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Pool water thermodynamics

  1. Sep 4, 2009 #1
    I'd appreciate any comments regarding the following prosaic/practical questions on inground swimming pool water...what the heat transfer mechanisms might be.

    (1) Does a dark vinyl liner aid in warming pool water any more than a light liner?

    (I'm guessing it might but with roughly three ft depths at one end and about 8 ft at the other end I'm unsure how much infrared sunlight energy makes it through water. Nor do I understand how that energy is transferred directly to the water than than via the liner.)

    (2) We had to cut off about two feet from the end of our pool cover blanket, (due in ultraviolet deterioriation) which is thin plastic bubble type material, that floats on the surface and helps contain heat during cool evenings. So now there is about two feet of pool water surface exposed for the width of the pool.

    Which will retain heat the best: leaving the open portion of the blanket at the deep end or the shallow end? Why?
    (I have no idea on this one but it would seem as surface water cools at night it likely slowly sinks and sets up convection currents. Maybe the shallow end would cool faster and hence lose less heat to the air but this does not take into account the underwater transfer of heat between warmer water under the blanket and cooler exposed water...)

    (3) Does anyone know if pool water warms faster in direct sunlight with the pool blanket on or off? Believe it or not manufacturers don't seem to say anything about that. (This would seem to depend in part on ambient air temperatures so maybe this question should be limited to those temperatures where air is at least as warm as water..that way little heat loss from water to air would occur.)

    Appreciate any thoughts or insights.
     
    Last edited: Sep 4, 2009
  2. jcsd
  3. Sep 4, 2009 #2

    LeonhardEuler

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    For (1), it should help significantly. The peak intensity of sunlight occurs in the visible, not the infrared region of the spectrum, so whatever makes the pool appear darker to the eye is most likely going to make it absorb more sunlight.

    For (3), unless the cover is transparent, then it is definitely better to take it off. The way you described it, it is an insulating material, and if it is opaque, then it blocks all the incoming sunlight. Whether it absorbs or reflects most of it, most is still not going to get to the pool. With a layer of insulating material between the heated outer surface of the pool cover and the water, most of the heat captured by the surface will be lost to convection and go to the air. Without the cover, you absorb at least a decent amount of heat into the pool (assuming the liner is not highly reflective).

    If the cover is transparent, then it's a little more difficult. On the one hand you block heat transfer by convection from the air to the pool. On the other hand, you stop evaporation which cools the water. The optimal strategy depends on the temperature and humidity. The net direction of heat transfer for the uncovered pool will depend on the so-called "wet bulb temperature" at those temperature ("dry bulb temperature") and humidity conditions. You can find this information on a psychrometric chart.
    http://en.wikipedia.org/wiki/File:PsychrometricChart-SeaLevel-SI.jpg
    This is probably overkill. I would just take the cover off.

    I'm not sure about (2).
     
  4. Sep 4, 2009 #3

    alxm

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    (1) I don't think it would help significantly. The peak intensity of sunlight may be in the visual range, but unless a substantial portion gets re-emitted as IR/heat, it's not going to make much difference. Dark things get hotter, but not a lot hotter. As anyone can tell from everyday experience, the material (and its ability to absorb and retain heat) tends to be more significant than its color.

    (2) I don't think it's significant either. I don't think I've ever noticed a horizontal heat gradient in a pool. The vertical one, OTOH, is quite obvious.

    (3) Off, but I wouldn't be sure. The pool cover is to insulate against heat loss to the surface through conduction/air convection, but also evaporative cooling. If the air is warmer than the water, it's doing the opposite of what it's supposed to in the former case. Evaporative cooling will always be there, though, as long as you have less than 100% humidity.
     
  5. Sep 4, 2009 #4
    Yes, I forgot to mention that I realize question #2 alternatives are an insiginficiant difference...what bothered me was that I had no logic to apply one way or the other...one alternative is likely in theory better than the other....

    Also, thanks for reminding me about evaporation....pool covers do obviously almost eliminate evaporative cooling...
     
    Last edited: Sep 4, 2009
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