Those Other Guys, Pt 1

Having made comment about others' concerns over tech startups like Theranos ability to fleece investors and the public, I thought I would make some comments regarding Energous, another wireless power company getting some press. I'll point out here that I have no inside knowledge of the workings of Energous, just that I've spent a lot of time in the last few years looking at the physics of transferring power via waves (acoustic and electromagnetic waves have a lot of similarities in the maths and fundamental behaviours, much of what I say below can be applied to ultrasound phased arrays), and how investors in tech companies think.

For the less technically minded among you, I'll summarise in the next part the key points from here, and you can just skip straight to part II.

Energous uses RF electromagnetic waves, similar to those used in the higher speed wifi signals, to transfer the energy. At the 5.8GHz they claim, wavelength is going to be around 5.2 cm (divide the speed of light at 3e8 m/s by the frequency of 5.8e9 Hz). Why is this important? Here's an image from their patent filings, with the power transmitter in the top left, labelled 706:

It's clear from the patent they intend to use a phased array, similar to what is done in radar, and what uBeam has claimed to do. Essentially, it's a large regular grid of transmitters, like a chessboard, and by sending the signals from each small piece at slightly different times, you can send a beam to a chosen location. An image below shows a beam from an array from a standard computer simulation package. This is a well known phenomena and has been for many years, there are few surprises.

Notice as well as the main lobe (big red bit where most of the energy is going), there are lots of little peaks going in other directions called side lobes. These are an inevitable consequence in any practical system, and involve energy wasted and going places you don't want. You can also get even worse lobes called 'grating lobes' if you don't put those transmitters close enough together, less than half a wavelength or around 2.5 cm. Let's assume, like any good engineer would if designing a phased array where you are sending power and don't want to cook anything off to the sides, that you go with 2.5 cm spacing.

Now Energous have been claiming 'hundreds of small elements' so in the great power analysis here let's assume around 500, or about a 22 by 22 grid, which at 2.5cm spacing is 55cm on each side. Wow, that's a big panel. Let's make it closer to the size of a speaker, which is what the press is describing here and make it to 100, that's a 25 by 25cm plate, still pretty sizeable. That's not out of scope - if you look at the scale of array that Ossia, also using RF to do charging, is showing in their patents. (Ossia work at half the frequency, so wavelength is 2x bigger).

Anyway, Energous claim to create 'pockets of 3D energy' which is marketing speak for 'beam'. No pocket of energy magically appears with none elsewhere, like a magnifying glass with sunlight, it concentrates more and more until your point focus is very intense. How small you can make that focus is related to the wavelength L (smaller wavelength for smaller focus) and transmitter size relative to wavelength D (bigger transmitter is smaller focus), and how far away you are trying to focus F (further away is larger focus). Generally, your focus (pocket of energy) is given by L*F/D. That's not exact, but is close enough for a blog, and the fact that the energy slowly dies off, like in the image below, would certainly be a safety concern. Not something I want to be near even away from that pocket.

So using the above numbers, at 5 meters range (just beyond the 15 feet they state), the focus is 0.05 * 5 / .25, or around 1 meter in size. That's a big 'pocket'. Even at 1 meter range that's still a 20 cm focus which is larger than any phone, and will catch your hand, head, and anything else nearby. In an IEEE Spectrum article they state the pocket is larger than the receiver, so not unbelievable. (Note to uBeam: Energous seem to know how to handle a skeptical technical press, despite similar incredulity from the experts)

The 'mini-transmitters' they talk about are ridiculous though - do the above calculation with a USB sized device. It's no wonder it would literally take years to charge a phone with it, even before you consider you need to reach a level of 0.5 to 1 Watt these days to trigger charging.

And then there's directivity where the amplitude falls off with angle as you move away from the beam pointing straight ahead - by the time you get to 60 degrees, you're at half-power, and down to 0 when pointing to the side. This simply doesn't mesh with Energous' claims of a '30 foot diameter bubble'. In summary - with a phased array it's easy to point straight ahead, and harder as you move to the side. This means it gets less efficient - a significant problem when imaging, a killer if you're transferring power.

I won't go into other numbers showing that their claims of power delivery capability are not just difficult but defying laws of physics - the only thing more ridiculous would be claiming that ultrasound could charge a device that's in your pocket. Anyway, you can find information demolishing Energous' claims here including mention of the regulatory issues they face with the FCC, though everyone seems to forget the FDA can choose to become involved in any radiation emitting device (and ultrasound is radiation, FYI).

Confused, bored, lost? No? Then you probably knew most of this anyway, are technically proficient, and were already skeptical of Energous. Yep? That's what 95% of people who read this will feel like, and switch off. And that's the point.

That's the tech side - next part, the business side and their novel approach to fundraising.