Similar to a solar cell...

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Externet
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Similar to a solar cell...

Post by Externet » Tue Apr 25, 2006 10:10 am

Hi everyone.
¿Does it exist? a PN junction made of some materials/crystal/ semiconductors/chemicals/whaetever; that will knock off its electrons by exposure to radiofrequency instead of sunlight?

A radiowave cell. What would it take to engineer such animal to generate some electricity? It is not for faint radiowaves from broadcasting, but high intensity fields :?

Miguel

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Post by JPKNHTP » Tue Apr 25, 2006 10:30 am

-JPKNHTP
-God Bless

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Post by ecerfoglio » Tue Apr 25, 2006 10:33 am

To extract energy from a high intensity radio field you only need an antena.

If you want to obtain DC just rectify the antena's output with a diode and add a filter capacitor. You can use a schottky diode to lower its voltage drop and power loss.

With "faint radiowaves from broadcasting" you can use a similar aproach. In fact the old "crystal radios" where only an antena, tuned circuit, detector and headphones, and they took all their power from the radio field.

Originaly the detector was a mineral crystal ("galena"), but in later versions ("experimental" radios I have assembled as a kid circa 1970) we used germanium diodes.
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Post by L. Daniel Rosa » Tue Apr 25, 2006 12:03 pm

I think the size of the junction may have to be larger than some fraction ofthe wavelength. The antenna part takes care of that. This isn't too far off a microwave detector I made from one schottkey and one LED. The trick was that the leads of the schottkey were trimmed to be a quarter wave dipole and the LED was as close as possible. The whole thing was glued on a card.

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Post by rshayes » Tue Apr 25, 2006 2:41 pm

There are some trinary alloys whose band gap can be changed by changing the proportions of the various materials, and that cuold , in theory, be made with a band gap close to zero. In practice, the process for growing these materials is probably not consistent enough.

Even if the material could be made, the voltage of such a "solar cell" would be in the 5 microvolt range for RF energy in the gigahertz range.

Antennas with rectifiers can be used, and at one point NASA proposed orbiting power stations that transmitted power to earth using microwave energy received by arrays of antennas and rectifiers. The proposed arrays would have covered several square miles.

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Post by Chris Smith » Tue Apr 25, 2006 4:43 pm

Even a exposed AIR type capacitor will collect RF.

If you live near high tension wires, you can collect free power making a very large crude air type cap, and then filter it using diodes and other capacitor or battery.

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Post by Externet » Tue Apr 25, 2006 5:36 pm

Thanks, gentlemen.

--->rshayes: May you explain the band gap thickness being smaller the more convenient ?
For visible light, which is of much higher frequency, the solar cell works. Don't know the thinness of its optimal band gap, but just cannot understand its relationship.

¿ 5 µV ? Well, I suppose depends on the EM field intensity the "radiowave cell" be exposed to, but say if 1 Watt of RF in the GHz range is entirely focused to such hypothetical cell, would its DC generation be that small ?

--->ecerfoglio: Am not aiming to a "rectenna" which is its name, but something more of a cell-like surface device, not about capturing any broadcast radio signals, but conversion to DC by semiconductor style chemistry.

If a solar cell gets ~1KW/m², with an efficiency of ~10%, something useable comes out of it, as it is a reality in practice.

If a "radiowave cell" could be manufactured to get 1 to 10 % efficiency, such as to be used in the higher energy yielding radiofrequency spectrum, the infrared, as a thermovoltaic cell, it would be great.
Does anyone know of advances on such technology?

-Nothing to do with Seebeck effect, as that is differential-
More like the infrared photons acting on a PN junction :???:

Miguel

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Post by jwax » Tue Apr 25, 2006 7:02 pm

RF simply does not contain enough energy to warrant harvesting. Your solar power of 1000 watts/sq meter is "wideband" light- couple hundred nanometers up to several microns wavelength. It is HOT and energetic.
Wimpy RF, MHz to GHz, contains orders of magnitude less energy than the visible hundreds of Terahertz of light.
Now a gamma cell, that could be a producer! :shock:

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Post by Chris Smith » Tue Apr 25, 2006 7:28 pm

Only directed MW energy is worth harvesting, at a loss to the sender.

25% to 35% is closer for today's standard for light efficiency gathering.

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Post by Externet » Tue Apr 25, 2006 7:38 pm

Dammn compfuser !! :x cannot open these ! :x :x

http://www.patentstorm.us/patents/7002071.html

http://www.osti.gov/bridge/purl.cover.j ... bviewable/

Will have to find a way to get in there.
Miguel :sad:

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Post by rshayes » Wed Apr 26, 2006 1:25 am

The energy of a photon is proportional to its frequency and inversely proportional to its wavelength. In a silicon solar cell, the peak response occurs at a wavelength around 9 x 10^-7 meters. Assuming 3 GHz radiation, the wavelength is 1 x 10^-1 meters. The difference in energy per photon is about 100,000. A photon detector, such as a solar cell, would have to have a band gap 100,000 times less than that of silicon. The potential energy that an electron would gain by crossing the band gap would also be lower by the same factor. This would imply that the output voltage of a photovoltaic cell would also be lower, or about 5 microvolts. The number of photons for a given power input would be much higher, so the cell would be capable of delivering high photocurrents. Unless the cell resistance was very low, possibly nanoohms, the voltage drop due to the cell resistancewould reduce the 5 microvlot to zero for relatively low currents low currents.

In addition, you would probably have to operate the cell somewhere close to 0 kelvin.

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Post by jwax » Wed Apr 26, 2006 3:57 am

That OSTI article describes a novel generator that uses a "Plasma Filter". Basically, a bandpass optical filter that covers the actual photocell. My question is, why a filter? Granted, it is blocking wavelengths that do not contribute to electro generation, but why block it? Is efficiency gained by blocking non-generating waves?
Great question Miguel! I'll email those papers if you still can't open them!
John

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Post by Externet » Wed Apr 26, 2006 8:43 am

Hi.
I will drill my way to get into that new patent; or let you know if not. I was able to open the Department of Energy document with the compfuser at my work place.

Seems as rshayes says, the wavelenghts that cannot be converted into DC by the gap are filtered out by reflection to avoid some sort of interference with the effect.

Miguel

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Post by jwax » Wed Apr 26, 2006 5:57 pm

Found out one reason for the filtering is to block IR wavelengths that only generate efficiency-robbing heat.

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Post by Gary » Tue May 02, 2006 5:19 pm

Using an antenna to harvest electricity may be free, but not without cost. And it could be illegal. You could probably use an old wire fence near a high voltage transmission line, disconnect the ground and get power, but it would be stealing, and you would be caught. Someone's thought of that long ago. There are ways of detecting these things.

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