Shifting medium is a kind of an anisotropic but homogeneous metamaterial designed by transformation optics. An object or free space enclosed by the shifting medium could be moved to a certain distance away from the original position. In this paper, we propose a cone-shaped shifting medium shell to move an internal coil to the given position. In this way, the two coils in a wireless power transfer system could be equivalently moved closer; thus, their magnetic coupling is enhanced. The theoretical models and numerical simulations are presented and analyzed to validate the effects of the shifting medium shell. Both ohmic loss and magnetic loss are also considered for practical concerns. Finally, we discuss the simplification of such a shifting medium to facilitate its fabrication.

In this paper, we experimentally validate the effects of different structures of metamaterial on efficiency enhancement in a four-coil magnetic resonant wireless power transfer system operating at 6.78MHz. First, we propose a concise design of magnetic metamaterials. A 6×6 planar array metamaterial slab is simulated to investigate its effect on power transmission between transmitter and receiver. The proposed metamaterials are fabricated for their application in the wireless power transfer system. Transmission and reflection coefficients of the system with 1-dimentional (including 1-slab and 2-slab), 2-dimentional, and 3-dimentional metamaterials at different positions are measured to investigate their influence on power transmission and to optimize the position of metamaterials in each case. Finally, the platform with metamaterials is utilized to light a 15W bulb to explicitly demonstrate the efficiency enhancement.

In an example scenario of magnetic energy harvesting, a spherical superscatterer is introduced to enhance coupling in a two-coil system. Although a three-dimensional (3D) model is preferred to fully model behavior in this example, to reduce computational complexity, an extension of transformation optics (TO) is proposed to reduce a 3D model to a two-dimensional (2D) axisymmetric model. The simulation results show details of a quasi-3D model of the superscatterer coupling enhancement of a two-coil system.

Over the past few years, various metamaterials have been designed and applied in antenna systems. In this paper, a new structure of magnetic metamaterials is proposed for megahertz wireless power transmission, which can be manually adjusted to work at a variable frequency ranging from 10∼30 Mhz. The S parameters, resonant frequency, effective permeability of metamaterials are computed for analysis. The different substrates and layouts of metamaterial slab are also numerically evaluated by full-wave simulation. The specific magnetic metamaterial slabs with 17.6 MHz are fabricated and experimentally verified by the network analyzer. Finally, these slabs are applied in a more practical two-coil coupling system to evaluate their performance enhancement.

Superscatterer is typically made of complementary media for expanding the scope of scattering. The fact that superscatterer can make object appear bigger than its geometric size provides the possibility of equivalently enlarging and moving coils in inductive coupling system. In this paper, we demonstrate that the superscatterer without perfect electrical conductor can also expand the distribution of magnetic field with a source inside. Based on transformation optics, a spherical superscatterer is designed and simulated especially in near field. A model of two-coil inductive coupling system is numerically analyzed to prove the enhancement on mutual coupling by superscatterer. Finally, we also discuss the simplification of such superscatterer for fabrication concerns.

The design methods and most related theories of metamaterials applied in wireless power transfer are elaborated.Three important theories,i.e.the negative refraction effect,the coupling model of magnetic dipoles,and the coordinates transformation theory,are introduced to explain the mechanism of improving the performance of wireless power transfer by metamaterials.In addition,two design methods,including the principle model method and the S-parameters retrieval method,are presented to illustrate the realization of specific electromagnetic constitutive parameters.The principle model method is theoretically derived for simple micro-structures,which has some inspiration significance for acquiring the electromagnetic parameters.And S-parameters retrieval method is more suitable for practical designs and complicated metamaterial micro-structures with the help of finite element simulation and retrieval equations.

Conditions for load-independent output voltage or current in two-coil wireless power transfer (WPT) systems have been studied. However, analysis of load-independent output current in three-coil WPT system is still lacking in previous studies. This paper investigates the output current characteristics of a three-coil WPT system against load variations, and determines the operating frequency to achieve a constant output current. First, a three-coil WPT system is modeled by circuit theory, and the analytical expression of the root-mean-square of the output current is derived. By substituting the coupling coefficients, the quality factor, and the resonant frequency of each coil, we propose a method of calculating the frequency for load-independent output current in a three-coil WPT system, which indicates that there are two frequencies that can achieve load-independent output current. Experiments are conducted to validate these analytical results.

Magnetic lens based on metamaterials has helped to increase the inductive coupling of two-coil system in wireless power transfer. By coordinate transformation, the spatially mapped metamaterials are proposed in this paper for a new magnetic concentrator in two-coil system to improve the mutual coupling. To achieve such metamaterials, the virtual rectangular domain is spatially mapped into a deformed spherical shell. The effects of such mapped spherical shell, functioning as magnetic concentrator, are simulated and evaluated. The fabrication and simplification of this magnetic concentrator are also considered. Finally, this model of spherical shell is compared with that of a traditional magnetic concentrator to demonstrate its advantage.