[tzs]

These people got 53% efficiency at 1 meter, which gives some hope for ultrasonic power transfer: Roes, M.G.L.; Hendrix, M.A.M.; Duarte, J.L., "Contactless energy transfer through air by means of ultrasound," IECON 2011 - 37th Annual Conference on IEEE Industrial Electronics Society , vol., no., pp.1238,1243, 7-10 Nov. 2011

Abstract: An alternative approach to the wireless transfer of energy is proposed, employing acoustic waves in air. Unlike conventional methods, acoustic energy transfer is able to achieve energy transfer at high efficiencies over distances that are large in comparison to the dimensions of the transmitter and the receiver. This paper gives an overview of the principle and explains the different loss mechanisms that come into play. A theoretically limit on the achievable efficiency is calculated. It exceeds that of a comparable inductively coupled system by an order of magnitude. First preliminary measurements indicate that AET is feasible, although the measured efficiency is lower than the predicted theoretical limit.

[jacquesm]

Ok, I read the paper. You got that completely wrong, the 53% is theoretical performance at 1 meter, the actual peak performance measured was 16% at an extremely small distance and two full orders of magnitude less (so 1%) at a distance of 100 mm.

The maximum output they measured was 37 uW, so 1000 of these would output ~37 mW, with an input power 1000's of times higher.

If anything this paper is a nice example of how theory and practice differ. It also highlights another big loss factor for ultrasound power transmission, the angle of incidence, the power falls of as the co-sine of the angle between the transmitter and the receiver (maximum at 0 degrees, minimum at 90 degrees).

[jacquesm]

Grr. I can't access that paper but thank you for digging that up. If Animats' calculations above are accurate though then uBeam is at a very small fraction of that 53%.

Is there anything in that paper that could explain the difference?

[tzs]

If you make a free account at deepdyve.com, you can read a free online preview of the paper for 5 minutes. The paper is only six pages long so that might be enough time to find an answer the question. (Or enough time to take six screenshots...).

I don't know any more because that was how I found that and skimmed the paper about three years ago, for a prior discussion of uBeam here on HN.

[m00n]

Not sure, if linking to the pdf is frowned upon here: sci-hub.cc and paste the paper name, voilá.

[jacquesm]

Thank you!