You may someday be able to just walk into a room and have your mobile device recharge while it sits in your pocket if researchers from Disney Research perfect their new charging-at-a-distance scheme. They say quasistatic cavity resonance (QSCR) can let special structures such as cabinets, rooms, and warehouses, generate magnetic fields that safely deliver kilowatts of power to mobile receivers inside the structure.
Writing in the online journal PLOS One, they describe a theoretical model of a quasistatic cavity resonator and results of tests they conducted verifying power transfer efficiency. Their experimental demonstration shows that a 54 m3 QSCR room can deliver power to small coil receivers in nearly any position with 40% to 95% efficiency. And a detailed safety analysis shows that up to 1,900 W can be transmitted to a coil receiver.
The room the Disney researchers constructed for their studies is basically an RF screen room. It is made of aluminum sheets. The QSCR apparatus created near-field standing waves that filled the interior of the room with uniform magnetic fields, allowing for strong coupling to small receivers inside. Basically, researchers excited the room at its resonant electromagnetic mode such that induced currents flowed through the walls, ceiling and floor got channeled through discrete capacitors in pole located at the center of the room. These oscillating currents in turn generated magnetic fields that permeate the interior of the structure, thus enabling wireless power transfer to the receivers while simultaneously isolating the potentially harmful electric fields in the capacitors. Researchers say this high Q-factor structure efficiently stores electromagnetic energy, and the discrete capacitors allow the resonant frequency to be lowered to a point where the cavity enters the deep sub-wavelength regime, effectively separating the magnetic field from the electric field.
Researchers say the magnetic fields are highly uniform and decay at a relatively slow rate towards the walls, making it possible to strongly couple to coil receivers thousands of times smaller than the size of the QSCR. Furthermore, deep sub-wavelength operation results in a magnetic-field/electric-field ratio that is on average 100 times greater than in free space, allowing for a substantially higher level of power to be safely transferred.
The main caveat to the QSCR technique is that the walls have to be conductive. Since coupled resonators only share energy efficiently with objects of the same resonant frequency, interactions with common everyday objects and materials is minimal, allowing for typical home and office furnishing to be included in the chamber. Researchers say this unexplored form of wireless power offers a seamless charging experience where a user’s device can be charged when entering a QSCR enabled space as easily as data is transfer through the air.
The QSCR room in Disney’s experiment had a central copper pole with a 7.2-cm diameter, with 15 high-Q discrete capacitors totaling 7.3 pF inserted across a 2.5-cm gap in the pole. The resulting resonance was at 1.32 MHz. A six-turn, 16.5-cm wide square coil receiver measured wireless power transfer efficiency. A 28 cm, eight-turn, spiral drive coil was used to stimulate the room.
In another demonstration, researchers furnished the QSCR room with standard bookshelves, chairs and tables. Ten electronic devices stationed around the room were augmented with wireless power receivers. A signal generator, power amplifier and drive coil injected 15 W of RF power into the room, enough to simultaneously power all ten devices.
Disney researchers say one of the key benefits of using magnetic fields in the low megahertz frequency range is that they do not interact with common everyday materials. Metal objects such as phones, lamps and office furniture do not strongly couple to the QSCR and importantly do not suffer from eddy current heating, which is typical in low frequency inductive systems.
Researchers also say the high Q-factor and sub-wavelength operation of the QSCR room permits the inclusion of windows and doors, without significantly altering performance. In the long term they think the requirement of metalized walls, ceilings and floors can be significantly reduced by optimizing the QSCR, and retrofitting of existing structures will be possible via modular panels or conductive paint.