Disney Research’s Potential Solution to Safe Wireless Power Transfer: Quasistatic Cavity Resonance

What if you could walk into a room and have all of your devices automatically charge? It's a dream many have attempted but none have safely achieved. However, a new wireless power delivery system from Disney Research can safely transfer kilowatts of power to mobile devices a room.

Disney Research engineers have developed a wireless power delivery system which can safely transfer kilowatts of power to mobile devices within a room. Using small coil receivers, the new method achieves an efficiency of 40% to 95% within anywhere in the 54-

m3

test room. The research team has demonstrated the feasibility of safely delivering 1.9 kilowatts of power which is sufficient to simultaneously charge 320 USB-powered devices.

Wireless Power Transfer (WPT)

WPT is the transmission of electrical energy through electrical, magnetic, or electromagnetic fields rather than through wires and cables. The technology is particularly attractive in cases where it is hazardous, inconvenient, or even impossible to use interconnecting wires. WPT is widely used in electric toothbrush chargers, implantable medical devices like artificial cardiac pacemakers, RFID tags, and more.

The basic concepts of wireless communication and wireless power transmission are very similar in that they both use the same fields and waves. However, in wireless communication, the goal is to transfer the data—as long as the signal-to-noise ratio of the received signal is acceptable, the transmitted power is not important. In contrast, WPT aims at maximizing the power transmission efficiency and/or the coverage range while ensuring the health and safety of the general public.

Currently, the closest we have to WPT includes inductive chargers that utilize the Qi system. Image courtesy of Qi Wireless.

WPT over Long Distances

Many of the commercial applications of WPT are currently limited to near contact transfer distances. There are two main challenges in increasing the coverage range: directing transmissions and safely utilizing powerful enough electric fields.

Firstly, to efficiently deliver the power, we need to implement highly directional transmitters. Otherwise, with an omnidirectional transmission, only a small portion of the transmitted power will hit the receiver. High directionality usually translates to large devices.

For example, a 1978 NASA study investigated WPT for solar power satellite application and concludes that, with a microwave system at 2.45GHz, the diameter of the transmitting antenna and the receiving rectenna needed to be 1 km and 10 km, respectively.

Secondly, in order to achieve power delivery over longer distances, we usually need to resort to radiative transfer methods which tightly couple electric and magnetic fields. Unfortunately, the long-term exposure to large electric fields causes serious injuries to the living tissues. Hence, we face a serious challenge: while safety concerns mandate small electric fields, long distance transmission requires powerful emission of electromagnetic waves.

The Disney Research Invention

To circumvent the above problems, Disney Research engineers have recently proposed a system which can safely transfer kilowatts of power to mobile devices within a room.

The main contribution of this research is delivering a sufficient amount of power while taking the safety concerns into account. To this end, they let the current flow through the walls, ceiling, and floor of a metallic chamber. Stimulating the resonant electromagnetic mode of the structure, researchers generate uniform magnetic fields that permeate the interior of the room. S
ince the generated fields are uniform, they decay at a rate less than

1ρ

(

ρ

 is the distance from the central pillar which is illustrated in Figure 1). Hence, receiving coils—which are 1000s of times smaller than the structure—experience a strong coupling and efficiently receive the power.



To separate the potentially harmful electric fields from the magnetic fields, Disney engineers channel the current through some discrete capacitors. In this way, the resonant frequency of the chamber is significantly lowered so that the cavity enters the deep sub-wavelength regime. Operating in the deep sub-wavelength regime, the cavity produces magnetic fields, which are 100 times stronger than the produced electric fields. As a result, the new method successfully avoids the hazardous effects of large electric fields without compromising the transmitted power.

​The following figure shows the concept of the proposed method for a rectangular cavity. The central pillar places the discrete capacitors in the path of the current to force the deep sub-wavelength operation. Moreover, this figure shows the variation of magnetic fields with

ρ

. As we get closer to the central pillar, the magnetic field becomes stronger. However, compared to the conventional methods the produced magnetic fields are almost uniform.

Figure 1. (a)The conceptual representation of the quasistatic cavity resonance and (b) the produced magnetic fields (red, large; blue, small). Image courtesy of PLOS ONE.

Figure 2 shows the test room for the Disney experiments. The floor, ceiling, and walls are made of painted aluminum sheets and the floor is covered with a gray carpet. While the test room has a door, the experimental results are still in good agreement with the theoretical predictions of the research.
Figure 2(b) shows the discrete capacitors, which total 7.3pF, incorporated into the central copper pillar of the test room. These capacitors bring the resonance frequency down to 1.32 MHz. The square coil receiver is shown in Figure 2(c).

Figure 2. (a) the employed test room (b) discrete capacitors used in the central pillar (c) the square coil receiver. Image courtesy of PLOS ONE.

To investigate the safety of the proposed technique, the research team followed two guidelines. The first guideline mandates keeping the measured electric field below 614 V/m (RMS) for frequencies below 1.34 MHz. The second safety metric is Specific Absorption Rate (SAR), which is a measure of how much power is absorbed by biological tissues. Simulation results of the SAR analysis verify the feasibility of safely delivering 1.9 kilowatts of power which is sufficient to simultaneously charge 320 USB powered devices.

Although the first attempt in WPT is attributed to Nicola Tesla at the turn of the 20th century, the advancement of the technology has not been sufficient yet and users have not been able to ubiquitously recharge their devices through a true wireless system. However, the Disney research is like a big stride in making widespread WPT a reality.

For a detailed explanation of the technology please refer to this paper.