“The prospect of having a room where a variety of devices could just receive power anywhere is really compelling and exciting,” says Joshua Smith, a professor of computer science and electrical engineering at the University of Washington, who was not involved in the new study. “And this paper takes another step toward making that possible.” In the study, the researchers describe a custom test room of about 18 cubic meters (roughly equivalent to a small freight container), which Sasatani built from conductive aluminum panels with a metal pole running down the middle. The team furnished the room with a wirelessly powered lamp and fan, as well as more prosaic items, including a chair, table and bookshelf. When the researchers ran an electric current through the walls and pole in a set pattern, it generated a three-dimensional magnetic field within the space. In fact, they designed the setup to generate two separate fields: one that fills the center of the room and another that covers the corners, thus allowing any devices within the space to charge without encountering dead spots.
By carrying out simulations and measurements, Sasatani and his co-authors found their method could deliver 50 watts of power throughout the room, firing up all of the devices equipped with a receiving coil that they tested: a smartphone, a light bulb and a fan. Some energy was lost in the transfer, however. Delivery efficiency varied from a low of 37.1 percent to a high of about 90 percent, depending on the strength of the magnetic field at specific points in the room, as well as the orientation of the device. Without precautions, running current through the room’s metal walls would typically fill it with two types of waves: electric and magnetic. This presents a problem, because electric fields can produce heat in biological tissues and pose a danger to humans. So the team embedded capacitors, devices that store electric energy, in the walls. “It confines the safe magnetic fields within the room volume while confining hazardous parts inside all the components embedded inside the walls,” Sasatani explains.
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