Large-Scale, Commercial Wireless Inductive Power Transfer (WIPT) for Fixed Route Bus Rapid Transportation
by Matt Jurjevich
An Introduction to Inductive Charging
There is a new charging technology making its way on to our streets or, should I say into our streets. It’s called Wireless Inductive Power Transfer (WIPT) or just “Inductive Charging”, and it’s poised to change the way we supply power to our electric vehicles forever. The focus of this article is to provide an overview of how WIPT is already working to revolutionize Bus Rapid Transportation Systems across the world, and assist in the inevitable transition to electrified public transportation systems.
The fundamental idea behind charging while “en route” is not new, it has actually been around since the 1800s . Why engineer a massive fuel tank into a vehicle, then fill it with fluid, or even pack it with batteries, when you could simply supplement fuel as it drives? We typically see this form of en-route powering in metropolitan areas where trolleys, subways and light rail systems operate from supply from overhead catenary power lines or powered rails (like a powered third-rail system). However, these systems are imperfect, being too costly, unsafe or compromised by severe weather conditions. Most catenary power line systems have frankly been dismantled because they are an eye sore or because of the public concern for safety  and contact powered rails are very dangerous. The next generation of en-route charging has arrived.
What would happen if we eliminated the contact required to power overhead or “removed the power rails”, and disconnected and buried charging ubiquitously in the roadway? Not just eliminating the eyesore, but also tackling another issue with overhead catenary power, that of significant friction wear (example video here). Inductive Charging requires no physical contact between the power source and recipient, therefore, eliminating all mechanical wear and tear on the system . Inductive Charging is possible in both a “static” or “dynamic” scenario where either the vehicle is stationary above a charging pad (when charging), or moving along, above an in-road power strip and charging while moving (dynamically) . For the focus of this article, we will discuss the differences in several static methods now in prototype or beta stages for commercial vehicle charging as well as with products currently available in today’s marketplace (see Table 1 below) including offerings from WAVE (Wireless Advanced Vehicle Electrification), OLEV’s (On-Line Electric Vehicle), Momentum Dynamics, Conductix-Wampfler, WiTricity, HaloIPT, Bombarbier Evatran and HEVO, just to name a few. These range in; the rate power can be theoretically transferred (from 7 kW to 300 kW), the maximum magnetic-gap distance (from a few centimeters to a foot) and the efficiency of AC-DC power source (from 80 % to around 90%). All of these technologies hold the same promise – to extend the range of an electric bus to run literally on an infinite loop, 24-hours a day, 7-days a week, charge neutral without returning to a conductive charge location without wasting much energy in the transfer.
Table 1: Basic Comparison of Some Wireless Inductive Power Transfer Providers:
|Attribute:||Magnetic Air Gap
> 10 inch?)
|High Power Transfer – demo’d (theoretical)||Charging
|WAVE IPT||Yes, 8 in. clearance||Yes, 50 kW (175 kW)||Yes, >90%|
|OLEV Technologies||Yes, 7.8 in. clearance||Yes, 83 kW (180 kW)||No, 85%|
|Momentum Dynamics||Yes, 12 in. clearance||30kW and 50 kW||Yes, >90%|
|Conductix-Wampfler||No, “within a few inches”||Yes, >100 kW||No|
|WiTricity||Yes, 7 in. clearance||No, 7kW (30 kW)||Yes, >90%|
|Halo-IPT (Qualcom)||Yes, “SUV clearance”||No, 3.3kW (20 kW)||Yes|
|Bombardier/ Primove||No, “a few centimeters”||180 kW (200 kW)||No, <85%|
|Evatran/ Plugless||No, 7 in. clearance||No, 3.3 kW||?|
|HEVO||No||No, 10 kW||?|
One Example to Demonstrate Technology:
One of the first Wireless Power Transfer demonstrations in the US is the inductive technology by WAVE IPT out of Salt Lake City, Utah. WAVE’s energy transfer is by electricity. Maxwell’s laws tell us that anytime there is a current, it also induces a magnetic field in the space surrounding the flow of the current and vice versa such that anytime a magnetic field moves it induces a current in a nearby wire. Magnetic induction is the principle upon which WAVE’s WPT systems is built.
The figure below illustrates how induction can be used to transfer power wirelessly. First, a power source such as the electrical grid is transformed to have certain desirable characteristics of current, voltage, and fluctuation speed (frequency). This power travels through wires just as in most electrical applications. The flow of current in the wire induces a specially shaped and fluctuating magnetic field that permeates the WAVE WPT hardware and the air, water, road materials, etc. Most materials are not susceptible to this magnetic field and are not damaged or altered by them. However, if another set of wires and field susceptible materials are brought into the field, it now induces a current within them (in a “secondary coupling” pick-up). That current carries energy from the original power source and can be used as a battery charger.
Figure 2: WAVE’s Basic System Diagram
Matt Jurjevich is a Market Research Analyst for BYD America Corporation. He most recently studied at the Columbia Business School and is a graduate of Saint Francis University where he studied Economics in the colleges Honors Program. Matt is an associate editor of the Transportation Electrification Initiative Newsletter.
About the Newsletter
The Transportation Electrification eNewsletter studies topics that span across four main domains: Terrestrial (land based), Nautical (Ocean, lakes and bodies of water), Aeronautical (Air and Space) and Commercial-Manufacturing. Main topics include: Batteries including fuel cells, Advanced Charging, Telematics, Systems Architectures that include schemes for both external interface (electric utility) and vehicle internal layout, Drivetrains, and the Connected Vehicle.