
Bye-Bye, Batteries -  Radio Waves as a Low-Power Source - donohoe
http://www.nytimes.com/2010/07/18/business/18novel.html?_r=1&src=twr
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sasmith
Should I be considering this differently from solar? They're both harvesting
ambient EM radiation; and the energy density of solar is much higher. I guess
that radio goes through walls and never sleeps, so that's a plus. Anyway, I
thought that the mentioning of a solar calculator was quite appropriate and
warranted further discussion.

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sliverstorm
It sounds like the device on the hard hat is more like a passive RFID chip;
It's hard to be sure, but the article says all the dangerous equipment has
transmitters of their own.

Of course, the effective difference between the passive RFID chip and a device
that simply consumes ambient radio-spectrum EM radiation is small, mostly a
paradigm change.

Your initial question begets an interesting question- since light is EM
radiation, just like radio waves, shouldn't we be able to pick up light with
an antenna? If we can figure that out, we can forget about solar cells with
their sad efficiency levels.

edit: with some quick research, the answer is obvious; while it seems
extremely weird to imagine, light can be absorbed by an antenna- and the first
and foremost reason this isn't being done already is because the appropriate
antenna would be ~700 nanometers long.

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kragen
You can absorb EM radiation just fine with an antenna that's "too long". It's
when it's too short that you run into problems.

500nm-wavelength light oscillates at about 600 terahertz, with a period of
about 1.7 femtoseconds. If you want to rectify that and turn it into DC
current so you can run current semiconductor devices, you need a diode that
can switch on once and off once in that period of time. So your forward
recovery time plus your reverse recovery time needs to total less than 1.7
femtoseconds. Among other things, I think this implies that the depletion
region in the diode needs to be less than 0.9 femtoseconds in width --- at the
electron drift velocity of the semiconductor, which I think is typically
around 12 orders of magnitude less than c, although in silicon it can be as
high as only three orders of magnitude less than c. Which means that your
depletion region needs to be 3 orders of magnitude smaller than the
wavelength. Unfortunately the wavelength we're talking about here, at around
1000nm, is only four orders of magnitude bigger than a smallish atom, at
0.1nm. So you're pushing up against the bounds of possibility here with an
insulating depletion region of a few atoms in thickness.

Forward and reverse recovery times for silicon diodes vary widely. Typical
values for discrete components are measured in the tens to hundreds of
nanoseconds. Schottky diodes bring that down to tenths of nanoseconds. One
nanosecond is one million femtoseconds, so that's still five orders of
magnitude too slow.

Anyway, I don't know anything about this stuff, really.

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sliverstorm
Seems like it'd be easier to just drive a 600 terahertz motor, if such a motor
can be made, and use that to drive a good old fashioned 60hz generator.

The silicon would be a much nicer solution though. Thank you for the details
on the diodes, I forgot about that part. You're probably right on the diodes
being the hold-up.

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kragen
Now that's an interesting idea. The light contains an oscillating magnetic
field that you could use directly to spin a permanent magnet, if you had one
that was small enough. (Any atomic nucleus would do, but you can't connect a
shaft to it.) Maybe you could build a multipole nanomotor so that the rotor
itself doesn't have to spin at 600THz; if you have 100 poles, which is not
that far out of what people commonly do with macroscopic stepper motors, then
you could get the rotation down to only 6THz. (But then you're only
potentially absorbing light at the rim of this rotor.)

Gearing that down to 60Hz at the nanoscale — without losing most of the energy
to friction — could still be a significant challenge. I don't know of any
hundred-billion-to-one gearboxes.

The basic difficulty with the nanomotor approach, I think, is that electrons
are lighter than nuclei, so it's easier to get them to oscillate over useful
distances in any particular frequency range, and this is especially tricky in
the terahertz to petahertz frequency range. A nucleus, under the influence of
the same electrical field as an electron, will accelerate about three or four
orders of magnitude more slowly.

Ultimately this should be a scale advantage for mechanical computation, since
it means you can localize an atom to a much smaller region, given a certain
momentum uncertainty, than an electron. The atom can't tunnel as far, so it
can store a bit reliably in a much smaller region. I don't think we're there
yet.

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sliverstorm
Is it necessary to make it a nanomotor? Friction is not a concern once you
have things suspended by magnets in a vacuum. If power is an issue for driving
the larger rotor, instead of using just one antenna use an array. It's
typically better to have one giant Engine than many small ones.

Forget gearboxes, a belt drive would be superior until you start cranking out
huge power, and in that case you could try chains instead. Also, don't forget
that a 60hz motor does not have to spin at 60rpm to generate 60hz.

~~~
kragen
The difficulty with making it larger than nanoscale is that the centripetal
acceleration becomes very great, which makes holding a large rotor together
tricky; you need very strong materials. Actually, I did the calculations, and
for visible light, it isn't even feasible at the nanoscale.

The centripetal acceleration of the rim of a rotor of radius r is rω².
Rotating at 600THz (i.e. 600 trillion rotations per second), ω = 600T2π/s ≈
3.8 × 10¹⁵/s. If your radius is 1mm, then your acceleration is about 1.4 ×
10²⁷ G. The smallest rotor you can make out of atoms is probably around 0.1nm,
which reduces the acceleration to only about 1.4 × 10²⁰ G. If your rotor was,
say, an orthohydrogen H₂ molecule, with a distance between the nuclei of about
62pm, and thus a radius of about 31pm, the acceleration is about 4.5 × 10¹⁹ G,
which would be a weight of about 74 micrograms pulling on that single covalent
bond. The nuclei would be whirling around the covalent electron cloud that
bound them together at about 11.7 kilometers per second, and each of them
would have about 1.13 × 10⁻¹⁷ J of kinetic energy. Unfortunately, hydrogen's
ionization energy is about 2.2 × 10⁻¹⁸ J, so that's about five times as much
energy as you'd need to rip the molecule apart. I think. It could work out in
the infrared, maybe. You'd just need a way to get the molecule started
spinning.

So I guess you'd have to make your rotor a lot smaller than a diatomic
molecule, or a lot stronger than a mere covalent bond.

Generally a 60Hz generator must spin at 360rpm _or slower_ to generate 60Hz.

~~~
kragen
Um, 3600.

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Groxx
important bit: simple processing & sound using _ambient_ radio waves in a
construction hat. Which is _very_ cool. Other cited uses cover small
temperature sensors and possibly replacing AAA battery powered devices.

Add in a healthy dose of sparkly-futurist-hope (devices that run FOR EVAR),
and some actual data on another experimental device:

> _The device collects enough power to produce about 50 microwatts of DC
> power, Dr. Smith said. That is enough for many sensing and computing jobs,
> said Professor Otis. The power consumption of a typical solar-powered
> calculator, for example, is only about 5 microwatts, he said, and that of a
> typical digital thermometer with a liquid crystal display is one microwatt._

Didn't see anything on the size of the "device", but that's still pretty cool.

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typedef_void
Didn't Tesla claim to have developed wireless power a long time ago. Those
more familiar with Tesla/Physics ... is this the same technology?

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retube
The technology in the linked article would appear, at least superficially, to
be similar to RFID. Although one difference might be that RFID works at a
fixed frequency (to match the harmonic frequency of the receiving radio
circuit) whereas this apparently works with "ambient" radio, implying a broad
spectrum of frequencies, which is quite clever.

Either way, the broadcast signal is losing power at a rate proportional 1/d^2.
So these devices necessarily have to work at very low power. What Tesla did -
or is supposed to have done - was to figure out how to transmit power without
the 1/d^2 loss. I.e, much like a collimated laser beam, he could transmit
power over large distances and power high current devices.

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Rhapso
It is interesting, that we are only now scratching on the surface of what
Tesla did 100 years ago.

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tomjen3
There is no kidding that he was off the charts brilliant, but he was also -
especially towards the end - insane.

That is why it is so hard to figure out what Tesla actually did or didn't do.

