Reinventing solar power

  1. Hello,

    I am not sure if I am posting in the right place, but here are my thoughts.

    I don't exactly understand how solar cell works, but it has to do with photons knocking out electron which makes the current flow.

    So I thought about the WiFi based charger. It generates electricity from WiFi waves, right? Which are electromagnetic, like visible light. In that case, could we use same principle to generate electricity from visible light waves?

    Any thought appreciated.
     
  2. jcsd
  3. I don't understand what you are asking. PV cells generate electricity from the visible light spectrum. Are you looking for another device that can as well but based on different principles?
     
  4. Basically I am asking two thing: does WiFi signal based charges generates electricity differently than PV cells and if so could we use same principle that WiFi charger uses (since WiFi signal is electromagnetic wave and visible light is electromagnetic waves) to generate electricity from visible light (or even all electromagnetic waves that travel).

    Sorry if I don't express my self understandably - English isn't my native language.
     
  5. Acutally you "generate" electricity within all types of antennas.

    But the energy in a wifi signal (2.4GHz) is much lower than a visible light wave. E=h*v

    A powerfull wifi router has a output of 1 Watt, in all directions. How much power do you think you can generate with a antenna/panel e.g.?
     
  6. SirAskalot, I am not sure I understand your question. So the idea is to transform that energy (what ever amount it is) to electricity. And the PV cells are not antennas for visible light, right? So is it possible to create one, and would it "generate" electricity?
     
  7. Wasnt really a question, just a comparison on how little energy/power you can produce. <1W aint really somthing to shout hurray about.

    The question is whether you find a semiconductor in the PV that has a lower energy gap than the energy of the wifi photon. Else it wont "knock of a electron". I dont know the answer to this.
     
  8. berkeman

    Staff: Mentor

    A tuned antenna will generate a terminal voltage in response to incoming EM waves. However, the amplitude of that AC terminal voltage is related to the wavelength and the incoming power.

    For WiFi antennas, the terminal voltage is tiny (mV to uV), so you cannot derive useful power from WiFi background radiation.
     
  9. sophiecentaur

    sophiecentaur 13,574
    Science Advisor
    Gold Member

    The nearest (if not the only) example of a device that is worked solely by received radio waves is the old Crystal Set. It necessitates a long wire, not because of the long wavelengths involved but because the RF fields around your house are a very few millivolts per metre, even from a large broadcast transmitter. The induced RF voltage is then tuned to match the wire to the detector (and to give you some selectivity) and the signal is rectified. Your ears are just sensitive enough to make use of the tiny amount of energy received.
    The signal levels from any WIFI / Bluetooth or mobile phone are so minute that they are only detected after amplification - no chance of driving anything with it - or even charging batteries, I'm afraid. After all, these systems run on the minimum power possible so that the devices can run for hours on batteries.
     
  10. This is actually a very interesting question. I know how PV cells work, in terms of quantum mechanics (photon hits electron, electron leaves orbit, etc.) I also know how antennas work, in terms of classical EM (changing magnetic field induces a potential). But... how do antennas work in terms of quantum mechanics? What is the fundamental difference between PV and antennas?
     
  11. I always thought regular RF antennas generated electricity through the changing magnetic and electric fields of the signal wave, in contrast with the photon interaction of PV - correct me if im wrong. Almson proposes a good question.
     
  12. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    You're correct.
    You can't detect visible wavelegths directly with an antennea because the frequncy is too high. (It's all to do with resistance and skin effects and other transmission line theory stuff that I forgot)
    But the efficiency of photon->electron in a semiconductor is pretty good, not sure a radio type receiver would be any better
     
  13. My idea was that if there is a charger that can charge phones with WiFi waves so do PV cells use the same principle for getting electricity?
    If not then is it possible to make some kind of device that would do that.

    And from that there was an idea to make a device which would get electricity from all EM waves.
     
  14. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    No - wifi chargers are basically radios. In a radio the electromagnetic field in the air makes current flow in an antenna and you detect this tiny current to receive the radio signal (or try and use it to charge a battery)
    In a solar cell a single photon knocks an electron out of an atom and makes electricity directly.

    The problem of doing it the other way around is wavelength - as you go to higher frequencies of EM the wavelength gets shorter, thats why you see giant radio towers for AM radio (100Khz), but have a small car antenna for FM (100MHz) and the antenna for your cell phone (2GHz) is tiny.
    When you get to light you would have an antenna impossibly small and the current would have to flow in a surface layer much smaller than an atom - all sorts of QM effects kick in and transmission line theory fails.

    Similarly if you try and make a semiconductor detector for radio you would need a huge crystal so that a metre long wave would be absorbed and you would need electrons impossibly weakly held to the atom so that they could be knocked off by low energy radio photons.

    There is a very interesting area where these two technologies overlap - it's currently somewhere in the microwave bands.
     
  15. But visible light wave length is much longer than size of an atom, so may be we just don't have the right nano technology?
     
  16. sophiecentaur

    sophiecentaur 13,574
    Science Advisor
    Gold Member

    That's rather an over simplification. Electrons don't "leave orbits" when light hits a PV cell - or any other solid, in fact. You don't have orbits or specific energy levels and a photon doesn't react with one electron (as in the simple Hydrogen atom model). An electron will move because of the interaction of a photon with the larger structure of the semiconductor and the energy is in Bands rather than Levels This is also what happens when a radio wave interacts with a metal antenna. The difference, for a PV cell is that electrons tend only to flow in one direction and it gives you DC. With visible light, the energy associated with the photons is enough to 'get over' the voltage required to get past the PN barrier (the diode forward voltage drop). In a metal, the electrons move very easily (the band gap is tiny) and the low energy photons (if you really insist on using the idea for RF) can still shift them in either direction.
     
  17. I think I'm getting the picture. Photons interact with electrons the same way whether for radio waves or light waves, but a traditional antenna is not possible for light because the AC current would be ridiculously high frequency. So high that it just wouldn't be transmitted through copper wires (primarily because of skin effect). A PV cell is a neat trick, however, because it converts the AC current into DC right at the point of reception, bypassing problems with transmitting petahertz AC.

    But... transmission of petahertz electricity might not be insurmountable using exotic materials. The author of the linked-to paper suggests that capturing light energy using conductors, not semiconductors, may be feasible. Very interesting stuff.
     
  18. berkeman

    Staff: Mentor

    I still remember the patent that I read many years ago, where the author patented using nano dipoles to capture solar energy for conversion to DC power. It still amazes me that the patent issued -- there were no diode technologies available at that time (or now, AFAIK) for rectifying the light-frequency AC to DC, and the rectification losses were ignored in the patent calculations. Sigh.
     
  19. To elaborate, I'll quote wikipedia:
    So at very high frequencies, only a small part of a solid copper wire will be transmitting the electricity. The resistance goes up as most of the copper goes unused. However, the problem becomes one of geometry. If you give the wire enough surface area to volume (er... perimeter length to cross-section), then the conductor material can be utilized effectively. The ideals shape would be a bundle of tiny hollow tubes made of extremely thin walls. Ie, it would look like carbon nanotubes!

    Still, we should calculate what is the skin depth for a 0.5 petahertz current. The wiki article gives a simple formula, but I'll just use a javascript applet: http://daycounter.com/Calculators/SkinEffect/Skin-Effect-Calculator.phtml Note that it assumes the material is copper, not carbon, but it's good enough to ballpark. For 0.5 petahertz, the skin depth is 3 nanometers. That's teeny, but perfectly in line with the thickness of nanotube walls! So there we go, transmitting light as electricity is another application for carbon nanotube wires whenever (if ever) we can reliably make them.

    Awesome.

    EDIT: We still need to rectify this AC. But if a PV can do it at the source, why can't it be done downstream with the same semiconducting material? This is something I don't know much about.

    Off-topic: I don't think inventing something that requires an unavailable (but not crazy) component is grounds for rejecting the patent. Of course, you're wasting money on the application hoping the patent won't expire before the damn thing comes into existence.
     
    Last edited: Jun 27, 2010
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