Study On Wireless Power Transfer Based On Transform Optics And Metamaterials

Electromagnetic(EM)field is ubiquitous in life,the manipulation of which is a popular topic in optics,electromagnetic and engineering.Since the foundation of Maxwell equations,scientists have proposed several schemes to control the EM field.Due to the limitation of frequency,these schemes are viable only for some specific applications.Transform Optics(TO)is an emerging approach to manipulate EM field in recent years,which is valid from DC to optical frequency.The Metamaterials(MM),designed by TO,have been applied in antenna system and demonstrated their good performance.As Wireless Power Transfer(WPT)delivers energy essentially based on magnetic coupling in space,we could improve its efficiency by proper approach for EM manipulation.In this paper,we focus on the feasibility of TO as well as MM applied in WPT and present preliminary experimental verification for further research.First,the theory of TO is analyzed using coordinate transformation and conformal transformation respectively,and EM tensors are exhibited under different coordinates.In this paper,we not only discuss the physical interpretation of these two transformations,but also make a comparison on their advantages and disadvantages.To demonstrate the effects of two transformations,we illustrate the principles of some novel EM devices including: invisible cloak,beam modulator,illusion optics and collimator.The effects of controlling EM field by these devices are numerically verified by simulations and could be utilized for particular application in WPT.For example,we can employ invisible cloak to magnetic shielding and use collimator for directional transmission.In addition,we also interpret the magnetic MM based on SRR model and deduce its frequency-response function of effective permeability.Second,the new magnetic concentrator designed by coordinate transformation is proposed in this paper.The idea is to map the magnetic field near the Transmitting coil(T coil)to the spherical region encircling Receiving coil(R coil).By numerical computing in near field,the concentrator is proved to make the magnetic field in the vicinity of R coil more convergent.Thus,the flux passing through R coil could be increased and the mutual inductance and efficiency could be enhanced.Considering the complex parameters of such concentrator,we use multi-layer and multi-sector structure to simplify the anisotropic material and achieve the homogeneous and isotropic material,which facilitates its fabrication.Compared with traditional EM concentrator,the new magnetic concentrator proposed in this paper has a simpler structure and takes up less space,which could ameliorate the flux distribution near the R coil.Besides,the superscatterer(SS)without PEC boundary is designed by TO to equivalently expand and move the coil inside.Specifically,the equivalent position of coil could be outside the SS and the equivalent size of coil could be larger than the SS.In this way,we are allowed to harness a more compact emitter to charge electronic devices in a wider and longer range,which is like throwing a sprat to catch a herring.Finally,two magnetic MM with resonant frequency at 17.6MHz and6.78 MHz are fabricated to evaluate the efficiency with different layouts of MM.The results measured by VNA indicate that the efficiency has a largest increment when two slabs of MM are symmetrically placed in WPT system,which is also verified in experiments based on inductive coupling and magnetic resonance.As for the inductive coupling WPT system,we conclude that the MM could not only suppress the decay of evanescent wave,but also sever as filter to ameliorate the distortion of voltage waveform at load.In the WPT system based on magnetic resonance,we successfully charge a 15 W incandescent lamp with two slabs of MM.The lamp is placed more than 60 cm away from the T coil,which is a valuable reference for mid-range wireless power transmission.