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Photovoltaic cell efficiency question.

Discussion in 'Photovoltaics' started by gary0232, Mar 9, 2009.

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  1. gary0232

    gary0232 Guest

    If current pv efficiencies are 18% where does the other 82% go. I'm sure the
    answer is reflection and heat, but I was curious about the percentages. It
    appears most of the sunlight just reflects off those shiny surfaces.
     
  2. Eeyore

    Eeyore Guest

    Some reflects but plenty will turn into heat which sadly reduces the efficiency
    of the panel.

    Graham
     
  3. KTo

    KTo Guest

    Hay Gary,
    about 10% of the 82% is reflected, the other 70% is turned into heat.
    On a good sunny day the temperature of the solar panel will be as high as
    80°C.
    cheers
    Kees
     
  4. gary0232

    gary0232 Guest

    I wonder if anyone thought to modify the panel so water going to your hot
    water heater could flow across the underside of the panel.
    Not only would this same on your hot water heater costs, you would be
    increasing the efficiency of your pv's by keeping them cool.
     
  5. Eeyore

    Eeyore Guest

    Just forget the PV solar bit and use solar thermal to heat your water.

    Graham
     
  6. Guest

    Just trickle a bit of water over the face.

    Yup. At the same time, you can also double sun power with a mirror.
    Just a fact of life.
    OR with a tiny water flow :) Try numbers...
    Does it ever rain on your planet? :)

    Nick
     
  7. daestrom

    daestrom Guest

    ==============================================
    What determines the peak frequency of the silicon freq resp? The
    dimensions of the silicon molecules? In other words, how do you dope
    up the molecule to move the resonance from 800nm to 500nm? Smaller
    wavelenth, faster frequency... you'd need to make it lighter somehow.
    Dope it with something lighter like Boron? The PV wafer already has a
    P layer and an N layer. How about turn it over, so the other layer is
    exposed to the sun?



    I don't believe its the atomic weight but the energy needed to 'lift' an
    electron out of its orbit. Once freed from an atom, the electric field of
    the depletion zone at the junction moves it across to the other side so it
    can't recombine directly back to the atom it came from. So it has to flow
    through the external circuit to get back to the atom it came from. (well,
    actually it just bumps another electron that bumps another, and another...
    until finally one bumps back into the original atom of silicon).

    The energy to 'lift' an electron has to come in one 'packet' of light, it
    can't work it's way out gradually (quantum mechanics). So a single photon
    has to have that much energy. (Photons with shorter wavelength (towards the
    blue end of the spectrum) have more energy.) This is known as the
    'photo-electric effect'. If the photon is only slightly more energy than
    needed, the electron is 'lifted' but with a bit extra 'speed' when it's
    free. The extra energy is quickly dissipated as heat as the electron
    'collides' with other atoms.

    If a photon has a lot more energy than needed, then it may 'lift' the
    electron out of the atom's orbit and re-emit a low energy photon that
    carries off the left-over energy. This is 'Compton scattering'.

    So the issue is to find a way to make it take less energy to free an
    electron from silicon. Kind of against 'natural law' of silicon. Of course
    different doping can change the electric field in the depletion zone and
    that can help a bit. Or try something besides silicon. But the next larger
    electron 'shell' gets pretty vague and messy.

    daestrom
     
  8. Eeyore

    Eeyore Guest

    I vaguely recall this at school. I think it's bandgap energy dependent on
    the material used and the wavelength of the light shining on the surface.
    Shorter wavelengths have more energy in each photon, hence why UV would be
    great if it came though the atmospohere ( but not so good for us of course
    ).

    http://en.wikipedia.org/wiki/Photo_voltaic#Simple_explanation

    Graham
     
  9. Eeyore

    Eeyore Guest

    It's not a resonance.

    Graham
     
  10. Eeyore

    Eeyore Guest

    http://en.wikipedia.org/wiki/Planck's_constant

    Graham
     
  11. Mike

    Mike Guest

    So if you make your pv's track the sun, precisely what issue stops you
    cooling the rear face with water?

    I certainly can't think of one.


    --
     
  12. Eeyore

    Eeyore Guest

    Depends if you have to pay for the water and where does it go afterwards ? Down
    the drain ?

    You have inadvertently just made the case for solar thermal water heating. A far
    more attractive source of 'green energy'.

    Graham
     
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