Researchers at Chalmers University in Sweden say they have developed a wireless charging technology that can replenish batteries with up to 500 kW of power without connecting them to a charger via a cable. They say the new charging equipment is complete and ready to start commercial scale production. The technology is not necessarily intended for charging personal passenger cars, but it could be used by electric ferries, buses, or autonomous vehicles used in mining or agriculture to charge without using a robotic arm or plugging in.
Yujing Liu, a professor of electric power in the electrical engineering department at Chalmers, focuses primarily on renewable energy conversion and electrification of the transportation system. “You can have a system built into the wharf that charges the ferry at some stops, at the same time as passengers get on and off. Automatic and completely independent of weather and wind, charging can take place 30 to 40 times per day. Electric trucks will need to be charged with such high power that the charging cables will become very thick and heavy, making them difficult to handle.”
Wireless Charging Advances
Liu says the rapid development of certain components and materials in recent years has opened the door to new charging possibilities. “A key factor is that we now have access to high power semiconductors based on silicon carbide, so-called SiC components. As power electronic products, these have only been on the market for a few years. They allow us to use higher voltage, higher temperature, and much higher switching frequency, compared to classic silicon based components,” he says. This is important because it is the frequency of the magnetic field that sets the limit to how much power can be transferred between two coils of a given size.
“Previous systems for vehicle wireless charging have used frequencies of around 20 kHz, much like a normal stove top. They became bulky and the energy transfer was not very efficient. Now we work with frequencies that are four times higher. Then induction suddenly becomes attractive,” explains Liu. He adds that his research group is in close contact with the two leading manufacturers of SiC modules in the world, one in the US and one in Germany.
“With them, rapid product development takes place towards even higher currents, voltages, and effects. Every two or three years, new versions are launched, which can withstand more. These types of components are important enablers with a wide range of applications in electric vehicles, not just for inductive charging.”
Another recent technological leap concerns the copper wires in the coils that send out and receive respectively the oscillating magnetic field that forms the actual bridge for the energy flow across the air gap. Here, the goal is to use as high a frequency as possible. “Then it does not work with coils looped with ordinary copper wire. It would lead to very large losses at high frequency,” Liu says.
Instead, the coils are now made up of braided ‘copper ropes’ consisting of up to 10,000 copper fibers only 70 to 100 microns thick — about the size of a strand of human hair. Such braids of so-called litz wire that are suitable for high current and frequencies have also only been commercially available recently. A third example of new technology that makes high power wireless charging possible is a new type of capacitor that adds the reactive power necessary for the coil to create a sufficiently powerful magnetic field.
It’s Not For Everyone
Liu emphasizes that charging electric vehicles requires several conversion steps both between direct current and alternating current and between different voltage levels. “So, when we say that we have achieved an efficiency of 98 percent from direct current in the charging station to the battery, that figure may not mean much if you do not carefully define what is measured. But you can also put it this way. Losses occur whether you use ordinary, conductive charging or charge with the help of induction. The efficiency we have now achieved means that the losses in inductive charging can be almost as low as with a conductive charging system. The difference is so small that in practice it is negligible, it is about one or two percent.”
CleanTechnica readers like specifications, so here’s what we know, thanks to Electrive. The research team at Chalmers claims its wireless charging system is 98% efficiency and is capable of DC transfer rates of up to 500 kW per two square meters with a 15-centimeter air gap between the ground pad and the onboard pad. This corresponds to a loss of a mere ten kW or 2% of the theoretical maximum charging power.
Liu is sanguine about this new wireless charging technology. He doesn’t think it will replace how we charge electric cars, for instance. “I drive an electric car myself and do not see that I would have any use for induction charging in the future. I drive home, plug in… it’s no problem.” He also doubts it will replace conventional charging methods that rely on a cable. “One should probably not claim that the technology itself is more sustainable. But it can make it easier to electrify large vehicles and thus speed up the phasing out of, for example, diesel powered ferries,” he says.
There is a big difference between charging a car and charging a ferry, an airplane, a train, or mining equipment. Most cars are parked 95% of the time. Most commercial equipment is in constant service and can’t wait hours to recharge. Liu sees the benefits of the new inductive charging technology being applicable to those commercial scenarios. Nobody really needs to charge their EV at 500 kW in their garage.
The point of this research is less about wireless charging itself and more how technology keeps coming up with newer, cheaper, more efficient ways of doing things that will move the EV revolution forward more quickly. Think of it like the heady days of the personal computer when the latest and greatest machines were obsolete before you got them home from Circuit City. (Remember them?) Electric transportation is experiencing a similar burst of creativity today. Such a beautiful thing!
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