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A hypothetical machine (fluid expansion)

  1. Jul 14, 2012 #1

    Imagine that we have a fluid heated up by solar energy (every day during sunlight). Due to higher temperature it would undergo thermal expansion and do some mehanical work while expanding, right? And during night it would cool down and hence compress and do some negative mechanical work. Now, my friend has an idea of constructing a shading system (practically a set of "shading units" that cover people from sun and open at day and close at night). What are the possible complications that might arise?
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  3. Jul 15, 2012 #2


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    Your friend is obviously thinking, which is always a good thing. In this case, however, his approach would be counterproductive. The most efficient use of solar energy would be based upon accumulating all heat or light available, which "shuttering" would decrease.
  4. Jul 15, 2012 #3


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    Perhaps that's the point. Perhaps this invention is intended to keep houses cool by automatically closing shutters on windows in strong sunshine?

    I think it might work but I suspect it would be hard to control the exact time at which the shutters open or close. Sunshine isn't allways a good measure of temperature either. Many countries have a lot of sunshine in winter when it's cold with snow on the ground. Do you want the shutters open or closed then? Open to allow heat in but closed if the sun is too bright?
  5. Jul 15, 2012 #4
    Also, the coefficients are thermal expansion (volumetric coefficents) are small numbers. Let's take for instance ethanol (which expands quite a lot) whose β=750*10^(-6) K^-1.

    Do you agree that thermal expansion will be too negligible to produce any significant work, like closing/opening the shutters?
  6. Jul 15, 2012 #5


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  7. Jul 15, 2012 #6


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    With boiling fluids, this is the concept of solar thermal energy, and solar updraft towers use expanding air. Both have demonstration projects, but they are still experimental.

    You can use them for shutters, too, but I would expect that the combination "power plant somewhere + motor where you need it" is usually better, apart from some simple systems like the greenhouse thing.
  8. Jul 15, 2012 #7
    They already use fluid cylinders on solar panel frames to track the sun and to open and close vents in green houses to maintain even temps. It would be easy to convet this to the operation of shutters to block the sun or to let the sun in for "passive" solar heating and cooling.
  9. Jul 15, 2012 #8


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    I want to take this opportunity to apologize to Fermat for misunderstanding his/her original question. I took it to mean that the shutters were to be used as a "strobe" system to maximize the number of times that a photoreactive mechanism would be cycled.
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