Induction, but not as you know it

Passengers Standing On Busy Commuter Bus

On the face of it, ‘wireless electricity’ sounds like futuristic magic.

But we’ve already had electric toothbrushes charging wirelessly since the early ’90s, and the latest generation of smartphones – such as the Samsung Galaxy S6 – have been drawing electricity from our businesses and homes without cables for months.

The next logical step was to pair this wireless technology with the growing use of electric road vehicles.  It’s already been put to the test in Milton Keynes and Gumi, South Korea, and now Transport for London is experimenting with inductive charging for London’s red buses.

How does it work?

At a basic level, the technology behind wirelessly charging toothbrushes, phones and electric buses is the same.

Electricity passing through an electromagnetic coil in the charging station creates a magnetic field.  This magnetic field acts upon a coil in the device – whether it’s a phone or a bus – to create an electric current.

The coil in the device is attached to a battery-charging circuit within the device, and the electricity ‘transferred’ is stored in the battery, charging it up.

Since the electric current isn’t directly moving from the charger to the device by a wire, we say that the current in the charger is inducing the current in the device – that’s why it’s called inductive charging.

So what’s that got to do with electric buses?

Without this inductive technology, electric buses need to stop at a charging station (usually at the depot) and plug in.

But by building electric coils under the ground at bus stops, electric buses fitted with inductive charging will be able to get a quick top-up every time they stop to let passengers on and off.

Of course, it’s not a perfect solution.  Buses will have to position their receiving coils over the underground coil with some accuracy, and the quick stops of the average bus route will only allow for small injections of power.

But when compared to making a long trip back to the depot, it could mean that electric buses will be able to run for longer, and hybrid buses won’t have to rely on their fossil-fuel engines quite so much.

But is it any better than traditional charging?

In terms of sheer power transferred, inductive charging just isn’t as efficient as conventional cables.  The trial in South Korea, for example, found a charging efficiency of just 85 per cent – some energy is lost as heat during the transfer.

This lower efficiency can also lead to slower charging times when attached to similar power supplies.

But despite this, inductive charging could save transport companies time and money.  Every small top-up happens at the same stops that the bus makes along its usual route, so they’re able to charge their batteries while still staying on schedule, and there’s no need for the driver to get out and spend time fiddling with physical cables.

There’s also likely to be less money spent on maintenance over time.  There are no electric connections exposed to the corrosive effects of oxygen or moisture, and there’s no physical wear and tear from plugging things in and out.

Finally, the ability to charge more frequently means that buses can get by with much smaller batteries.  The electric buses in South Korea, for example, use batteries that are roughly a third of the size of those found in the average electric car.  That could free up space for more passengers, or it could mean a higher level of comfort for the same number of people.

Would you like to see more electric buses on the roads?  Or do you think that wireless charging is just another smartphone fad?  Join the conversation on LinkedIn and Google+.

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