Stanford scientists have managed to transmit electricity wirelessly to a moving LED lightbulb, a technology that could one day be used to charge electric cars on the highway.
The long-sought dream of a life on the run—unplugged—is a step closer to reality, thanks to a team of Stanford University scientists, who have developed a way to wirelessly delivery electricity to moving objects.
The team built on existing technology developed in 2007 at MIT for transmitting electricity wirelessly over a distance of a few feet to a stationary object. In the new work, results of which were published in the journal Nature, the Stanford team transmitted electricity wirelessly to a moving LED lightbulb.
The demonstration only involved a 1mW charge, whereas electric cars often require tens of kilowatts to operate. The team said it is now working on increasing the amount of electricity that can be transferred, and tweaking the system to extend the transfer distance and improve efficiency.
Wireless charging would address the limited driving range of plug-in electric cars. Tesla Motors expects its upcoming Model 3 to go more than 300km on a single charge and the Chevy Bolt, which is already on the market, has an advertised range of 383km. But electric vehicle batteries generally take several hours to fully recharge. A charge-as-you-drive system would overcome these limitations.
“In theory, one could drive for an unlimited amount of time without having to stop to recharge,” said Shanhui Fan, a Stanford professor of electrical engineering and senior author of the research. “The hope is that you’ll be able to charge your electric car while you’re driving down the highway. A coil in the bottom of the vehicle could receive electricity from a series of coils connected to an electric current embedded in the road.”
Wireless technology could also assist GPS navigation of driverless cars. GPS is accurate up to about 10 metres. For safety, autonomous cars need to be in the center of the lane where the transmitter coils would be embedded, providing very precise positioning for GPS satellites.
Mid-range wireless power transfer, as developed at Stanford and other research universities, is based on magnetic resonance coupling. Just as major power plants generate alternating currents by rotating coils of wire between magnets, electricity moving through wires creates an oscillating magnetic field. This field also causes electrons in a nearby coil of wires to oscillate, thereby transferring power wirelessly. The transfer efficiency is further enhanced if both coils are tuned to the same magnetic resonance frequency and are positioned at the correct angle.
However, the continuous flow of electricity can only be maintained if some aspects of the circuits, such as the frequency, are manually tuned as the object moves. So, either the energy transmitting coil and receiver coil must remain nearly stationary, or the device must be tuned automatically and continuously—a significantly complex process.
To address the challenge, the Stanford team eliminated the radio-frequency source in the transmitter and replaced it with a commercially available voltage amplifier and feedback resistor. This system automatically figures out the right frequency for different distances without the need for human interference.
“Adding the amplifier allows power to be very efficiently transferred across most of the three-foot range and despite the changing orientation of the receiving coil,” said graduate student Sid Assawaworrarit, the study’s lead author. “This eliminates the need for automatic and continuous tuning of any aspect of the circuits.”
The group used an off-the-shelf, general-purpose amplifier with a relatively low efficiency of about 10%. They say custom-made amplifiers can improve that efficiency to more than 90%.
Assawaworrarit tested the approach by placing an LED bulb on the receiving coil. In a conventional setup without active tuning, LED brightness would diminish with distance. In the new setup, the brightness remained constant as the receiver moved away from the source by a distance of about 0.91 metres (3 feet). Fan’s team recently filed a patent application for the latest advance.