The wireless transfer of electric current that charges your electric toothbrush is highly efficient: The receiver coil in the handle of the toothbrush fits tightly around the transmitter coil in the charger, making the process about as lossless the operation of the ubiquitous transformer.
However, using wireless power transfer for electric cars by charging them with transmitter coils embedded in the pavement is more problematic. The receiver coil in the car has to be placed as close as possible to the ground; still, only a part of the transmitted energy reaches the receiver coil.
Now researchers from North Carolina State University and Carnegie Mellon University say they have hit upon a way to boost the efficiency of the energy transfer in that situation. They reported, in a paper published in the online edition of the journal IEEE Antennas and Wireless Propagation Letters, that by placing a magnetic resonance field enhancer (MRFE)—a loop of copper wire resonating at the same frequency as the AC current feeding the transmitter coil—between the transmitter and receiver coil, they could boost the transmission efficiency by at least 100 percent. “Our experimental results show double the efficiency using the MRFE in comparison to air alone,” David Ricketts of NC State, said in a press release. The MRFE increases the strength of the magnetic field that reaches the receiver coil, resulting in an increase of the transmission efficiency.
Previously, the team had investigated the use of metamaterials to enhance the magnetic field. “We performed a comprehensive analysis using computer models of wireless power systems and found that MRFE could ultimately be five times as efficient as using metamaterials and offer 50 times the efficiency of transmitting through air alone,” Ricketts says.
For their experimental setup, the team used two coils of 4.25-centimeter- diameter copper wire with six turns for the transmitter and receiver coils. The coils were separated by 12.2 cm and the transmitter coil was powered with a 2.94-megahertz signal. They measured the transmission efficiency by placing a metamaterial between the transmitter and receiver coil and comparing it with a setup where a single, 12-cm-diameter copper-wire loop replaced the metamaterial. They found that the copper wire version improved the efficiency by a factor of almost two.
These laboratory experiments, though much smaller systems than would be used in the future applications the researchers envision, clearly indicate how transmission efficiency could be tweaked. “This [research] could help advance efforts to develop wireless power transfer technologies for use with electric vehicles, in buildings, or in any other application where enhanced efficiency or greater distances are important considerations,” Ricketts says.