Quasistatic Cavity Resonance for Ubiquitous Wireless Power Transfer

Abstract

Wireless power delivery has the potential to seamlessly power our electrical devices as easily as data is transmitted through the air. However, existing solutions are limited to near contact distances and do not provide the geometric freedom to enable automatic and un-aided charging. We introduce quasistatic cavity resonance (QSCR), which can enable purpose-built structures, such as cabinets, rooms, and warehouses, to generate quasistatic magnetic fields that safely deliver kilowatts of power to mobile receivers contained nearly anywhere within. A theoretical model of a quasistatic cavity resonator is derived, and field distributions along with power transfer efficiency are validated against measured results. An 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. Finally, a detailed safety analysis shows that up to 1900 watts can be transmitted to a coil receiver enabling safe and ubiquitous wireless power.

Fig 1. Canonical example of a quasistatic cavity resonator.

(a) Geometric setup along with reference geometry and coordinate systems. A square coil receiver is depicted within the QSCR. (b) Magnetic field, H. Color is magnitude (red, large; blue, small); Arrows are magnetic field vectors.

Fig 2. Top view of the QSCR.

Locations and polarity of image currents due to the conducting walls (dotted circles). Solid circle in the center is the real pole current, and the black square is the QSCR outline from above.

Fig 3. Photographs of the experimental setup.

(a) Image of the QSCR wireless power room as viewed from the outside (b) Close up image of the central copper pole and discrete capacitors inserted across the gap. (c) Photo of the multi-turn, square receiver coil used to measure WPT efficiency.

Fig 4. Measured and theoretical results.

Measured, simulated, and analytically computed magnetic fields, (a), and electric fields, (b), when 15 W is transferred to a receiver at 50% efficiency. (c) Analytically computed WPT efficiency, G_max between the QSCR room and the receiver of Fig 3c. The blue dotted line shows where the data in panel (d) is taken. (d) Line-slice plot of G_max vs. distance from center of wireless power room.

Fig 5. SAR simulation.

(a) Setup of SAR simulation in finite element software. (b) Horizontal slices of local SAR values when the pole carries a 140 A current.

Fig 6. Safe input power thresholds.

Maximum permissible power levels (green region) as a function of transfer efficiency. Red line shows where SAR limit is exceeded when the human body model is 46 cm away from the central pole, and the black line is the action level or where the E-field magnitude exceeds 614 V/m at 46 cm away from the pole.

Fig 7. Example devices being powered in the experimental quasistatic cavity resonator room.

(a) Photo showing simultaneous powering of multiple devices in a realistic living room type environment. (b) Close-up photo of a 5 W fan being powered wirelessly near the room’s wall. The receiver coil is hidden inside the casing, wound around the perimeter. (c) Close-up photo of a mobile phone being powered wirelessly within the room.