Researchers at the US Department of Energy’s Oak Ridge National Laboratory (ORNL) have demonstrated a 20kW bidirectional wireless charging system across an 25cm (11in) air gap.
The system was installed on a UPS medium-duty, plug-in hybrid electric delivery truck, where the larger air gap opens up wireless charging for a new class of larger vehicles with higher ground clearance. “There’s no off-the-shelf solution that can deliver 20 kilowatts across an 11-inch air gap with these efficiencies,” said Omer Onar, who led the team at ORNL.
The system used two electromagnetic coupling coils and transferred electricity from the power grid to the vehicle battery terminals at more than 92 per cent efficiency. The technology takes energy from the grid and converts it to a DC voltage. Then a high-frequency inverter using wideband gap devices generates an AC supply, which in turn creates a magnetic field that transfers power across the air gap. Once the energy is transferred to the secondary coil across the air gap it is converted back to DC, charging the vehicle’s battery pack.
The higher power level is also important for commercial vehicles. At 20kW, it would take about three hours to charge the vehicle’s 60kW battery pack, compared to five to six hours using the existing onboard charging system.
The bidirectional design also supports use of the vehicle’s batteries for energy storage. This would give energy flexibility to a fleet owner’s business, and help better manage on-site generation such as solar power. Another key part of the demonstration was that the bidirectional technology is fully compliant with grid power quality standards.
“Scaling the technology to a fleet of 50 trucks gives you megawatt-scale energy storage,” said Onar. The system incorporates ORNL’s custom electromagnetic coil design and controls system, as well as wide bandgap power conversion systems. The technology was tested using grid and battery emulators before integration into the vehicle.
The technology “represents an integrated, holistic solution for vehicle electrification that also advances the next-generation smart grid,” he added. “The system expands the possibilities for fleets who want convenient, efficient EV charging as well as electricity storage solutions.”
Oak Ridge researchers first demonstrated a 20kW wireless charging system on a light-duty passenger vehicle in 2016, and that technology has been scaled up to 120kW. This wider gap technology for commercial vehicles is undergoing further testing and analysis.