Extending renewable energy’s reach

Inductive charging

A related EV feature is wireless charging—either statically when you park your car over a charging area or more interestingly—and futuristically—as you drive along. The technology responsible for wireless EV charging is based on coupled magnetic circuits. A recent article¹ discussed a BMW inductive charging system with a 3.3-kW capability. As stated in the article, “Inductive charging re-energizes an EV’s battery with a magnetic field rather than a wire from car to power source. It’s achieved by fitting a primary coil in a floor-plate over which a car can park and a secondary coil on the underside of the car itself. An alternating magnetic field is generated between the two coils, which creates electricity that is then sent to the BMW’s on-board battery.”

According to the article, the BMW system is claimed to cause less electromagnetic radiation than a kitchen hotplate, and the field strength is well within regulatory limits. Optimum alignment is required and “… a parking assist in the electric BWM will tell drivers where to park.”

The Plugless Level 2 EV charging system from Evatran transfers 3.3 kW via a floor-mounted parking pad and an adapter that is permanently installed under the car. Charging automatically starts when the pad and adapter are aligned and will not occur if metallic objects are close to the parking pad and/or the EV also is plugged in to charge. Rather than the alignment indicator being located in the car as BMW has done, the Evatran control panel is mounted on a wall or an optional pedestal.

A June 2015 Frost & Sullivan report² forecast a 126.6% CAGR for inductive charging between 2012 and 2020. “OEMs such as Renault, Nissan, Daimler, Volvo, BMW, and Toyota are working on the development of inductive charging for future EVs, and more than 10 automakers have announced trial tests,” said Frost & Sullivan Automotive and Transportation Senior Research Analyst Prajyot Sathe. “As a result, inductive charging will soon be available in cars either as an additional feature or as an inbuilt feature.”

The report continued, “While in the short term 3.3 kilowatts inductive charging will be widely accepted to enable residential and semi-public charging, with time, vehicles will tilt toward 6.6 kilowatts to enable faster charging,” said Sathe. “Inductive charging in stationary applications will also be most sought after in the near term, whereas dynamic or on-the-move charging will gain traction post-2020.”

Whether all of the systems being developed can be grouped under the inductive charging term isn’t clear. The biggest problems with simple inductive coupling are the restrictions on the distance between and alignment of the two coils. In contrast, a resonant system greatly relaxes both constraints.

For example, one research paper³ commented, “… it is well documented that there are substantial limitations to an inductive power transfer scheme. The transfer distances are typically below 20 cm.” This paper discussed the practicality of embedding charging coils in a roadway and concluded, “For safety reasons and in order to ensure that the road can still be used for other kinds of vehicles, the source needs to be buried below the pavement. Thus, the transfer distance in the inductive power transfer scheme is, in fact, not sufficient. The lateral tolerance of these schemes is also quite stringent, typically on the order of 10 cm.”

On the other hand, these constraints are easily overcome in a controlled, static situation such as described in the BMW article. Vehicle ground clearance isn’t an issue if the floor-mounted coil can be mounted at the correct height relative to the car’s coil. In the United Kingdom, WrightBus is testing an inductive charging system that boosts the bus battery charge at each end of a 15-mile route.

As described in a BBC News article,⁴ “There, the bus parks over plates buried in the road. The driver then lowers receiver plates on the bottom of the bus to within 4 cm of the road surface, and the bus is charged for around 10 minutes before resuming service. The system uses a process called inductive charging. Electricity passes through wire coils in the road plates, generating a magnetic field. This field induces a voltage across coils in the bus plates, and the vehicle’s batteries are charged.” The article concluded, “The new vehicles … will operate as part of a five-year trial programme led by the European division of Japanese company Mitsui and U.K. engineering group Arup.”

The Wireless Power Consortium has developed the Qi charging standard supported by 232 companies. Having a common standard is very important for consumer products such as smart phones. A lot of the convenience of wireless charging is the freedom to charge just about any phone wherever a wireless charging capability is available. Obviously, this wouldn’t work unless a standard could be agreed upon. For cars, it still makes sense to have a stand­ard—for example, in a parking garage that offered charging. For home use, a standard only would become important if you had several cars or bought a new one of a different brand.

The Wireless Power Consortium uses the inductive charging terminology when they mean transformer coupling—a phone sitting directly on a charging mat with a separation of only a few millimeters between the primary and secondary coils. For larger distances, the consortium uses the term resonant mode operation.

Resonant power transfer

Unlike a transformer, which requires the primary and secondary coils to be very close to each other, a highly resonant wireless power transfer (HR-WPT) system still can achieve good efficiency even with tens of centimeters between primary and secondary coils. Both transformers and HR-WPT systems operate in the near- field region with primary-to-secondary separation much less than a wavelength.

Although wirelessly transferring power has been a persistent research goal for a long time, it was MIT Professor Marin Soljačić’s 2006 work and subsequent 2007 publication of his experimental results that energized product development. WiTricity was formed in 2007 to commercialize the HR-WPT technology he and colleagues had developed.

A recent WiTricity paper⁵ described the advantages this technology might afford to charging of autonomous undersea vehicles (AUVs). The authors speculated, “… one could imagine a fleet of AUVs monitoring an exclusion zone around a submarine or carrier group. There AUVs could survey an area, returning regularly to a ‘mothership’ which they might follow alongside to recharge and transfer collected data.” Toyota has invested in WiTricity, and reference 1 includes a photo of a typical EV or hybrid charging application capable of transferring 3.3 kW.

In a resonant power system, the primary and secondary quality factors (Qs) need to be very high. Basically, Q is equal to energy stored divided by energy lost during one complete oscillating cycle. Some writers describe these kinds of systems as having evanescent fields. By using that term, they mean that very little of the oscillating energy is radiated. Instead, it is maintained between the two resonators: The oscillating field generated by the primary circuit excites the secondary circuit resonance, extracting energy from the primary circuit and transferring it to the load.

In experimental work done at Stanford University, researchers achieved 94% transfer efficiency between two resonators operating at 8.38 MHz and separated by 60 cm.⁶ There were no nearby large metallic objects to influence this result as would be the case when wirelessly charging an EV. To investigate a more practical situation, large aluminum plates were added to the test setup, and the researchers found that similar large efficiencies still could be attained. The trick was to either arrange the plates symmetrically so that the resonators were each affected to the same degree or to adjust the electrical parameters of one of the resonators so they closely matched those of the other resonator. Matching the resonators, even if geometric symmetry wasn’t possible, was the key to high performance.

And, Fulton Innovation, a member of the Alticor family of companies, has developed eCoupled technology. As described on the company’s website, “… eCoupled technology includes an inductively coupled power circuit that dynamically seeks resonance, allowing the primary supply circuit to adapt its operation to match the needs of the devices it supplies. It does so by communicating with each device individually in real time, which allows the technology to determine not only power needs, but also factors such as the age of a battery or device and its charging lifecycles, in order to supply the optimal amount of power to keep a device at peak efficiency.”

Fulton doesn’t describe in detail what “dynamically seeking resonance” means, and there are a number of ways the primary resonance can be made to match that of the secondary. In reference 3, the comment is made, “… in the resonant inductive power transfer scheme … there is always a frequency where 100% power transfer takes place, as long as one is in the strong coupling regime where the coupling coefficient characterizing the wireless power transfer dominates the output coupling rate of the resonator.”

Although this may be true, varying the operating frequency to achieve primary-secondary matching might not be practical. In a paper⁷ that addressed this issue, the authors cited government regulations that “… strictly limit the radiating electric field strength of wireless applications outside specific bandwidths, which will be exceeded by these frequency tuning algorithms.” Instead, this paper advocates the use of fixed-inductance low-pass π adaptive impedance matching networks working in conjunction with the primary and secondary resonators to maximize efficiency at a constant frequency.

Two algorithms were proposed to accomplish adaptive matching. In the first one, selections from a list of values can be chosen for the source and load capacitors. As stated in the paper, “The low-pass π-match topology allows for a fixed inductor value to be placed in the high-current path, and variable source and load capacitor values can control the impedance matching capabilities of the network. Additionally, the Q of the π-match network provides an extra degree of freedom to achieve wideband or narrowband impedance matching.” The second algorithm uses constrained nonlinear optimization to compute component values that maximize the system’s power transfer.

Summary

Wireless power transfer is a reality. How much power can be transferred across what distance and the cost to do so are the remaining concerns that research and commercial products will address over time. The U.K. bus trial has a few more years to run, during which the inductive car charging systems mentioned in the Frost & Sullivan report will enter service. How quickly the field of wireless vehicle charging grows may depend as much on adopting a common standard—as Qi provided for consumer devices—as it does on technical advances.

References

1. Kalogianni, A., “Cut the cord. BMW’s inductive EV charging system charges by proximity,” digitaltrends.com, Jan. 6, 2015.
2. Crowe, P., “Inductive EV Charging to Grow 126 Percent by 2020,” hybridcars.com, June 2014.
3. Fan, S., et al, “Safe Wireless Power Transfer to Moving Vehicles,” Stanford University, May 2012.
4. Bowdler, N., “Wirelessly charged electric buses set for Milton Keynes,” BBC News.
5. Kesler, Dr. M., and McCarthy, C., “Highly Resonant Wireless Power Transfer in Subsea Applications,” WiTricity, April 2014.
6. Yu, W., et al, “Wireless power transfer in the presence of metallic plates: Experimental results,” AIP Advances, June 2013.
7. Waters, B., et al, “Adaptive Impedance Matching for Magnetically Coupled Resonators,” PIERS Proceedings, August 2012.