Research On Shifting Medium And Metamaterials In Wireless Power Transfer

In recent years,the research and application of wireless power transfer have been accelerated by increasing demand in market.However,the effects of traditional ways to improve power transmission efficiency are approaching their limits since they cannot overcome the constitutive property that efficiency decreases exponentially as transmission distance increases.Manipulating electromagnetic fields by metamaterials is a novel way to improve power transmission efficiency.In this paper,I research the transformation optics and metamaterials in the near-field wireless power transfer,and experimentally validate the effects of metamaterials.First,the transformation optics based on coordinate transformation and conformal mapping are analyzed,and the media tensors in different coordinates are calculated,respectively.Taking the invisibility cloaking as an example,the two methods are compared concerning on their application,electromagnetic tensors,fabrication,et al.Second,the shifting medium,which can optically move an inner space or object,is designed by the coordinate transformation.Two models of 3-dimensional shifting medium are demonstrated: the cone-shaped shifting medium shell which can move coils,and the shifting medium layer which can move and expand coils.The electromagnetic tensors for each model are derived.The shifting medium shell is homogeneous,which is an obvious advantage.The electromagnetic fields and mutual inductance between coils are simulated in both models.The results reveal that,both models can equivalently move(or move and expand)coils,thus enhancing magnetic coupling.The effects of material loss and impedance matching are also considered.Third,for practical concern,an optimization method to simplify materials is proposed based on effective medium method and Nelder-Mead algorithm.In this way,the side shell can be substituted by 2 positive materials,and the bottom shell can be substituted by 5 materials,among which only 3 are negative.All the materials are homogeneous and isotropic.This method needs no active materials and makes the fabrication easier.Moreover,two types of low-frequency magnetic metamaterials are designed according to the split-ring resonator.One is a unit cell working at 4MHz with tunable effective permeability.By adjusting the width of its middle plate,three negative permeability required by the shifting medium shell could be obtained.The other one is a unit cell working at 6.78 MHz with negative unity permeability.Simulation results illustrate that the metamaterial slab consisting of 6.78 MHz unit cells can refocus magnetic flux and enhance the magnetic field near the receiving coil.Main factors influencing effective permeability and loss tangent are also discussed.The 6.78 MHz metamaterials are manufactured and assembled into one-slab,two-slab,2-dimensional,and 3-dimensional structures.Their effects on power transmission and optimal positions in a 6.78 MHz resonant wireless power transfer setup are measured by a network analyzer.The results show that all structures perform best in the middle position between coils,and the 3D one obtains the greatest efficiency enhancement.Finally,a 15 W bulb over a 60 cm distance is lit by power experiments.Power transmission efficiencies with different metamaterial structures are measured and validate the results given by the network analyzer.The 3D structure improves the efficiency from 16% to 40%,which is the greatest improvement in our experiments.