Fundamental Research On Free-piston Stirling Power Generating System

As a clean and fuel adaptive alternative power plant,the Stirling cycle mechanical equipment has become a research hotspot in the field of aerospace power supplies in the early twenty-first century.A free-piston Stirling engine connected with a linear electrical machine converting mechanical power into electrical power which implements thermo-mechanical-electrical energy conversions is called a free-piston Stirling power generating system.Based on the operating characteristics of the free-piston Stirling power generating system,research is carried out from the aspects of dynamic system stability,thermodynamics-based thermoelectric conversion efficiency,optimized design of cylindrical permanent-magnet synchronous linear motor,system control strategy and experiments.Firstly,the dynamic model of free-piston Stirling power generating system is built,and the stability of the thermo-electro-mechanical coupled system is studied by theoretical analysis and numerical simulations.The amplitude ratio and phase relation of the displacer and piston for both fixed and damping housings are investigated,which provides basis for preliminary design.Based on the system dynamics model,the stability of linear and nonlinear thermal-mechanical-electrical coupled systems under fixed and damping housings is studied,and the corresponding stability criteria are deduced.The system dynamics simulation is carried out,and the influence law of motor load and damping stiffness on system stability is given,which provides a reference for system design.Secondly,a thermodynamic model is built to analyze the thermodynamic process of free-piston Stirling power generating system.The influences of key parameters on system efficiency,and efficiency optimization methods considering multiple parameters are investigated.The initial design method of free-piston Stirling generation system combining thermodynamics and dynamics is proposed and studied.The effects of key parameters of regenerator,heat exchanger and heater on system efficiency are analyzed by 1-D or multi-D thermodynamic numerical analysis software.The system parameters and partial heat exchanger are optimized by 2-D and 3-D software.Consequently,the system efficiency and performance are improved by optimized design.Thirdly,to improve the performance of the cylindrical permanent-magnet synchronous linear motor,key parameters are adjusted which optimizes the motor structure and improves the efficiency and power density.By the analysis of the influence of parameters,such as the mover pole-arc coefficient,stator tooth width and mover radius,on main performances,like the back EMF waveform,harmonic distortion,and thrust fluctuation etc.,the general method for optimizing the structural parameters of the cylindrical permanent-magnet synchronous linear motor is given.By analyzing the influence law of radial and axial lengths and the additional magnetic block of the end teeth on the stator force and load thrust fluctuation,the optimization method of the end tooth structure for the linear motor is obtained.By the analysis of the efficiency and power density,a comprehensive optimization method of the cylindrical permanent-magnet synchronous linear motor is proposed.Afterwards,to improve the output frequency response and stability,the model of a free-piston Stirling power generating system is established,and control strategies with respect to different operating modes are studied and optimized.Based on the results of thermodynamic and dynamic coupling analysis and modeling,combined with the mathematical model of the linear motor,the model of free-piston Stirling power generating system is built by the isothermal analysis.Current-stroke and current-voltage double closed-loop control strategies are employed for starting and generating modes,respectively.To improve the output voltage load response in generating mode,decoupled state feedback is studied for current control,and the influence of full decoupled d-axis and q-axis currents is analyzed with respect to the output voltage adjusting time and fluctuation amplitude under the variations of piston displacement and output load.With regard to the dynamic regulation lag in the decoupled current control caused by system parameter variations,an optimized control strategy based on internal model control is proposed,which improves the performance of the decoupled-state feedback current control that relies on system parameter accuracy,and further reduces output voltage ripple in generating mode.Finally,in order to validate the motor optimization and system control strategy,a test bench is built for the free-piston Stirling power generating system.The no-load and load characteristics of the linear motor,and the controller performance for both starting-up and generating modes are tested.To evaluate the performance of the optimized linear motor,performance characteristics such as no-load EMF and load output voltage are tested and analyzed under different operating conditions with respect to frequency,piston stroke and load.Experimental results are consistent with theoretical analysis and simulations.The stability of stroke control for starting mode is tested under different reference stroke values.The steady state and transient voltage control performances are tested for generating mode with fixed and varied output load,and evaluated by observing the piston stroke,DC bus voltage and load current.