| Nowadays,about 60 to 70 percent of the large-scale public buildings electricity load in summer is consumed by the HVAC system,in which about 50 to 60 percent is used for generating chilled media by the chiller,and about 30 to 40 percent is used by the chilled water and cooled water pump.Compared with water circulating system,there are complex two-phase flow characteristics inside the chiller,which will seriously affect the dynamic performance and stability of the refrigeration system.The energy consumption of chiller is more than half of the total energy consumption of refrigeration system.Therefore,it is of great significance to reduce the energy consumption of chiller.However,due to the immature analysis of the internal characteristic of heat exchanger,most chillers are still working in the non-ideal conditions and their operating efficiency can be further enhanced.In order to solve this problem,the saturated boiling theory is used to the stability analyze of the chiller in this paper,aiming to describe the effect of internal characteristics of the heat exchanger on the system stability quantitatively,based on which an optimization method for minimizing superheat and robust control algorithm are presented,the main research work is as follows:1.If the superheat in evaporator of the chiller is lower than a certain critical value(minimal stable superheat,MSS),the system will have a serious shock phenomenon.This paper presents an improved switching dynamic model that is able to simulate the the critical stable state of electronic expansion valve(EEV)controlled refrigeration system for the first time.Firstly,on the basis of the traditional dynamic model and considering that the heat transfer mechanism of the evaporator is the key factor causing the stability of the system,the evaporator is divided into the nuclear boiling and convective boiling condition according to the theory of saturated boiling: in the nuclear boiling condition,the heat transfer coefficient of the two-phase region is the sum of the nuclear boiling and convection boiling components,otherwise the heat transfer coefficient is only composed of convection boiling component.Subsequently,a composited switching criterion is proposed,which includes the criterion of heat flux and superheat.The dynamic simulation of the critical stable state of the electronic expansion valve controlled refrigeration system is realized successfully by periodic switch of the heat transfer condition of evaporator,and the comparison results between simulation and experiment show that the improved switching model has better accuracy.Finally,the effect of periodic perturbation of the heat transfer on the system stability is further analyzed by the dynamic simulation.2.Considering the system operating efficiency is high at low superheat value,based on the quantitative analysis of internal heat transfer characteristics of the chiller,an optimization method which can minimize the superheat is proposed in order to find the further energy saving potential of the refrigeration system.Firstly,the influence of internal parameters on the system operating efficiency and the feasibility of the optimization method containing minimal superheat constraint are analyzed by experiments.Secondly,the models of the steady state model of the refrigeration system and the energy consumption models of the main energy consuming parts are established,and the optimization Constraints including the minimal superheat constraint are given at the same time.Consider that the MSS is affected by the system characteristics and external conditions greatly,based on the heat flux criterion in the previous paper,the minimal superheat constraint is translated into the heat flux constraint to improve the stability of system.Subsequently,the minimum energy consumption of the whole system is used as the target and the genetic algorithm is used to cauculate the optimized set point of the evaporation pressure,condensation pressure and superheat.Finally,the experiment results show that the proposed method can effectively expand the optimization range,and achive 1.92% daily average energy saving than existing optimization methods under typical conditions in summer.3.Although the refrigeration system in the optimal setting has a high efficiency,the superheat setting value is close to the MSS line.The external disturbance or a wide range,rapid change of thermal load may cause the parameter perturbation of the system,and there are inevitably model uncertainties such as unmodeled dynamics.The traditional feedback control method,whose performance is conservative,cannot meet the need of the refrigeration system.To this end,a robust control method of the refrigeration system is designed by combining the disturbance observer and nonlinear control method.Firstly,in order to facilitate the design of nonlinear controller,the refrigeration system simplified nonlinear model is deduced based on a series of simplifications.Secondly,the nonlinear disturbance observer is designed to estimate the system composite disturbance,which primarily contains the system parameter perturbation and model mismatch.The sliding mode control algorithm is designed to compensate the disturbance in the evaporator subsystem.Subsequently,considering the partial disturbance in the condenser is non-matching form,which cannot compensated directly,the basic structure of the condenser controller is established by the systematic design process of the backstepping method whereby compensating the non-matched disturbances via virtual control.Thirdly,based on the Lyapunov theory,the tracking error of the superheat and evaporation temperature is proved to be asymptotically stable,and the tracking error of the condensing temperature is bounded stable.Finally,the simulation and experimental results show that the robust control algorithm with disturbance observer has excellent disturbance rejection ability,which can effectively guarantee the efficient and stable operation of refrigeration system. |
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