Wireless power transfer technologies for electric vehicles: Principles, challenges and opportunities

Wireless power transfer (WPT) technology is a promising solution for charging electric vehicles, which have many advantages such as zero emissions, low noise, and convenience. This paper reviews the current state-of-the-art of WPT technology for electric vehicles, focusing on the classification, principle, advantages, and disadvantages of different WPT methods, such as capacitive WPT, inductive WPT, microwave WPT, and laser WPT. The paper also discusses the future trends and challenges of WPT technology, such as coil design, international standards, safety issues, multiple charging modes, and compensation networks. The paper aims to provide a comprehensive overview of WPT technology for electric vehicles and to inspire further research and development in this field.


Introduction
With the strengthening of environmental awareness and the development of technology, pure electric vehicles are gradually becoming a new choice for people's transportation [1].These vehicles do not require fuel and utilize renewable energy, have zero emissions, low noise, low operating costs, and fast acceleration.Electric vehicles make a huge contribution to environmental protection through their clean, efficient, and economical advantages.
The basic structure of an electric vehicle consists of a power system, an electric drive system, and a vehicle control system.The power system includes the power battery, which is responsible for storing and outputting electrical energy; the drive motor system includes the motor controller and the motor, which convert electrical energy into mechanical energy; and the vehicle controller is responsible for coordinating the operation of various components.
However, the traditional plug-in charging method of electric vehicles has problems such as inconvenient use, contact wear, poor compatibility, and safety hazards, which restrict the development of electric vehicles.Wireless charging technology for electric vehicles is a new type of charging method that can be divided into dynamic and static modes [2].As shown in Figure 1, the dynamic mode refers to the electric vehicle receiving electrical energy from coils buried under the road surface during driving to achieve continuous charging.The static mode refers to the electric vehicle receiving electrical energy from coils installed on parking spaces during parking to achieve fast charging.Compared with traditional plug-in charging methods, wireless charging technology has many advantages.Firstly, it can improve charging efficiency and convenience, avoiding the cumbersome operations of plugging and unplugging plugs and finding charging stations [3].Secondly, it can reduce the impact on the environment, reduce the use and maintenance costs of hardware facilities such as wires and sockets, and also reduce safety hazards such as fires and electric shocks.Finally, it can promote the development of intelligent transportation systems, monitor and optimize vehicle status, road conditions, and energy management through wireless communication and data transmission.Due to its many advantages, wireless charging technology has become a highly anticipated solution and inevitable requirement for the development of the future electric vehicle industry.
The concept of wireless energy transmission dates back to the late 19th century and was first proposed by Nikola Tesla [4].He used a device called a Tesla coil that allowed high-frequency electrical energy to be transmitted through the air.After several stages of research and development, it has now become a rapidly developing field.In recent years, with the continuous advancement of technology, wireless power transfer technology has been widely researched and applied.At present, wireless power transfer technology has been applied to drones, smartphones, smart homes and other fields, and in the field of electric vehicles, wireless power transfer and even wireless charging roads have also been realized.Wireless charging technology gradually becoming a necessary component of future technologies.
This paper reviews the development of WPT technology, the main classification of current WPT technology and the comparison of WPT technology.The advantages and disadvantages of wireless charging technology are analyzed and possible solutions are proposed, and the future development trend of wireless charging technology is foreseen in conjunction with the current situation of wireless charging technology applications for electric vehicles.

Classification
Wireless power transfer technologies can be mainly divided into radiative WPT (microwave, laser), inductive WPT (magnetic induction, magnetic resonance) and capacitive WPT according to the fields.As shown in Figure 2, these technologies have different principles and characteristics and different application scenarios.3, the capacitive WPT topology mainly includes power electronic converters (including rectifiers and inverters), compensation networks (including capacitors and inductors), power generators and power receivers.When a current passes through the high-frequency oscillating circuit at the transmitting end, a high-frequency oscillating electromagnetic field is generated.This electromagnetic field passes through the air or other medium to the receiving metal plate.When an electromagnetic field is induced, a voltage is generated that can be used to drive a rectifier circuit to convert the wirelessly transmitted energy into direct current.Different system coupling structures can form different capacitance-coupled wireless power transmission systems.In practice, because of the capacitor's equivalent resistance and transmission distance, capacitive WPT has some problems with power loss and voltage drop.To solve these problems, a capacitive WPT compensation circuit can be used.In a wireless capacitive power transfer system, the topology of the compensation circuit is an important factor.Different compensation circuit topologies can affect the power efficiency and frequency characteristics of the system.When designing a capacitive WPT system, it is necessary to select an appropriate compensation circuit topology to improve the performance of the system, meets the specific frequency requirements, and achieve efficient power transmission.Compensation circuits can be divided into non-resonant topologies and resonant topologies, including those based on PWM converters, power amplifiers and full-bridge inverters [5].Among them, the most popular compensation networks for capacitive wireless power transmission are shown in Figure 4. Figure 4(a) shows the structure of a Double-side L compensation circuit, which is widely used in low-power and high-power scenarios and has a relatively simple structure.The disadvantage is that the value of the inductor needs to be selected precisely to ensure that the circuit can work properly.If the load changes or the coupler capacitance changes, the circuit may need to be retuned to ensure stability and efficiency.Figure 4(b) shows the structure of a Double-side LC compensation circuit with the addition of equivalent coupling capacitance.Its advantage is that it can be applied to long-distance and high-power transmission, but its disadvantage is that the system power is inversely proportional to the coupling coefficient [6], which shows that with the increase of distance, the power increases but the efficiency decreases (which may cause damage to the system).Figure 4(c) shows the structure of a Double-side LCLC compensation circuit.The advantage is that the system power is inversely proportional to the coupling coefficient in Figure 4(b), thus improving the coupling coefficient, efficiency and power.
The capacitive coupler is a component composed of multiple metal plates.Each pair of plates has a coupling capacitor.Different structures of capacitive couplers will produce different coupling models.Therefore, in the design and optimization process of capacitive couplers, it is necessary to consider the size of capacitors, coupling structure, frequency response and other factors to achieve good power transfer performance.According to the different structures of the coupler, there are two-plate coupler structure, four-plate parallel structure, four-plate overlapping structure, six-plate structure, repeater structure, etc. [5].

Advantages. (a)
Low cost and small size: Aluminium plates are cost-effective, conductive, lightweight and do not require thickness, making them cost lower compared to other technologies where only aluminium plates are required for capacitive wireless power transmission.
(b) Low metal eddy current loss: If a metal conductor is present in the vicinity of the capacitive wireless power transfer application, the metal eddy current loss will be very small, resulting in less heat generation and reducing the probability of damage to equipment and even a fire.
(c) High safety: when transmitting energy, only an electric field is generated within the range of the capacitor, making it safer than conventional electromagnetic induction WPT.
(d) Ignoring metal barriers: effective energy transfer is achieved even in the presence of metal barriers [7].
(e) High misalignment tolerance: when the metal plates are not aligned and misaligned during charging, high efficiency is maintained.

Disadvantages. (a) Parasitic capacitance:
In vehicles, there is a large amount of parasitic capacitance due to the presence of many electronic devices and wires, which can affect the performance of wireless chargers [8].
(b) Low power density: In comparison, capacitive couplers are typically lower in power density than inductive wireless transmission technologies, which means that to achieve the same power output, capacitive couplers require a larger area and will take up more space.

Inductive wireless power transfer
2.2.1.Principle.As shown in Figure 5, the inductive wireless power transfer topology mainly consists of power electronic converters (including rectifiers and inverters), compensation networks, coupling coils, etc.When a high-frequency AC signal passes through the transmitting coil, an alternating magnetic field is generated within the coil.This magnetic field passes through the air and generates a current in the receiving coil by induction, thus enabling wireless charging.In inductive WPT technology, the magnetic flux between the coils is the main factor affecting the transmitted power.If the relative position between the coils changes, it will significantly affect the performance of the wireless transmission, and the size and shape of the coils will affect the coupling efficiency and the efficiency of the power transmission.Instead of matching the power supply frequency to the intrinsic frequency of the coil, magnetic inductive wireless power transmission techniques reduce the reactive power in the system by using compensating elements.In addition to magnetic induction WPT, magnetic resonance WPT is also a part of the inductive WPT technology.
Magnetic resonance WPT is a special form of electromagnetic resonance that occurs when the inductance and capacitance in a coil reach a specific ratio and resonate at a certain frequency.In inductive wireless power transmission, magnetic resonance can be used to achieve higher transmission efficiency and distance by adjusting the ratio of inductance and capacitance in the transmitting and receiving coils so that they are magnetically coupled at the resonant frequency.Therefore, magnetic resonance can improve the transmission efficiency and distance of inductive wireless power transmission and is one of the important optimisation tools in inductive wireless power transfer technology.

Figure 6.
Compensation topologies of mono-resonant. Figure 6 illustrates the four basic compensation circuits.The SS structure shown in Figure 6(a) has the primary voltage and current in phase and the secondary voltage and current in phase, but the phase relationship will be 90° ahead with respect to the primary voltage and current and allows for a constant current output from the voltage source.The SP structure shown in Figure 6(b) has the primary current in phase with the secondary current and the secondary voltage in phase with the primary voltage, and can achieve a constant voltage output from the voltage source.The output voltage of the PS structure shown in Figures 6(c) and 6(d) can achieve either a constant voltage or a constant current output, depending on the relationship between the capacitor Cp (or equivalent inductor Lx) and the compensation inductor Lp.SS and SP are more suitable for changeable load conditions because they guarantee resonance and enable more stable and efficient energy transfer.However, the compensation topology of SS and SP is more complex compared to that of PS and PP and requires more circuit elements and control circuits to implement.
The multi-resonant topology for Magnetic resonance WPT is a circuit design solution that achieves high efficiency.By using multiple resonant circuits at the transmitter and receiver ends in a multiresonant topology, the transmission efficiency and distance of magnetic resonance WPT can be increased, as well as the stability and reliability of the transmission.accommodate different input supply voltage variations.In LCC resonant WPT systems, the current source introduces an additional magnetic field, resulting in the need for a larger inductor in the resonant circuit.In order to optimise the transmission efficiency of LCC resonant WPT systems, it is often necessary to use additional parallel resonant capacitors to regulate the resonant frequency.
2.2.2.Advantages.(a) Higher power density: Inductive WPT has a higher power density compared to other WPT technologies, which allows it to transmit more energy in a smaller space, thus reducing the space occupied during charging.(b) Inductive WPT is easy to operate and has a wide range of applications, providing a viable solution for wireless charging and data transmission for a variety of electronic devices.(b) Higher coil quality: To obtain higher transmission efficiency, inductive WPT requires a high quality of coils.It is necessary to select high-quality materials for the coils, to consider the balance of self and mutual inductance of the coils, and to ensure that the coils at the transmitting and receiving ends are matched.
(c) Low efficiency: Once deviated from resonance, it leads to a significant reduction in the system's performance in terms of power and efficiency [9].8, microwave WPT utilises a high-frequency microwave signal, which is generated at the transmitter end by a microwave energy generator.This signal is emitted into the air through a microwave antenna and propagated to a phase-shifter array microwave antenna at the receiver end.The microwave antenna at the receiving end converts the microwave signal to a DC signal and provides the electrical power to the required equipment via a circuit.

Advantages. (a) Long-range transmission: Microwave signals have relatively little attenuation
during transmission, allowing for transmission over longer distances.In addition, by using larger antenna gain and focusing techniques, the transmission range can be further extended.

Disadvantages. (a) Difficulty in achieving high power:
To achieve high-power transmission, microwave WPT requires the use of bulky transmitting and receiving devices.These devices need to include high-power microwave transmitters and receivers, power amplifiers, filters and other components, which are relatively heavy and bulky.
(b) Environmental adaptation issues: When transmitting over long distances, meteorological conditions such as clouds, fog and atmospheric refraction may have an impact on the performance of the transmission.
(c) Safety issues: Microwaves have high energy density and strong penetrating power, and therefore may cause harm to humans and equipment when transmitting at high power.In addition, microwave WPT may also cause some interference to the surrounding environment.9, the principle of laser WPT is to use the laser beam generated by the laser to modulate the information signal onto the carrier of the laser and transmit the laser beam through free space to the receiving end.The receiving end uses a corresponding optical detector to convert the optical signal into an electrical signal, which is demodulated and processed to restore the original information signal.The laser is one of the most critical parts of the transmitter side of a laser WPT system.It mainly uses a laser generator to convert electrical energy into laser energy and then sends the laser beam out.The laser generator in a laser is usually a semiconductor laser or a solid-state laser, which enables high power, high speed, high efficiency and low noise.In addition, lasers are characterised by high directionality and narrow beam angles, giving them the advantages of high speed, high bandwidth, low interference and security.
The photovoltaic cell is one of the most critical parts of the receiving end of the laser wireless power transmission system.The current structure of photovoltaic cells is divided into photovoltaic eye type, flat plate type, convergence type and so on.It converts the energy in the laser beam into DC electrical energy for the power supply or charging.When the laser beam shines on the surface of the PV cell, the laser photons will excite the electrons, causing them to jump to the conduction band and generate current.

Advantages. (a) Low interference:
The signal of laser WPT is not susceptible to other electromagnetic interference as it only interacts with air molecules during transmission and is not subject to interference from other wireless devices.
(b) Good directionality: the transmission path of laser WPT is a straight line and does not require consideration of issues such as multipath propagation and reflections.This type of transmission reduces signal jitter and delay, thus improving signal stability and reliability.(b) Environmental adaptation issues: meteorological conditions such as clouds, fog and atmospheric refraction may have an impact on the performance of the transmission when the transmission distance is long.
(c) Safety issues: Laser WPT requires high-precision pointing and tracking technology, which may result in laser beams being misdirected at the human body in the event of system failure or misuse, thus causing harm to the human body.

Comparison
Among them, inductive WPT is more widely used.Inductive WPT and capacitive WPT is more costeffective [1], microwave WPT and laser WPT is limited by practicality, security and other issues, and there are no significant application results.The comparison between different WPT technologies is summarized in Table 1.
To address the problem of parasitic capacitance in capacitive WPT, non-contact measurement methods can be used to detect and eliminate the effect of parasitic capacitance on wireless charging systems.By changing parameters such as electrode shape, size, position and spacing, the electric field uniformity and concentration can be improved, thus increasing the power density.However, optimising the electrode structure also requires consideration of a number of factors, such as safe distance, degree of matching and reliability.
For inductive WPT, methods such as multiple transmitters and multiple receiver repeaters are used to extend the transmission range.This is a method of using multiple transmitters or receivers or intermediate nodes to increase the signal coverage area and strength, and it can effectively overcome the problems posed by distance limitations in the single point-to-single point transmission mode.However, this approach can also increase system complexity and cost.
Future wireless power transfer technologies could be continually optimised in the following directions: (a) Coils: Coils are one of the most important factors affecting system performance in wireless power transfer technology.New coil structures, optimised coil sizes and optimised coil arrangements can be adopted to achieve charging technology with higher transmission efficiency and longer transmission distances.
(b) International standards: At present, there are mainly A4WP, Qi, PMA and other alliances that set their charging standards.In order to facilitate the rapid development and application of wireless charging technology, there is still a need to develop a globalised and unified international standard.
(c) Safety: The current wireless charging technology may cause harm to the human body such as electromagnetic radiation, so it is necessary to continuously optimize the design, explore the wireless transmission method to ensure human health and develop relevant safety standards.
(d) Multiple charging modes: In future application scenarios, wireless charging technology can be used to lay wireless charging coils under roads, thereby enabling charging while electric vehicles are in motion, including new charging methods that are dynamic (e.g.driving roads) and a combination of dynamic and static (e.g.bus stop roads).
(e) Compensation networks: The efficiency of the wireless power transfer technology is improved by optimising the topology, the power electronics, the control algorithms and the transmission environment to improve the performance of the compensation networks.

Conclusion
In summary, this paper focuses on the development and application of wireless power transfer technology for electric vehicles.It first introduces the basic structure and advantages of electric vehicles, then elaborates on the classification and principles of wireless charging technologies, including capacitive WPT, inductive WPT and radiative WPT, and reviews in detail the necessary constituent structures such as power electronic components, various compensation circuits and so on.The article also analyses the advantages and disadvantages of each technology and looks at future trends in wireless power transfer technology.

Figure 1 .
Figure 1.Two scenarios of wireless charging.Compared with traditional plug-in charging methods, wireless charging technology has many advantages.Firstly, it can improve charging efficiency and convenience, avoiding the cumbersome operations of plugging and unplugging plugs and finding charging stations[3].Secondly, it can reduce the impact on the environment, reduce the use and maintenance costs of hardware facilities such as wires and sockets, and also reduce safety hazards such as fires and electric shocks.Finally, it can promote the development of intelligent transportation systems, monitor and optimize vehicle status, road conditions, and energy management through wireless communication and data transmission.Due to its many advantages, wireless charging technology has become a highly anticipated solution and inevitable requirement for the development of the future electric vehicle industry.The concept of wireless energy transmission dates back to the late 19th century and was first proposed by Nikola Tesla[4].He used a device called a Tesla coil that allowed high-frequency electrical energy to be transmitted through the air.After several stages of research and development, it has now become a rapidly developing field.In recent years, with the continuous advancement of technology, wireless power transfer technology has been widely researched and applied.At present, wireless power transfer technology has been applied to drones, smartphones, smart homes and other fields, and in the field of electric vehicles, wireless power transfer and even wireless charging roads have also been realized.Wireless charging technology gradually becoming a necessary component of future technologies.This paper reviews the development of WPT technology, the main classification of current WPT technology and the comparison of WPT technology.The advantages and disadvantages of wireless charging technology are analyzed and possible solutions are proposed, and the future development trend of wireless charging technology is foreseen in conjunction with the current situation of wireless charging technology applications for electric vehicles.

Figure 2 .
Figure 2. Classification of wireless power transfer technologies.

Figure 3 .
Figure 3.The basic structure of capacitive WPT.In practice, because of the capacitor's equivalent resistance and transmission distance, capacitive WPT has some problems with power loss and voltage drop.To solve these problems, a capacitive WPT compensation circuit can be used.In a wireless capacitive power transfer system, the topology of the compensation circuit is an important factor.Different compensation circuit topologies can affect the power efficiency and frequency characteristics of the system.When designing a capacitive WPT system, it is necessary to select an appropriate compensation circuit topology to improve the performance of the system, meets the specific frequency requirements, and achieve efficient power transmission.Compensation circuits can be divided into non-resonant topologies and resonant topologies, including those based on PWM converters, power amplifiers and full-bridge inverters[5].Among them, the most popular compensation networks for capacitive wireless power transmission are shown in Figure4.

Figure 4 .
Figure 4. Compensation topologies of capacitive WPT.Figure4(a) shows the structure of a Double-side L compensation circuit, which is widely used in low-power and high-power scenarios and has a relatively simple structure.The disadvantage is that the value of the inductor needs to be selected precisely to ensure that the circuit can work properly.If the load changes or the coupler capacitance changes, the circuit may need to be retuned to ensure stability and efficiency.Figure4(b)shows the structure of a Double-side LC compensation circuit with the addition of equivalent coupling capacitance.Its advantage is that it can be applied to long-distance and high-power transmission, but its disadvantage is that the system power is inversely proportional to the coupling coefficient[6], which shows that with the increase of distance, the power increases but the efficiency decreases (which may cause damage to the system).Figure4(c)shows the structure of a Double-side LCLC compensation circuit.The advantage is that the system power is inversely proportional to the coupling coefficient in Figure4(b), thus improving the coupling coefficient, efficiency and power.The capacitive coupler is a component composed of multiple metal plates.Each pair of plates has a coupling capacitor.Different structures of capacitive couplers will produce different coupling models.Therefore, in the design and optimization process of capacitive couplers, it is necessary to consider the size of capacitors, coupling structure, frequency response and other factors to achieve good power transfer performance.According to the different structures of the coupler, there are two-plate coupler structure, four-plate parallel structure, four-plate overlapping structure, six-plate structure, repeater structure, etc.[5].

Figure 5 .
Figure 5.The basic structure of Inductive WPT.Instead of matching the power supply frequency to the intrinsic frequency of the coil, magnetic inductive wireless power transmission techniques reduce the reactive power in the system by using compensating elements.In addition to magnetic induction WPT, magnetic resonance WPT is also a part of the inductive WPT technology.Magnetic resonance WPT is a special form of electromagnetic resonance that occurs when the inductance and capacitance in a coil reach a specific ratio and resonate at a certain frequency.In inductive wireless power transmission, magnetic resonance can be used to achieve higher transmission efficiency and distance by adjusting the ratio of inductance and capacitance in the transmitting and

Figure 7 .
Figure 7. Compensation topologies of multi-resonant.Figure7illustrates the two basic compensation structures.the LCL topology does not require an external shunt resonant capacitor and therefore does not suffer from mandatory output OVP problems and also has lower light load losses than LCC, although LCL may require a higher cost.the LCC structure allows for a constant current output.the LCC structure has a wider voltage range and can

Figure 7
Figure 7. Compensation topologies of multi-resonant.Figure7illustrates the two basic compensation structures.the LCL topology does not require an external shunt resonant capacitor and therefore does not suffer from mandatory output OVP problems and also has lower light load losses than LCC, although LCL may require a higher cost.the LCC structure allows for a constant current output.the LCC structure has a wider voltage range and can

Figure 8 .
Figure 8.The basic structure of Microwave WPT.

Figure 9 .
Figure 9.The basic structure of Laser WPT.The laser is one of the most critical parts of the transmitter side of a laser WPT system.It mainly uses a laser generator to convert electrical energy into laser energy and then sends the laser beam out.The laser generator in a laser is usually a semiconductor laser or a solid-state laser, which enables high power, high speed, high efficiency and low noise.In addition, lasers are characterised by high directionality and narrow beam angles, giving them the advantages of high speed, high bandwidth, low interference and security.The photovoltaic cell is one of the most critical parts of the receiving end of the laser wireless power transmission system.The current structure of photovoltaic cells is divided into photovoltaic eye type, flat plate type, convergence type and so on.It converts the energy in the laser beam into DC electrical energy for the power supply or charging.When the laser beam shines on the surface of the PV cell, the laser photons will excite the electrons, causing them to jump to the conduction band and generate current.

2. 4 . 3 .
Disadvantages.(a) Controlling phase: control of phase requires high precision tracking techniques, which require the support of hardware and software technologies including high precision optics, control algorithms, sensors, etc.

Table 1 .
Comparison between different WPT technologies.