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

Wireless power transfer system (WPTS)-based wireless electric vehicles, classified into roadway-powered electric vehicles (RPEVs) and stationary charging electric vehicles (SCEVs), are in the spotlight as future mainstream transportations. RPEVs are free from serious battery problems such as large, heavy, and expensive battery packs and long charging time because they get power directly from the road while moving. The power transfer capacity, efficiency, lateral tolerance, electromagnetic field, air-gap, size, weight, and cost of the WPTSs have been improved by virtues of innovative semiconductor switches, better coil designs, roadway construction techniques, and higher operating frequency. Recent advances in WPTSs for RPEVs are summarized in this review paper. The fifth- and sixth-generation online electric vehicles, which reduce infrastructure cost for commercialization, and the interoperability between RPEVs and SCEVs are addressed in detail in this paper. Major milestones of the developments of other RPEVs are also summarized. The rest of this paper deals with a few important technical issues such as coil structures, power supply schemes, and segmentation switching techniques of a lumped inductive power transfer system for RPEVs.

Modern Advances in Wireless Power Transfer Systems for Roadway Powered Electric Vehicles

Roadway-powered electric vehicles (RPEVs) are attractive candidates for future transportation because they do not rely on large and heavy batteries but directly and efficiently get power while moving along a road. The inductive power transfer systems (IPTSs) that have been widely used for the wireless powering of RPEVs are reviewed in this paper. The development history of the IPTS is tracked from the origin of the RPEV in the 1890s to the recent RPEV. Throughout its 100-year history, the size, weight, efficiency, air gap, lateral tolerance, electromagnetic force, and cost of the IPTS have been substantially improved, and now RPEVs are becoming more widely commercialized. Important milestones of the developments of the IPTS and RPEVs are summarized in this paper, focusing on recent developments of on-line electric vehicles that were first commercialized in 2013.

Advances in Wireless Power Transfer Systems for Roadway-Powered Electric Vehicles

More than a century-old gasoline internal combustion engine is a major contributor to greenhouse gases. Electric vehicles (EVs) have the potential to achieve eco-friendly transportation. However, the major limitation in achieving this vision is the battery technology. It suffers from drawbacks such as high cost, rare material, low energy density, and large weight. The problems related to battery technology can be addressed by dynamically charging the EV while on the move. In-motion charging can reduce the battery storage requirement, which could significantly extend the driving range of an EV. This paper reviews recent advances in stationary and dynamic wireless charging of EVs. A comprehensive review of charging pad, power electronics configurations, compensation networks, controls, and standards is presented.

Wireless Power Transfer for Vehicular Applications: Overview and Challenges

In the inductive power transfer systems (IPTSs) of wireless electric vehicles (WEV), the electromagnetic field (EMF) should be lowered for the safety of pedestrians. In general, the EMF should be canceled for every space, time, and load condition of interest. Three generalized design methods for cancelling the EMF of WEV are proposed in this paper. By adding active EMF cancel coils to each primary main coil and secondary main coil, respectively, the EMF generated from each main coil can be independently cancelled by their corresponding cancel coils. Moreover, the EMF can be successfully mitigated if a dominant EMF source only is cancelled with 3-dB margin, which can be applied to any resonant type wireless power transfer systems. Furthermore, no significant power drop may occur if the cancel coils are placed aside from magnetic coupling path. Design examples are shown for U-type and W-type IPTS as well as a wireless stationary EV charger. Experimental verifications are shown for a recently developed I-type IPTS, which has a narrow rail width structure with alternating magnetic polarity along with a roadway. The proposed design methods have been demonstrated, without the loss of generality, to only the secondary coil where relatively large EMF is generated due to high ampere turns. An optimum spacing for cancel coils from main coils and an optimum number of turns are determined. Through experiments, additional EMF mitigation techniques such as the magnetic mirror method, separating pick-up rectifiers, and passive Al plate are provided. Thus, the EMF at 1 m distance from the center of a pick-up becomes under 44 mG even for the maximum power of 12 kW.

Generalized Active EMF Cancel Methods for Wireless Electric Vehicles

An ultraslim S-type power supply rail, which has a width of only 4 cm, for roadway-powered electric vehicles (RPEVs) is proposed in this paper. The cross section of the core has a thin S-shape, and a vertically-wound multiturn coil is displaced inside the core. In this way, the most slim power supply rail is designed, which is crucial for the commercialization of RPEVs. The construction of roadway infrastructure, which is responsible for more than 80% of the total deployment cost for RPEVs, can be much easier when the width of the power supply rail is so small. To increase portability and to minimize construction time, a foldable power supply module is also proposed in which flexible power cables connect each foldable power supply module such that no connectors are needed during deployment. An effective winding method for minimizing the cable length is proposed, and an optimum core thickness of the proposed power supply rail is determined by FEA simulations and verified by a prototype power supply module. By virtue of the ultraslim shape, a large lateral displacement of 30 cm at an air gap of 20 cm was experimentally obtained, which is 6 cm larger than that of the I-type power supply rail. In addition to the larger lateral displacement, it is estimated that the S-type one has lower EMF than the I-type one because the width of the S-type one is narrower than that of I-type one. The maximum efficiency, excluding the inverter, was 91%, and the pick-up power was 22 kW.

Ultraslim S-Type Power Supply Rails for Roadway-Powered Electric Vehicles

Self-decoupled dual pick-up coils for large lateral tolerance and low electromagnetic field for pedestrians are proposed. Analytical models are developed that are applicable to any self-decoupled coils, regardless the coil types such as single/dual pick-ups and core/coreless coils. An optimum decoupling distance between adjacent pick-up coils is determined and found to be independent of the existence of a core plate. Maximum load power over a large lateral tolerance is obtained for the optimum decoupling distance. The proposed models are so general that they can be applied to any self-decoupled pick-up coils for stationary charging and dynamic charging systems. Moreover, the self-decoupled coils are compatible with any compensation method such as serial, parallel, and serial–parallel. A prototype system of 1.5 kW and $Q = 60$ for roadway powered electrical vehicles was implemented and showed fairly good agreements with the theoretical models and simulations. The measured lateral tolerance was 90 cm, which is about 1.5 times of the coil width.

Generalized Models on Self-Decoupled Dual Pick-up Coils for Large Lateral Tolerance

Roadway powered electric vehicles (RPEV) are attractive candidates for future transportations because they are free from large and heavy batteries but directly get power efficiently while moving along a road. The inductive power transfer systems (IPTS) that have been widely used for the wireless powering of RPEV are reviewed in this paper. Main milestones of the developments of the IPTS and RPEV are summarized in this paper, mainly about the recent developments of on-line electric vehicles (OLEV) that have been commercialized first in 2013.

Beom W. Gu

Papers

Advances in Wireless Power Transfer Systems for Roadway-Powered Electric Vehicles

Generalized Active EMF Cancel Methods for Wireless Electric Vehicles

Ultraslim S-Type Power Supply Rails for Roadway-Powered Electric Vehicles

Generalized Models on Self-Decoupled Dual Pick-up Coils for Large Lateral Tolerance

Trends of Wireless Power Transfer Systems for Roadway Powered Electric Vehicles