Masahiro Hanazawa at Toyota Central R&D Labs and Takashi Ohira at Toyohashi University of Technology (Toyohashi Tech) are proposing a novel solution for powering EVs capable of running unlimited distances without being hampered by battery limitations. Their basic concept stems from electric railways, where each car of the train is powered from an overhead wire.
To avoid the use of dangerous contacting devices such as pantographs for on-road transportation, the duo propose up-converting the energy from power lines into radio frequency (RF) using high-speed inverters implanted along tracks in the road. The RF voltage is applied to a balanced metal track embedded under the surface of the road. The EV picks up the RF voltage via electrical capacitance between the metal and a steel belt installed inside of the tires of the EV.
The researchers conducted feasibility experiments to test their ideas, and to explore the RF frequencies where such power transfer is effective and practical. In the experiments, the researchers put small metal plates on the floor and inside a tire, and positioned another metal plate above the tire. Finally, they measured the electrical impedance between the two plates. This set-up should be equivalent to double the impedance between a plate and a steel belt.
Experimental results showed the impedance to depend linearly on the RF frequency. Then the researchers designed and implemented a 50 ohm reactance circuit to match this, where 50 ohm is the standard impedance for RF transmission lines. This experimental set-up enabled a high transmission efficiency with sub 1 dB loss.
Although these were low power experiments, they demonstrate the feasibility of energy transfer from the road to a running automobile, the researchers said. If this energy transfer could be increased to tens of kW on express ways, then in the future it may be possible to take EV from a house to the nearest interchange with a small battery and then cruise on the expressway via this feeder system.
Masahiro Hanazawa and Takashi Ohira, “Power Transfer for a Running Automobile”, IEEE MTT-S International Microwave Workshop Series on Innovative Wireless Power Transmission, IMWS-IWPT2011, pp.77-80, Kyoto, May 2011.