Imagine never having to plug in an electric car to recharge,but instead simply take the highway on-ramp to get a range boost.
Researchers from Stanford University have published a new demonstration of highly efficient wireless charging that could allow the technology to one day be scaled up to boost driving range of electric vehicles on highways of the future.
Wireless, or inductive, charging – the same technology that is nowadays often used for electric toothbrushes and some smartphones – is under development and being piloted by some car makers already.
But current electric car inductive technology has its limitations: it relies on charging pads that must be aligned perfectly with the oscillating magnetic field that transmits the current to optimally recharge the vehicle, and of course the subsequent downtime to recharge.
The culprit causing such system sensitivity is the typical amplifier used in inductive charging technology, that becomes very inefficient if a device is not sitting in just the right place.
The new demonstration, published in Nature Electronics, is based on a new circuit configuration that allowed the researchers to make use of another type of amplifier known as a “switch mode” amplifier, which is far more efficient but requires precise conditions in order.
The new system demonstrated by Stanford electrical engineer Shanhui Fan and Sid Assawaworraritwho shows up to 92% efficiency can be achieved, with the lab prototype used in the demonstration transmitting 10 watts over a distance of up to 1 metre.
Initially the system may be suitable for smaller but power hungry electric devices such as drones or robots but according to Fan, there are no fundamental reasons the system couldn’t be scaled up to transmit hundreds of kilowatts needed to charge an electric vehicle.
“This is a significant step toward a practical and efficient system for wirelessly re-charging automobiles and robots, even when they are moving high speeds,” Fan was quoted as saying by Stanford News.
“We would have to scale up the power to recharge a moving car, but I don’t think that’s a serious roadblock. For re-charging robots, we’re already within the range of practical usefulness.”
According to Fan, the time it takes the power to be transmitted is a few milliseconds – ample time to recharge an electric vehicle crossing above an inductive pad embedded in a highway of the future. The only limitation would be the vehicle’s own maximum charge rate.
For those concerned about health risks, Fan says the magnetic field needed to recharge an electric vehicle is well within established safety guidelines.
It may be several years until recharging highways become a reality. The retrofitting costs for highways would need to be justified by a high percentage of electric car drivers.
The abstract of Fan and Assawaworraritwho’s demonstration is as follows:
Stationary wireless power transfer has been deployed commercially and can be used to charge a variety of devices, including mobile phones and parked electric vehicles.
However, wireless power transfer set-ups typically suffer from an inherent sensitivity to the relative movement of the device with respect to the power source.
Nonlinear parity–time symmetric circuits could be used to deliver robust wireless power transfer even while a device is moving rapidly, but previous implementations have relied on an inefficient gain element based on an operation-amplifier circuit, which has inherent loss, and hence have exhibited poor total system efficiency.
Here we show that robust and efficient wireless power transfer can be achieved by using a power-efficient switch-mode amplifier with current-sensing feedback in a parity–time symmetric circuit.
In this circuit, the parity–time symmetry guarantees that the effective load impedance on the switch-mode amplifier remains constant, and hence the amplifier maintains high efficiency despite variation of the transfer distance.
We experimentally demonstrate a nonlinear parity–time symmetric radiofrequency circuit that can wirelessly transfer around 10 W of power to a moving device with a nearly constant total efficiency of 92% and over a distance from 0 to 65 cm.