| In the context of the urgent transformation of energy structure and China’s proposal of the "dual carbon" plan,the utilization of renewable energy is an important development direction in the field of air conditioning.Among them,the most widely used and technologically mature solar energy is utilized.In order to combine solar energy with air conditioning systems and ensure the efficiency of unit use,a solar electric hybrid drive absorption compression heat pump unit and key technology research project have been carried out.The most important part is the absorption heat pump system with R134a-DMF as the working pair.The research group has successfully built and operated the test bench system and completed the static mathematical model of the unit.However,due to the instability of solar energy,the stability research of the absorption heat pump unit under the disturbance condition is the most important part.Based on this,this thesis establishes the dynamic model of the absorption heat pump unit and studies the dynamic characteristics of the system.The main research work of this article is as follows:(1)Based on the operating principle of absorption heat pump and the principles of mass conservation,energy conservation,and material conservation,reasonable assumptions and simplifications were made for various physical processes.Considering the thermal inertia of the main components and the storage of solutions in the components,a transient model of each component of the absorption heat pump was established with time as the independent variable,including the generator,absorber,condenser,evaporator,solution heat exchanger,and electronic expansion valve.Detailed analysis was conducted on the physical processes of the internal and external media of each heat exchanger,and appropriate empirical formulas were selected.The calculation of the internal and external heat transfer coefficients and the thermal inertia of the heat exchanger were carried out using MATLAB software.The heat transfer coefficients and thermal inertia of each heat exchanger were calculated.(2)Collect and fit the physical properties parameters of the working fluid,and write sub functions of the physical properties parameters for querying and calling in the module and overall program.Directly write sub functions for the physical parameters of existing empirical formulas;For the physical parameters that can be queried in REFPROP software,import their data into MATLAB,use Curve Fitting Tool to fit them,and import the fitted function into the sub function file;For the R134a-DMF binary mixture solution,the NRTL thermodynamic model was first used for fitting in Aspen Plus software,and the data was imported into MATLAB for fitting and sub function writing.(3)The absorption heat pump system is divided into six major component modules.Each module is mathematically described based on the physical process and converted into a set of nonlinear differential equations.The MATLAB module subprogram is written.The numerical calculation method is used to solve each module subprogram through the ode45 solver in MATLAB to form a module model.Serialize the subroutines of each module in the order of physical processes to form the entire machine subfunction file.Similarly,solve the entire machine subfunctions using ode45 to form the entire machine model.Both steady-state and dynamic validation were conducted on the establishment of the dynamic model,and the results showed good accuracy of the model.(4)Using the established R134a-DMF absorption heat pump dynamic model,conduct disturbance condition research,change the temperature of the heat source water,cooling air,and chilled water,observe the heat exchange,outlet temperature,and internal average temperature changes of each component,as well as the response time.When the inlet water temperature of the heat source steps from 80℃ to 90℃,the heat exchange between the generator,absorber,condenser,and evaporator increases,with the generator having the longest response time and the cooling COP increasing from 0.27 to 0.39.When the cooling air temperature steps from 18℃to 24℃,the heat exchange between the generator,absorber,condenser,and evaporator decreases,with the generator and condenser having the longest response time and the cooling COP decreasing from 0.27 to 0.21.When the freezing water temperature changes from 14℃ to 12℃,the heat exchange rate of the condenser increases,while the heat exchange rate of the generator,absorber,and evaporator decreases.The response time of the generator and condenser is longer,and the cooling COP decreases from 0.19 to 0.17.The results indicate that the generator takes a long time to reach stable state again after being disturbed.The main reason for this is due to changes in the concentration of the solution output from the absorber and pressure changes.In practical applications,it is necessary to adjust the pressure of the generator to ensure fast and stable operation of the unit.At the same time,it was found that the heat exchanger selection of the test unit was improper,and the COP was small when the absorption type was operated alone,which could not meet the design requirements of the project unit.Therefore,the heat exchanger needs to be re selected based on the working fluid characteristics and flow characteristics of R134 a DMF solution and R134 a refrigerant.The dynamic model studied in this thesis can be used for subsequent system optimization,establishing linear programming,dynamic programming and multi-objective programming,and can provide reference for the design,configuration,performance analysis,integrated intelligent control and other key research issues of R134a-DMF absorption heat pump units,with farreaching research significance. |
PDF Full Text Request