Stanford University develops wireless charging system for electric vehicles to extend driving range

How pleasant it would be if there was a highway where electric cars could be charged automatically while driving, and the battery in the car would have more power when arriving at the destination than when setting out! A research team at Stanford University in the United States is designing and developing this efficient charging system. By burying a metal coil every few feet under the road surface, a large amount of electricity can be transmitted wirelessly using magnetic fields. Although the principle of this technology is simple, it has great potential and can greatly extend the driving range of electric vehicles, ultimately bringing about changes in high-speed transportation. A recent paper in the Journal of Applied Physics Letters provides a detailed introduction to this technology and its application prospects.

Unlimited mileage

Current electric vehicles have limited range. For example, the all-electric Nissan Leaf can travel no more than 100 miles on a single charge, and it takes several hours to fully charge the battery. A system that charges while driving would overcome this limitation.

The concept of “driving to charge” is exciting because it means your car could go on indefinitely without having to recharge, said Richard Sassoon, co-author of the paper and managing director of Stanford’s Global Climate and Energy Project (GCEP). “When you arrive at your destination, you may have more power in the battery than when you set out.”

The wireless charging system being designed by Stanford University is expected to solve the problem of charging electric vehicles with wires. Its long-term goal is to develop an all-electric highway that can wirelessly charge cars and trucks on the road. "Our vision is to allow you to charge your car while driving on any highway," said Shanhui Fan, an associate professor of electrical engineering. "But laying such facilities on a large scale requires refurbishing the entire highway system, even beyond the transportation field."

Technical principles of wireless charging system

The technology behind wireless power transmission is called "magnetic resonance coupling connection." Two copper coils are tuned to the same frequency to form resonance, and are placed a few feet apart. When one coil is connected to a current, a magnetic field is generated to resonate the second coil. This magnetic resonance allows electrical energy to be transmitted invisibly through the air, from the first coil to the magnetic induction coil.

As early as 2007, MIT researchers used magnetic resonance to light up a 60-watt light bulb, proving that electricity can be transmitted between two stationary coils 6 feet apart, even if there are people or other obstacles in between. "This experiment also proves that the magnetic field will not affect people standing between the coils, which is very important from a safety perspective." Fan Shanhui also pointed out that "wireless charging transmission can only be transmitted between two resonant coils, and any objects with different frequencies will not be affected." MIT has set up a branch to develop a stationary charging system, which can currently charge cars parked in garages or on the street wirelessly with a power of 3 kilowatts.

The working principle of the charging system is to bury a series of coils connected to electric current under the highway surface, and install an induction coil at the bottom of the car. When the car passes through the highway, it will resonate and generate a magnetic field to continuously transmit electricity to the battery.

How pleasant it would be if there was a highway where electric cars could be charged automatically while driving, and the battery in the car would have more power when arriving at the destination than when setting out! A research team at Stanford University in the United States is designing and developing this efficient charging system. By burying a metal coil every few feet under the road surface, a large amount of electricity can be transmitted wirelessly using magnetic fields. Although the principle of this technology is simple, it has great potential and can greatly extend the driving range of electric vehicles, ultimately bringing about changes in high-speed transportation. A recent paper in the Journal of Applied Physics Letters provides a detailed introduction to this technology and its application prospects.

Unlimited mileage

Current electric vehicles have limited range. For example, the all-electric Nissan Leaf can travel no more than 100 miles on a single charge, and it takes several hours to fully charge the battery. A system that charges while driving would overcome this limitation.

The concept of “driving to charge” is exciting because it means your car could go on indefinitely without having to recharge, said Richard Sassoon, co-author of the paper and managing director of Stanford’s Global Climate and Energy Project (GCEP). “When you arrive at your destination, you may have more power in the battery than when you set out.”

The wireless charging system being designed by Stanford University is expected to solve the problem of charging electric vehicles with wires. Its long-term goal is to develop an all-electric highway that can wirelessly charge cars and trucks on the road. "Our vision is to allow you to charge your car while driving on any highway," said Shanhui Fan, an associate professor of electrical engineering. "But laying such facilities on a large scale requires refurbishing the entire highway system, even beyond the transportation field."

Technical principles of wireless charging system

The technology behind wireless power transmission is called "magnetic resonance coupling connection." Two copper coils are tuned to the same frequency to form resonance, and are placed a few feet apart. When one coil is connected to a current, a magnetic field is generated to resonate the second coil. This magnetic resonance allows electrical energy to be transmitted invisibly through the air, from the first coil to the magnetic induction coil.

As early as 2007, MIT researchers used magnetic resonance to light up a 60-watt light bulb, proving that electricity can be transmitted between two stationary coils 6 feet apart, even if there are people or other obstacles in between. "This experiment also proves that the magnetic field will not affect people standing between the coils, which is very important from a safety perspective." Fan Shanhui also pointed out that "wireless charging transmission can only be transmitted between two resonant coils, and any objects with different frequencies will not be affected." MIT has set up a branch to develop a stationary charging system, which can currently charge cars parked in garages or on the street wirelessly with a power of 3 kilowatts.

The working principle of the charging system is to bury a series of coils connected to electric current under the highway surface, and install an induction coil at the bottom of the car. When the car passes through the highway, it will resonate and generate a magnetic field to continuously transmit electricity to the battery.

The Stanford team is working on further improving the MIT system to increase the power to 10 kilowatts and the transmission distance to 6.5 feet, so that it can charge cars driving on the highway. When accelerating or driving upward, the car needs the battery to provide additional power.

"The asphalt on the road may have some minor effects, but it is the metal elements in the car body that cause huge disturbances to the electromagnetic field." Fan Shanhui explained, "How to find the most optimized power transmission solution when large metal objects appear is the main goal of our research." In order to find the best solution, they built a computer model of the system. After a lot of calculations and simulations, they bent the coil into a 90-degree angle and mounted it on a metal plate, which was able to transmit 10 kilowatts of power to the resonant coil 6.5 feet away. "Through computer simulation, we can know exactly what will happen to a real device."

Fan Shanhui introduced that the charging efficiency of this wireless transmission solution can reach 97%, but to actually charge the car battery, a series of coil arrays must be buried under the road surface.

A bright future

The research team recently filed a patent application for the wireless system, and the next step is laboratory testing and ultimately use in real driving environments. Before it can be used, the researchers must ensure that the system will not affect the driver, passengers and the various instruments that control driving, navigation, air conditioning and other operating functions.

"We need to determine as early as possible that the system is harmless to people, animals, automotive electronics or the credit cards in your wallet." Sven Baker, co-author of the paper and executive director of the Stanford Center for Automotive Research, said that although 97% power transmission efficiency is already extremely high, they also need to study whether the 3% loss is converted into heat. If it is radiation, it will cause potential harm.

In addition, future wireless charging technology may also assist GPS navigation in self-driving cars. "GPS has a basic accuracy of 30 to 40 feet. It can tell you where the car is on the earth, but it can't tell you if it is safe, because you have to make sure the car is driving in the lane." Baker explained. In the system they designed, the magnetic field can also be used to control driving, because the coil is buried in the center of the lane, which can help the car accurately locate without additional cost.

Currently, researchers have begun to study the optimal layout of roadbed transmission and whether steel or other metal in the road surface affects the transmission efficiency.

Road experts envision that in the future, automated highway systems will be able to use solar power or other renewable energy sources to wirelessly charge self-driving electric vehicles on the road. This will not only reduce traffic accidents and increase traffic speed, but also reduce greenhouse gas emissions.

In the past, people believed that electricity transmission must rely on wires, wall sockets, etc., but now we can rethink how to power cars, homes and factories. Fan Shanhui said: "In the future, electricity can be transmitted in a vacuum without wires and sockets. Our research has taken a step in this direction."