1522 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 5, MAY 2011 A Comparative Study Between Novel Witricity and Traditional Inductive Magnetic Coupling in Wireless Charging S. L. Ho , Junhua Wang , W. N. Fu , and Mingui Sun Department of Electrical Engineering, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, PA USA A non-radiative energy transformer, commonly referred as Witricity and based on ‘strong coupling’ between two coils which are sepa- rated physically by medium-range distances, is proposed to realize efficient wireless energy transfer. The distance between the resonators can be larger than the characteristic sizes of each resonator. Non-radiative energy transfer between the first resonator and the second resonator is facilitated through the coupling of their resonant-field evanescent tails. The proposed system operates as traditional induc- tive magnetic coupling devices when the operating frequencies are not the resonant frequency. Corresponding finite element analysis (FEA) and experiments have been carried out to facilitate quantitative comparison. Compared with typical magnetic inductive coupling energy transmission devices, the efficiency of the proposed system is much higher. This investigation indicates that it is feasible to use wireless energy transfer technology to recharge batteries, particularly in implant devices. Index Terms—Inductive magnetic coupling, resonant wireless energy transfer, strong coupling, witricity. I. INTRODUCTION T RADITIONAL plug and socket charging method cannot keep up with demands of consumers for safety and fast charging. Thanks to the advent in power electronics, inductive charging, also known as wireless charging, has found much suc- cesses and is now receiving increasing attention by virtue of its simplicity and efficiency. The most important distinctive struc- tural difference between contactless transformers and conven- tional transformers is that the two ‘coils’ in the former are sep- arated by a large air gap. Compared with plug and socket (i.e., conductive) charging, the primary advantage of the inductive charging approach is that the system can work with no exposed conductors, no interlocks and no connectors, allowing the system to work with far lower risk of electric shock hazards. As the charging system is often fully enclosed, wireless charging can be realized in waterproof packages and as such, wireless charging is attractive in situa- tions at which rechargeable devices need to be frequently used near or even under water as well as in humid conditions. It is also a very attractive option in the medical field when the re- placement of batteries for implant devices is very costly. The risk due to surgical operations is highly reduced if the batteries for the implant devices can be charged externally through wire- less charging techniques. Physical separation between the primary and secondary windings incurs proximity-effect winding losses. Poor coupling can result in poor transmission performance and low efficiency. Due to the large air gap between the primary and secondary windings, contactless transformers have large leakage induc- tances, small mutual inductance and low efficiency [1]–[3]. Compared to direct contact charging, inductive charging effi- ciency is lower and resistive heating is higher. Realization of Manuscript received May 30, 2010; accepted November 02, 2010. Date of current version April 22, 2011. Corresponding author: J. Wang (e-mail: junhua. wong@polyu.edu.hk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2091495 Fig. 1. Basic components of the Witricity system. low frequency inductive charging in electrical devices will lead to slow charging and heat generation. This paper reports the study on a topical mode of energy transmission using resonant technique, commonly known as Witricity (short form of wireless electricity) [4]. Detailed theo- retical and numerical analyses reveal that Witricity is efficient and practical for mid-range wireless energy exchange. Unlike conventional inductive coupling methods, there are only very small energy dissipations in off-resonant objects for systems working on Witricity principle [5]. In this paper, a novel system based on Witricity is proposed for charging rechargeable batteries. The first resonator receives energy from an external power supply. It also includes a second resonator which is physically separated from the first resonator to supply useful working power to an external load. The dis- tance between the two resonators can be larger than the charac- teristic sizes of each resonator. It transfers non-radiative energy between the first resonator and the second resonator through the coupling of their resonant-field evanescent tails. Fig. 1 shows the basic components of the Witricity system. II. THEORY AND DESIGN OF THE PROPOSED WITRICITY SYSTEM As a new wireless power transfer technology, Witricity is based on the concept of near-field and strongly coupled mag- netic resonance. The fundamental principle is that resonant ob- jects can exchange energy efficiently, while non-resonant ob- jects only interact weakly. Fig. 2 shows the basic design of the Witricity system consists of source and device resonators, a driving loop, and an output loop. The source resonator is coupled to the driving loop which is linked to an oscillator that supplies energy to the system. The 0018-9464/$26.00 © 2011 IEEE