January 15, 2020

In-Motion Wireless Charging - Revolutionizing the Social Value of Electric Vehicles

At Tokyo Motor Show 2019, held at Tokyo Big Sight, NSK exhibited a number of new and future technologies powered by its vision of "Setting the Future in Motion."

One of them was in-motion wireless charging, a groundbreaking technology poised to deliver a whole new kind of energy infrastructure. We talked with Assistant Manager Daisuke Gunji, one of the key NSK engineers involved in the development of this technology.

Charging Electric Vehicles (EVs) While Driving from Coils Embedded in the Road

── What exactly is in-motion wireless charging?

Simply put, it is a system for sending electricity from coils embedded in the road to a moving vehicle; the vehicle is then powered by that electricity. The advantage of this technology is that it enables EVs to have smaller batteries.

Right now, if you want to increase how far an EV can travel, you have to install bigger batteries. The lithium ion batteries used in EVs are really wonderful, but they are heavy compared to gasoline and are still quite expensive. In addition, limited battery materials is another issue we will face as EV spread and battery production drastically increases, as well as the large volume of CO2 emitted when making them. It would be best if EVs needed fewer, smaller lithium ion batteries.

Electric trains, on the other hand, run without carrying batteries, right? That's because they receive electricity from power lines. If we could do the same thing with EVs, their batteries could be smaller. But unlike trains, cars don't follow a set of tracks and can move around freely, so we can't power them from fixed lines. That's why we want to try and send electricity wirelessly.

── Would coils need to be embedded all along the roads that cars drive on?

No. To consider where coils would need to be embedded, we drove cars around town near our research center to investigate where the cars were on the road and for how long.

We found that about a fourth of the time on the road is spent within 30 meters in front of a traffic light. So, if you drive for an hour, about 15 minutes is spent waiting at traffic lights. It might be even longer in big cities like New York or Tokyo. In other words, if we install coils under the 30 meters in front of traffic lights, it's very likely that EVs would get sufficient energy to run on a smaller battery.

If everything goes to plan, EV battery charge levels would hardly change at all. In other words, you wouldn't even have to bother with plugging your car in to charge it. After all, it's a pain to have to charge an EV after every drive, right? With in-motion wireless charging, though, we could get EVs that don't need to be plugged in. The whole current concept of an EV, as a car that can't go anywhere unless the user puts energy into it, will change, and people will be able to keep driving continuously without range limits. This technology will also be extremely important with the emergence of unmanned, autonomous driving vehicles.

In fact, our vision is even a bit bigger than that. The output of renewable energies like solar and wind power fluctuates dramatically depending on the weather. This raises the tricky question of how to control that fluctuation across the utility grid so that renewable energy can be used stably. One solution is using the batteries of lots of EVs driving around town to store power.

And again, I'm not just talking about EVs that plug into a charging station; the ideal is that we could use all the EVs driving around town to store energy. That's only possible with in-motion wireless charging. In the future, in-motion wireless charging technology should also be able to help address demand response, which is about adjusting the demand for power as needed.

Efficiently Receiving and Using Energy from a Location Close to the Road Surface

── The in-motion wireless charging system proposed by NSK is not only wireless, it is also an in-wheel motor. Why did NSK, which is known for bearings, develop an in-wheel motor?

Given our business, it is more likely that we would end up manufacturing parts used in an in-wheel motor rather than the motor itself. However, to understand the parts, we need to know about the whole unit. Our development went as far as to actually install whole units into a test car, and we intend to offer the knowledge gained along the way to our customers.

We started working on the development of an in-wheel motor 10 years ago. Moreover, we have been conducting joint research into wireless charging with the University of Tokyo. EVs are not only environmentally friendly; they also have excellent maneuverability. The pursuit of these benefits is what led to our in-wheel motors, and combining those with in-motion wireless charging really sets apart NSK's technology.

── What is the advantage of combining an in-wheel motor and in-motion wireless charging?

At the moment, other in-motion charging systems besides ours have coils under the vehicle body. With that setup, gap between ground coil and receiver coil will fluctuate. Efficiency of wireless charging deteriorates the farther away you get from the ground coils.

And so, if we put coils in the wheel hub, which is closer to the road surface, the distance becomes shorter and more stable. By doing this, we can increase the efficiency of wireless charging.

We have confirmed in experiments that our system can charge at 92.5% efficiency. So, another advantage is that, with good efficiency, the coils embedded in the road only have to be short, which means the installation cost will be lower.

── How is the electric power used?

When the vehicle is in motion, the power is used by the motor, and when it is not used in the motor, power is sent to be stored in the vehicle's batteries. With in-motion charging, we can set the system so that it doesn't receive power when electricity is not needed, and we can also make it receive only the amount needed.

Since the alternating current received wirelessly has a high frequency, there is considerable loss if the cable to the converter circuit is long. Since we want to make that distance as short as possible, it is really important to convert to direct current somewhere close-that is, inside the wheel.

In terms of performance, we're talking 25 kW per wheel with a motor. So, installed in two wheels, you get about the performance of a subcompact car, and installed in four wheels, you get about a small passenger car.

One of the research topics is how fast the vehicle can be moving and still charge in-motion. One coil embedded in the ground can't be made that long. If it's too long, it won't match the size of the receiver coil and power ends up being wasted.

And, the faster a car goes when in motion, the shorter its time over the ground coil. For example, if a car drives over a one-meter long ground coil at a speed of 36 km per hour, or 10 meters per second, it will go by in just a tenth of a second.

The system we are researching assumes power will only be transmitted when the system detects that a car is directly over the coil. It's difficult because we're dealing in the realm of a few tenths of a second. In any case, we want to avoid transmitting power when there is no car overhead so that power isn't wasted.

On the whole, practical implementation of in-motion wireless charging with an in-wheel motor is an extremely tough challenge. By intentionally researching the difficult stuff first, we can create versions with no in-wheel motors and also "reutilize" our know-how to in-wheel motor designs where there is no wireless charging system.

A Real Determination to Contribute to Society, Not Just Mere Technology Research

── Do you find working on this project to be rewarding?

Wireless charging in-motion is not just about cars; it connects to the bigger question of how we handle energy throughout the whole of society. Electrifying cars is not the end of the story. The result is a future vision for a world with lower CO2 emissions and the ability to secure a good energy balance across our infrastructure. One piece of that puzzle is the technology of in-motion wireless charging.

Aging populations and shrinking workforces in many countries are increasing the need for autonomous driving vehicles. I believe that in-motion wireless charging will be a key technology for supporting autonomous driving vehicles.

Even though it's only one piece of the puzzle, I personally feel it is tremendously worthwhile to be taking on the challenge of developing a foundational technology that will solve a social issue and impact the future of society. I'm confident that everyone on the whole development team and throughout NSK shares this same feeling.