E-Roadways: How Do Inductive-Charging Roads for EVs Work?

A flurry of research and pilot projects worldwide are exploring the dream of “e-roadways,” stretches of road equipped to wirelessly charge compatible electric vehicles on the move using dynamic inductive charging. The roads are an industrial-strength relative of the kind of inductive Qi charging technology that lets you wirelessly charge your devices. But how do these inductive-charging roads work for EVs?

Related: Which States Have the Most Public EV Chargers?

The appeal goes beyond potentially cutting back anxiety over EV range and necessary charging stops. Induction charging also could allow EVs — particularly big trucks and buses — to be practical with smaller batteries, resulting in less battery weight to haul around and be potentially more energy-efficient, cheaper vehicles with greater payload capacity. While the technology is getting a lot of attention, almost all inductive-charging roads in operation now are tests, temporary or pilot projects aimed to gauge whether the performance, costs and benefits justify widespread adoption.

The Basics of Inductive Charging

The basic principles of inductive charging are based on the facts that an electric current through a wire creates a magnetic field, which could transmit or induce electricity. Around the turn of the 20th century, electrical pioneer Nikola Tesla demonstrated it was possible to use alternating current in a coil to transmit power wirelessly across an air gap to a separate coil through a magnetic field (i.e., “magnetic resonant inductive coupling”).

With much further development, these basic principles still underlie modern wireless charging, which involves two coils resonating or vibrating at the same frequency. For an EV, power is transferred by creating a magnetic resonance field between a grid-powered transmitter coil on the ground and a receiver coil under the vehicle. The energy crosses the gap between the pads and is then converted from AC into direct current on the vehicle to charge the battery.

Some of the most significant advances in improving the transmission efficiency, power and practical distance for magnetic resonance wireless charging were made at MIT in the early 2000s. MIT’s research led to WiTricity, a startup created to further develop and commercialize their inductive-charging technology. The company has since become one of several startup leaders in wireless EV charging technology, with investments from major companies including Toyota.

Charging Pads Are a First Step

Most wireless inductive-charging applications in the real world so far have been charging pads. Meant to be used for stationary vehicles ranging from golf carts and factory equipment to light-duty EVs, trucks and buses, charging pads provide wireless charging for a home garage and public lots and can even be embedded in roads where a vehicle might stop or park. Most charging pads have power delivery on par with a wired Level 2 charger.

All wireless inductive charging also requires the vehicle to be equipped with compatible equipment to receive the power. SAE International is developing standards for the global interoperability of wireless charging systems for various power levels and types of vehicles; the association has already published the first combined standard outlining the supply and vehicle equipment for wireless EV charging with up to 11 kilowatts. Initial guidelines written by the SAE could lead to standards for high-power charging for heavy vehicles, as well.

Several premium carmakers have put a toe into wireless charging with partnerships, investments and test projects. In fact, after buying a German wireless charging company last summer, Tesla confirmed in December that the company is working on a wireless inductive home charger.

Meanwhile, several startups have also developed their own higher-power wireless induction systems intended for parked vehicle use, mostly for truck and bus fleets. InductEV, for example, currently has more than a dozen projects, including those for bus systems in Washington state and Indianapolis with pads to charge buses at stops on their routes. The company says that with multiple charging pads and more receivers on the bigger vehicles, it could deliver up to 450 kW of power wirelessly. Additionally, WiTricity has a pilot project at California’s port of Long Beach for vans.

Hitting the Road

Dynamic induction charging built into a road for EV charging works on the same principles but with a moving vehicle using what is essentially a line of interconnected charging pads. The inductive e-roadway is fitted with a series of charging coils; as a compatible vehicle passes over each charging coil, it gets a jolt of power, which adds up to significant charging as the vehicle rolls over the series of coils. The Energy Department’s National Renewable Energy Laboratory has a video illustrating how EVs and plug-in hybrids could use e-roadways (as well as wireless charging at home).

The various test projects underway worldwide offer varying power levels, designs, spacing and control software for the charging pads. Some designs use multiple receivers on larger vehicles to span multiple road coils at a time in order to get higher levels of power. As with wired charging, there would need to be communication between the chargers and the vehicle to allow the wireless power transfer, to position the vehicle in the lane (though advances in technology make precise alignment less critical) and to handle billing for the user.

While e-roadway designs could include charging for personal EVs, much of the initial attention has focused on commercial use for trucks or buses with defined routes, as well as for future autonomous EVs, such as shuttles, that could benefit from also being able to charge themselves without intervention.

Road to the Future?

Most e-roadways being built today are in search of results that would justify commercial building. European countries generally have been more active in testing of e-roadway technology in preparation for the European Union’s plan to ban sales of new gas-powered vehicles in 2035. Meanwhile, Sweden is building the world’s first permanent electric charging highway.

In November, what backers say is the first stretch of U.S. public road with inductive charging made its public debut. A quarter-mile of 14th Street in Detroit’s Corktown (home to the Ford-sponsored Michigan Central area of mobility businesses) was completed, though it is open for use only by test vehicles. This joint project of the Michigan Department of Transportation, Detroit, wireless charging company Electreon and other technology partners will retrofit a planned milelong lane of the road, as well as a stretch of Michigan Avenue in Detroit.

Among other U.S. projects:

• Florida has a test project to install induction charging on a 5-mile stretch of State Road 516. It is sponsored by the Central Florida Expressway Authority, which is collaborating with the Aspire center and Norway-based induction charging company Enrx.
• The Pennsylvania Turnpike has a project underway to test stationary wireless charging pads in parking spots at service plazas and is working with the University of Pittsburgh on plans to test a dynamic induction charging stretch of highway.
• Purdue University and the Indiana Department of Transportation began construction in April of a quarter-mile test section of U.S. Highway 231/U.S. Highway 52 in West Lafayette. It would provide high-power induction charging at highway speeds and will be tested with a heavy truck equipped with a 750-kilowatt-hour wireless charging system developed by a partnership led by Indiana-based manufacturer Cummins (better known for its diesel engines).

Competing E-Roadway Technologies

While inductive-charging e-roadways are getting the most attention for the dynamic charging of vehicles, there are a few more old-school conductive (i.e., physical contact) charging technologies being tested.

The oldest method is a catenary system using pantograph connectors on a vehicle, which stay in contact with overhead wires for conductive power transfer. These systems have long been used for rail trolleys or heavy locomotives, including on the electrified Northeast Corridor passenger rail system from Washington, D.C., to Boston. But they also are used for vehicles on roads, such as the trolley buses in San Francisco; some trolley buses also include a battery that allows limited operation off the wire.

More recently, overhead wires have been studied with pilot projects in Germany and Sweden, and Siemens also created a demonstration project in Carson, Calif. These projects used hybrid heavy trucks with batteries, and in some cases engines, to allow operation off the wire. While this is a more proven technology, it requires costly infrastructure and maintenance, plus specially equipped vehicles, and it leaves the streetscape visually cluttered.

A second method is ground-level conductive charging that involves a power rail on the road surface or in a groove in the road; this also requires physical contact with a connection on the vehicle. Such systems already are in use for railed trams and are sold as a modern alternative to overhead wires. They’re similar to the third-rail systems used by subways and other light-rail mass transit but with a design that makes them safe for open use in roads. The system’s power rail has charging segments that transmit power only when the vehicle is directly over them, making it safe to be exposed in the roadway. French company Alstom has participated in a project with Sweden and Volvo Trucks to adapt its ground-level tram-charging system for road vehicles.

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Is the Future Wireless?

Much work remains to be done on research for higher induction power levels, reliable low-maintenance equipment and standards for interoperability that would make an induction highway for public use practical (we still have global differences on simpler wired charging systems and plugs, after all). There are other questions, as well.

Cost

The potential benefits of e-roadways for commercial trucks and buses in exchange for the substantial investment in the roads seem the clearest, as electrifying heavy vehicles eliminates their high emissions. A more limited network of routes and fewer lanes would be needed to have impact, and road charging could have payback in cutting the cost and weight of batteries needed for a given load capacity.

The case is less obvious for personal EVs, however. Adding wireless charging capability would increase the cost and weight for the vehicle, but likely without making lighter batteries (and less range) much more appealing for users who wouldn’t want to give up any of their current ability for general use. The primary appeal of induction charging roads for personal EVs would be for long-distance trips without charging stops. For both types of users, automakers and users would need to see the benefits versus the cost to get enough buy-in to justify building the roads.

Maintenance

As we know from other infrastructure projects (e.g., subways, highways and bridges), governments and taxpayers are more willing to support building something new rather than spending money year in and year out to keep it working in top form. Beyond equipment upkeep, an e-roadway would be subject to the wear and tear any road suffers from heavy traffic and weather (particularly snow removal and salting).

Health

Wireless charging poses fewer traditional hazards than charging with wires and plug handles. But for decades, health concerns have been raised about radiation from electromagnetic fields, particularly cancer risks. Concerns have also been raised about extremely low-frequency fields (such as those created by power lines and appliances) and higher radio-frequency fields (like those created by cellphones, computer monitors and TVs). Such radiation is regulated with prescribed safe levels, and health authorities have assured there is little or no risk from safe levels, but questions remain.

Miles of inductive-charging roads would seem likely to raise concern, even though the magnetic field level is only high in the area directly between the charging pad and a vehicle’s receiver — not nearby. Current designs also incorporate safety tech that shuts off power if something inanimate or live (such as a cat) comes between the charger and vehicle. Even then, the cat would not feel anything, but absent the shut-off feature, it could be exposed to the high radiation between the pad and vehicle.

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