Wireless Power Transfer (WPT) technology can accelerate EV adoption
To reach global climate goals, it’s a crucial factor to make mobility more sustainable. Electric vehicles (EVs) can make a significant contribution to reducing emissions coming from the transportation sector. The fast growth of the EV market brings new challenges for many sectors, among them the charging infrastructure. A completely new approach is needed to efficiently charge growing EV fleets, as the technology is vastly different and there is no fast solution similar to internal combustion engine (ICE) refuelling. Charging is a critical aspect of EV acceptance, and Wireless Power Transfer (WPT) technology can accelerate EV adoption by providing a safe, efficient, and convenient charging solution that helps increase public acceptance.
Figure 1 – Car driving on a charging pad installed on the ground.
An old technology resurfaces in an efficient way
The principle of wireless power transfer for EV charging (inductive coupling) is based on electromagnetic coupling and magnetic resonance and was first studied by the famous scientist Nikola Tesla some 130 years ago.
Before that, other famous scientists such as Oersted, Ampere, Faraday and Maxwell discovered and described how a magnetic field is created and behaves around an electric current passing through a wire. Ampere’s law explains how a magnetic field perpendicular to the current is distributed when an alternating current flows through a closed circular loop. The resulting magnetic flux generated by this loop (or transmitter coil) then depends on the frequency, current density, and area of the closed loop. An example is shown in Figure 2.
Figure 2 – The principle of magnetic resonant coupling
When a second (receiving) coil is placed in the vicinity of this alternating magnetic field, the principle of electromagnetic induction (Faraday’s law of induction) causes an electric current to flow in the wire of the receiving coil. This allows electrical energy to be transferred through a magnetic field without physical contact. The quality of the coils’ magnetic coupling depends on the receiver coil’s relative position, as shown in Figure 2. The coupling factor describes how much of the total flux is captured by the receiver and is, therefore a measure of coupling quality, where k=1 is maximum coupling.
The use of WPT for EV charging inevitably results in a high air gap between the primary and secondary coils and therefore low magnetic coupling. However, if the coils are tuned with capacitors to form two resonant circuits at the same frequency (see Figure 2), the power transfer capability, first discovered by Nikola Tesla, can be significantly increased. In summary, the principle of resonant coupling allows very high efficiencies (comparable to conductive charging) even at very low magnetic couplings.
The lack of litz-wire, suitable power electronics and efficient switches made this technology impractical at Tesla’s time. With the recent development of high frequency switching devices, it is possible to supply high frequency energy to the transmitter coil and rectify the induced current to charge the battery with direct current without much loss.
State of the Art
Much research and development has been done on this technology over the last 20-30 years. With recently published standards, such as SAE (J2954) and IEC (61980), wireless charging up to 11kW is regulated and ready for implementation and mass market distribution.
Before the technology is ready to move from prototypes and individual deployments to cost-effective solutions for global markets, several technical issues need to be addressed. Siemens has therefore teamed up with WiTricity and MAHLE to work together on these issues.
Interoperability and Standardization
The published standards include many definitions such as power (WPT1-3, 3.7 – 11 kW) and different vehicle to ground distances (Z1-Z3, 100 mm – 250 mm), interoperability and efficiency requirements, and address safety issues such as leakage field emission (ICNIRP, CISPR), foreign object detection (FOD), living object protection (LOP) and much more. These aspects all need to be considered in the design of a WPT system and result in many existing, mostly proprietary, systems that can meet these requirements.
However, some white spots have been identified in the standards that need to be addressed for the technology to be successful.
As an infrastructure provider, Siemens is motivated to provide a truly interoperable solution. The public charging infrastructure needs to support both low power (3.7 kW) and high power (11 kW and potentially more) systems. Interoperability must also be ensured over a range of air gaps and positional parking offsets for each EV. This results in a range of magnetic coupling factors that need to be covered by the charging station, adding complexity to the design process. Together with its partners, Siemens engages in continuous interoperability tests to ensure maximum cross-functionality.
In addition, to ensure efficient and fast wireless power transfer, the secondary coil must be placed as close as possible to the primary side. This requires good and reliable alignment and positioning technology, which is currently lacking in standards. Two existing technologies – LF (low frequency or also called magnetic vectoring) and UWB (ultra-wideband, chip-based solution) – are currently on the market and need to be evaluated in terms of their performance with respect to requirements such as accuracy over short and long distances, interoperability, interference, and sensitivity to harsh weather conditions (e.g., snow).
Siemens and its partners are not only working on solutions for the car positioning, but they are also involved in standardization groups like DKE and SAE to drive a common solution forward.
Moreover, other aspects of WPT systems, such as communication from the charging station to the vehicle, need to be properly addressed in existing standards (e.g., ISO 15118-20). Requirements for transmitting parameters, response times, or open/closed-loop operation need to be clarified and included in the standard to enable fully interoperable WPT systems.
Bidirectional Power Transfer
In addition to convenience and safety, WPT for EV charging can be an enabler for autonomous driving. Future visions like automated parking require a fully automated charging process, which is impossible with conductive charging since a person is always needed to connect the charging cable.
An automated charging process can also contribute to grid stability in Vehicle2Grid (V2G) solutions. Therefore, it is important to also focus on bidirectional power transfer when developing and creating wireless charging solutions.
The magnetic coils are naturally capable of transferring power in both directions, and the power electronics need to be adapted accordingly for bidirectional power transfer with topologies such as active rectifiers on the secondary side. Here, again, standardization of definitions for bidirectional power transfer is lacking, as well as the communication aspects mentioned above. Existing standards (e.g., ISO 61980-2) must be extended to allow bidirectional WPT system communication.
Summary
The technology of WPT for EV charging is very advanced and represents a sustainable solution and accelerator for EV adoption in the future. To be successful, remaining gaps such as bidirectional power flow, communication, and positioning technology, need to be addressed and standardized.
The alliance of Siemens, WiTricity and MAHLE is working on efficient, interoperable, and standards-based bidirectional solutions. Moreover, they are involved in standardization processes to drive the technology towards the future.
With this, Siemens underlines its engagement in innovative technologies.
Thomas Wuerz, Head of Wireless Charging, Siemens Smart Infrastructure eMobility
Denis Kraus, Simulation Engineer Wireless Charging, Siemens Smart Infrastructure eMobility
