Monday, April 30, 2012

Thermal Solar Pool Heater - summary

Since there's plenty of interest in the thermal pool heater that we made last summer, I thought I'd try and do a post just to summarize what we're using, and how it's holding up.  (My original post is here.)

Here's a silly MS Paint schematic of our system:
Simple schematic showing intake, pump, coils (2xparallel of 3 in series) and return.

Main components are the coils and pump.  The pump was picked up off this surplus site is:
Diamond brand; Model No 312233-01; Consuming 120 Volt and maxing at 7 Amps (nameplate), yielding 1/2 HP, in a CSA Enclosure 3, Made in Italy.  I see that it's available as of 04/30/12 for $95, though I seem to remember paying less.
This pump is MORE than enough power, but with two smaller pumps, we didn't quite have enough juice, so we figured we'd go a bit overkill on this one.  For those wondering, we don't submerge this pump, but rather, it sits by the side of the pool, and so the electric cord (which goes through a watertight gland) doesn't come into contact with the pool water).

Our roof is about 15 feet above the pool water level (note that we crest the apex of the roof, and the coils are on the far side, for sun exposure).

The pump is controlled on a time-of-day timer, though someday I'd like to trigger it with a light-activated system, or possibly a water-temperature-activated system.  On cloudy days, I try and remember to flip the switch on the pump itself, to prevent it from cooling the pool.
The timer is an all-weather system, with a rain/splash guard, not meant for submersion.
The one I bought is $11 from Amazon, click here to see it.
 The coils are arranged into two banks of three coils in series, the banks are in parallel, to slow the flow of water somewhat, to increase heating.  Our flow rate is measured at 15 L/min which is almost 240 gallons per hour.  In the summer, I've measured the temperature from the roof at over 130 F during startup, and continuous around 110, while incoming water is 80 degrees, so I don't feel the need to slow down the flow.  The coils are detailed in my past article, but since posting there, We spray painted the PVC pipe black, as well as the zip ties, to reduce with UV exposure weakening.  Also, a commenter recommended making the PVC crosses at different diameters (6 inches per side), so that it's easier to thread the flexible pipe onto the coil.  LOVE that idea, thanks Ariel!

What we failed to do was to glue the straight PVC pipes into the joiners at the middle of each coil, so that when we went to reposition them a bit, the coils became un-attached.  So if you do this, go ahead and use a primer and glue for PVC.  Totally easy, and worth the $5.
All of the water-filled connections are holding up just fine after all that daily cycling.  I had serious doubts about the push-to-seal type of joints employed by this pipe (remember, it's DIG corp, SKU # 642176 at Home Depot, $80 for 1000 ft).


Finally, go check out the 

Separately, we have a photovoltaic (PV) solar panel powering an attic fan to keep the attic cool in the summer.  This is a related effect to the above project, where the cool water from the pool actually cools the roof as well.  Bonus!  We enacted both projects at the same time, so it's hard to say which had a larger effect on the summer temperature in the house, but I would bet money that the fan had a larger effect -- it was noticeable.



19 comments:

  1. I tried to get my neighbor interested in this idea, but in the end he went for the gas heater. ugh.

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  2. The gas market collapsed the past two years, so it's a good time to have gas.

    Then again, I'm fairly sure the cost of sunshine is going to be stable for the next few millenia.

    -HB

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  3. The idea of "slowing down the flow" to raise the outlet temperature is actually the oposite of what you want to do to maximize the energy transfered to the pool. Heat transfer is a function of the temperature difference. Therefore the higher flow rate and subsequent lower average temperature profile through the exchanger will result in higher energy transfer, not lower. Although it will be cooler to the touch coming out, it will lead to faster temperture gains in the long run.

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  4. @Anyonymous - thanks for the input. I do remember m*C*deltaT, but I'm curious if there's an upper limit here. The pump we have is quite over sized and really passes water quickly, and so we felt like a bit more 'dwell time' might help heat transfer. I think you're right, on hot days, we'd want to go back to a series setup, as it comes out quite hot.

    -HB

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  5. I see,this seems a legit looking job for home solar,thou,I think it will really get complicated.

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    1. RomanzJustin - I think you're right, in this case a solar PV solution as you're advocating can get very complicated. With PV panels you'll probably want an inverter and grid-hookup & metering, and then a lossy electric pool heater on top of all of that.

      With the setup above it's a self contained unit, no electrical work, and it's even quasi-uninstallable, if need be. :)

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  6. I'm very interested in doing this type of thing myself, and your version looks very easy and affordable. How did you attach the coils to your roof? Sorry if I missed that somewhere.

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    1. See my reply, below! And go for it, and post your results here, so we can see your work!

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  7. Joelsephus, I don't think I actually mentioned it - my apologies. There's no hard fastener. Being that the coils are so dense / heavy and that they are not a large wind catcher due to being low profile, there's nothing except gravity holding them in place. It hasn't been a problem, even when they were empty coils (no water), there was enough weight in windy conditions. What I did have a challenge with was slow creep, down the low incline of the roof, and so I just tied the coil frames to each other, across the apex of the roof (pulling against each other). This slow creep happened over weeks/months, and I think mostly due to the heat cycling which caused some cyclic expansion / contraction. (Movement was measured in inches over months.)

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  8. Thanks Holden, that's helpful. With regard to the expansion/contraction, do you have any concerns regarding the system wearing holes in the shingles over time? I've seen that mentioned with regard to some improperly installed professional systems, and wondered if it would be an issue with yours. The solution they recommended was to attach a sheet of tin under the potential wear spots. Thoughts?

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    1. Joelsephus, I hadn't really thought about that at all, and haven't noticed any wear on the shingles, as the tubing is a tough, but not abrasive plastic. Of course, your materials may be different. For the simplest approach I would think it would suffice to do as I have, and after one season, simply inspect. If you have to put something under at that point, it is possible to drain and re-fill the tubes.

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  11. Any idea where can i buy a 1/2 hp pump for a good price in Ontario, Thanks
    Oscar

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  12. Thanks for this write up on your system!

    I was wondering if you can share your experience on how loud this pump is. I put together a similar but on MUCH less grand scale last year and in principle it worked very well. But I used a cast iron transfer pump and while it moved water just fine it also moved a lot of rust with it. It was also very loud. I'm definitely looking for a SS pump for this season and would love to hear about the one you've selected as far as noise. Is it loud? Have you done anything with placement to deal with noise if it was an issue? Thanks again!

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    1. Regarding the noise, I don't have a sound / dB-meter to quantify it, but it's the kind of thing that I don't mind having turned on while I read a book 5 feet away, though if I had some friends over for a meal on the back patio, I'd probably turn the pump off for total silence or to play some soft music instead. I would definitely not classify it as "very loud" but rather "very quiet". I'd say try something for fountains, as they tend to be artistic pieces which need to be quiet anyway.

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    2. Perfect, thanks!

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