| Currently,indoor humidity and temperature control for most public buildings in China are achieved with a central air conditioner cooling water system.Energy consumed by air conditioner systems accounted a significant proportion in the total building energy consumption,of which,energy consumption of cooling water distribution system is the main component of the energy consumption of an air conditioner system.Reducing its energy consumption by means of optimized operation has been always a hot topic in the field of energy saving,in which researches on optimized operation studies for water chiller units,pumps and other equipment in the system is not rare.However,mutual coupling effect will affect the overall energy-saving effect when each equipment of a system is operating in its own optimized way.The best energy-saving effect can be achieved only when considerations are given to the overall system optimization.The primary pump system comprised of double water chillers and two pump units is the most basic form of central cooling water distribution systems adopted in public buildings,and also a form with very extensive applications.This paper takes such system as the research object of this paper,reveals influential factors for energy consumption of equipment in a central cooling water distribution system,energy-saving effects of overall optimization of water systems,overall optimization of subsystems and local optimization of equipment as well as energy-saving priorities of system through researches with regard to local optimization of water chiller units and other equipment,respective global optimization of water chiller units,chilled water subsystem and cooling water subsystem up to overall optimization of cooling water distribution system.Water pumps,motors and VFDs are incorporated,defined as water pump units,and used for optimization of central cooling water distribution system.Experiments and researches are necessary for obtaining characteristics functional equations of flow ratio and ratio of revolutions,of lift ratio and ratio of revolutions and of power ratio and ratio of revolutions with regard to description of basic characteristics of water pump units.A multifunctional experimental platform for hydraulic conditions of air conditioning water system had been established in the HVAC laboratory of Harbin Institute of Technology to support these researches.This platform can be used for performance tests of water pumps and individual equipment of water pump units,or hydraulic tests for closed water systems of primary pumps,and also used for experiments and researches on open water systems of primary pumps after corresponding transformation.Three formulas for characteristic functions of water pump units are obtained through tests: linear function equations of flow ratio and ratio of revolutions;quadratic functional equations of lift ratio and ratio of revolutions,;cubic functional equations of power ratio and ratio of revolutions,.Computational accuracy of above equations is higher than that of those typical monomials with three pump groups which are commonly used at present.Water chillers are equipment with largest energy consumption in a central cooling water distribution system.It is found through analyses that,the temperature of fed chilled water,the temperature at inlet of cooling water and the partial load ratio are three influencing factors for COP of screw water chiller units.This paper uses a regression analysis method,established COP models for water chiller with bivariate water temperatures as well as forecasting COP models suitable for screw water chiller units with variable temperature at both outlet of chilled water and inlet of cooling water..Verification on models with sample data and comparison between them and typical NG models suggest that such mathematical models for energy consumption of screw water chillers have a higher accuracy.A physical model for chilled water subsystem is established on the basis of a primary pump system of central air condition cooling water distribution system,a constant flow operation mode is taken as the basic operation mode to set up a mathematical model for annual energy consumption of a variable flow chilled water subsystem with an objective function of minimum energy consumption.It is found through analyses that,primary influential factors for operating energy consumption of a chilled water subsystem include the temperature of fed chilled water,the number of water chillers in service,fixed/variable frequency mode of water pump units and the number of units in service.As for overall optimization of chilled water subsystem,the energy-saving rate of the water chiller unit vs.system is 2.44% due to the coupling effect between water chillers and chilled water pump units,which is lower than the 3.84% of the water chiller unit during its locally optimized operating;the average of energy-saving rate of water pump unit vs.system is5.26%,which is lower than the 5.88% of the water pump unit during its locally optimized operating;however,the effect of overall optimization of the system at this time is the best,the average of energy-saving rate is 7.70%.A cooling water subsystem is optimized by means of system optimization for chilled water subsystem.It is found from analyses that,major influential factors for its operating energy consumption of cooling water subsystem include the number of water chillers in service,fixed/variable frequency mode of water pump units and the number of units in service,operating mode of cooling towers and the number of towers in service.There isn’t any operating coupling phenomenon between various equipment of cooling water subsystem if the temperature at inlet of water chillers is given,that is,the energy-saving rate of locally optimized equipment is equal to the energy-saving rate of equipment in an overall optimized cooling water subsystem.The energy-saving rate of water pump unit vs.system is 12.32%,of cooling tower is 4.91%,now the energy-saving rate for overall optimization of system is 17.23%.Overall system optimization models for overall optimization with an objective function of minimum energy consumption are established also on the basis of the primary pump system of a central air conditioners cooling water distribution system,and in combination with physical models for chilled water and cooling water subsystems.Its results suggest that when the central cooling water distribution system for air conditioners is operating with overall optimization,the energy-saving rate of chilled water subsystem vs.the whole system is same as that of the chilled water subsystem during its overall optimized operating,their average are 6.84%;the energy-saving ratio of cooling water subsystem vs.the whole system is 2.02%,which is higher than the 1.94% obtained when only overall optimization of the cooling water system is adopted for operating,their difference is little;effect of overall system optimization is the cooling water subsystem best,its average of energy-saving rate is 8.86%,higher than the sum of the energy-saving rate of overall optimization of chilled water subsystem and the energy-saving rate of overall optimization of cooling water subsystem.The thought of overall system optimization presented in this paper and those mathematical models for overall energy-saving optimization of central cooling water distribution system for air conditioners established thereof can be used not only for overall energy-saving optimization of cooling water distribution systems with double chillers and double pumps,but also as the foundation for subsequent optimization for complex systems with multiple units and pumps. |
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