Overview about Wireless Charging of Electrified Vehicles – basic principles and challenges
By, Dr. Joachim G. Taiber
Republished with permission from Emobility Tec –March 2014 Edition
There are many hurdles that are related to the introduction of full electric vehicles – first of all the limited range which is due to the limited energy density of existing battery chemistries inferior to gasoline or diesel-powered vehicles. But if we assume a practical radius of action within the available range, the vehicle has to be recharged in a useful timeframe in order to stay operational. Whereas conventionally powered vehicles can be fully refueled within a few minutes with electric vehicles the electric energy needs to be transferred via cable in a process that may take hours.
Although the energy transfer process can be done in a safe manner for human beings due to standardization of the electric energy transfer, there are significant limitations associated with conductive charging with respect to parallelization and flexibility. In analogy we could imagine how difficult it can be to find electric plugs at airports in order to charge mobile devices. Either the plugs cannot be found at desired locations or they are occupied. Limitations with respect to reachability and availability of the charging infrastructure are the largest inhibitor of a faster implementation of a transportation system based on electrification.
By introduction of wireless charging substantial benefits can be achieved with respect to user interaction, availability, reachability and automation compared to wired charging.
First of all, we would like to explain the physics of wireless charging. We need two electromagnets (coils) that are positioned in a certain distance to each other. The road-side electromagnet is called primary coil and the vehicle-side electromagnet is called secondary coil. If current is flowing through the primary coil, a magnetic field is created which will cause via the principle of Faraday Induction a flow of current in the secondary coil. The efficiency of induction based energy transfer is going down quickly with increasing distance between the coils. But the efficiency can be substantially increased if the frequencies of the electromagnetic fields of the primary and secondary coil can be brought into resonance, i.e. if the stimulating frequency and the Eigen frequency are identical. This condition is called inductive resonant energy transfer. In order to further optimize the efficiency of the transferred energy with achievement of the resonance frequency, parameters such as the coupling factor between the coils (depends on coil geometry and coil distance), coil inductivity and quality factors of the electromagnetic resonance circuits need to be considered and adapted accordingly.
Wireless charging systems following the principle of inductive resonant energy transfer can achieve the best energy transfer rates and efficiency rates with increased coil distances, reduced electromagnetic inference risks and more compact geometrical dimensions in the (lower) kHz frequency band.
• As shown in Fig. 1 the most important components of a wireless charging system are:
• Utility interface
• Inverter and controller (off-board)
• Coupled coils
• Power electronics (on-board)
• Communication interface between road-side and vehicle side radios
|Figure 1: Components of a wireless charging system (Quelle: CU-ICAR- Clemson University International Center for Automotive Research, http://www.cuicar.com)|
The functional principle is as follows:
As soon as the vehicle is identified and localized from the road-side controller the energy transfer from the grid to the vehicle will be synchronized by the inverter. The AC current that flows in the road-side primary in resonance frequency induces AC in the vehicle-side secondary. The AC is either on the vehicle side converted to DC and directed to the Lithium-Ion battery or is directly utilized to drive the electric motor.
We distinguish the following charging categories: stationary, quasi-dynamic and dynamic. With stationary charging (see fig. 2) the electric energy is transferred to a parked vehicle (typically without passengers on board). It is important to keep the geometrical alignment of primary and secondary within certain tolerance values in order to ensure a sufficient efficiency rate of the energy transfer.
|Figure 2: Principle of stationary wireless charging (source: Oakridge National Lab)|
With quasi-dynamic wireless charging the energy is transferred from the road-side primary coil system of limited length to the secondary coil of a slowly moving or in stop-and-go mode moving vehicle (with passengers).
With dynamic wireless charging (see fig. 3) the energy is transferred via a special driving lane equipped with a primary coil system at a high power level to a secondary coil of a vehicle moving with medium to high velocity.
|Figure 3: Principle of dynamic wireless charging (source: Oakridge National Lab)|
It is interesting that driver assistance systems can play a role in combination with wireless charging. For example with stationary wireless charging a system could be developed where the vehicle is parked automatically and at the same time primary and secondary coils are brought into perfect alignment. With quasi-dynamic and dynamic charging the vehicle speed as well horizontal and vertical alignment could be automatically adapted via the driver assistant systems in order to optimize the efficiency rate of the energy transfer and to synchronize the energy transfer via the coil systems as needed and adjusted to the charging infrastructure, The communication system plays a central role to exchange standardized control commands in real-time between the grid ad the vehicle control systems. In particular from a safety perspective vehicle in neighbor lanes and other vehicle users of the primary coil system of the charging lane need to be monitored in real-time.
From a perspective of safety for human beings the shielding of electromagnetic fields in the active charging zone between primary and secondary needs to be considered with very high priority. Compliance to thresholds values defined by international standards need to be achieved.
A specific challenge for automotive OEM’s and suppliers are the geometrical dimensions of the vehicle-side charging system as it needs to be integrated into an existing vehicle platform package. Only future vehicle generations can be designed to fully consider wireless charging options.
Furthermore with the selection of the power frequency spectrum for resonant inductive energy transfer dependencies such as interference risks with other vehicle systems but also resulting geometrical dimensions and systems costs need to be considered.
In particular with high energy transfer power levels the wireless charging system needs to be cooled which comes with additional cost, weight and need for space.
The development of wireless charging systems for vehicle is currently at the beginning of its market cycle and the focus is on unidirectional stationary charging of electrified vehicles (full electric and Plug-In Hybrid). The SAE committee in charge for wireless charging (J2954) recommends for light duty vehicles the following power levels: 3.7 kW and 7.7 kW, a power level for fast charging in the amount of 22 kW is currently in discussion. As permitted operational frequency band SAE recommends 81.38-90.00 kHz.
The fundamental motivation for vehicle users to operate wireless charging systems is the convenience and the capability to automate the charging process. In the longer term, scenarios can be envisioned where quasi-dynamic and dynamic charging will enable a substantial range extension for zero-emission driving. This requires investments in infrastructure which could be amortized via telematics-based road charging systems. For the automotive OEM this could result in cost savings for energy storage solutions and related weight reductions and energy efficiency gains.
In the future of automotive zero-emission driving – in particular in urban high density areas – as well as automated driving will play a more important role. The internet of things will be highly relevant for transportation and wireless charging can become very relevant in the vehicle-to-infrastructure connectivity context. IEEE established as part of its transportation electrification initiative an expert group of industry and academic partners (EVWPT – electric vehicle wireless power transfer) where future scenarios – in particular for dynamic wireless charging – and recommendations for future standardization aspects are being developed.
Prior to coming to Clemson in 2010, Dr. Joachim Taiber was heading the Information Technology Research Office at the BMW ITRC (Information Technology Research Center) at CUICAR (Clemson University International Center of Automotive Research). He designed and implemented an open innovation business model with leading information & communication technology companies as a first of its kind in the automotive industry at this center. Since the ITRC started its operation in 2005 he completed more than 50 research projects in close cooperation with universities, especially in cooperation with faculty members from ECE, School of Computing and Mechanical Engineering at Clemson University. He also created a 4G wireless communication infrastructure at a high-speed track which is considered the first of its kind in the automotive industry.