Omer Onar, leader of the Vehicle Power Electronics group at Oak Ridge National Laboratory (ORNL). “In order to fully automate the charging process, you need to have wireless charging.”
To that end, Onar and his colleagues, in collaboration with Volkswagen Group of America, have created a 270-kilowatt wireless power transfer system that has successfully charged a Porsche Taycan.
The breakthrough was a refinement of—and a jump in power from—a similar 120-kilowatt demonstration system they developed for the DOE in 2018. The receiver coil on that version weighed a bit more than 114 pounds, was two inches thick, and more than two-and-a-half feet wide. Because the size and heft of that prototype would be a deal killer for car manufacturers doing everything to keep their vehicles light, the DOE asked Onar and his team to come up with something smaller, sleeker, and more powerful.
The coils of existing wireless charging systems pulse power, from zero to peak, across an air gap between the transmitter and the receiver. That means they have a “very low space time utilization of the magnetic field,” Onar said. In other words, they’re limited in power rating, which makes charging a slow process.
To overcome this, the ORNL researchers turned to a coil geometry that energizes three phases to generate rotating magnetic fields. The coils are designed to have three sections, shifting every 120 degrees. Each phase carries about a third of the power. “So, when you sum up those powers at any time from the three phases, you always have constant power transfers delivered from primary to secondary,” Onar explained.
The polyphase geometry also reduced magnetic field emissions. With a single circular coil, the magnetic field lives at the outer edge. With the 120-degree design, it sits at the center, so the system requires no shielding to remain compliant with international guidelines for electromagnetic field emission limits.
The result is a coil that can charge a car’s battery to 50 percent in 10 minutes. After that, the power has to be turned down, so things charge a bit slower. “You may spend another five minutes or so to push another 20-kilowatt hours or so,” said Onar. “So, in more or less 15 minutes, you can reach a 75 or 80 percent state of charge.” That’s enough to drive two or three hours on the interstate.
There’s an air gap of 4.75 centimeters between the transmitter and receiver coils of the charger—a typical ground clearance in light-duty passenger vehicles. Despite this seemingly large leap, the system is 95 percent efficient—in the same ballpark as the plug-in chargers used today.
With the success of their recent demonstrations, the charger is ready to be moved out of the realm of handcrafted prototypes and into mass manufacturing. And there’s not much standing in the way of that happening quickly, according to Onar.