Emanuel G. Marques, Valter S. Costa, André M. S. Mendes, Marina S. Perdigão
©SHUTTERSTOCK.COM/ROSESTUDIO
Electric vehicle (EV) technology has proven to be a propulsion technology of the future, but urgently needs to address challenges, such as lower-priced, reasonably sized EVs for higher market penetration, higher lifecycle efficiency, and increased power density. Range extension, in particular in urban scenarios, is critical. Inductive power transfer (IPT) technology simultaneously solves the electric hazard risks of conventional power cord battery chargers, but especially EV limited autonomy and related anxiety and even security. The current main focus of IPT research has shifted toward challenges imposed by the dynamic operation mode, with cost-effective analysis of the best dynamic configuration and optimization of the transmitter pads, leakage flux control techniques, or even the state-of-the-art solution with compact in-Wheel approaches. The presented analysis, from past to future trends in this field of expertise, reveals the amount of effort that has been put forward to guarantee a mature transition to a greener and more sustainable future in electric transportation.
EVs have clear advantages over internal combustion engine vehicles, like reduced noise, full torque capability of the motor from a standstill position, and smaller carbon footprint. However, they are still limited in range by the battery’s storage capacity. The current state of EV lithium–ion battery technology has a specific energy (energy per unit mass) that places them behind that of gasoline by a factor of almost 100. Manufacturers work around this limitation by using larger battery packs, a costlier and bulkier solution to a limitation that has followed EVs since their first appearance. Also, lithium–ion batteries can take up to several hours to charge, which will undoubtedly affect the driving habits of users. Table 1 shows some battery EVs (BEVs) and EV truck models and their main battery-storage capacities. On average, a BEV consumes 185 Wh/km, whereas EV trucks require 1,250 Wh to travel the same distance. These values, together with the battery capacity, define the EVs range.
Table 1 Battery specifications of different BEVs.
All current hybrid EV (HEV) and BEV models have built-in sockets that can charge the battery pack from a few kilowatts, in domestic chargers, up to 250 kW in supercharging stations. The vehicles can be charged from ac or dc power supply with different voltage and current values. The ac chargers are mainly found in domestic and public locations like malls and parking lots; dc charging systems require a dedicated infrastructure and they are usually mounted at public charging stations. The dc chargers automatically adjust the voltage ratings in accordance with the battery packs. Additionally, they are faster than ac chargers since they bypass the vehicle onboard battery charger and charge the battery pack directly. The supercharger of Tesla is one example where this strategy is adopted, allowing a maximum charging power of 250 kW.
The high power rating superchargers require large power demands from the grid simultaneously. This scenario forces a restructuring of the electric power lines to accommodate several chargers. Unfortunately, the increase of EVs and the limited number of chargers will create long queues in the charging areas. Additionally, risk of shock hazard and electrocution also cause concern in current chargers due to the need of human intervention in the charging process.
Wireless power transfer (WPT) technology enables the energy transfer between two systems without any contact. The concept dates from the late 19th century, when Prof. Heinrich Hertz demonstrated for the first time electromagnetic wave propagation in free space using a spark to generate high-frequency power and to detect it in the receiving end. Nikola Tesla, in 1899, conducted a series of experiments in Colorado Springs when he devised the best approach for WPT and marked the beginning of WPT technology [1].
IPT is a near-field technology that enables power transfer using a varying magnetic field and a prominent replacement to plug-in chargers. The power transfer is accomplished by a transmitter coil that generates a variable magnetic field, which induces a voltage across the receiver coil, according to Faraday’s Law. IPT systems gain popularity and its applicability to EVs has been investigated by the scientific community since the early 21st century. Since then, several works have presented efficiencies as high as 97% for hundreds of kilowatts and have shown them to be equiparable to conductive chargers [1], [2]. This article discusses the main research areas of study in IPT systems, including past trends of research. Then, recent developments are reviewed along with future prospects. To conclude, the current research groups and companies are presented.
The main development areas in IPT systems are related to the electrical configuration of the compensation networks and design optimization of the magnetic coupling structure.
An IPT system, in its simplest form, uses two magnetically coupled coils, also known as power pads, to transfer energy between the offboard and onboard sides, as illustrated in left side of Figure 1. The offboard side typically comprises a high-frequency power supply with a power factor correction stage, a resonant compensation network, and the transmitter power pad of a magnetic coupler (MC). The onboard side includes the receiver power pad of the MC, a resonant compensation network, the onboard converter, and battery pack. A distinctive characteristic of EV IPT systems is the spatial freedom of the receiver pad toward the transmitter pad due to the vehicle’s movement. The relative positioning of both pads has a direct impact on the coupling factor and, subsequently, on the system power transfer capabilities. Therefore, high operating frequencies, together with the resonant compensation networks, are employed to increase transfer capabilities, and at the same time minimizing the volt-ampere rating and switching losses of the power supply. The mobility of the receiver pad together with the high-frequency operation creates additional concerns when compared with plug-in chargers, like position detection, stray magnetic fields compliance, and foreign object detection between the transmitter and receiver pads. Figure 1 summarizes the main concerns in both offboard and onboard sides.
Figure 1 Overview of a typical IPT system with its main components. ICNIRP: International Commission on Nonionizing Radiation Protection.
IPT systems are classified according to the vehicle’s movement during the charging process into static IPT (SIPT) and dynamic IPT (DIPT) modes. In the first mode, the vehicle is charged in a fixed position, whereas in the second mode, the vehicle is moving along the roadway. Regardless of the vehicle’s position, four main research areas are found for IPT systems: MCs, resonant configurations, circuit analysis, and controllers and control. Figure 2 illustrates the main research areas and subcategories of IPT systems. The first advancements in EV IPT occurred for the SIPT mode. The charging process with the vehicle in a fixed position eliminates the dynamic effect of a variable coupling, simplifying the analysis. The well-established electrical analysis of resonant converters was extended to IPT systems, where the voltage, current, and transconductance functions are derived from the selected compensation networks. In addition, impedance matching with the switching frequency of the power converters is analyzed to minimize switching losses in the semiconductors. On the other hand, the study of new resonant configurations with optimized control strategies and the optimization of MCs drastically increased over the last decade, with new developments being reported in the literature on a weekly basis. The following subsections detail the main findings in resonant configurations, controllers and control, and MC research areas.
Figure 2 Main research areas of IPT systems. MIMO: multiple input/multiple output; MISO: multiple input/single output.
Early single-coupling IPT systems had limited power transfer capabilities caused by the poor coupling between the transmitter and receiver power pads of the MC coupler. Placing a capacitor in each side, either in series or parallel, cancels the reactive part of the power pads and enhances the power transfer. Four classical resonant configurations can then be derived: series–series (SS), series–parallel (SP), parallel–series (PS), and parallel–parallel (PP) [2]. The positioning of the capacitor changes the intrinsic characteristics of the circuit and their response to coupling, load, and frequency variations. The SS configuration offers output current independence of load and resonant frequency, and it is preferable for high-power applications. PS and PP configurations, on the other hand, limit the input current in the event of total absence of the vehicle [2].
Hybrid configurations use multiple reactive components to form high-order resonant configurations like the LCL–LCL, LCL–LCC, or S–SP [3]. These configurations provide additional advantages when compared with the classical configurations, like extended voltage or current source characteristics under specific conditions. As an example, the LCL configuration in the offboard side ensures a constant current in the transmitter power pad independently of load and coupling variations. The S–SP configuration offers mutual coupling and load-independent voltage gain. In addition, it can realize good output voltage stability and low circulating losses under the condition of wide parameter variations.
Both classical and hybrid configurations still limit the maximum power transfer capabilities during charging scenarios, where the vehicle has large ground clearances and is laterally displaced from the transmitted power pad. The low mutual coupling value enforces higher current values in the offboard side. Moreover, high current values translate to higher magnetic fields, which may lead to adverse problems in the human body. Intermediate coil (IC) systems, also known as multicoil resonators or relay coil IPT systems, place resonators between the transmitter and receiver pads to enhance the magnetic link. The resonators are formed by a magnetic-coupled coil and a capacitor, usually tuned at a higher frequency than the operating frequency.
Several studies demonstrate the benefits of ICs over single coupling IPT systems in terms of efficiency, less component stress, and better flux leakage control. They are placed in proximity or even in a coplanar fashion with the transmitter coil. In this configuration, ICs often replace the ferromagnetic core in the transmitter pad, making them ideal for dynamic applications. The inclusion of an IC with a classic or hybrid resonant configuration modifies its intrinsic characteristics altogether. As an example, the SS configuration with an IC exhibits both load-independent voltage and current source characteristics in different operating frequencies, while the transmitter current is shifted to the intermediate circuit, thus reducing the switching losses [3]. Recent works use several ICs to increase robustness of EV IPT systems against the unavoidable lateral displacements. This means that the total number of admissible configurations increases drastically each time a new IC or a variant of a hybrid resonant configuration is proposed, meaning that multiple resonant configurations can satisfy a specific EV IPT application. Therefore, the selection of the most adequate resonant configuration can be decided in the smallest detail, like voltage stress across the capacitors, or even the configuration with the minimum number of components.
The electronic converts in IPT systems are highly dependent on the resonant topologies and the magnetic properties of MCs. The employment of silicon carbide MOSFETs allows operation frequencies in the range of 80.12–90 kHz, according to the SAE J2954 guideline, and power levels in the hundreds of kilowatts. Simple unidirectional IPT configurations employ an H-bridge inverter on the offboard side and an H-bridge diode rectifier with a dc–dc converter on the onboard side, as illustrated in the common topologies in the center of Figure 1. Bidirectional configurations employ an H-bridge or matrix converters on both sides with synchronous or independent control strategies. Figure 3 depicts a common bidirectional dual active bridge configuration with a wireless link to synchronize the control in both converters [5]. Alternatively, estimation algorithms with an additional sensor coil could be employed to replace the wireless communication. The phase-shift control is often used to regulate the output voltage of the converters and, at the same time, regulate the current and/or voltage at the battery terminals.
Figure 3 Overview of a dual active bridge control scheme for IPT systems.
MCs are considered the key element in IPT systems as they are responsible for the energy transfer without physical contacts using a variable magnetic field. The MC resembles a conventional 50-Hz transformer with a transmitter and receiver pads. The relative positioning of the receiver pad, however, varies due to the vehicle’s movement and ground clearance, and it has a direct impact on the coupling factor. The MC coupling typically ranges between .05 and .3, and so, MCs are also referred to in the literature as loosely coupled transformers. The degrees of freedom that the receiver pad may be subject to are [6]:
Figure 4 illustrates the different degrees of freedom in different perspectives. The vertical displacement and tilt degrees of freedom depend on the vehicle’s type (Sedan, SUV, etc.) and, in normal operation, these values remain approximately constant. The lateral displacement and rotation degrees of freedom, on the other hand, depend on the driver’s ability to park the vehicle or drive it in a straight line. The guideline SAE J2954 suggests that a minimum lateral tolerance of ±150 mm is sufficient for an average driver to drive/park the vehicle correctly. Extreme charging positions with large vertical and lateral displacements reduce the coupling factor of the MC and increase the leakage magnetic fields. Over the years, many researchers have addressed these issues by proposing new coil and core arrangements with better materials and shield techniques [7], as well as a variable inductor in the resonant circuit for misalignment compensation or booster in the IC.
Figure 4 DIPT system, top view (left side) and lateral cross-section view (right side). BPP: bipolar pad.
MCs commonly have a unitary size ratio between the transmitter and receiver pads, especially in SIPT [8]. The transmitter pad configuration of DIPT systems, on the other hand, can either be formed with an elongated track shape or a set of smaller transmitter pads [9], [10]. The first option offers a continuous power transfer with a constant coupling factor. The large inductance of the track, however, reduces the overall coupling factor and requires higher voltage levels to drive the necessary transmitter current. Additionally, compliance with the International Commission on Nonionizing Radiation Protection (ICNIRP) guidelines to human exposure leakage magnetic field levels is only achieved at wider distances. The second option places several discrete transmitter pads along the roadway where the power transfer to the vehicle occurs, as illustrated in Figure 4. Since both approaches exhibit merits, there is still an open discussion on the appropriate solution for EV charging applications. However, the segmented solution is gaining terrain.
The impact of the EV movement in the mutual inductance (MTxiRx) profile between the transmitter and receiver pads in DIPT systems is visible in Figure 4. The bell-shaped pattern of MTxiRx is caused by the lateral displacements between both pads. This behavior occurs for both SIPT and DIPT systems and the difference resides in the range variation of MTxiRx. For SIPT systems, MTxiRx varies between a perfectly aligned charging position, which corresponds to the peak of the bell-shape curve, and a minimum MTxiRx in the worst charging position in terms of vertical and lateral displacements. This minimum value also ensures that the rated characteristics of the overall system are not exceeded. On the other hand, DIPT systems exhibit the same maximum value as static systems, but the minimum value is zero. This no-coupling charging position MTxiRx = 0H occurs right before the receiver pad enters the first transmitter pad (first point of the red curve in Figure 4) and soon after exiting the last transmitter (last point of the green curve in Figure 4). Therefore, DIPT controllers must cope with no-coupling scenarios and ensure that the limits for a safe operation are not exceeded.
A power pad, in its basic form, is formed by a single coil, a ferromagnetic core, and shield. The main flux pattern depends on the coil placement in the power pad. The middle section of Figure 1 illustrates two distinctive patterns using different coil placements. The first scenario creates a perpendicular flux to the pad surface, whereas the second scenario creates a flux component parallel to the pad surface. Over the years, several geometries with different flux patterns have been presented in the literature, as illustrated in Figure 5. The Auckland research group in New Zealand optimized the circular pad (CP) geometry back in 2009 by fracturing a ferromagnetic disk into several ferrite bars. The CP, however, is only compatible with perpendicular flux geometries. To overcome this limitation, the same research group proposed several geometry alternatives, including the solenoid pad in 2010, and the double-D pad (DDP) and bipolar pad (BPP) in 2011. The new geometries use two coils to increase the lateral displacements tolerance by creating a parallel flux pattern to the transmitter pad, as illustrated in Figure 5. These new geometries are compatible with geometries with both perpendicular and parallel flux patterns, and are commonly designated in the literature as interoperable geometries. The BPP and DDP have a similar construction and coupling profiles, but the BPP uses less material.
Figure 5 Chronological evolution of MC geometries. BPP: bipolar pad; CFLP: concrete ferrite-less pad; CP: circular pad; DDP: double-D pad; FLCP: ferrite-less circular pad.
In 2014. the Korea Advanced Institute of Technology (KAIST) introduced the asymmetric coils geometry for stationary charging. The proposed geometry uses different transmitter and receiver pad size ratios to achieve large lateral displacement tolerances. The same group focused on DIPT applications applied to buses using elongated transmitter pads. The ultraslim S-type geometry, introduced in 2015, uses S-shape ferromagnetic cores along the roadway to channel the magnetic fields of an elongated wire. In the same year, the research group from the Swiss Federal Institute of Technology (ETH) Zurich optimized the rectangular pad (RP) geometry with a stripped ferromagnetic core. The optimized RP showed equivalent performance when compared with the DDP in terms of vertical and lateral displacements for output powers up to 50 kW.
One major concern in DIPT systems is the cost of manufacturing the transmitter pads. In 2015 the Auckland research group proposed a concrete ferrite-less pad (CFLP) geometry that uses a “pipe” coil instead of a ferromagnetic core to channel the flux in the backside of double-D coils. In 2017, a research group from University of Coimbra in Portugal proposed a variant of the CP without the ferromagnetic core. The geometry, referred to as a Ferrite-less CP (FLCP), uses a cone-shaped coil to channel the magnetic flux in the backside of the transmitter coil. The coupling profile shows only a reduction around 15% when compared with the CP in both vertical and lateral displacements. Newer ferrite-less are become an attractive solution to face the increase of ferromagnetic material costs [11].
Figure 5 summarizes chronologically the geometries as they appear in the literature. The greater number of contributions was made between 2010 and 2015. Since then, the number of new proposed geometries has reduced substantially, and the well-established geometries are now an object of optimization in terms of leakage magnetic flux control and size versus power transfer capabilities with large lateral tolerances.
The advancements in IPT systems made in the last years have paved the way to commercially available SIPT solutions. Their applicability ranges from simple parking lot IPT chargers for standard EVs to high-power (≈200 kW) solutions in transportation sectors, like city buses. Now, there are still many challenges to transit from SIPT into commercial DIPT systems.
DIPT presents additional challenges relative to SIPT, among them, the choice of pad topology (elongated or segmented), vehicle displacement due to inherent vehicle movement, air gap and lateral displacements, and EV speed. Figure 6 presents past, current, and future developments regarding DIPT. When DIPT applications first appeared, long track coils were used, where a single transmitter with a length greater than the receiver on board the EV is used, typically around 10- to 100-m long, enabling charging multiple vehicles simultaneously. Optimized core designs, like the ultraslim S, exhibit low leakage electromagnetic field (EMF) and large tolerances to displacements. Nonetheless, long track coils lead to redundant unwanted leakage EMF, since the track is still driven by a current during the vehicle’s absence. Alternatively, the long track coil can be divided into multiple subtracks with sizes that vary between 1 and 3 m [12], [13]. The smaller subtracks are also designated as segmented pads with geometries similar to the MCs of SIPT. The supply configurations and the benefits of elongated versus segmented pads are a focus of research. In the segmented configuration, each pad can be controller individually using a switch box or by the offboard inverter, as illustrated in Figure 7.
Figure 6 Dynamic IPT research areas and future trends.
Figure 7 Different elongated and discrete dynamic IPT configurations.
Other configurations use intermediary coupler circuits (ICC) between an elongated track and multiple subtracks to change the operating frequency between both circuits while, at the same time, store energy to balance the power demand from the grid. Each configuration illustrated in Figure 7 has merits in terms of construction savings, maintenance, and fault tolerance, but the best configuration for wide adoption is still the subject of investigations.
Among the future trends in DIPT, different research topics are the aim of research: road infrastructure, i.e., road adaptation for wireless charging implementation and the use of magnetizable concrete technology to boost the system coupling factor; EV detection systems to know the EV position and activate/deactivate different segmented transmitter pads using optic and magnetic sensors or through mapping methodologies and artificial intelligence models; EMF shielding solutions, with the use of intermediate coils, for example, to shield leakage flux; and durability and cost of the DIPT infrastructure.
One of the near-future trends is an interconnected infrastructure with all vehicles in an Internet of Things (IoT). New advancements are disclosed every day in the area of telecommunications and autonomous driving, but IoT holds the key for the optimization and storage of renewable energy. EVs will be the largest energy storage units interconnected and available around the globe. The development of power-demand algorithms together with IoT would provide almost unlimited power with grid equalization, even in the most remote areas.
One milestone of IPT technology is its applicability into heavy-duty vehicles like trucks or off-road vehicles. Unfortunately, these vehicles have typical ground clearances between 350 and 550 mm. This limitation requires high driving current on the offboard side to transfer significant amounts of power. However, the high-leakage magnetic fields pose significant concern to the human body. In 2021 a research group from the University of Coimbra proposed a double coupling in-Wheel Inductive Power Transfer (WIPT) system, illustrated in Figure 8. The energy transfer from the offboard side to the onboard side occurs via the wheel using two consecutive MCs without any physical contacts. The proposed solution avoids the need of slip-rings to transfer the energy between the wheel and the onboard side. In addition, the aluminum rim shields the leakage flux lines above the receiver coils and avoids the use of additional shielding materials [14].
Figure 8 Double coupling inWIPT.
In-Wheel IPT systems just follow the tendency of moving the powertrain and batteries charger from the vehicle into the wheels, leaving only the battery itself within the vehicle [15]. The development of new airless tire designs with sustainable and nonmagnetic materials (glass fiber and resins), like the Uptis model from Michelin, strengthen the viability of in-WIPT systems. These new airless tire designs also eliminate the risk of pressure increase in the tire caused by the Joule losses of the coils placed within the tire. This solution, however, presents new challenges, among them the sizing of both MCs, since traditional IPT approaches do not take into account the curvature of the coils. Moreover, the rotational effect requires additional research regarding coil support, temperature dissipation, and power flow regulation in both static and dynamic modes.
IPT technology will be a billion-dollar market in the following years and many companies and research groups are keen in developing solutions for both static and dynamic applications. Table 2 lists some research groups and companies working in IPT solutions for EVs. Witricity is a Massachusetts Institute of Technology-based company composed by scientists that demonstrated nonradiative energy transfer back in 2007. Witricity acquired Qualcomm Halo, a spin-off company from Auckland University in New Zealand, and holds the majority of intellectual property of IPT technology. The ENRX group has solutions for heavy-duty vehicles in both static and dynamic conditions up to 180 kW. WAVE by Ideanomics ensures an up time above 95% in transportation buses with their 250-kW IPT systems. The Swiss-based Brusa company offers an 11-kW static charger, whereas Wiferion was recently acquired by Tesla to incorporate IPT technology into their fleet.
Table 2 Commercial and research groups using IPT technology.
EV technology has proven to be a propulsion technology of the future, but urgently needs to address challenges, such as lower-priced, reasonably sized EVs for higher market penetration, higher lifecycle efficiency, and increased power density. Nonetheless, this will not be enough if the issue related to reluctance to EV adoption due to the lack of charging stations, despite number increase, or especially limited range is not solved. What is more, gender is one of the different socio-demographic factors (level of education, income, age, mobility pattern, etc.) pushing toward EV adoption. Women seem to prefer the benefits of an EV (environmental impact, fuel economy, ease of operation) but still rank lower on their potential EV interest, despite being the important users in an urban driving scenario. Recent accounts suggest that EV charging operators need to prioritize women’s safety to not lose or miss potential users. Lack of lighting and the positioning of public EV chargers in more isolated places do not contribute to the notion of security deemed so important by women. Therefore, range extension, in particular in urban scenarios, is critical. IPT technology solves simultaneously the electric hazard risks of conventional power cord battery chargers, but especially EV-limited autonomy and related anxiety and even security. The current main focus of IPT research has shifted toward challenges imposed by the dynamic operation mode, with cost-effective analysis of the best dynamic configuration and optimization of the transmitter pads, leakage flux control techniques, or even the state-of-the-art solution with compact in-Wheel approaches. The presented analysis, from past to future trends in this field of expertise, reveals the amount of effort that has been put forward to guarantee a mature transition to a greener and more sustainable future in electric transportation.
This work was supported by Ph.D. scholarship SFRH/BD/138841/2018 and Instituto de Telecomunicações Project UIDB/50008/2020 and Project UIDP/50008/2020. In addition, this work was also supported by Project inWheel Inductive Power Charging for Sustainable Mobility (2022.06192.PTDC). All projects are funded by the Portuguese Foundation for Science and Technology (Fundação para a Ciência e a Tecnologia). The authors thank Alex Gruzen and Eric Cohen of Witricity for providing specification details of their commercial products.
Emanuel G. Marques (egmarques@co.it.pt) is a researcher with the Instituto de Telecomunicações, University of Coimbra, 3030-290 Coimbra, Portugal. He received his Ph.D. degree in electrical engineering from the University of Coimbra. His research interests include magnetic elements modeling, resonant converter applications, and inductive power transfer systems applied to electric vehicle charging applications.
Valter S. Costa (valter.costa@co.it.pt) has been working toward a Ph.D. degree in electrical engineering with the Instituto de Telecomunicações, University of Coimbra, 3030-290 Coimbra, Portugal. His research focuses on dynamic inductive power transfer systems for electric vehicle battery charging applications. He is a Student Member of IEEE.
André M. S. Mendes (amsmendes@ieee.org) is an associate professor and the director of the Power Electronics Laboratory in the Department of Electrical and Computer Engineering, University of Coimbra, 3030-290 Coimbra, Portugal, and a researcher with Instituto de Telecomunicações, 3030-290 Coimbra, Portugal. His research interests include fault diagnosis and fault tolerance in electric drives and power electronic converters, wireless power transfer for electric vehicles. He is a Member of IEEE.
Marina S. Perdigão (perdigao@isec.pt) is an adjunct professor in the Coimbra Institute of Engineering, Polytechnic Institute of Coimbra, 3030-199 Coimbra, Portugal, and a researcher in the Instituto de Telecomunicações, 3030-290 Coimbra, Portugal. Her research interests include high-frequency switching converters, resonant converters, dc–dc converters, and power electronics for renewable energies. She is a Member of IEEE.
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Digital Object Identifier 10.1109/MVT.2023.3318840