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Taking heat off of Sunlight?

  1. May 15, 2009 #1
    If I should reflect our suns' rays off of a parabolic surface, and placed a one way mirror at a distance away, short of the focus point. Let's say a 10cm disk is placed at that point parallel to the parabolic rim. The light would tend to reflect back at the parabolic surface but would the majority of the heat that is also refracted/reflected back to the parabolic surface be cool?

    Thanks
     
  2. jcsd
  3. May 15, 2009 #2

    russ_watters

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    That doesn't make any sense. How can heat be cool? Perhaps you could reword the question...
     
  4. May 15, 2009 #3
    yes the heat is reflected with the light....that's where the power resides....
     
  5. May 16, 2009 #4

    Redbelly98

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    It sounds like "heat" means infrared radiation here. It all depends if the mirror reflects infrared radiation or not.

    p.s.
    Infrared radiation is not really the same as heat, though it can be converted to heat upon absorption. For that matter, the same is true of visible light or any part of the electromagnetic spectrum.
     
  6. May 17, 2009 #5
    Interesting, so what type of suface treatment is needed to have the infrared radiation go through a one way mirror? Are there one way infrared morrors on the market?

    What I'm trying to do is relfect light minimizing heat and have more light hit a small surface, a PV cell 50mmx50mm square in this case. The suns rays hitting a 50x50 PV cell will, in my case produce 1/2V @ 180ma. If I concentrate more light on it, it should do better but I will need to disapate the heat.

    Thanks again
     
  7. May 17, 2009 #6

    diazona

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    You'd probably be better off checking the specifications for your PV cell to see what wavelengths (or frequencies) of radiation will make it produce power, and then looking for a filter that blocks everything else. For all you know (well, for all I know), the cell might produce power just as well from infrared radiation as it does from visible light, and in that case if you block the infrared light you're just wasting useful energy.
     
  8. May 17, 2009 #7
    Is there really such a thing as a "one-way" mirror?
     
  9. May 18, 2009 #8

    Redbelly98

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    No. That's a misnomer.

    The "one-way" effect comes about from it being much darker on one side than the other.
     
  10. May 18, 2009 #9
    Hi there,

    Infrared radiation is the same as visible light, just with a longer wave length. Therefore, if infrared radiation can be considered as heat, so does visible light.

    In this case, heat should not be the word use. Since heat refers to transfer of energy between two body.

    Cheers
     
  11. May 18, 2009 #10

    Redbelly98

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    If this is a typical silicon PV cell, it will absorb wavelengths shorter than 1.1 μm. Not sure offhand what fraction of the sun's energy this is.
     
  12. May 19, 2009 #11

    Redbelly98

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    From eyeballing the red area in this graph, looks like about 75-80% of the solar spectrum is less than the 1.1 μm cutoff of a silicon PV cell.

    Solar_Spectrum.png

    While rejecting the 25% or so unusable part of the spectrum could offer some heat reduction, one has to consider if it is worth the expense.

    A question I have is, to what extent are the longer wavelengths absorbed and heat the cell? Or is silicon simply transparent there? If that's the case, then there really is no need to worry about heating by the longer wavelengths.

    EDIT: the above graph is from here:
    http://en.wikipedia.org/wiki/Solar_radiation#Composition
     
  13. May 19, 2009 #12
    I found this on:

    http://en.wikipedia.org/wiki/One_way_mirror#Unusual_types_of_mirror Quote:

    "Cold mirrors are dielectric mirrors that reflect the entire visible light spectrum while efficiently transmitting infrared wavelengths. Conversely, hot mirrors reflect infrared light while allowing visible light to pass. These can be used to separate useful light from unneeded infrared to reduce heating of components in an optical device."

    It looks like I'm looking for a cold mirror. Does anyone know where I can get a hold of a cold mirror?

    Thanks again
     
  14. May 23, 2009 #13
    If you just want to prevent infrared light from reaching the PV cell then it would seem like your best bet would be something which is transparent to visible light and opaque to infrared. However, as has been said infrared light is no more so heat than any other wavelength, including visible light. Blocking infrared will just lessen the amount of power the PV generates, you might as well just put shades on it.

    Note that in your quote above it says those mirrors are useful to block unused light from optical devices. A PV cell is not an optical device. Ideally you would want to find out what wavelengths your PV cell uses (it likely uses infrared) and block the rest. I doubt though that it would be worth the cost to do this.
     
  15. May 23, 2009 #14

    russ_watters

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    Well, if you can find the maximum wavelength your solar cell can turn into electricity and if that wavelength matches with the cutoff frequency of some material (ie, ordinary window glass...?), you could just put such a thing up above the solar panel.

    But I suspect the reason you don't see this being done is that the cost outweighs the benefit.
     
  16. Mar 19, 2010 #15
    Could someone tell me a good way of creating an inexpensive filter that attenuates UV radiation to around 1/10,000 but affects visible light only by a tenth. I need this as a sheet which I have to place on a sensor. And the given budget is $5.
     
  17. Mar 19, 2010 #16

    ZapperZ

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    Er... any polycarbonate clear plastic is a good UV attenuator. Even ordinary clear glass absorbs almost all of the UV.

    P.S. You really should start a new thread since this is off-topic to the current thread.

    Zz.
     
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