Wireless Charging Infrastructure for Electric Vehicles

Overview: Wireless power transfer (WPT) eliminates galvanic connection and manual charging of electric vehicles to provide a convenient and unobtrusive charging method in harsh environments. This article will discuss the various infrastructures of wireless charging.

The absence of the need to physically move the charging cable will help to promote automatic wireless charging, which will make it more appealing for use in public transportation. The magnetic-resonant coupling (MRC) based inductive power transfer (IPT) has been recognized as the most practical and frequently used for wireless EV charging applications out of several coupling mechanisms for WPT techniques, such as inductive coupling, capacitive coupling, magneto-dynamic coupling, and microwaves, as listed in Table 1.

Table 1. Classification of WPT Technologies for Wireless EV Charging. Source: IEEE Open Journal of Vehicular Technology

Park-and-Charge of Stationary Charging

The IPT technology is enhanced to be plug-less, in which the primary coil is installed on the floor of a garage or parking lot, and the secondary coil is installed on the vehicle, as shown in Fig. 1. This will make the park-and-charge (PAC) process for EVs easier. The driver doesn't need to worry about those unwieldy and hazardous charging cables. This system is very simple to use, and after the driver parks the electric vehicle correctly, the charging process begins automatically. 

This plug-less PAC not only improves user convenience but also provides a way to avoid charging plug standardization. In a wireless power transfer system based on magnetic resonance coupling, primary and secondary coils with the same resonant frequency can efficiently transfer power with high power density while dissipating relatively little energy in non-resonant objects like car bodies or drivers.

Fig. 1. Typical configuration for park-and-charge IPT system. Source: IEEE Open Journal of Vehicular Technology.

The most recent WPT for PAC research and development is active and diverse, including compensating for magnetic coupler misalignment, implementing bidirectional WPT between chargers and EVs, and combining power transfer and information transfer within the same channel. These cutting-edge works are listed below:

1. The magnetic coupler design is crucial for realistic PAC and efficient WPT. For instance, the compound primary pad has been proposed to offer a uniform magnetic flux density at the majority of the charging area, thereby resolving the misalignment issue by using uneven pitch distances of the spiral winding. In the meantime, a bipolar primary pad has been created that works in conjunction with straightforward secondary pads to transfer power with a wide lateral tolerance.
2. The advancement of vehicle-to-grid (V2G) technology is promising because EVs can act as mobile power plants to support and stabilize the power grid with renewable energy. A bidirectional power interface has been created by integrating the WPT into the V2G to enable the simultaneous charging and discharging of numerous EVs. For wireless V2G, a variety of bidirectional resonant inverters have recently been developed in an effort to increase power output, control power flow, and fault tolerance.
3. In order to transfer power from the charger to the vehicle and communicate data with the onboard battery management system, simultaneous wireless power and information transfer (WPIT) technology is actively being developed. The triangular current waveform's fundamental component is used to transfer power in a WPIT system, while its third-order harmonic component is chosen to transfer information.

Move-and-Charge of Dynamic Charging

The electric vehicle prefers to be wirelessly charged while moving rather than stopping or parking. A receiver is mounted at the bottom of the EV, and a network of power transmitters is embedded beneath the road (referred to as the charging zone or lane). The long-term issues with EVs could potentially be solved fundamentally by this move-and-charge (MAC) technology. The EV can be conveniently charged at the charging zone while driving, automatically extending the driving range, and there is no need to install as many batteries, both of which significantly reduce the initial cost of the vehicle. 

The system configuration for MAC is more difficult than for PAC. The transmitters that are installed below the surface of the road come in pad or rail designs. The pad design has numerous primary pads that are all the same size or smaller than a vehicle. The primary pads can be separately excited using power switches and sensors based on one power inverter per section, as shown in Fig. 2(a). These pad-based transmitters inevitably require numerous primary pads, power inverters or power switches, and sensors, which results in high installation complexity and exorbitant investment costs. 

Contrarily, the rail design, as shown in Fig. 2(b), uses only a primary rail, or rather, a long primary coil, and a power inverter to supply multiple EVs. This design has the distinct advantage of having a much lower investment cost and a much simpler installation process than the pad design. The rail design typically adopts the configuration of a sectionalized roadway, in which it uses one power inverter per section to supply multiple EVs in order to provide for more adaptable maintenance and scalability.

Fig. 2. Coil configuration of the move-and-charge system. (a) Multiple coil pads as the primary transmitter. (b) Single coil rail as the primary transmitter. Source: IEEE Open Journal of Vehicular Technology.

Research and development of MAC technology have been overwhelmingly successful because it has the potential to fundamentally resolve the long-term issues with EVs. These cutting-edge MAC works are outlined below:

1. The rail-based MAC system has been successfully implemented as part of the Online Electric Vehicle (OLEV) project run by KAIST. It has resolved a number of MAC issues, including cost-effective improvement, continuous power transfers, and high-frequency current-controlled inverters. The system efficiency of the OLEV can reach up to 83% at an output power of 60 kW with a resonant frequency of 20 kHz, thanks to innovative coil designs and road construction methods. For the MAC system, various magnetic couplers utilizing rails, such as the U-type, E-type, W-type, I-type, and S-type rails, have been well developed.
2. Due to the EV's mobility, the performance of current WPT techniques is inevitably impacted by the misalignment between the primary and secondary coils of the pad-based MAC system. The alternate winding design, which is used to gaplessly assemble primary coils to increase magnetic flux density, and the vertical-and-horizontal secondary coil, which improves the capability of acquiring energy, particularly in the area of the coils gap, are the key components of the homogeneous WPT technique that has been proposed. As a result, it can significantly enhance the power transfer efficiency of this MAC system with pads.
3. The energy encryption technique has been proposed to prevent unauthorized EVs from stealing wireless power when they are operating on the rail-based MAC system and to enable authorized EVs to perform charging. The primary rail is synchronously tuned to have the resonant frequency match with the operating frequency while the operating frequency is purposefully adjusted to follow a predefined sequence over a predefined frequency band (referred to as the security key). As a result, the transmitted energy is encrypted. The authorized EV can receive the desired energy when the secondary coil is also synchronously tuned to have the same resonant frequency in accordance with the security key; otherwise, without knowing the security key, the unauthorized EV cannot decrypt the encrypted energy or receive the desired energy.

Summarizing with key points:

Some of the takeaways from the article are as follows:

• Wireless power transfer eliminates galvanic connection and manual charging for convenient and unobtrusive charging in harsh environments.
• The magnetic-resonant coupling-based inductive power transfer coupling mechanism is the most practical and widely used for wireless EV charging.
• PAC system is easy to use and starts charging automatically after the driver parks the electric vehicle.
• The latest WPT for PAC research and development includes compensating for magnetic coupler misalignment, bidirectional WPT between chargers and EVs, and power transfer and information transfer in the same channel.
• As the WPT system's energy carrier, the transmitter and receiver pads are essential for transmitting and receiving magnetic flux lines.
• MAC system wirelessly charges EVs while driving. The EV has a receiver and a power transmitter network under the road (referred to as the charging zone or lane).
• In MAC technology, pad or rail transmitters are installed under roads. Because it could solve EVs' long-term problems, MAC technology research and development have been successful.

This blog post is based on a full research article from the IEEE Open Journal of Vehicular Technology.