Recent advances in wireless power transfer technologies for electric vehicles

In today’s society where electric vehicles are widely used, it is particularly important to make electric vehicles more efficient and convenient to charge. Wireless power transfer technology is becoming increasingly popular as a new type of charging technology. Different from conventional charging strategies, wireless power transfer becomes a hot topic in industry and academia due to the application possibility. It can not be ignored that this technology can also effectively reduce energy consumption. And the technology contributes to improve some environmental issues. Without doubt that it is a kind of beneficial clean energy source in today’s society. In this paper, the state-of-the-art wireless power transfer technologies are comprehensively review in terms of operation characteristics and optimization strategies. Meanwhile, a comparative analysis is given with different application scenarios, including the cost and power conversion efficiency. At the same time, the future perspectives for developing the wireless power transfer techniques are given.


Introduction
With the goal to save energy and reduce consumption and reduce environmental pollution, many automobile companies have increased their investment in electric vehicles and research.The United States Environmental Protection Agency informed that nearly 28% of the greenhouse gas emissions in this country in 2018 were produced from the transportation sector [1].Whether static wireless charging or wired charging, there are some problems, such as charging with frequency, short range, consumption of high power, large cost and so on.Therefore, Wireless power transfer (WPT) technology is applied to provide a power source for electric vehicles.
The development of new energy and energy saving and emission technologies is the key to the realization of low-carbon economy.Electric vehicles can better solve the problems of motor vehicle emission pollution and energy shortage.Now we are vigorously promoting the low-carbon economy.When developing the economy that has low carbon, the thing that matters most is to make full use of new energy, and electric vehicles are vigorously promoted.Besides, the process of the charging is limited by the battery that is physical constraints, such as the maximum charging current, power and voltage [2].Because of limited battery capacity and charging of infrastructure, the problem of charging has become the principal obstacle for the process of electric vehicles.However, technology of charging that is wireless can overcome security problems such as wired charging interface restrictions, so wireless charging technology has over time become the main means of electric vehicle charging.Wireless charging technology is both convenient and easy to use, and breaks space constraints.Therefore, users can use a single transmitter to charge multiple devices simultaneously, and there are no special requirements for the location of the charging device.
Wireless energy transmission technology based on electromagnetic coupling for medium to realize the transmission of electric energy, and charge the electric vehicle wirelessly.The framework of WPT system is shown in Figure 1.

Rx Coil Filter
Figure 1.The structure of WPT system.Transmission way of wireless charging systems is mainly about: alternating current passes through a helix to form a magnetic field; at the receiving side the coil senses the magnetic field and generates AC current; AC current is converted into DC voltage through rectifier bridge; and DC voltage to charge the battery.
WPT comprises two main methods: near-field and far-field transmission [3].The WPT systems has the classification: Inductive coupling power transfer (ICPT), capacitive coupling power transfer (CCPT), magnetic resonant inductive coupling power transfer (MRICPT), microwave coupling power transfer (MCPT), as shown in Figure 2. Figure 2. Category of wireless power transfer systems.This article introduces the different types of WPT and their working principles and functions.And the article compares and analyzes the characteristics of different types of WPT technologies and their applications in real life.Besides, outlining some future trends is included.

Inductive coupling power transfer
Inductive coupling is the process of transferring power that is generating a varying magnetic field to a primary coil [4].Once power frequency AC power gets to DC power between the rectification and the filtering, through high-frequency inverter it is inverted.The high frequency AC power generated by the inverter device is input to the coil which is primary at the end of the primary transmitter, and the magnetic linkage generated by high-frequency AC in the winding which is primary is cross-linked with the secondary winding at the end of the receiver, thus generating the induced electro-motive force.Until passing the high-frequency rectification and DC chopping and other regulating circuits, the induced force that is electromotive can provide power to the load.The beginner of the winding and the other winding can be equivalent to a group of separable loose coupling transformers.When the high frequency current passes over the primary winding, the same frequency electric power will be induced in the secondary side, so that the inductive coupling of the two can be used to complete the electric energy transmission.Figure below illustrates this principle.Four basic compensation structures as follows in figure 4.

Capacitive coupling power transfer
The process of contactless CCPT systems for power that is low and applications which are low air-gap have been received the most attention [5].The capacitive coupler is composed of several metal sheets which are useful for generating electrical fields, which has the function to transfer power.The working principle of capacitive coupling is to make the strong current and weak current systems be combined and isolated through capacitor coupling, provide path of high frequency signal, inhibit low frequency current, enter the low current system, and assure personal safety.Apart from the above functions, the coupling capacitor with extraction device can also pull out the power frequency voltage for protection and closing, which acts in the position of voltage transformer.Constructing these capacitors between the transmitter and the receiver with two parallel and close plates [6].Considering that capacitive coupled wireless power transmission has an insignificant eddy current loss, truly lower cost, more convenient advantages in terms of weight, and superior dislocation performance.The CCPT have the function of displacement that is used to current in two capacitors with four copper plates and one dielectric layer.So, CCPT can take the place of conventional inductively coupled wireless power transmission.Comparison of different CCPT compensation topologies, as shown in Figure 5.

Magnetic resonant inductive coupling power transfer
The power transfer in MRICPT which is depend on magnetic resonance inductive coupling consists of a power transmission device and a power reception device.Where the two devices are set to the same frequency or resonate to a specific frequency, they can switch power from each other.This is a technology that is currently under review, which can be used for long-range loading, and the charging distance can reach thousands of kilometers.Magnetic resonance coupling radio energy transmission technology uses the magnetic field cofrequency resonance at the transmitter and receiver to transfer energy in space.For this, more power can be transferred between the two coils which are L1 and L2, longer in distance [7].When we use a radio frequency energy to excite the transmitter, a non-power field will be generated in the space around the transmitter.In this field, the magnetic field and electric field at any time at any position are orthogonal, and the phase difference is 1/2 pi, and the magnetic field strength is clearly superior to the electric field strength, this space electromagnetic field can store energy, but the power flow density of the synthesized electromagnetic wave is zero, and it will not transmit any energy, that is, this field will not radiate outward and will not lose inward.When we place the receiver with the same resonant frequency in this field, the same frequency magnetic resonance will be generated between the receiver and transmitter, and the energy will be coupled through the transmitter to the receiver in field that has the form of magnetic, thus realizing the spatial transfer of energy.
In addition, by adding a resonant capacitor to the capacitor, the transmitted energy can be increased by inductive coupling.A model of inductive coupling based on electromagnetic resonance is established in Figure 6.

Microwave coupling power transfer
Microwave coupling power transfer is a promising technology representing the combined application of electrical and microwave science [8].The system uses antennas to transmit and intercept electromagnetic waves in free space, and then uses rectification technology to convert electromagnetic waves into DC current.Especially suitable for long-distance transportation, but also suitable for energy transmission.The MCPT with diode and triode power that is microwave supply is more suitable for situations with small to transmission distance that is medium.Besides, despite the urgent need to develop remote and high-power MCPT systems for widespread applications, there are few reports.
Microwave coupling power transmission converts electromagnetic waves to current, then passes the current through the charging circuit.A system for transmitting radio waves is consisting primarily of a microwave transmitter and a microwave receptor.The microwave transmitting device transmits radio waves, and the microwave receiving device picks up power from radio waves and a load-dependent rule to achieve stable DC power.The system of microwave coupling power transmission is as below.The transmitting part mainly comprises a DC power supply, DC RF Inverter & Transmit Coil.The receiving part is called rectifier antenna, mainly including receiving antenna, low-pass filter, rectifier diode and DC filter.

Characteristics comparison
From the perspective of specific technical principles and solutions, there are currently four basic types of wireless charging technology: power transfer that is depend on inductive coupling, power transfer that is depend on capacitive coupling, power transfer that is depend on magnetic resonant inductive coupling and power transfer that is depend on microwave coupling.These technologies are suitable for fields that are near or far, respectively.
Compared to the indicative coupling power transfer, capacitive coupling power transfer has more advantages and progressiveness.
Capacitive coupling power transfer couple electric energy in the form of microwave waves to two sets of asymmetric dipoles in the vertical direction to generate electricity, which is transmitted wirelessly to the receiving antenna at the remote end.Then used after rectification, modulation, and other processing.
Although this technology has low energy efficiency, it is the most convenient to use.In an ICPT system, the circuit which is with the series-series compensation, it is similar to the circuit with double-sided LC compensation in the CCPT system [9].Both the input and output characteristics of two-stage LCC compensation in ICPT system are consistent with those of two-stage LCLC compensation in CCPT system [10].
The primary side uses a series resonant ICPT system, and the efficiency of the system is only 50% under the optimized mutual inductance coupling conditions obtained with the maximum transmission power as the design goal.
Although there are many similar between the two wireless charging technologies, CCPT and ICPT, there are also some differences between them, and different applications can be used to distinguish the two technologies.In practical applications, ICPT systems can cause significant temperature.It will be improved, no matter in the high and low power occasions.Especially for the application of vehicle charging, the magnetic field will cause the surrounding metal to produce a huge amount of heat, thus causing a fire.This is a place where fire is likely to break out.However, the CCPT device uses the electric field for energy transfer, there is no need to consider the current loss.For that, it is not necessary to avoid metal areas.
To solve this problem, cross-layer coupling technology is introduced into ICPT, an additional transformer is considered.But the transformer increases the cost.Besides, CCPT system do not need to consider this problem.
The transmission power of ICPT is large, but the transmission distance is relatively short.The local BYD company in China used magnetic resonant inductive coupling power transfer technology in 2015.In the main winding, the AC current of a certain frequency is induced by electromagnetic induction, so that the clock in the auxiliary winding forms a kind of electric energy from transmission to reception.MCPT has broad application prospects and special applications, such as solar power plants (SSPS).Collecting solar energy through the geostationary orbit SSPS satellite and transmitting it to the ground in the form of microwaves to provide clean electricity to the ground is of great help in solving the human energy crisis.In addition, small unmanned aerial platforms near the ground can achieve environmental monitoring, weather monitoring, radar detection, etc.It can also be used to serve space science research.Provide energy for various satellites and spacecraft in space.In areas with complex terrain and difficulty in erecting transmission lines, such as barren mountains and deserts, microwave wireless transmission systems can be used to transmit electric energy as a supplement to traditional transmission lines.
Magnetic resonant inductive coupling power transfer technology is the technology that is currently the most mature and popular, and its principle is somewhat similar to that of a transformer.
Microwave coupling power transfer is a relatively mature technology.It is similar to the ore radios used in the early days.
The efficiency of MCPT is lower than the one obtained by inductive or capacitive coupling [11].The range of the transmission distance of magnetic resonant coupling WPT technology is from tens of centimeters to several meters.And the range of the transmission power can be tens of watts to thousands of watts, and the efficiency of transmission which is about power is between 40% and 90%.It has obvious advantages in various aspects.
During the working process of wireless charging by microwave radiation, electrical energy is emitted in the form of microwaves by the transmitting antenna.
Table 1.summarizes the main characteristics comparison of different technologies in the paper.Charging time-For the electric vehicle charging market, energy loss means extending charging time.In order for wireless charging to win the electric vehicle market, its battery charging time must be lower than the time it takes to fill the fuel tank.
Application-Wireless charging technology is playing an increasingly important role in our lives.Wireless power transfer technology has irreplaceable and important applications in some special fields, such as medical implant devices, wireless sensor networks, deepwater working electronic equipment, mechanical and electrical engineering construction in the construction industry, and so on.The combination of mature solar photovoltaic power generation technology and wireless charging technology can provide power for a variety of mechanical and electrical devices such as street lights, LED signs, surveillance cameras, etc. on highways.This approach can also improve the safety of power supply.
Power-There are still many difficulties to overcome in high-power wireless power transfer technology.Its input efficiency is very poor and takes a long time to recharge, and it is a waste of time.High power remote mechanical equipment consumes a lot of energy and wastes a lot of energy.

Conclusion
Improving the technology from the above perspectives is beneficial for increasing its practicality and enhancing its development.There is no doubt that wireless power transfer technology is conducive to the charging of electric vehicles.It has the characteristics of convenience and speed.And can reduce environmental pollution, is a clean energy.It is clear that reduce the use of fuel and effectively reduce greenhouse gas production.It also reduces charging costs.According to the different working principles, this article mainly introduces four wireless power transfer technologies.In the paper, the four wireless power transfer technologies are compared through different application scenarios and functional characteristics.It can be found that the characteristics of different technologies are different.The limitations of the four technologies are also compared.

Figure 6 .
Figure 6.A model of inductive coupling based on electromagnetic resonance.

Table 1 .
Characteristics comparison of different technologies.Considering expansion and development of the application of wireless power technology, it is necessary to improve it in order to ensure its long-term development and increase practicality in the future.Besides, discussions for the future optimization of wireless power technology are as follows.