What if your smartphone or laptop started charging as soon as you walked in the door? Researchers have developed a specially built room that can transmit energy to a variety of electronic devices within it, charging phones and powering home appliances without plugs or batteries.

This system “enables safe and high-power wireless power transfer in large volumes,” says Takuya Sasatani, a project assistant professor at the University of Tokyo’s Graduate School of Engineering and lead author of the new study, which was published this week in Nature Electronics. The room relies on the same phenomenon as short-range wireless phone chargers: a metal coil, placed in a magnetic field, will produce an electric current.

Existing commercial charging docks use electricity from a wall outlet to produce a magnetic field in a small area. Most recent smartphones are equipped with a metal coil, and when such a model) is placed on the dock, the interaction generates enough current to power the phone’s battery. But today’s commercial products have a very limited range. If you lift a phone off the dock or swathe it in a case that is too thick, the wireless power transfer ceases. But if a magnetic field filled a whole room, any phone within it would have access to wireless power.

“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.”

Credit: Takuya Sasatani, et al., Nature Electronics

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.

The researchers also tested the room’s safety by running computer simulations, measuring what the human body would be exposed to in a digital model of the powered room. Authorities such as the Federal Communications Commission have established standards for how much electromagnetic radiation the human body can safely be exposed to, and the simulation suggested the absorption of energy in the test room would remain well below acceptable limits. “We’re not saying blanketly that this technology is safe under all uses—we’re still exploring,” says study co-author Alanson Sample, an associate professor at the University of Michigan’s electrical engineering and computer science department. “But it gives us some confidence ... that there’s still lots of area for us to be well underneath that threshold of power, where we can still charge your cell phone just as easily as you walk into a room, without having to worry about those safety issues.”

Beyond phones, Sample suggests a dedicated wireless charging room would allow a variety of electronic devices—sensors, mobile robots or even medical implants—to function in the background, recharging themselves without a wired connection and letting humans largely ignore them. The technique could also be applied to more specialized situations. “I can imagine this being really useful for highly instrumented, expensive spaces like, for example, an operating room,” Smith says, “where you can imagine having various instruments and devices just be able to be powered without needing cords.”

But those applications are still far in the future. “It’s just too burdensome to put aluminum sheets all over your walls—that benefit doesn’t make sense yet,” Sample says. “We’ve just developed a brand-new technique. Now we have to go figure out how to make it practical.” He plans to continue researching whether coating existing rooms with conductive material or building specialized walls that contain conductive layers could enable the construction of wireless-charging rooms that also comply with building codes. Meanwhile Sasatani hopes to improve the efficiency of power transmission in the room and eliminate any lingering spots that the charge does not reach.

Wireless charging is an extremely competitive concept, with multiple start-ups vying to transmit power via electromagnetism, lasers or sound waves. “A lot of people are interested in beam-forming type approaches, where you actually generate a propagating radio wave and steer it around,” Smith says. “The advantage of the approach here in this paper is that the fields are predominantly magnetic, which is safer and allows higher power for the same safety level, compared to actually transmitting a propagating radio wave, where you have roughly equal electric and magnetic fields.” On the other hand, he points out, a charging beam would not require a custom-built metal room with a pole running down the middle. Each technique might have its own uses.

“There are other charging mechanisms that are more far-field, that give you much longer range,” Sample says. “But there’s really not a mechanism that gives you, say, 10 watts of power anywhere in a space.”