https://www.designnews.com/electronics-test/power-out-thin-air/153947192358622
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Power Out Of Thin Air
Wireless charging of electric buses proves its worth
By: Kevin Clemens
Electronics & TestBattery/Energy Storage, Sustainability, Automotive,
Automotive and Mobility
May 01, 2018
One of the ongoing debates about the general acceptance of electric
vehicles (EVs) is the need for wireless charging. Proponents of such
systems point to the ability to park over a pad embedded in a parking
spot and pick up 20-30 miles of range while grocery shopping. They say
that this so-called "opportunity charging" helps to reduce range
anxiety, making EVs more attractive. If you put the same charging pad in
your garage, you simply park over it at night and the car automatically
adds the electricity needed for the next day’s commute.
The convenience of wireless charging comes with a price tag, however.
Wireless charging adds a need for costly equipment both in the home
garage and onboard an already expensive electric vehicle. Current owners
of EVs find that remembering to plug in their charger at night is a
small price to pay for never having to stop at a gas station. They
always have a full “tank” in the morning.
Wireless charging an EV works in a similar way to wirelessly charging a
cell phone. Electricity is transferred from the coil in the charger on
the ground to a coil located on the bottom of the vehicle through
induction. It is important that the coils are parallel and closely
aligned to one another. The air gap between the coils is also critical
(4-6 inches is typical). If everything lines up properly, charging
efficiencies for inductive charging can be as high as 85-90 percent.
One application where wireless charging can show real advantages is with
heavy vehicles—and most particularly city buses. A Pennsylvania company,
Momentum Dynamics, has just announced the commissioning of a
200-kilowatt wireless charging system for a bus fleet in Wenatchee,
Washington. The system is operational on a BYD K95 electric bus operated
by Link Transit.
The 200-kilowatt charger adds enough energy to the bus battery during a
five minute stop to allow it to complete another route cycle. The
charging system is embedded into the pavement at a Link Transit transfer
station stop. Each time the BYD bus parks over the charger, its battery
is automatically recharged.
“The simplicity along with the seamless charging allows Link Transit to
operate all of our urban service routes continuously without the need
for midday charging,” said Todd Daniel, Technology Manager for Link in a
press release. “Link is planning to expand this technology for our
longer rural routes in the near future, which will assist in the
agency’s goal of 100% fleet electrification,” he added.
The technology involved has proven to be quite adaptable. “On this BYD
bus, the distance between the charging coil mounted flush with the
cement and the vehicle coil is approximately 7 inches,” Ben Cohen, vice
president of engineering for Momentum, told Design News. That distance
is typical for low-floor municipal buses, but can vary up to a distance
of 12 inches on other types of vehicles. Cohen reported that the overall
charging efficiency is 92%. Almost all of those losses were from the
power electronics—virtually no losses were coming from the wireless
power transfer.
The cost of the Momentum Dynamics wireless equipment is similar to the
cost of equipment on the vehicle used for fast DC charging. Cohen told
Design News that those costs are dramatically affected by product
volume. The company is also working on a larger 300-kilowatt wireless
charger that should be available in 2019. By next year, Momentum also
plans to have rolled out wireless charging to several other bus lines in
the US and Europe.
Senior Editor Kevin Clemens has been writing about energy, automotive,
and transportation topics for more than 30 years. He has masters degrees
in Materials Engineering and Environmental Education and a doctorate
degree in Mechanical Engineering, specializing in aerodynamics. He has
set several world land speed records on electric motorcycles that he
built in his workshop.
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Len Moskowitz
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