The Route to Wireless Charging of Electric Vehicles
Wireless charging offers a number of benefits to both electric vehicle owners and infrastructure providers in terms of simplicity, reliability, and ease of use.
According to Allied Market Research, the value of the global electric-vehicle market will grow from $162.34 billion in 2019 to $802.81 billion. That’s a lot of cars that will need charging (EV charging), which is why a growing number of companies are actively looking at simplifying the process by delivering cable-free charging solutions (Wireless charging).
Indeed, analysts are predicting significant growth for the EV wireless charging market. Research company Market Research Future, for example, predicts a CAGR of 48.82% from 2021 to achieve a value of $258.65 million by 2027. That said, the market for EV wireless charging is still incredibly small if compared with more traditional wired chargers. This is because the undeniable advantages cannot compensate for the technical difficulties typical of wireless charging technologies for electric and hybrid vehicles.
Wireless charging offers a number of benefits to both EV owners and infrastructure providers in terms of simplicity, reliability, and ease of use. These include removing the need to store, remember, and plug/unplug heavy and dirty charging cables, eliminating the risk of accidental or intentional damage to wires or charging stations and providing immunity to dirt and water ingress. Adding charging to spaces in parking lots makes EV charging much more accessible, while wireless solutions mean that charging can start immediately without the driver leaving the vehicle. And of course, when it comes to autonomous EVs, these will, by definition, require wireless charging.
Thanks to the benefits outlined above, a number of automotive manufacturers and companies have already developed, or are in the process of developing, wireless charging solutions.
BMW, for instance, got as far as offering a wireless charging system for its 530e plug-in hybrid saloon back in 2018, introducing it as part of a lease deal on the car in Germany, the U.K., the U.S., Japan, and China. Available to customers as an option, BMW wireless charging consisted of an inductive charging station (“GroundPad”), which could be installed either in a garage or outdoors, and a secondary component (“CarPad”), which was fixed to the underside of the car. The system had a charging power of 3.2 kW, enabling the 530e’s batteries to be fully charged in about three-and-a-half hours.
Plugless Power and WiTricity offer aftermarket solutions to upgrade EVs for wireless charging, with the latter also acquiring Qualcomm’s “Halo” wireless car charging technology, which was originally used to recharge the safety and medical cars at Formula E races.
Other companies involved in wireless EV charging include Continental AG, Elix Wireless, Hevo Power, Toshiba Corporation, and ZRE Corporation.
The challenges of EV wireless charging
The key challenges facing designers of EV wireless charging schemes are summarized in Figure 1.
Addressing these challenges requires a rethink of the traditional approach to wireless charging architectures.
As Figure 2 illustrates, from a system perspective, a conventional wireless charging design comprises at least three different blocks made up of five distinct stages:
- A mains rectifier to rectify the input line voltage. For low-power applications, this may simply be a diode bridge, but in more complex designs such as EV charging, a power-factor–corrected stage is also required.
- A transmitter voltage regulator (typically a buck converter with two active elements and one high-current magnetic element) to manage the power to be transmitted when the main wireless power link works at fixed frequency (in order to manage EMI, this is a common situation in the automotive field).
- A DC/AC transmitter coil driver stage to energize the coil. This resonant stage may be a half-bridge with two active elements or a full-bridge with four active elements. In both cases, operation sees a load-dependent efficiency curve that achieves zero-voltage switching (ZVS) only in full-load conditions, with a degradation of efficiency for lighter loads or when the coupling between receiver and transmitter is far from nominal. Other elements such as a tuning network can be added to improve the efficiency in every working point.
- A wireless receiver AC/DC coil full-bridge rectifier stage to rectify the voltage at the receiver coil.
- An optional DC/DC receiver voltage regulator (again, typically a buck converter with two active elements and one magnetic element and typically for high-power applications) to regulate the output voltage.
This means that a traditional wireless charging system for EV and plug-in hybrid vehicles has between 12 and 16 active devices, at least two magnetic components, two coils, and an AC cable. This number of elements not only contributes to a high bill-of-materials cost but also impacts overall system efficiency.
Now, however, there is a new approach that provides the foundation for addressing the challenges of conventional architectures.
Figure 3 summarizes the key elements of the new architecture, which is a hybrid AC power solution that is both a power supply and a wireless charger. Known as E2WATT and based on a proprietary inductive technology, this architecture facilitates higher power densities and larger distances than previous wireless charging designs and supports efficiency levels comparable with the best conventional AC wired adapters. As a result, it offers a route for taking wireless charging from a few watts to tens of kilowatts.
The new architecture reduces to just two the total number of stages required to implement a complete, end-to-end wireless charging system. This is achieved thanks to a topology that merges multiple stages into a multi-functional, single-stage, zero-voltage–switching, zero-current–switching (ZVS-ZCS) wireless power supply.
The two stages of an E2WATT wireless charging system are:
- A transmitter (AC/AC) stage supplied directly from the mains. This stage is modeled as a bridgeless AC converter that is able to drive the coil using an innovative, proprietary half-bridge architecture based on gallium nitride power ICs and two diodes. A design that allows fine control of energy transmitted through the coils guarantees both good voltage regulation and, if necessary, supports power-factor control.
- A single-stage AC/DC receiver half-bridge that requires just two active devices and acts simultaneously as secondary-side rectifier and non-dissipative output regulator.
In the transmitter stage, thanks to a combination of ZVS and near-ZCS conditions and robustness to input and load variations, the switching power losses are close to zero. And because the transmitter stage is not resonant, ZVS operation can be reached for any load condition. As a result, transmitter efficiency remains high over a wide range of output powers.
The result is a system that allows transmission of higher power levels and offers efficiency as good as that of a wired converter. Because of this, the architecture lends itself to deployment in static EV wireless charging designs.
Dynamic EV charging
If static wireless charging is complicated, dynamic wireless charging in which the vehicle is charged while being driven is very complicated.
Despite this, analysts such as Market Research Future have gone so far as to say the dynamic wireless electric charging segment is likely to dominate the market in the future. Whether this will be the case, what is clear is that there is growing interest and investment in this area as governments look to meet net-zero commitments through emissions-free driving.
Dynamic charging works by lining roads with wireless charging coils that charge EVs while they are on the move. The advantages are the obvious economy of time, cars that require smaller batteries, and the removal of wires. In addition, If the wireless power is bidirectional, the wireless link can supply the car (as it accelerates) or transfer energy back to the grid (as it brakes), further reducing the need for large batteries.
Many countries are conducting tests of dynamic wireless charging schemes. For example, in 2021, Sweden successfully tested the wireless charging of a fully electric long-haul truck, and in Italy, Stellantis installed dynamic wireless power transfer (DWPT) technology at a closed-circuit built near Italy’s A35 autostrada. Called Arena del Futuro (“Arena of the Future”), this road strip is a 1,050-meter track that has been powered with a 1-MW DWPT system, and a Fiat 500 Electric and an Iveco E-Way bus will be used to test the strip’s inductive charging capacity.
As well as inductive solutions, it is worth considering capacitive charging solutions that have the possibility of providing the positional freedom needed to optimize the charging capability of dynamic wireless charging schemes.
Capacitive systems have potential advantages over inductive systems because of the relatively directional nature of electric fields, which reduces the need for electromagnetic field shielding. Also, because capacitive wireless power transfer (WPT) systems do not use ferrites, they can be operated at higher frequencies, allowing them to be smaller, thinner, and less expensive. Capacitive WPT could thus make dynamic EV charging a reality.
However, because of the very small capacitance between the road and vehicle plates, effective power transfer can occur only at relatively high frequencies in comparison with inductive wireless power, making the design of these systems extremely challenging. With the recent availability of wide-bandgap (GaN and silicon carbide) power semiconductor devices that enable higher-frequency operation, high-power medium-range capacitive WPT systems are becoming viable (Regensburger et al. 2017; Zhang et al. 2016).
Eggtronic has been prototyping a new position-free capacitive technology that could address these challenges by ensuring both zero-voltage and zero-current operations to dramatically reduce the dynamic losses in power semiconductors (one of the key issues as working frequencies increase). Based on a thin transmission layer made of several low-cost capacitive pads that work independently of each other and that are coupled with thin receiving pads embedded into the system being charged, this system has the possibility of transferring thousands of watts with a low cost per square meter. An innovative control algorithm, based on changing dynamically the number of transmission pads activated, means the natural frequency of the system is tuned continuously to control the power transferred to the vehicle working always at fixed frequency. In comparison with traditional inductive wireless, this is another advantage of capacitive wireless power technology.
Charging distance
Most of the wireless charging systems today are based on multiple coils (for increasing position freedom and improving coupling) and dynamic tuning (based on extra reactive components).
By contrast, there are some patents and studies based on reducing the distance between the transmitter and the receiver with a mechanism that brings the transmitter and the receiver closer together during the transmission of power.
If this can be done automatically, it will bring benefits both for static and dynamic wireless power in terms of efficiency, charging speed, and cost. It also reduces the possibility of poor EMI performance and eliminates the need to sense whether foreign objects are stuck between the receiver and the transmitter.
The main blocking point for this approach is the need for an integrated mechanical, electronic (and, in the case of dynamic wireless power, also aerodynamic) co-design.
Standards
As well as addressing the technical challenges of static and dynamic wireless EV charging, it is important that systems are harmonized to ensure the true interoperability needed to accelerate the technology’s rollout. Indeed, the lack of one universal standard remains one of the challenges of existing wired charging infrastructures, and it is the reason that the Society of Automotive Engineers (SAE) has created the first global standard for wireless electric car charging.
Known as SAE J2954, this standard applies to charging systems up to 11 kW and fulfills requirements for autonomous cars to charge themselves without human interaction. This standard is a starting point, as the wireless power solutions for electric and plug-in hybrid vehicles are still at too early a stage to crystallize the rules. As we move forward, constant updates will be needed to deliver the requisite compatibility without becoming a bottleneck for performance and functionality.
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