TOKYO -- Supplying power to electric vehicles is a conundrum researchers have been dealing with for years.
Batteries are an obvious way to power vehicles. But they are heavy and expensive, and can only get vehicles so far before they need recharging.
Another option is power lines, like the overhead wires trams and trains use.
And then there is the idea of supplying energy wirelessly. Electric roads may in the future power cars as they drive along.
Such roads would be liberating. If they can be developed, electric vehicles would likely leave hybrid and gas cars behind. A technology being developed in Japan has the potential to make electric roads a reality.
In the floor
Power on electric roads could be delivered to motors through tires.
The tire idea was first proposed by professor Takashi Ohira of the Toyohashi University of Technology in 2011. His university team then joined forces with general construction company Taisei to develop the technology.
A demonstration last October at Ceatec Japan, the nation's largest information technology and electronics exhibition, introduced the system. The team showed how the technology can move a small electric cart. The cart only traveled for 5 meters, but it did so without a battery.
Its tires were made from rubber, an insulator, but reinforced with metal belts and wires. That is the secret to Ohira's success. He and his team use electric-field coupling to power the vehicle. An ultrahigh frequency current of several megahertz generates an electric field between two metals that are close together but not in contact.
For the setup at Ceatec in October, an inverter turned a regular-frequency current into an ultrahigh frequency current, which was sent running through a metal track buried below the floor. Above the floor, electric-field coupling generated a current in the metal belts and wires in the tires, which was delivered to the motor to propel the cart.
This January, Ohira and Taisei went a step further with experiments where they managed to power a vehicle traveling at 6kph around a 20-meter course.
Supplying a vehicle with power from the ground while it is running eliminates the need to recharge batteries. As long as vehicles are on top of electrified floors or roads, they "can continue to run and run," Ohira explained.
The goal is to first promote the technology to power vehicles that run set routes inside buildings such as airports and factories. "We want to have a commercial version ready in fiscal 2016," said Tetsuo Endo, a manager at the Taisei Technology Center.
It is relatively easy to install the electrode tracks indoors, and the experiments performed to date can be commercialized.
Plans are also afoot to tackle the great outdoors. First, an outside course will be built this year. Work will then begin on developing the means to power a small electric vehicle traveling at 30kph. In the future, highways could be fitted with electrode plates to power vehicles.
Large batteries that need constant recharging are an impediment to the widespread adoption of electric vehicles. To free electric cars from these shackles, various companies and academic groups are investigating wireless power supply technologies.
At the forefront is a method based on electromagnetic induction, developed in the U.S. Power is transferred by induction between two coils of wire: When a current passes through one coil, electricity is generated in a second one nearby.
Toyota Motor tested this kind of inductive charging in Japan, the U.S. and Europe in February 2014; U.S. company Qualcomm has tested 50 vehicles on public roads in the U.K.
Inductive charging has been put to practical use already for consumer electronics products. Power can be supplied wirelessly across distances of several dozen centimeters.
However, this technology is not meant to be a source of constant power. It would still require batteries. In addition, the power transmitter and receiver units are relatively heavy pieces of equipment.
Electric field coupling is a more useful technology for a number of reasons. It does not involve the use of coils, there are fewer costs associated with that infrastructure or for modifying the design of electric vehicles.
The technology is not without its problems. Money will need to be spent to install electrode tracks under roads.
In addition, the supply of power is interrupted whenever tires veer away from the metal tracks buried in the road. A possible way to deal with that is by using capacitors as supplementary charging devices in the cars. Capacitors charge and discharge quickly and are far lighter than batteries.
When asphalt gets wet, another problem emerges. The electric fields from the current passing through the metal plates in the road get weaker. Solving that will require measures such as the use of highly water-permeable asphalt and drainage equipment.
In addition, studies are required to determine what kinds of effects the electromagnetic radiation will have on components in electric vehicles and regular cars.
If these issues are resolved, electric-field coupling could change the way we drive.