Dr. Burak Ozpineci, Section Head, Vehicle and Mobility Systems at Oak Ridge National Laboratory, is developing a dynamic wireless charging system that charges electric vehicles while driving. The same technology can be installed in your garage, so you never have to remember to plug it in.
Tech Briefs: Did you develop the three-phase rotating field design as part of your wireless vehicle charging project?
Dr. Burak Ozpineci: Actually, I didn’t personally come up with the idea. I have outstanding colleagues working on wireless charging and I also have outstanding colleagues working on electric motors. Electric traction motors are three phase systems in which you generate a rotating magnetic field to impart motion to the rotor. So, they said: Why not use the same technology in wireless charging — we can get higher-power, greater power-density coils that way.
We demonstrated a 120 kW, 85 kHz system a few years ago, for which the coils weighed about 110 pounds. When OEMs are already trying to save weight, you cannot convince them to put a 110-pound coil in the vehicle — it's like adding another passenger. So that's when we came up with the idea of using three-phase.
If you have a single-phase coil, there are power pulsations. However, to get the same average power, if you use three-phase coils, you have much smaller pulsations per phase and correspondingly lower coil currents. If the current is lower, you can have smaller and lighter coils.
We use a layered construction, with a three-phase coil at the bottom and another three-phase return coil on top — similar to a motor design. There is a rotating magnetic field generated. In this way, power is transferred more uniformly with a much higher surface power.
Tech Briefs: The 110-pound weight seems high to me for an 85-kHz system.
Ozpineci: At these power levels you generate a lot of heat. We used generously sized copper windings to minimize that heat. Our goal was to demonstrate the technology. When they are commercialized, companies might not want to use as much copper as we do. They might make some tradeoffs to reduce the size and weight of the coils.
Tech Briefs: How do you produce the 85-kHz power that drives the coils?
Ozpineci: We take the three-phase power from the utility and convert it to DC. Then we have an inverter that converts it to high frequency AC — basically square waves. That is followed by a resonant network that generates the sinusoidal 85 kilohertz waveform, which is fed to the underground transmitter coil. The receiving coil in the vehicle is also tuned to that frequency — it too has a resonant network — which is followed by a rectifier that converts the high frequency AC back into DC to feed the battery.
Tech Briefs: Do you use filters to reduce the ripple?
Ozpineci: We use DC link capacitors to reduce the ripple.
Tech Briefs: How large is the transmitting package?
Ozpineci: We need some kind of cabinet to include the power conversion system. In addition to the cabinet, we also have resonant capacitors and a ground side transmitter coil. As we go to higher power levels, the transmitting package will be larger.
Tech Briefs: Where would this cabinet be installed for the dynamic on-the-road applications?
Ozpineci: The cabinet would be on the side of the road. And we would have power lines going under the pavement to the coil. Eventually, what we want to do is to connect the cabinet directly to the medium voltage power lines. Because if you don't, you would have to deal with 60Hz transformers, which are large. If we connect to the medium voltage lines, we can use an 85 kHz solid-state transformer, which is much smaller than a 60 Hz transformer. That would allow us to have a much smaller cabinet.
Tech Briefs: What voltage do you deliver to the vehicle?
Ozpineci: We go directly to the main battery, which will be around 800 volts in the upcoming electric vehicles. And we will go to still higher voltages for newer vehicles. For passenger vehicles, right now, if you want to achieve extremely fast charging at 350 kilowatts, they're going to be around 800 volts. For electrified trucks, it looks like battery voltages could be between 1000 and 1500 volts.
Tech Briefs: Does this battery technology exist now?
Ozpineci: The Porsche Taycan, which is their electric vehicle, and sells for about $100,000, has an 800 Volt DC battery and is built to accept up to 350 kW charging right now. And Teslas can accept 250 kW.
Tech Briefs: So, are those the power ranges you’re aiming for now?
Ozpineci: Yes, for passenger vehicles, we're aiming at around 300 or 350 kW for stationary applications. For trucks, we want to go to higher power levels.
Tech Briefs: How much have you achieved so far?
Ozpineci: We have demonstrated 120 kW wireless charging through 6 inches, and this fall we're going to try 300 kilowatts. These are stationary applications. For dynamic wireless charging, we're going up to 200 kilowatts.
Tech Briefs: Are you going to demonstrate your charger with existing vehicles?
Ozpineci: Yes, we want to have a demonstration in the fall. Right now, we have a Hyundai Kona vehicle, in which we’re going to install wireless charging. We’ve already demonstrated it in a UPS delivery truck, and we have a few other vehicles coming up for both stationary and in-motion charging.
Tech Briefs: For people to install your charger systems in a home garage, would they have to install a new power line?
Ozpineci: No, not necessarily — I have an electric vehicle and when I go home, I plug in.
Tech Briefs: How much power do you need?
Ozpineci: The maximum at home is typically 11 kW, but normally it’s about 7 kilowatts. I plug in when I arrive, but I’d rather not have to bother with that. I'd rather just park my car and charge automatically without me knowing it — that's the vision I have for the future. If we can have in-motion charging, if we can have charging in our garage, if we are charging while we’re at work, we would never have to worry about the range of our vehicle.
Tech Briefs: For the dynamic charging, won't the charging rate vary with the speed of the vehicle?
Ozpineci: Yes — the slower the better if you want to get more charge.
Tech Briefs: I imagine these systems would probably be installed on Interstates.
Ozpineci: Yes, we're designing for Interstate speeds. A demonstration that was done before ours, used a 20-kilowatt system for about a mile. They didn't have any gaps — they covered the roadway from the beginning to the end with coils. We're thinking that if we multiply the power by 10, we only need 10% of the road covered. So, that's why we’re designing for about 200 kilowatts covering about 10% of the road. We think it would be easier to have a single drop with high power, rather than long stretches of 20 kW power distribution.
Tech Briefs: How do you plan to prevent interference from stray magnetic fields?
Ozpineci: That's a good question. The electromagnetic fields generated by these coils can induce voltages on metals. In the pick-up under the vehicle, most of the magnetic field is between the two coils, but there will always be some leakage. We're looking at different shielding technologies to eliminate that. We put the shields between the coil and the vehicle, to protect against stray induced voltages.
The International Commission on Non-Ionizing Radiation Protection (ICNIRP) specifies the strength of leakage fields at various distances from the source. At each point, the measured electric and magnetic fields must be below a certain value, so we follow their guidance. We measure the fields at different locations to make sure that they are lower than the ICNIRP numbers. We did that for 120 kilowatts, but now when we go to 300 kilowatts and even higher power, we’re going to get closer and closer to the limit. So, before we go there, we decided to do some shielding work to reduce the electromagnetic emissions.
Tech Briefs: I read that you could use the car as an energy storage apparatus to feed power back into the grid. How would you do that?
Ozpineci: So, the same thing in reverse. What we're doing right now is feeding power from the grid into the battery. If we design it to be bi-directional, we can get the energy from the battery and send it back to the grid.
When the high frequency AC is received in the vehicle’s coil, it is converted to DC. That conversion can be done with a diode rectifier, but then you wouldn’t have any control. However, if you use controlled rectifiers, you could drive them in reverse to convert DC power into AC. Both transmitter side and receiver side would then more or less look the same, so you could send power either way. You could even use these vehicle systems to power microgrids. We demonstrated this at 20 KW on a UPS delivery truck last year.
Some car manufacturers have been announcing that you could send power from your car back to your home to use in case of emergency — we were promised this years ago, but now it seems like it's going to happen sooner rather than later.
An edited version of this interview appeared in the July 2021 issue of Tech Briefs.