Research On Some Key Technologies About The Application Of Metamaterials In Wireless Power Transfer

It is an ultimate target for researchers to increase the power transfer distance and efficiency of a wireless power transfer(WPT)system.Most researchers focus on improving power electronic devices and transmitting/receiving coils.Few other methodologies are taken into account.In this thesis,a new idea is presented that the characteristics of the media in the space are designed artificially to control the magnetic field distribution for a better power transfer efficiency and a longer distance of a WPT system.Metamaterials is able to control the electromagnetic field in a specific frequency band.Therefore,it is a novel and challenging task to apply metamaterials into a WPT system for the unusual field control purpose.Since it is a brand-new intersection study,related research literatures are seldom available.This thesis is attempt to discuss some key problems about applying metamaterials in WPT system from the viewpoint of electrical engineering.It mainly focuses on the feasibility and validity of the metamaterials in WPT system designed by negative refraction and electromagnetic coordinate transformation theories.It provides novel idea and methodology in improving WPT system.The main works in this thesis are concluded below:1.Study the theoretical basis of metamaterials applied in WPT.It includes negative refraction and electromagnetic coordinate transformation.(1)According to negative refraction theory,this thesis explicitly indicates two situations,which are double-negative isotropic metamaterials and uniaxial anisotropic metamaterials.Negative refraction theory of doublenegative isotropic metamaterials is derived from the definitions of refraction index and chiral coefficient of coordinates.Negative refraction theory of uniaxial anisotropic metamaterials is derived from transmission and reflection coefficients of incident plane electromagnetic waves on layered medium.Both theories can illustrate the enhancement of the coupling between transmitting and receiving coils in a WPT system and be verified in simulation in COMSOL.(2)According to electromagnetic coordinate transformation theory,this thesis indicates an integrated design methodology and offer a novel magnetic concentrator and a superscatterer as examples.Simulations in COMSOL are done to verify the designed metamaterial devices are able to enhance the coupling between transmitting and receiving coils in a WPT system.Meanwhile,practical implementation are taken into account.A new layered simplification method is proposed.Simulation results verify that the simplified metamaterial device with multi-layer isotropic medium is able to improve the electromagnetic fields as the original precise metamaterial device can do.2.Study the practical implementation basis of metamaterials applied in WPT.It includes the design of microstructure of metamaterials and the principle of microstructure array.(1)The design of microstructure is a fundamental problem in metamaterial application.Compared with existing literatures about the design in microwave or even higher frequency,this thesis proposed a methodology of microstructure design in 6.78 MHz,which is a commonly used operating frequency in WPT system.Taking a metamaterial slab through negative refraction theory as an example,this thesis proposed a complete design process including microstructure estimation,retrieval method from S parameters of full-wave electromagnetic simulation in HFSS and experimental verification.(2)The analysis of the operating principle of microstructure array is an attempt to explain the operating principle of metamaterials.Many researchers are puzzled by the difference between metamaterials and relay coils and seldom literatures have deeply studied in this problem.This thesis has referred to the concept of magnetoinductive waves.A mathematical model of wave is derived from a mutual inductance and self-inductance model,which is familiar to electrical engineers.After that,the magnetic field distribution of a metamaterial and relay coils are explained through the idea of standing waves,which is formed from incident wave and reflection wave.The simulations of a 1-D coil chain and a planar coil matrix are made to verify the validation and correction of the proposed theory.The simulation results are correspond to the experimental phenomenon.3.Based on the Maxwell equations in time-harmonic electromagnetic field,the basic principle of WPT and the process of electromagnetic energy conversion are reviewed.The core idea of metamaterial enhanced WPT is emphasized: the medium parameters are designed artificially to improve the distribution of the electromagnetic field and to increase the coupling of primary and secondary coils in WPT system.