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Solar heat exhanger on a pool

  1. Aug 27, 2016 #1

    DaveC426913

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    My longtime friend moved recently and his new place has an in-ground pool. He's a bit* of an Engineering nerd he has decided to build a solar heater from scratch. Him being a neophyte at the art of pool science though, I felt it my duty - despite being someone who never got his own custom exchanger working - to share my wisdom.

    *a lot

    Some ideas I'm pretty sure of, some less so. But it's got my curiosity up. Looking for feedback.

    In descending order of confidence:

    4. His heat exchanger core is 50' of black garden hose. Obviously, thinner, more conductive tubing would be better. I would go with 1/4" copper tubing myself. Greater surface area per unit volume, better thermal conductivity. We all agree, but it's out of budget for now.

    3. He intends to secure his heat exhange array to a large sheet of plywood.

    Truth be told, we both agree that it would be good to have a heat sink (something more heat absorbent than plywood), and embed the array in a 1/2" or so of conductive substrate of some sort. (Heat exchange by direct conduction will be far more efficient than heat exchange via radiation from a reflective surface). However, such a solution is also not in the budget at this time, so plywood back it is.

    He intends to paint the plywood black. My suggestion is to paint it silver, or better yet, line it with tinfoil. Failing an actual heat sink, you don't want your backing to absorb rays, you want it to reflect rays back to the hose.

    2. They are using black hose, to absorb heat. I suggested clear hose would actually let more solar radiation in. Upon reflection though, I recanted. Clear hose will also let much of that same solar radiation right back out the other side. Even with a reflector, much radiation will never make it into the system.

    1. They are making what I consider and excessively complex setup that takes cold water in to the array and recirculates it - in a secondary loop off the main plumbing, and powered its own secondary pump - within the heat exchanger until it has reached a certain temperature, then releases it back into the pool.

    Being a nerd with an electronics bent, he has constructed a control box from an arduino, a temperature probe and an actuator valve, that automagically opens and shuts when the water temp reaches the desired values. I am impressed to say I have seen it working, and - for this reason alone, I'd like to see his solution functional - however, I don't think he is gaining anything with his secondary loop.

    Water running continuously through the system will draw heat from the solar array just as efficiently, no matter how fast it flows. (In fact, the faster the better, to a point) i.e. even if a single pass raises the temp by, say, only one degree, it will be made up for by the sheer throughput. Heat transferred is heat transferred, whether it be in 5 gallons of throughput or 50. Matter of fact, 50 is better - the higher he raises the temp., the more heat loss from the system.

    0. Finally, something I've been toying with on my own: exchanging heat directly on the pump. It puts out a lot of waste heat. I'm thinking a coil of pipe off the main line, running through some sort of liquid-filled bag or pillow**, draped *directly* over the pump should transfer some heat, and do so with little fuss and muss.

    ** self-contained fluid-(prob'ly water)-filled bag acts as the substrate/heat sink. Form-fits to pump, with flexible copper pipe running in one corner, winding around lots of times, and out another. i.e. Fluid contents of bag are static, and not part of pool water.
     
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  3. Aug 28, 2016 #2

    mfb

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    If the hose orientation is fixed, you can use a clear hose and paint the lower side black. Paint at the inner surface is probably even more effective but I don't know how to get that reliably and without fancy chemicals.

    The secondary cycle looks odd, unless there is a reason to not mix water running through the hose with pool water (e.g. some additives used in the heating circuit).

    If the whole thing is efficient, the thermal power of the pump should not be a large contribution, but if it can be included easily, why not.
     
  4. Aug 28, 2016 #3

    DaveC426913

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    I rejected the clear hose though because, even with reflecting from the backing, solar energy is lost. Any light that passes out of the hose is lost heat. (And since I can see something other than utter black, light is escaping)

    Sorry. Their secondary cycle is not isolated. They are heating pool water directly, after bringing it in to the heat exchanger. The switches let hot water out nd cold water in.

    I may have obfuscated that by mentioning my own system, in item 0.
     
  5. Aug 28, 2016 #4

    mfb

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    A completely clear hose is certainly not a good option. But a clear upper side and a black lower side could be more effective than all black. You can some absorption directly in the water, and everything else still gets absorbed at the black side. Reflection at the clear surface might be higher.
    Then that concept makes even less sense.
     
  6. Aug 28, 2016 #5

    DaveC426913

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    Why? I mean beyond than the reasons I stated in the OP item 1?
     
  7. Aug 28, 2016 #6

    mfb

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    Well, I would see some advantages of a separate second circuit, but I don't see any advantages of such a messy system that just makes everything more complicated and reduces the efficiency.
     
  8. Aug 28, 2016 #7

    DaveC426913

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    This is what I'm interested in.

    I'm not sure his logic is sound re: efficiency of the recirculating phase, as opposed to just once-through. Looking for confirmation (not that I'm going to tell him. He's pretty happy).
     
  9. Aug 28, 2016 #8

    mfb

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    Sure, a higher water temperature in the system will decrease the efficiency.
     
  10. Aug 28, 2016 #9

    rbelli1

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  11. Aug 28, 2016 #10

    DaveC426913

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    Yes. Indeed, that is the basis for my argument.
     
  12. Aug 29, 2016 #11
    Forget about heat transfer from the pump motor. Even if the pump was wasting 500W to heat (which it does not), that would only be 3,400 BTU, which is enough to raise 12,000 gallons of water 1.3˚F per day at 8 hours of pump run time, and that assumes a perfect heat transfer.

    As for the flow rate, we tell people with commercial pool heating panels all the time that more flow is better, as heat exchange (efficiency) increases as Delta T increases.
     
  13. Aug 29, 2016 #12
    I forgot to mention... Don't let copper touch pool water. It will corrode in no time.
     
  14. Aug 29, 2016 #13

    DaveC426913

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    I should have provided some numbers.
    Again, this relates to my pool, not my friend's
    My pool is only 3,000 gallons. And I only need to heat it up a degree or two (C). It spends much of the summer just a degree or two below tolerable temp for my delicate nether regions.

    Funny thing about pump-heating - my hot tub does it that way. It does not have a heater, all it has is a couple of coils around the pump. And it reaches 105F. (Sure, it's 1/10th the volume, and has a lid, but still...)
     
  15. Aug 30, 2016 #14

    jim hardy

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    Fair Anne would NOT be happy with an ugly coil of pipe.

    Science wise It'd sure work better to collect heat over area larger than the surface of the pipe and conduct it into the water without a plastic pipe wall in the way..

    Most impressive solar pool heater i ever saw was much simpler.
    The guy built a decorative structure with a shed roof to provide a shady area adjacent the pool. He put his Tiki bar, a small fridge, lights, and and some lounge chairs under it.
    The roof he made from corrugated aluminum painted black
    and covered that with a piece UV resistant plastic sheet made for greenhouses.
    At the high end of shed roof between it and the plastic sheet he put a "header", just a piece of PVC pipe with holes drilled to distribute water into the corrugations ,
    and collected the warmed water at bottom with a plain old rain gutter that drained right back into the pool. He plumbed the header into the pool pump, also with PVC.

    That gave him tremendous area to collect heat with great thermal conduction right into the water. The plastic sheet prevents evaporation and keeps out the bird droppings.
    And he got a nice looking conversation starter not an ugly coil of pipe.

    Just a thought, one that seems to make sense from a heat transfer point of view.


    old jim
     
    Last edited: Aug 31, 2016
  16. Aug 30, 2016 #15

    jim hardy

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