Wireless charging technology (WCT) for electric vehicles is a non-contact power transfer technology used for charging electric vehicles. It has advantages such as safe operation, intelligent charging, and flexible configuration.
This paper reviews the technology system, categories, and technical characteristics of wireless charging for electric vehicles . Research hotspots include: power electronic topology, magnetic coupling element structure, energy transfer level, modeling approaches, and biosafety. The paper summarizes the research progress on these hotspots and outlines the practical applications achieved by relevant automotive companies and laboratories.
The future development trends of this technology include: innovation and optimization of power electronic topology and control algorithms, biosafety, and the application of new materials, while application trends include: charging while in motion, assisted driving, and bidirectional power transmission such as V2X (vehicle-to-grid, V2G) and vehicle-to-home, V2H.
In recent years, wireless charging technology (WCT) for electric vehicles has received increasing attention. Wireless charging systems have no exposed ports, require no manual operation, do not occupy ground space, and can charge while stationary or in motion. Therefore, compared to wired charging methods, they offer advantages such as operational safety, intelligent charging, and flexible configuration options. Furthermore, they are expected to reduce the amount of battery power and overall vehicle weight, thereby reducing energy consumption.
Wireless charging technology enables efficient, non-contact power transfer over a certain spatial distance. In 1893, scientist Nikola Tesla first used wireless power transfer to illuminate phosphorescent lighting at the Columbian Exposition, after which wireless power transfer technology became a research hotspot in the transportation sector. In 1894, M. Hutin obtained a patent for a wireless charging system for rail transit. In 1974, D.O. Votto proposed a design for a wireless charging system for electric vehicles with a charging current of 2000A and a frequency of 10kHz. Lawrence Berkeley National Laboratory in the United States conducted two wireless charging research projects in 1976 and 1992, testing mobile charging vehicles with power outputs of 8kW and 60kW respectively. Although these projects did not achieve true commercial application, wireless charging technology subsequently developed rapidly in the automotive industry. The establishment of the Wireless Power Consortium's "Qi" standard in 2008 marked the true entry of wireless charging technology into commercial operation. In China, in December 2013, Southeast University successfully developed a wirelessly charged electric vehicle with a charging power of 3kW.
Wireless charging technology for electric vehicles is a type of wireless power transmission technology, and it has the following unique characteristics in terms of technical requirements:
Power rating: from several kilowatts to tens of kilowatts, and charging time is relatively short, therefore requiring a large charging system capacity;
Charging distance: 15~45cm in the vertical direction, the offset in the horizontal direction should be greater than 15cm, and the tilt angle margin should be 15° in the tilt direction.
Charging efficiency: Generally speaking, the charging efficiency from the grid to the vehicle battery needs to be greater than 85% to be practical.
System size and weight: Considering factors such as the size of the car chassis, load-bearing capacity, wheelbase, and specific power of the wireless charging system, the lateral dimension of the system should be between 40 and 80 cm, and the weight should be less than 50 kg.
Data communication: In order to realize the automatic operation of the charging system and the intelligent adjustment of charging parameters, and to cooperate with the implementation of driver assistance technologies such as automatic parking, the system should have data communication function.
With advancements in power electronics, battery, and electric vehicle technologies, wireless charging technology has developed rapidly in recent years, demonstrating significant advantages. This article summarizes and refines the wireless charging technology for electric vehicles from the perspectives of its technical system, categories, and characteristics. It analyzes the current research status of hot and challenging issues such as power electronic topology, magnetic coupling element structure, energy transfer characteristics, system modeling approaches, and biosafety. The article also summarizes the latest R&D progress of wireless charging technology from major automotive companies and related research institutions, and forecasts future development trends from both technical and application perspectives.
1. Wireless charging technology system, classification and characteristics of electric vehicles
1.1 Wireless Charging Technology System for Electric Vehicles
Figure 1. Structure of a wireless charging system for electric vehicles
A wireless charging system for electric vehicles is typically divided into two parts: a power supply and a power receiving system. Its system structure is shown in Figure 1.
Figure 2. Technical framework of wireless charging system for electric vehicles
The essence of wireless charging systems for electric vehicles is the conversion and control of electrical energy, with reliability, efficiency, and safety being fundamental requirements. Wireless charging technology is based on three scientific problems: optimization and coordinated control of power electronic topology, biosafety of electromagnetic energy transfer, and bidirectional coupling management of multi-source energy. Supported by circuit design and parameter matching optimization, EMC and radiation safety protection, nonlinear system analysis and control, and vehicle-related technologies, it has formed a multidisciplinary, deeply coupled and mutually influential technical system encompassing power electronics, electromagnetic fields, vehicle-related theories, electrochemistry, nonlinear system control, and data communication, as shown in Figure 2.
1.2 Categories and Performance Characteristics
Figure 3 Categories of Wireless Power Transfer Technologies
Wireless power transmission methods are divided into two main categories: far-field transmission and near-field transmission, as shown in Figure 3.
1.2.1 Far-field region
The far-field region is the area beyond the source of the electromagnetic field, defined as 2D^2/λ+λ, where D is the maximum diameter of the transmitting coil and λ is the wavelength of the electromagnetic wave. In the far-field region, the radiated field plays a dominant role, and the electromagnetic wave can be approximated as a plane wave. Operating frequencies above 300MHz can be analyzed using Maxwell's equations. However, since the antenna size is comparable to the wavelength, the lumped parameter method cannot be used to analyze the operating process. Based on their different principles, far-field wireless power transfer technologies can be divided into microwave and laser types.
Microwave wireless power transfer technology enables long-distance transmission and miniaturized design. However, because radiated power is inversely proportional to the square of the transmission distance, and high-power transmission in the far field is subject to legal restrictions, this technology is generally used in low-power, long-distance devices, such as RFID cards. High-power microwave wireless charging is only used in special industries, such as military or aerospace, and is not suitable for wireless charging systems in vehicles.
Laser-based wireless power transfer technology can achieve longer-distance power transmission and smaller-size designs, and has lower electromagnetic interference to the surrounding environment. However, it has low conversion efficiency, and atmospheric absorption and scattering will cause additional losses. At a transmission power of several hundred watts, the transmission efficiency is less than 25%. In addition, it is harmful to the human body. Therefore, its application is limited to the military and aerospace fields, and it is also difficult to use it in wireless charging systems for electric vehicles.
1.2.2 Near Field Area
The near-field region is the area within 2D^2/λ+λ of the field source, encompassing both the radiative and induced near-field regions, with a boundary of 0.62(D^3/λ)^0.5. The near-field operating frequency range is 10kHz to 100MHz. Faraday's law of electromagnetic induction can be used to analyze this type of system. Since the size of the transmitting and receiving equipment is mostly less than λ/10, the lumped parameter method is suitable. Based on the coupling method, near-field wireless power transfer technology can be divided into magnetic field coupling and electric field coupling. Based on whether resonance occurs, magnetic field coupling includes inductive coupling and magnetic resonant coupling. Currently, near-field power transfer technology is widely used in vehicle wireless charging systems.
Inductive coupling wireless charging works similarly to a separable transformer without compensation. Due to the narrow air gap between the transmitting and receiving coils and the coils' association with ferromagnetic materials, the coupling coefficient is typically higher than 0.5. This technology has relatively high mutual inductance and leakage inductance between the coils, resulting in high transmission efficiency over short distances. However, it is very sensitive to distance and unsuitable for wireless charging over longer distances. Furthermore, the presence of ferromagnetic materials leads to larger winding size and mass, resulting in higher iron losses at high frequencies. Therefore, this approach is suitable for low-frequency operation where the charging distance is less than the coil size.
Magnetic resonant coupling wireless charging systems are a technical solution for achieving medium-distance (typically several times the size of the coil) wireless charging based on the magnetic field resonant coupling mechanism. Compared to inductive coupling wireless charging technology, its significant feature is the inclusion of a tuning network in the circuit topology, enabling leakage inductance compensation and frequency tuning, thus increasing the transmission distance. Furthermore, obstacles in the charging path, even at considerable distances from the coil, do not significantly affect wireless charging. In 2007, Professor Marin Soljacic's team at MIT used this technology to achieve a transmission distance of 2 meters and a power of 60W, with a coil-to-coil transmission efficiency of 40%–50%. Due to its significant advantages in charging distance, charging efficiency, and electromagnetic radiation, it has become a research hotspot in recent years.
In an electric field-coupled wireless power transfer system, the transmitter and receiver are connected to separate metal plates. To improve transmission efficiency, the plates must be made of a high-dielectric-constant dielectric material. Since the electric field is confined within the air gap between the plates, electromagnetic interference is low. However, to achieve efficient power transfer, the plate spacing needs to be very small, the plate area needs to be large, and the compensation inductance value needs to be high. Consequently, copper and iron losses are high at high frequencies, making the technology less feasible. In 2012, Toyohashi University of Technology in Japan conducted related research on a 1/32 scale vehicle model, but this technology has not yet been commercialized.
Table 1 lists the performance comparison of various wireless power transmission technologies. The comparison shows that magnetic resonant coupling and inductive coupling power transmission technologies have significant advantages over other wireless power transmission technologies in terms of transmission distance, volume, mass and cost. They are more suitable for the wireless charging technology requirements of electric vehicles with large air gap (15~45cm), high efficiency (>85%) and high power (kW level).
Table 1 Comparison of Performance of Various Wireless Power Transfer Technologies
2. Research Hotspots in Wireless Charging Technology for Electric Vehicles
2.1 Power Electronic Topology and Operating Characteristics
Figure 4. Power electronic topology and power representation of a typical wireless charging system for electric vehicles.
The goal of an electric vehicle wireless charging system is to transfer electrical energy from the power grid to the vehicle's battery. Therefore, its power supply end is connected to the power grid, and its load end is connected to the vehicle's battery. The power electronic topology and energy representation of the transmission process are shown in Figure 4. The energy transmission process includes both wired and wireless transmission.
In the wired power transmission process (black lines in the diagram), the alternating current (AC) from the power grid is filtered out by an electromagnetic interference filter before entering the rectifier and becoming direct current (DC). A power factor correction unit improves the power factor and enhances power quality, while the power amplifier circuit converts the DC into high-frequency AC. This AC is then fed into an LC resonant circuit composed of a tuning network and an excitation coil, forming a sinusoidal AC current. Simultaneously, a high-frequency alternating magnetic field is generated in the space surrounding the excitation coil. At the receiving end, the AC current induced in the load coil is rectified and filtered before flowing into the battery to charge it.
In the wireless transmission of electrical energy, through inductive coupling, the high-frequency alternating magnetic field generated by the excitation coil induces an alternating current in the nearest transmitting coil, and the receiving coil also induces an alternating current in the nearest load coil. The frequency of the alternating current is the same in each stage. Through magnetic resonance coupling, the transmitting coil and the receiving coil with the same natural frequency achieve efficient coupling, thereby ensuring the medium-distance transmission of electrical energy.
It is worth noting that the power electronic topology in the wireless charging system for electric vehicles is similar to that of the main circuit topology of wired charging piles. The difference is that the latter changes the coil structure in the wireless transmission link to a transformer structure, so that the transmitting circuit and the receiving circuit are physically connected through the transformer, and then connected to the charging port of the electric vehicle after rectification and filtering.
For power electronic topologies, domestic and international research mainly focuses on two aspects: power amplifier circuits and tuning (compensation) networks.
Table 2 Comparison of Power Amplifier Circuit Topologies
In existing research, the power amplifier circuit topologies used in wireless charging systems for electric vehicles mainly include full-bridge inverter circuits and E-type power amplifier circuits. Their topologies and advantages and disadvantages are listed in Table 2.
Figure 5 Different combinations of tuned network topologies
Tuning networks are used to connect converters and coils, reducing wasted power and improving power transmission efficiency. Based on different structures, tuning networks can be divided into five types: S, P, CC, LC (CL), and LCC (CCL). Their circuit structures and the main combinations of transmitters and receivers in existing research are shown in Figure 5.
For the transmitting end:
The S-type topology has low input impedance, making it suitable for high-power devices and easy to implement voltage feedback regulation.
The resonant frequency of a P-type topology is coupled with the coupling coefficient and the load, making it susceptible to disturbances. It also has a relatively large input impedance, so it is rarely used in practice.
CC-type topology can improve the lateral offset margin between coils;
LC topology can achieve a higher power factor, thereby improving power transmission efficiency;
The LCC topology can further reduce switching losses, decouple the output current from the load, increase the circuit power factor, thereby reducing control difficulty and improving power transmission efficiency.
However, the more complex the circuit structure, the greater its cost and size, and the more difficult it is to match circuit parameters.
For the receiving end, the S-type topology can output a stable voltage, while the P-type topology can output a stable current. Furthermore, the CC-type, CL-type, and CCL-type topologies derived from the P-type topology can further decouple the output current from the load, improve the controllability of the output power, and help achieve a higher power factor, making it an ideal current source and thus increasing the system's transmission power. Meanwhile, as shown in Figure 5, several combined topologies have not yet been studied. Therefore, the topologies and combinations of tuned networks require further in-depth exploration.
2.2 Structure of Magnetic Coupling Element
Magnetic coupling elements are components in wireless charging systems for electric vehicles that enable the conversion between electrical energy and field energy. They typically consist of high-conductivity and high-permeability components. The high-conductivity component is the conductor of electrical energy; considering the skin effect and proximity effect at high frequencies, it is usually made of copper tubing or Litz wire. The high-permeability component is the carrier of field energy, forming part of the magnetic circuit. It can improve the coupling coefficient between the transmitter and receiver, reduce eddy current losses caused by the magnetic field within the vehicle's metal components, and enhance the system's power density; ferrite materials are typically used. The transmitter of the magnetic coupling element is usually fixed, while the receiver can be either fixed or mobile.
2.2.1 Fixed magnetic coupling element
Fixed structures typically fall into two categories: flat type, where a helical coil is arranged in parallel on a ferromagnetic material; and cylindrical type, where a solenoid coil is wound on a ferromagnetic material. The former is more widely used in practice.
Figure 6. University of Auckland single-sided multi-coil planar magnetic coupling element
In the design of planar magnetic coupling elements, research teams from the University of Auckland in New Zealand and Southeast University in China have made relevant research achievements. The research results of Professor JTBoys' team at the University of Auckland are representative. They started in the 1990s and proposed the first generation and the second and third generation planar magnetic coupling element structures in 2009 and 2013, respectively, as shown in Figure 6.
Because the substrate is ferrite, these coupling elements are typically brittle, requiring the gaps to be filled with soft plastic or rubber materials, and the sides and back to be wrapped with aluminum plates to reduce electromagnetic radiation. Structurally, circular charging plates allow vehicles to approach from any direction, offering greater flexibility in placement, while rectangular plates provide a larger coupling area. Analysis shows that using DD-shaped plates as the transmitting plate, mounted on the ground, and DDQ or BP-shaped plates as the receiving plate, mounted on the vehicle chassis, achieves the most ideal charging effect.
2.2.2 Movable magnetic coupling element
For mobile magnetic coupling elements, the transmitting end is usually a long, straight, energized track fixed to the roadbed, or it consists of multiple magnetic coupling elements arranged in series along the road, while the receiving end is a flat coupler installed on the bottom of the vehicle.
As a car travels along the road, current is generated in the onboard flat coupler, charging the onboard battery and thus achieving charging while in motion. The earliest real-vehicle testing of onboard charging based on rail power supply was conducted at Lawrence Berkeley National Laboratory in the United States. The three-phase rail-powered train from Bombardier in Canada and the distributed mobile charging method proposed by Ross also demonstrate some feasibility.
Figure 7. Track design for the OLEV wireless charging electric vehicle from the Korea Advanced Institute of Science and Technology (KAIST).
In China, Chongqing University and Tianjin University of Technology have also proposed wireless power supply solutions for electric vehicles and high-speed trains in motion, respectively. Among the commercially available products, the On-line Electric Vehicle (OLEV) researched by Professor ChunT. Rim's team at the Korea Advanced Institute of Science and Technology (KAIST) is representative, and the evolution of its wireless charging design is shown in Figure 7.
2.3 Energy Transfer Characteristics
The energy transfer technical indicators of electric vehicle wireless charging systems are mainly reflected in three aspects: transmission power (the maximum power that a single set of magnetic coupling elements can transmit) P, transmission distance (coupler spacing) S, and transmission efficiency η. Related parameters include the operating frequency f of the wireless charging system, coupling area (the maximum planar area of the magnetic coupling element) A, and offset margin ε (horizontal offset length divided by the maximum lateral outer diameter of the coupling element). Table 3 shows the energy transfer indicators achieved by various research institutions internationally in the field of electric vehicle wireless charging technology.
Table 3 Energy Transfer Level of Wireless Charging for Electric Vehicles
Based on the statistical results, the influencing factors and development trends of relevant characteristics are shown in Figures 8 and 9. Figure 8 shows the trend of transmission efficiency as a function of the ratio of coupler area to transmission distance. Figure 9 shows the trend of frequency and power of the wireless charging system over time.
Figure 8. Effects of coupler area and transmission distance on efficiency
As shown in Figure 8, the transmission efficiency tends to increase as the ratio of coupler area to transmission distance increases. This is mainly related to the increase in coupling coefficient. In recent years, with the advancement of related technologies, smaller coupler area and longer transmission distance have been achieved with the same transmission efficiency.
Figure 9 Operating Frequency and Transmission Power Limits
Three pieces of information can be obtained from Figure 9:
1) Most research results operate at frequencies of 20kHz and 100kHz, which is mainly related to electromagnetic radiation limits and the operating frequency of power electronic devices;
2) From a global perspective, as the frequency increases, the transmission power decreases, which is mainly related to the development level of power electronic device technology;
3) Over time, the product of power and frequency (P_out*f) increases, and its value also represents the improvement of the technology level of commercial power electronic devices.
The average values for different time periods are shown in Table 4.
Table 4 Average values of the product of power and frequency at different time periods
As shown in Table 4, the product of power and frequency increased by about 7 times between 2005 and 2015, and there is a continuing growth trend. This growth trend indicates that wireless charging devices are developing towards higher power density, longer transmission distance and miniaturization.
2.4 System Modeling
Models for wireless charging systems for electric vehicles include mutual inductance models, coupled-mode models, scattering matrix models, and bandpass filter models. Among these, most theoretical research both domestically and internationally focuses on mutual inductance models and coupled-mode models.
2.4.1 Mutual Inductance Model
Figure 10 Equivalent circuit of magnetic resonant coupling wireless charging system
Based on circuit theory, the magnetic resonant coupling wireless charging system can be uniformly equivalent to the circuit model shown in Figure 10. Considering the presence of a multi-coil structure in the intermediate transmission stage, the total number of coils is set to n. When the natural frequencies of all coils are the same, the transmission impedance is minimized, and the system coupling degree is maximized.
According to Kirchhoff's voltage law, the mathematical model of the equivalent circuit is expressed by the following formula:
In the resonant state, the inductive reactance and capacitive reactance of the coil cancel each other out, i.e., jωL_i = -1/(jωC_i), the imaginary part of the impedance is zero, and the system transmission efficiency η can be defined as the ratio of output power P_L to input power P_in, where I_i is the effective current value of coil i. Then:
Meanwhile, by solving the equivalent circuit numerical model, the system power characteristics and efficiency characteristics can be calculated, and the performance parameters can be optimized.
2.4.2 Coupled Mode Model
Coupled-mode theory is an important and accurate analytical method for describing the oscillation or transmission characteristics of high-frequency waves. It can be used to study the coupling between two or more electromagnetic wave modes. Taking a strongly coupled magnetic resonant wireless charging system with two coils as an example, its mathematical model based on coupled-mode theory can be expressed as follows:
The following assumptions must be met: 1) The resonant frequency band is sufficiently narrow; 2) The overall system performance is composed of isolated units that are coupled to each other. This coupling only causes minor perturbations to the operating state of each unit, and the overall performance of the coupled system is the result of the superposition of these minor perturbations from the independent units. Therefore, the following expression for the overall system efficiency can be obtained:
In the formula, the subscripts S, D, and W represent the transmitting coil, receiving coil, and load coil, respectively. Given an excitation source, the system transmission performance can be further solved by incorporating the coupling coefficient.
2.5 Biosafety Studies
Because wireless charging systems for electric vehicles operate at high frequencies, the constant alternation of electrical and field energy generates high-frequency alternating electromagnetic fields in the surrounding area. Therefore, whether the level of electromagnetic radiation affects biosafety is crucial for the widespread application of this technology. The Institute of Electrical and Electronics Engineers (IEEE) standard, "Human Safety Levels in Radio Frequency Electromagnetic Fields 3kHz to 300GHz," and the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines, "Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (1Hz to 100kHz or Below 300GHz)," specify limits on the intensity of electric and magnetic fields around high-frequency equipment.
Figure 11 Public Exposure Control Limits for Electric and Magnetic Field Intensities under No-Interference Conditions
Considering the different electromagnetic energy levels and my country's actual situation, China can use the public exposure control limits stipulated in the "Electromagnetic Environment Control Limits" (GB8702-2014) as the radiation limit standard for wireless charging systems of electric vehicles. Figure 11 shows the different limits for electric field strength (E) and magnetic field strength (H) for various standards.
As shown in the figure, for the existing wireless charging technology for electric vehicles, the higher the frequency (within 100MHz), the lower the control limit. Moreover, my country's limit requirements are more stringent than international standards, thus requiring higher electromagnetic radiation safety for wireless charging systems for electric vehicles.
Figure 12 SAR value simulation study based on human body model
Regarding the biosafety of electromagnetic radiation from wireless charging systems for electric vehicles, WuH.H et al. conducted electromagnetic field strength analysis on the Witricity system, testing the magnetic field strength of the knees, groin, chest, and head of the human body in the vicinity when the wireless charging system was operating. OCOnar et al. also conducted similar research, testing the magnetic field strength of the driver's side front wheel, floor, seat, and headrest during the wireless charging process of an electric vehicle. A.Christ et al., based on anatomical models of adults and children, measured the specific absorption rate (SAR) in different sagittal planes and analyzed the effects of radiation from a four-coil charging system on the human body, as shown in Figure 12.
Regarding electromagnetic radiation protection methods, existing research theories include active and passive protection. Active protection methods include adding electromagnetic interference protection coils and replacing self-resonant coils with capacitive coils to reduce the resonant frequency, while passive protection mainly relies on the application of ferromagnetic materials and other materials with good electromagnetic shielding performance.
3. Practical Application Results
3.1 Research and Development Progress of Wireless Charging Technology by Major Automakers
Internationally, major automakers are also accelerating technological research and development and product promotion for wireless charging systems for electric vehicles.
Toyota, Nissan, Honda, and other automakers have partnered with the American company Witricity to develop the WiT-3300 tablet-style wireless charging system. The system has a total transmission power of 3.3kW and a transmission efficiency of 90%, and can charge within a range of 100-200mm. This product will be installed in the 2015 Infiniti LE luxury electric vehicle and the 2016 PRUIS.
The BMW i8 series is equipped with Qualcomm's Halo wireless charging products. Their DD-type charging tablets can achieve a power transmission of 3~20kW at a working frequency of 20kHz, while the products suitable for charging while the car is in motion can achieve a power transmission of 20~30kW at a distance of 250~300mm.
Automotive parts manufacturer Bosch has partnered with American company Evatran to launch the PluglessL2 wireless charging system. This product enables wireless charging and assists drivers with automatic parking; it will be installed in the Nissan Leaf and Chevrolet Chevy Volt models.
Germany's Conductix, Canada's Bombardier, NASA's Ames Research Center, the United States' HEVOPower, and the United States' WAVE have also launched wireless charging products for mobile transportation.
In China, Dongfeng Motor Corporation, in partnership with ZTE Corporation, built China's first high-power wireless charging bus demonstration line in Xiangyang, Hubei Province, with a charging power of 60kW. Shudu Bus also collaborated with ZTE to launch the nation's first wireless charging urban micro-circulation bus solution. BAIC New Energy plans to equip its E150EV model with wireless charging equipment in the second half of 2015. Furthermore, automakers such as Yutong, Changan, and Chery have also invested heavily in wireless charging technology research and development.
3.2 Laboratory Apparatus and Prototype
In the research of kilowatt-level wireless charging systems for electric vehicles, foreign research institutions mainly include the University of Auckland in New Zealand, the Korea Advanced Institute of Science and Technology (KAIST) in Korea, Saitama University in Japan, Oak Ridge National Laboratory in the United States, Utah State University in the United States, and the University of Michigan-Dearborn Campus in the United States.
Professor JTBoys' team at the University of Auckland, New Zealand, started early, proposing a passenger transport vehicle design with a total transmission power of 17kW in 2000. In 2009, they proposed a planar magnetic coupling element design with a power of 2kW. In 2013, they improved the structure of the magnetic coupling element, increasing the magnetic field coverage area and offset margin, and reducing manufacturing costs. Their charging plate can be used for charging in both stationary and moving states.
The online electric vehicle (OLEV) project by Professor ChunT.Rim's team at the Korea Advanced Institute of Science and Technology (KAIST) enables vehicles to draw power from the road power grid while in motion. Since 2009, they have proposed five generations of design schemes, achieving the transmission of 27kW of power per unit within a 200mm spacing.
In 2011, Saitama University in Japan tested a coil structure with an H-shaped magnetic core, achieving power transmission of 1.5 to 3.0 kW within a 7 cm range;
Oak Ridge National Laboratory (ORNL) proposed ferrite-free and ferrite-containing magnetic coupling elements in 2011 and 2013, respectively, achieving energy transfer of 2.5kW and 3kW per unit. However, the former is larger in size, and its magnetic coupling element can also be used for charging electric vehicles in both static and moving states.
In 2012, Hunter H. Wu et al. from Utah State University tested a planar magnetic coupling element with a transmission power of 5 kW, achieving a system transmission efficiency of over 90%.
Professor Chunting Chris Mi's team at the University of Michigan-Dearborn designed and fabricated planar magnetic coupling elements with individual power of 8kW and 5.6kW in 2014 and 2015, respectively. The DC-DC transmission efficiency exceeded 95%. The experimental setup is shown in Figure 13. The product of power and frequency that this device can achieve is currently the largest.
Figure 13 Wireless charging experimental setup at the University of Michigan-Dearborn Campus, USA
In China, Southeast University, Chongqing University, Harbin Institute of Technology, and the Institute of Electrical Engineering of the Chinese Academy of Sciences have also carried out the design and development of wireless charging devices suitable for the kW-level power requirements of electric vehicles.
Southeast University has conducted extensive research on the design and efficiency optimization of dual-relay coil wireless charging systems.
Chongqing University proposed a power distribution scheme for an online power supply system for electric vehicles, which solved some practical problems in the application of wireless charging systems for electric vehicles.
Harbin Institute of Technology designed a 1.85kW wireless power transmission system using high-permeability flat magnetic core windings.
The Institute of Electrical Engineering, Chinese Academy of Sciences, has proposed a system design scheme based on capacitor optimization to improve the transmission efficiency and horizontal offset margin of wireless charging systems.
4. Development Trends
4.1 Technological Development Trends
1) Innovation and optimization of power electronic topology and control algorithms. Further improvements in the performance of wireless charging systems largely depend on the innovative design and optimization of power amplifier circuits and tuning (compensation) networks, and even more importantly, on improvements in control methods. Developing power electronic topologies with high power factor, low input impedance, and low matching difficulty, and proposing more precise and stable control methods, are of great significance for improving the offset margin, circuit stability, and power transmission efficiency of wireless charging systems.
2) Biosafety of Electromagnetic Energy Transfer. Biosafety is a significant public concern, and the promotion and application of wireless charging systems require the exploration of more intelligent and universal active protection methods against electromagnetic radiation.
3) Introduction of new materials and improvement of wireless charging constraint mechanisms. The introduction of advanced materials with superior parameters such as magnetic permeability and electrical conductivity helps to reduce system losses and improve power transmission efficiency. In recent years, the emergence and application of new materials such as unconventional electromagnetic materials (left-handed materials), magnetoelectric layered composite materials, and superconducting materials have made it possible to further reduce energy loss during the charging process and have also created space for improving the transmission performance of wireless charging systems.
4.2 Application Trends
1) Charging while the vehicle is in motion. One of the bottlenecks in the development of electric vehicles is the low energy density of batteries and the limited energy storage. Charging while the vehicle is in motion technology involves placing the power transmitting coil directly below the road surface, which can charge the vehicle while it is in motion, thereby replenishing the electrical energy consumed by the vehicle while driving and extending the vehicle's driving range.
2) Driver Assistance Technology. Integrating wireless charging with driver assistance technologies such as automatic parking and cruise control improves overall vehicle driving performance and the effectiveness of wireless charging.
3) V2X (Vehicle-to-Grid, V2G, Vehicle-to-Home, V2H, etc.) bidirectional power transmission. The intelligent bidirectional integration of electric vehicles and the power grid can play a role in peak shaving and valley filling of power regulation, making electric vehicles truly intelligent mobile energy storage devices and giving full play to their performance.
5. Summary
This paper reviews the current research status of wireless charging technology for electric vehicles. Based on a summary and refinement of the wireless charging technology system, categories, and technical characteristics, it outlines current research hotspots, including: power electronic topology, magnetic coupling element structure, energy transfer characteristics, system modeling, and biosafety. It also summarizes the R&D progress of wireless charging technology from major automotive companies and related research institutions. Future development trends of this technology include: innovation and optimization of power electronic topology and control algorithms, biosafety, and the application of new materials. Application trends include: charging while driving, assisted driving, and V2X (Vehicle-to-Grid, V2G; Vehicle-to-Home, V2H) bidirectional power transmission.
Wireless charging systems for electric vehicles are complex nonlinear magnetoelectric coupling systems. Further improvements in their performance require deeper analysis and exploration of fundamental scientific issues and common technological systems, necessitating the development of more universally applicable technical solutions and control strategies. Analysis of research progress reveals that many engineering problems remain to be solved; therefore, wireless charging technology for electric vehicles will remain a hot topic in the industry for the next few years.
