Scientists Prove You Can Charge Your Phone With Ultrasound - In Just 7 Weeks!

There's been very little out there regarding ultrasonic wireless power transfer that anyone interested in the numbers can really dig into, so anyone critical of the claims of some companies has very little to point to when discussing the topic. uBeam have been extremely tight lipped when criticized, often simply claiming that no-one understands the field, or their assumptions are wrong, but never actually correcting those assumptions or providing detailed alternative numbers. While I've shown some information, as has Dave Jones of EEV Blog (among others), as a "disgruntled former employee with an axe to grind" apparently I'm not to be trusted in my analysis. Things have just changed, however. A prestigious group of ultrasound researchers at Stanford has just published a paper on "Wireless Power Transfer to Millimeter-Sized Nodes Using Airborne Ultrasound" that goes into some depth on the topic. 

It's peer-reviewed, which means other scientists read it, critiqued, and have judged it meets a standard that it is a novel or major contribution to the field, and there are no significant mistakes they can find in the work. The publication is the IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, which is the highest ranked journal related to ultrasound (by Impact Factor). This is going to be hard to dismiss as partisan, biased, incorrect, with false assumptions, or incorrect numbers.

Even better, the authors have made it "Open Access" which means that anyone can download it without being a journal subscriber or paying for it. Please, if you're at all inclined, download it from the above link or at the one below to make sure the authors get credit for their great work.

http://ieeexplore.ieee.org/document/8006286/

I'll highlight a few points here from the abstract as a taster for you:

We propose the use of airborne ultrasound for wireless power transfer to mm-sized nodes... We show through simulation that ultrasonic power transfer can deliver 50μW to a mm-sized node 0.88m away from a ~50 kHz, 25 cm2 transmitter array... We also argue that longrange wireless charging at the watt level is extremely challenging with existing technology and regulations... 

To translate for everyone that's 50 microWatts from a 25cm2 panel, out to around 1 meter. Scaled to my estimate of uBeam's 45 by 45 cm transmitter that's around 4 milliWatts. A typical phone battery is around 5Wh so you're looking at 1250 hours to charge the phone with this method, if it were switched off. 

So there you go, proof that uBeam can work and charge your (switched off) phone in no more than 7 weeks! (Are there ways to go faster, yes, but 1250 times faster? While I'm painting closer to the worst-case picture, if you look at some of my earlier posts you can find different numbers that change the outcome to a higher number, but is it safe, efficient, practical, legal? It's now really up to uBeam to show.)

It's a well written paper, please read it if you have any technical background. For those of you who don't, they make a pretty reasonable statement that for "Internet of Things" (small devices that need very low power charging very intermittently) it's a possible solution, but phone scale devices (or larger) are unlikely to be practical. Further, they point out this is contingent on the 145 dB safety levels that the USA used to allow, but appear to not allow any longer. At ~115dB, the level almost every other country has always given as a limit, things get 1000x worse than this.

This is not the first time recently that peer-reviewed papers have called into question the use of high power ultrasound. Last year, the Proceedings of the Royal Society published a study of the potential negative health effects of the increasing use of high power air coupled ultrasound in our environment.

Over to you uBeam, it will be very interesting to see your response to this...