If electric cars could recharge while driving down a highway, it would virtually eliminate concerns about their range and lower their cost, perhaps making electricity the standard fuel for vehicles. Researchers have wirelessly transmitted electricity to a nearby moving object, which could advance wireless charging of vehicles and personal devices such as cellphones, and untether robotics in manufacturing.
The device builds on existing technology developed at MIT for transmitting electricity wirelessly over a distance of a few feet to a stationary object. For the new device, the team transmitted electricity wirelessly to a moving LED lightbulb. That demonstration involved a 1-milliwatt charge, whereas electric cars often require tens of kilowatts to operate. The team is working on greatly 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 a major drawback of plug-in electric cars: their limited driving range. Electric vehicle batteries generally take several hours to fully recharge, so a charge-as-you-drive system would overcome these limitations. 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.
Mid-range wireless power transfer 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. The continuous flow of electricity, however, 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 this challenge, the researchers eliminated the radio-frequency source in the transmitter and replaced it with a commercially available voltage amplifier and feedback resistor. This system automatically determines the right frequency for different distances without the need for human interference. Adding the amplifier and resistor allows power to be efficiently transferred across most of the three-foot range, and despite the changing orientation of the receiving coil. This eliminates the need for automatic and continuous tuning of any aspect of the circuits.
The approach was tested 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 three feet. The group used an off-the-shelf, general-purpose amplifier with a relatively low efficiency of about 10 percent. Custom-made amplifiers can improve that efficiency to more than 90 percent.