Electrical engineers at Duke University have determined that unique man-made materials should theoretically make it possible to improve wireless power transfers to small devices, such as laptops or cell phones, or ultimately to larger ones, such as cars or elevators.
This advance is made possible by the recent ability to fabricate exotic composite materials known as metamaterials that can be engineered to exhibit properties not readily found in nature. The metamaterial likely to be used in future wireless power transmission resembles a miniature set of tan Venetian blinds. It might be similar to the first demonstration of a cloaking device <http://www.pratt.duke.edu/node/2234> in 2006 Duke studies.
Theoretically, this metamaterial can improve the efficiency of recharging devices without wires. As power passes from the transmitting device to the receiving device, most if not all of it scatters and dissipates unless the two devices are extremely close together. However, the metamaterial postulated by the Duke researchers, which would be situated between the energy source and the recipient device, focuses the energy transmitted and permits the energy to traverse the space between with minimal loss of power.
"We currently have the ability to transmit small amounts of power over short distances, such as in radio frequency identification (RFID) devices," said Yaroslav Urzhumov, assistant research professor in electrical and computer engineering at Duke's Pratt School of Engineering. "However, larger amounts of energy, such as that seen in lasers or microwaves, would burn up anything in its path."
The metamaterials can help increase power levels without doing damage."Based on our calculations, it should be possible to use these novel metamaterials to increase the amount of power transmitted without the negative effects," Urzhumov said.
The results of the Duke research were published online in the journal Physical Review B. Urzhumov works in the laboratory of David R. Smith, William Bevan Professor of electrical and computer engineering at Pratt School of Engineering and developer of the metamaterial cloaking device.
Just as the metamaterial cloaking device appeared to make a volume of space "disappear," in the latest work, the metamaterial would make it seem as if there was no space between the transmitter and the recipient, Urzhumov said. Therefore, he said, the loss of power should be minimal.
Urzhumov's research is an offshoot of "superlens" research also conducted in Smith's laboratory. Traditional lenses get their focusing power by controlling rays as they pass through the two outside surfaces of the lens. But the superlens metamaterial directs waves to the volume between the lens's outside surfaces, giving researchers much greater control over whatever passes through it.
The metamaterial used in wireless power transmission would likely be made of hundreds to thousands of individual, thin, conducting loops arranged into an array. Each piece is made from the same copper-on-fiberglass substrate used in printed circuit boards, with excess copper etched away. These pieces can be arranged in an almost infinite variety of configurations.
"The system would need to be tailored to the specific recipient device, in essence the source and target would need to be 'tuned' to each other," Urzhumov said. "This new understanding of how metamaterials can be fabricated and arranged should help make the design of wireless power transmission systems more focused."
The analysis performed at Duke was inspired by recent studies at Mitsubishi Electric Research Labs (MERL), an industrial partner of the Duke Center for Metamaterials and Integrated Plasmonics, which is currently investigating metamaterials for wireless power transfer. The Duke researchers said that with these new insights into the effects of metamaterials, developing actual devices can be more targeted and efficient.
The Duke University research was supported by a Multidisciplinary University Research Initiative (MURI) grant through the Air Force Office of Scientific Research and the U.S. Army Research Office.
