While most wireless chargers are only capable of up to 20 kW speeds — far below even the slowest maximum charge rates of yesterday’s EVs, they also require a large footprint that doesn’t make installation at your local grocer or shopping spot easy or practical in most cases. That size doesn’t just mean its width and length, either. To get a proper connection, a wireless charger needs a specific air gap between the transmitter and the receiver to work as efficiently as possible due to size of the electromagnetic field created by the transmitter. The researchers at ORNL are no strangers to pushing wireless charging to its limits, being the first group to create the 20-kW wireless charging system in 2016. And it now has created a traditional wireless charger that is capable of 120 kW of power.
Tight air gaps and wieldy size are easy to achieve for, say, your phone, as you’re able to nearly place it directly on top of the transmitter. That’s not so easy with an electric vehicle, as most civilian EVs sit well off the ground (and at varying heights). Adjustable-height air suspensions that can squat a car down to a charging pad or movable receivers are expensive, complex solutions to a problem afflicting something that is generally already too complex and expensive. Never mind making the transmitter movable, and while we do see that with public transport bus stops, those buses are made to meet a single specification. What we need is a wireless car charging pad that is more compact, powerful and more efficient at larger air gaps — i.e., farther away from the car. And that’s just what ORNL achieved.
While there are a few details we’re still waiting to hear back from ORNL about (mostly on the modifications needed for the Hyundai Kona Electric used for the demonstration), here are the major points noted in a news post on its site. The transmitter is compact, at just over 14 inches in diameter, and utilizes an innovation ORNL created in 2022 called the “polyphase electromagnetic coupling coil” with rotating magnetic fields. Polyphase technology isn’t new or radical as it’s essentially the same technology used in electric motors and generators right now. It’s just that this is being applied to wireless charging but still offering much of the same advantages: more power out of a conductor the same size and same applied voltage as a single-phase version.
And it’s that reason the biggest innovation here is that this polyphase coil can produce bipolar electromagnetic fields (EMFs) on three-phase AC power with a 120-degree shift between each phase, allowing the transmitter to use three compact coil loops in two layers where most wireless chargers use single-phase AC power on a single “racetrack” transmitter. While that may sound easier, the single-phase racetrack transmitter produces a monopole EMF that reduces its efficiency, requires a larger space for the transmitter wiring and requires a tighter air gap. There’s more to the polyphase coupling than just being made more compact, such as each coil pair having an opposing polarity and negative interphase mutual inductances, but the big picture is that this new wireless charger is smaller, more efficient and more powerful.
How much more powerful? That traditional single-phase, monopole racetrack transmitter can only produce 20 kW of power with a five-inch air gap, achieving around a 90 percent efficiency on average when comparing its input power to its output power. This polyphase wireless transmitter can produce 100 kW charge rate with the same air gap at 96 percent efficiency. This is also in the same power delivery capacity as many early DC fast chargers, but without the plug and its bulky wiring. In theory, and following what we know about wireless power transmission, this polyphase transmitter could allow an increase in air gap with only a minor drop in efficiency when compared to the single-phase transmitter.