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Wirelessly Charged Buses Hit The Induction Highways

Jan. 27, 2014

A new spin on “electric avenue” is coming to Milton Keynes, U.K.: The city will introduce wirelessly charged electric buses that can do a full day’s work. They are the first of their kind to operate in the U.K. (Fig. 1), but similar systems are already in use in Germany, Italy, the Netherlands, and South Korea.

Wireless booster charging facilities, positioned at the start and end of the buses’ routes, come in the form of plates embedded in the road. After a night charging at the depot, the buses will receive booster charges throughout the day. Receiver plates on the bottom of the bus are lowered to approximately within 4 cm of the road surface. After around 10 minutes of charging, the bus resumes service.

The system uses inductive charging, which employs an electromagnetic field to transfer energy between two objects. Energy is sent through an inductive coupling to an electrical device; that energy charges the batteries or runs the device.

Induction chargers use an induction coil to create an alternating electromagnetic field from within a charging base station. A second induction coil in the portable device (i.e., the bus) takes power from the electromagnetic field and converts it back into electrical current to charge the battery.

 When it comes to charging buses, this system brings some important advantages. Foremost is that the system connections are all enclosed and, therefore, not susceptible to corrosion from water or the salt and chemicals used to de-ice roads. On top of that, the system needn’t be plugged and unplugged, thereby eliminating wear-related problems.

It’s not a perfect system, though. One detractor is inductive charging’s low efficiency rating. Another is that it creates heat, which raises resistance. However, newer design approaches have reduced power-transfer losses through the use of ultra-thin coils, higher frequencies, and optimized drive electronics.

Furthermore, the buses must stand still while recharging. It’s simply a fact of inductance-charging life, one that could only be resolved by returning to tram-like systems with overhead power cables—a nostalgically attractive but costly option.

In fact, induction charging itself isn’t cheap. Drive electronics and coils in both device and charger push costs higher.

 On these fronts, developers of South Korea’s buses went a stage further technically. The buses run on a 15-km section of road that enables charging without stopping. This obviously leads to more efficient bus timetable scheduling.

The Korea Advanced Institute of Science and Technology developed the Online Electric Vehicle platform. Their continually charging electric avenue system enables the use of much smaller batteries. It’s claimed they achieve 85% charging efficiency.

The charge plates under the road generally take up only between 5% and 15% of the total route. They remain switched off until the approach of induction-capable bus.

Taking a speculative look into the future, the next approach to charging electric vehicles could involve highways that are continuous solar panels (Fig. 2). Though that may sound a bit far-flung at present, there are already projects exploring this idea.

But why stop at surfacing just highways with solar panels? Driveways and parking areas also could provide a means of solar-charging vehicles. Just one point: What happens when the panels are covered in snow and ice? Easy, temporarily redirect some of the solar power to heat the roads and de-ice the surface.


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