Researchers from North Carolina State University have developed new technology and techniques for transmitting power wirelessly from a stationary source to a mobile receiver – moving engineers closer to their goal of creating highway “stations” that can recharge electric vehicles wirelessly as the vehicles drive by.
“We’ve made changes to both the receiver and the transmitter in order to make wireless energy transfer safer and more efficient,” saysDr. Srdjan Lukic, an assistant professor of electrical engineering at NC State and senior author of a paper on the research.
The researchers developed a series of segmented transmitter coils, each of which broadcasts a low-level electromagnetic field. The researchers also created a receiver coil that is the same size as each of the transmitter coils, and which can be placed in a car or other mobile platform. The size of the coils is important, because coils of the same size transfer energy more efficiently.
The researchers modified the receiver so that when it comes into range and couples with a transmitter coil, that specific transmitter coil automatically increases its current – boosting its magnetic field strength and the related transfer of energy by 400 percent. The transmitter coil’s current returns to normal levels when the receiver passes out of the range of the transmitter.
These modifications improve on previous mobile, wireless power transfer techniques.
One previous approach was to use large transmitter coils. But this approach created a powerful and imprecise field that could couple to the frame of a car or other metal objects passing through the field. Because of the magnetic field’s strength, which is required to transfer sufficient power to the receiver, these electromagnetic field “leaks” raised safety concerns and reduced system efficiency.
Another previous approach used smaller transmitter coils, which addressed safety and efficiency concerns. But this approach would require a very large number of transmitters to effectively “cover” a section of the roadway, adding substantial cost and complexity to the system, and requiring very precise vehicle position detection technology.
“We tried to take the best from both of those approaches,” Lukic says.
Lukic and his team have developed a small, functional prototype of their system, and are now working to both scale it up and increase the power of the system.
Currently, at peak efficiency, the new system can transmit energy at a rate of 0.5 kilowatts (kW). “Our goal is to move from 0.5 kW into the 50 kW range,” Lukic says. “That would make it more practical.”
The paper, “Reflexive Field Containment in Dynamic Inductive Power Transfer Systems,” is published online in IEEE Transactions on Power Electronics. Lead author of the paper is NC State Ph.D. student Kibok Lee. The paper was co-authored by Dr. Zeljko Pantic, a former Ph.D. student at NC State. The research was partially supported by National Science Foundation grant number EEC-0812121.
Credit: NCSU News Services press release “New Approach Advances Wireless Power Transfer for Vehicles” by Matt Shipman
This new fast-charging technology was developed by ABB, an energy engineering company, at N.C. State.
Ewan Pritchard, a professor from the Department of Electrical and Computer Engineering at N.C. State, demonstrated how the fast charger worked on a Nissan Leaf.
“When the charger was originally installed last year, it was the first one on the east coast,” Pritchard said. “What this does is it puts direct current straight into the batteries at 500 volts. So, by doing a very high current and high voltage, it allows the electrons to get in very quickly.”
The more conventional chargers are known as Type-2 chargers—these take about four hours to charge a vehicle.
In a Type-2 charger, the fast-charging station converts alternating current to direct current before the current reaches the car. The alternating current is then converted to direct current inside the battery, making the Type-2 chargers less efficient.
Despite being able to charge a car in less than an hour, the fast charger has some limitations. For example, the Chevy Volt cannot be charged with the fast-charger—the Type-2 charger must be used.
Additionally, the fast charger will be more expensive than the Type-2 charger.
The fast charger costs $30,000, whereas the Type-2 charger costs $2,000.
The Nissan Leaf that researchers put fast-charging technology in has a range of 88 miles before it runs out of energy, making it one of the longer range all-electric vehicles available today.
According to the EPA, passenger vehicles sold in 2012 had an average fuel economy of 23.8 miles per gallon. Driving 88 miles would cost about $13 if the gas price were $3.50 a gallon.
Comparatively, the same distance of 88 miles would cost $2.20 to power a Nissan Leaf.
While the new fast-charging station is quick, there are methods being developed that will speed up the process even further.
“It’s possible to speed up the charging time quite a bit more,” Pritchard said. “Rather than sending a current to the battery as a constant current, you can send it in as small rapid pulses. Using this method, we can get the charging time down to under 10 minutes.”
In addition to pulse-charging technology, further research is being conducted at N.C. State to expand the ways vehicles can be charged.
Later this year, Pritchard said he plans to take the electric car technology one more step by installing a wireless charging system on Centennial Campus at N.C. State.
“One thing that we are looking at doing is inductive power transfer,” Pritchard said. “That’s where you are able to charge the vehicle from underneath the road while you are driving. We intend to install some of that on Centennial campus. We are working on doing that later this semester.”
Notwithstanding the costs, ABB plans to install 201 fast-charging stations in the Netherlands.
Credit: “Electric car can charge four times faster than previous models” from The Technician Online by Sasha Afanasieva