An electric vehicle has a limited range, meaning that the battery needs to be recharged with some regularity in order for the vehicle to continue driving. It would therefore be ideal to be able to charge an electric vehicle while driving, so that the car has more or less unlimited range. A new discovery by researchers at Stanford University brings such technology one step closer.
The results of Stanford University's research have been published in the scientific journal Nature. Wireless energy transfer is not new and has been possible based on magnetic resonance coupling for some time. In this technology, electricity is passed through a coil, creating an oscillating magnetic field. This field can cause electrons in a nearby coil to also oscillate, allowing energy to be transferred wirelessly.
Transfer to moving objects
However, this technology has limitations. For instance, it is necessary to tune both coils to the same magnetic resonance frequency and position them at the right angle to transfer energy efficiently. This is not a problem in a fixed arrangement where both coils are positioned at a fixed distance from each other.
With a moving object, however, the distance of the coils relative to each other changes continuously. In practice, this means that this technique is only suitable for wirelessly transferring energy to a moving object if, among other things, the magnetic resonance frequency is continuously adjusted to the exact position of the coils, which is a time-consuming process. As a result, the technique has so far not been suitable for charging electric cars, for example.
Automatically adjust frequency
Professor Shanhui Fan and PhD student Sid Assawaworrarit from Stanford University have come up with a solution to this problem. The team fitted the coil used to transmit energy with a commercially available voltage amplifier and a feedback resistor. With this, they have developed a system that continuously calculates the correct frequency based on the distance between the two coils, without requiring input from a human.
"Adding the amplifier allows energy to be transferred very efficiently over a distance of almost a metre, regardless of the changing orientation of the receiving coil," explains Assawaworrarit. "This eliminates the need for automatic and continuous adjustment of aspects of the circuit."
Tested in practice
The method was tested in practice by placing an LED lamp on the receiving coil. In a conventional setup, the brightness of the light produced by this LED lamp would decrease as the distance between the sending and receiving coil increases. In the researchers' new setup, however, the brightness of the light remained the same up to a distance of almost a metre.
The researchers expect to further improve the efficiency of the system. The test setup used a commercially available voltage amplifier, which has an efficiency of only 10 per cent. The researchers expect that a custom-made amplifier could increase efficiency to more than 90 per cent, allowing greater distances to be covered.
By: Wouter Hoeffnagel
Source: Stanford University
Photo by: Mark Shwartz/Stanford University
