The team is now building a version that is capable of charging vehicles more quickly, while also charging multiple vehicles at the same time.
The new technology is called a medium voltage fast charger (MVFC).
Conventional, 50kW state-of-the-art chargers include a distribution transformer, which weighs 1000kg, and a separate fast charger unit, which weighs 200-600kg. To support the weight, this transformer-and-charger system usually needs to be installed on a concrete slab.
The transformer takes power from a utility medium-voltage line and steps down the voltage to 480V so that it that can be used by the fast charger. The fast charger takes the AC voltage and converts it to DC voltage that is compatible with the EV’s battery.
“Our 50-kW MVFC weighs only around 100kg and can be wall- or pole-mounted,” says Associate Professor Srdjan Lukic of NC State. “The MVFC does the work of both the transformer and the fast charger, taking power directly from a medium-voltage utility line and converting it for use in an EV battery.
“This new approach offers four times more power from the same system footprint, reducing the system installation costs at the same time,” says Research Professor Srdjan Srdic of NC State.
The researchers were able to make the technology so much smaller, in part, because they used wide bandgap semiconductor devices. This also made the technology more energy efficient.
At present, the best transformer-and-charger stations are reported as having an efficiency of up to 93 percent, meaning that at least 7 percent of the power is lost to heat during the charging process.
In testing, the researchers say their prototype MVFC had an efficiency of at least 97.5 percent, meaning an additional 4.5 percent of the power is used to charge the vehicle, rather than being wasted as heat. This reduces operating costs, increasing revenue without increasing the cost to consumers.
In other words, the researchers claim they were able to cut the wasted energy by more than 60 percent.
The current version of the MVFC charges at the same speed as existing charging stations. That’s because this iteration of the MVFC was designed to operate at 50kW, which is the power level of a typical fast charger.
However, the research team is in the process of building a next generation MVFC that handles much higher power, capable of charging more vehicles and charging them more quickly.
In the multi-port station design, a utility line is connected directly to a solid-state transformer, (SST), which is a power-electronics-based smart transformer. The SST then feeds a local DC microgrid, with battery storage systems and multiple charging nodes that vehicles can plug into.
“We are building five charging nodes into the prototype, but there could be twice as many or more,” Assoc Prof. Lukic says.
The multi-port MVFC will have a rating capacity of one megawatt, with each charging node capable of providing up to 350kW of power. The upgrade from 50kW to 350kW means that a vehicle can be charged up to seven times faster, according to the researchers.