherwise the entire system is running under the optimum chilledwater flow ratio of after the altered evaporator.At the request of the national energy saving policies, the two-stage pumpchilled water system under variable flow operation is not only reduce the totalenergy consumption of the system, but also can reduce the users and businessspending and it will be more and more used.