'Charging room' powers devices without wires

Researchers at The University of Tokyo (UTokyo) and the University of Michigan (U-M) have developed a system to safely deliver electricity over the air, potentially turning entire buildings into wireless charging zones. Described in the journal Nature Electronics, the system can deliver 50 W of power using magnetic fields.

The researchers said their technology is a major improvement over previous attempts at wireless charging systems, which use potentially harmful microwave radiation or require devices to be placed on or near dedicated charging pads. Instead, it uses a conductive surface on room walls and a conductive pole to generate magnetic fields. Devices harness the magnetic field with wire coils, which can be integrated into electronics like mobile phones.

According to study author Professor Alanson Sample, from U-M, the key to making the system work was building a resonant structure that could deliver a room-size magnetic field while confining harmful electric fields, which can heat biological tissues. The team’s solution used devices called lumped capacitors; placed in wall cavities, these generate a magnetic field that resonates through the room, while trapping electric fields inside the capacitors themselves. Previous wireless systems were limited to either delivering large amounts of power over a few millimetres or very small amounts of power over long distances.

Lumped capacitors set into wall cavities in the wireless charging room. Image credit: The University of Tokyo.

A second hurdle was how to generate a magnetic field that reaches every corner of the room — magnetic fields tend to travel in circular patterns, creating dead spots in a square room. In addition, receivers need to align with the field in a specific way to draw power.

“Drawing power over the air with a coil is a lot like catching butterflies with a net,” Prof Sample said. “The trick is to have as many butterflies as possible swirling around the room in as many directions as possible. That way, you’ll catch butterflies no matter where your net is or which way it’s pointed.”

To make that happen, the system generates two separate, 3D magnetic fields. One travels in a circle around the room’s central pole, while the other swirls in the corners, travelling between adjacent walls. This approach eliminates dead spots, enabling devices to draw power from anywhere in the space.

The team demonstrated the technology in a purpose-built aluminium test room measuring approximately 3 x 3 m, wirelessly powering lamps, fans and phones that could draw current from anywhere in the room regardless of the placement of people and furniture. The coil receivers needed to be aligned at right angles to the magnetic field to achieve maximum efficiency; however, power delivery efficiency exceeding 37.1% was still achievable anywhere in the room and also while a device was in motion.

The wireless charging room under construction at The University of Tokyo. Image credit: The University of Tokyo.

Tests with anatomical dummies showed that the system could deliver at least 50 W of power to any location in the room without exceeding US Federal Communications Commission (FCC) guidelines for electromagnetic energy exposure. Prof Sample said it’s likely, however, that it will be possible to deliver higher levels of power with further refinement of the system.

Prof Sample added that, further untethering phones and laptops, the technology could also power implanted medical devices and open new possibilities for mobile robotics in homes and manufacturing facilities. The team is also working on implementing the system in spaces that are smaller than room-size; for example, a toolbox that charges tools placed inside it.

“This really ups the power of the ubiquitous computing world — you could put a computer in anything without ever having to worry about charging or plugging in,” Prof Sample said. “There are a lot of clinical applications as well; today’s heart implants, for example, require a wire that runs from the pump through the body to an external power supply. This could eliminate that, reducing the risk of infection and improving patients’ quality of life.”

Furthermore, the system could easily be scaled up to larger structures like factories or warehouses while still meeting existing safety guidelines for exposure to electromagnetic fields. According to corresponding author Takuya Sasatani, from UTokyo, “Something like this would be easiest to implement in new construction, but I think retrofits will be possible as well. Some commercial buildings, for example, already have metal support poles, and it should be possible to spray a conductive surface onto walls, perhaps similar to how textured ceilings are done.”

Top image: The finished charging room, located at The University of Tokyo. Image credit: The University of Tokyo.

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