Starting from Tesla's principles of wireless power transfer a century ago, this critical review outlines recent magneto-inductive research activities on wireless power transfer with the transmission distance greater than the transmitter coil dimension. It summarizes the operating principles of a range of wireless power research into 1) the maximum power transfer and 2) the maximum energy efficiency principles. The differences and the implications of these two approaches are explained in terms of their energy efficiency and transmission distance capabilities. The differences between the system energy efficiency and the transmission efficiency are also highlighted. The review covers the two-coil systems, the four-coil systems, the systems with relay resonators and the wireless domino-resonator systems. Related issues including human exposure issues and reduction of winding resistance are also addressed. The review suggests that the use of the maximum energy efficiency principle in the two-coil systems is suitable for short-range rather than mid-range applications, the use of the maximum power transfer principle in the four-coil systems is good for maximizing the transmission distance, but is under a restricted system energy efficiency (<;50%); the use of the maximum energy efficiency principle in relay or domino systems may offer a good compromise for good system energy efficiency and transmission distance on the condition that relay resonators can be placed between the power source and the load.

A Critical Review of Recent Progress in Mid-Range Wireless Power Transfer

Due to limitations of low power density, high cost, heavy weight, etc., the development and application of battery-powered devices are facing with unprecedented technical challenges. As a novel pattern of energization, the wireless power transfer (WPT) offers a band new way to the energy acquisition for electric-driven devices, thus alleviating the over-dependence on the battery. This paper presents an overview of WPT techniques with emphasis on working mechanisms, technical challenges, metamaterials, and classical applications. Focusing on WPT systems, this paper elaborates on current major research topics and discusses about future development trends. This novel energy transmission mechanism shows significant meanings on the pervasive application of renewable energies in our daily life.

Wireless Power Transfer—An Overview

Wireless charging is a technology of transmitting power through an air gap to electrical devices for the purpose of energy replenishment. The recent progress in wireless charging techniques and development of commercial products have provided a promising alternative way to address the energy bottleneck of conventionally portable battery-powered devices. However, the incorporation of wireless charging into the existing wireless communication systems also brings along a series of challenging issues with regard to implementation, scheduling, and power management. In this paper, we present a comprehensive overview of wireless charging techniques, the developments in technical standards, and their recent advances in network applications. In particular, with regard to network applications, we review the static charger scheduling strategies, mobile charger dispatch strategies and wireless charger deployment strategies. Additionally, we discuss open issues and challenges in implementing wireless charging technologies. Finally, we envision some practical future network applications of wireless charging.

Wireless Charging Technologies: Fundamentals, Standards, and Network Applications

Starting from the basic principles of Tesla's wireless power transfer experiment in the 1890s, this review article addresses the key historical developments of wireless power and its modern applications up to formation of the world's first international wireless power standard “Qi” launched in 2010 for portable electronics. The scientific principles laid down by Nicolas Tesla for wireless power transfer, which still remain valid today, are first explained. Then, modern wireless power applications based on nonradiative (near-field) magnetic coupling for short-range applications are described. Some industrial application examples emerging since the 1960s are highlighted. The article then focuses on the comparison of the horizontal and vertical magnetic flux approaches developed in the early 1990s for low-power planar wireless charging pads. Several critical features such as localized charging, load identification, and freedom of positioning that are essential to wireless charging of portable electronic devices are explained. The core technologies adopted by the Wireless Power Consortium (WPC) for the “Qi” Standard in 2010 are summarized. Finally, the latest research and developments of wireless power transfer for midrange applications based on the domino-resonator concept and their future application potential are described.

Planar Wireless Charging Technology for Portable Electronic Products and Qi

Recently, a highly efficient midrange wireless transfer technology using electromagnetic resonance coupling has been proposed and has received much attention due to its practical range and efficiency. The resonance frequency of the resonators changes as the gap between the resonators changes. However, when this technology is applied in the megahertz range, the usable frequency is bounded by the industrial, scientific, and medical (ISM) band. Therefore, to achieve maximum power transmission efficiency, the resonance frequency has to be fixed within the ISM band. In this paper, an automated impedance matching (IM) system is proposed to increase the efficiency by matching the resonance frequency of the resonator pair to that of the power source. The simulations and experiments verify that the IM circuits can change the resonance frequency to 13.56 MHz (in the ISM band) for different air gaps, improving the power transfer efficiency. Experiments also verified that automated IM can be easily achieved just by observing and minimizing the reflected wave at the transmitting side of the system.

Automated Impedance Matching System for Robust Wireless Power Transfer via Magnetic Resonance Coupling

We experimentally demonstrate that the efficiency of wireless power transfer utilizing resonant coupling is improved by applying the “matching condition.” The `matching condition' is extracted from an equivalent circuit model of a magnetically coupled wireless power transfer system. This condition is achieved by varying the coupling factor between the source (load) and the internal resonator. Applying this technique results in efficiency improvements of 46.2% and 29.3% at distances of 60 cm and 1 m, respectively. The maximum efficiency is 92.5% at 15 cm. A circuit model based on the extracted parameters of the resonators produces results similar to the experimental data.

Experimental Results of High-Efficiency Resonant Coupling Wireless Power Transfer Using a Variable Coupling Method

We investigate a compact metamaterial for enhanced magnetic coupling in a resonator coupled wireless power transfer system operating at around 6.5 MHz. The metamaterial is constructed by realizing an array of three-turn spiral resonators on a thin slab. Although the metamaterial has its own loss, the experimental results show that the proposed metamaterial slab enhances the power transmission capability. The number of unit cells in the array is an important parameter, because exceeding a certain number of unit cells does not enhance the efficiency due to the loss of the slab. Furthermore, strong surface mode resonance is observed when two slabs are assembled with proper gap spacing between them. By using the optimization approach, we achieve a significant efficiency improvement at a mid-range distance. The measured efficiencies are 71.1% and 54.3% at a 0.6 and 1.0 m distance, respectively. At a 1.0 m distance, this efficiency performance corresponds to a 270% improvement compared to a case with no metamaterial slab. In addition, we experimentally confirm the threshold distances above which the metamaterial shows enhanced performance.

Experimental investigation of compact metamaterial for high efficiency mid-range wireless power transfer applications

To enable the geometrical freedom envisioned for wireless power transfer (WPT), fast dynamic adaptation to unpredictable changes in receiver position is needed. In this paper, we propose an adaptive impedance-searching system that achieves good impedance matching quickly. For fast and robust operation, the proposed method consists of three steps: system calibration, coarse search, and fine search. The proposed WPT system is characterized using distance variation and lateral and angular misalignment between coils. The measured results indicate that the proposed method significantly reduces searching time from a few minutes to approximately one second. Furthermore, the proposed system achieves impedance matching with good accuracy. The robust impedance-searching capability of the proposed system significantly improves power transfer efficiency. At 6.78 MHz, we achieve a maximum efficiency of 89.7% and a high efficiency of >80% up to a distance of 50 cm. When the center-to-center misalignment is 35 cm, the efficiency is improved from 48.4% to 74.1% with the proposed method. At a distance of 40 cm, the efficiency is higher than 74% for up to 60° of angular rotation. These results agree well with the simulated results obtained using a lumped-element circuit model.

A Dynamically Adaptable Impedance-Matching System for Midrange Wireless Power Transfer with Misalignment

We investigate a metamaterial structure to investigate the potential of extending the WPT to mid-range distances. Three-dimensional (3D) metamaterial structure is realized using 4×5×1-array of three-turn spiral resonators. By using the designed metamaterial structure, we achieve 33% and 7.3% efficiency improvements at 1.0 and 1.5 m distance between Tx/Rx resonators, respectively. EM simulations are also performed and verify that proposed metamaterial structure is capable of enhancing the efficiency at mid-range distances.

Experimental investigation of 3D metamaterial for mid-range wireless power transfer

Recently, metamaterials has been emerged as one of the potential candidate for enhancing the efficiency of resonator coupled wireless power transfer (WPT) systems. This paper presents results of experimental investigation on metamaterial enhanced WPT System. Isotropic bulk metamaterial structure has been fabricated by assembling 5 × 5 × 1 array of three turn spiral resonators. Measurements show that the designed metamaterial structure has a great potential in extending the power transfer distance of resonator coupled WPT System. Also we have identified that there is a certain threshold for distance between transmitter and receiver resonators beyond in which the power transfer efficiency is enhanced. The threshold is attributed to the loss of the metamaterials.

http://ieeexplore.ieee.org/iel7/7057737/7067568/07067650.pdf

Thuc Phi Duong

Papers

Experimental Results of High-Efficiency Resonant Coupling Wireless Power Transfer Using a Variable Coupling Method

Experimental investigation of compact metamaterial for high efficiency mid-range wireless power transfer applications

A Dynamically Adaptable Impedance-Matching System for Midrange Wireless Power Transfer with Misalignment

Experimental investigation of 3D metamaterial for mid-range wireless power transfer

Investigation on threshold distance for efficiency enhanced wireless power transfer using bulk metamaterial