Solar Pool Heater

[daveob]

OK. Here's a bit of brain exercise.

I plan to add a solar pool heater ( this weekend ), in the form of a 4 poly pipe coils on my carport roof - strong, flat, protected, full sun.

I have done a load of research and have the assembly all planned out, except for 1 small detail : the pipe diameter.

What are the pros / cons of using 12mm compared to 20mm poly pipe ?

I want to add 4 coils, each 100m of pipe.

My feed from the pump / filter is a 50mm pipe ( and so is the return pipe back to the pool ), and I plan to add 4 T-pieces to that line ( 1 T for each coil ). So the water from the pump will be split to the 4 coils.

I just can't figure out which diameter pipe to use.

My logic tells me that :
1. using a set outer diameter for the total coil (say 1.3m diameter), will result in the same area of pipe in the coil being exposed to sunlight.
2. the larger diameter pipe will put less back pressure on the pump.
3. the larger diameter pipe ( x 4 coils ) will have a better flow rate than the thinner pipe, thereby giving better thermal transfer to the water - research shows that the best thermal transfer to the water occurs when you can not feel the heat on the outside of the black pipe. If the pipe gets hot, then it means the solar energy, collected by the pipe, is dissipating to the surrounding air, instead of into the water.

Thoughts please.

[AndyD]

What are the pros / cons of using 12mm compared to 20mm poly pipe ?

I want to add 4 coils, each 100m of pipe.

If you have a coil consisting of 100m of 20mm pipe it will have a larger surface area than 100m of 12mm pipe. The amount of heat acquired by the piping would be proportional to its surface area therefore 20mm pipe would be better.

Are you going to connect the coils in parallel or in series? three coils of 12mm pipe in series would create a considerable restriction and your flow rate would be very low. I would go for three coils of 20mm or even 25mm in parallel in the interest of getting a decent flow rate at around 1 bar which is what most pool pumps will give you on a good day.

My feed from the pump / filter is a 50mm pipe ( and so is the return pipe back to the pool ), and I plan to add 4 T-pieces to that line ( 1 T for each coil ). So the water from the pump will be split to the 4 coils.

Sounds like a parallel arrangement which I would also favour.

My logic tells me that :
1. using a set outer diameter for the total coil (say 1.3m diameter), will result in the same area of pipe in the coil being exposed to sunlight.
2. the larger diameter pipe will put less back pressure on the pump.
3. the larger diameter pipe ( x 4 coils ) will have a better flow rate than the thinner pipe, thereby giving better thermal transfer to the water - research shows that the best thermal transfer to the water occurs when you can not feel the heat on the outside of the black pipe. If the pipe gets hot, then it means the solar energy, collected by the pipe, is dissipating to the surrounding air, instead of into the water.

If you're using set diameter coils then the surface area would be similar regardless of pipe diameter (see examples below). I agree you want a decent flow rate but it might be worth considering asheet of glass or perspex or even some clear building sheeting over the coils to reduce heat losses to the surrounding air.

[daveob]

I want to add 4 coils, each 100m of pipe.

Sorry AndyD, fingers working faster than the brain there for a moment. The 100m was for the 12mm pipe. The 20mm pipe would use less pipe for the same total coil diameter, but the surface area of the plastic would, as you correctly point out, be about identical.

The coils are to be connected in parallel to help reduce the friction, and resulting force that would be required to drive a single feed through 4 coils.

As far as the glass / clear material covering is concerned, the general thinking that I have see during research, is that from a cost / performance / weight perspective, it isn't worth it. Much better to keep a reasonable flow rate which would pull the heat from the pipe into the water. If more BTU is required, simply add additional coils.

Will be ordering the pipe today and shopping for fittings, so hope to post back around Tuesday with the results.

[wynn]

If you insert a ball-o-stop valve on the downstream side of each of your tees you can fine tune the restriction on the pool return to suite whatever pipe you install and also by only restricting the return by what the pump can bear.

The first tee will need less restriction than the last but every time you adjust further down the line you will have to readjust the first units until you have the right combination.

also bear in mind that if you have a 'baracuda' that will affect the feed to the pump.

[daveob]

Update : Project completed.

I decided to go with the 20mm ID pipe and, due to the limitation of the width of the flat roof, I added 4 coils, each about 1500mm diameter. Each coil used about 70m of pipe. The remaining off-cuts may very well be used later to add another 1 or 2 coils.

I added 4 ball valves ( for isolation in the event of a failure ) to the 50mm feed pipe, and connected each of the 4 coils to a ball valve.

The return from the 4 coils all feed back into a single 50mm which drains into the second inlet in the pool.

Murphys law, Saturday to Tuesday were all overcast days, so didn't seem to get much heat from the coils. In fact, the weather was so cool on 2 of the days, that I simply closed the master ball valve and by-passed the coils completely.

Wednesday was a great day. The pool started off at 22c in the morning. By mid-day, 23c and afternoon it got to 24c.

Overnight to this morning it lost 1c (back to 23c), but I think that a solar cover on the pool would reduce that. Just not sure if I want the extra cost and hassle for that just yet.

Again sunny today, and the water has gone from 23 to 25.5c. A bit more windy today, so will have to see how much of that is lost tonight.

I am hoping that the overnight loss is kept to about 1c a day, so the water has a constant daily increase of 1c.

Water coming back from the coils to the pool seems to be a constant 2c higher than the water going into the coils. Not certain what the flow rate is, but from the research I did, it appears that if you have a huge in / out difference, you're probably losing heat off the coils to the surrounding air. Based on this, I think the flow rate is about right.

With a warm sunny day predicted for tomorrow, I think there's a reasonable probability that we'll get to 26c after mid-day, so our first dip of the season should be a great event tomorrow.

Total cost of the setup was R2330 and took 2 full days to assemble. Monthly running cost : Zero.

With the 2c per day increase, it breaks down to about 0,5c per coil. Adding a second row of 3 coils would therefore add an extra 1.5c per sunny day, and with a 1c drop off at night, I am looking at a real increase of 2 - 2,5c per day - naturally, that will only be up to an optimum point. When we hit mid-summer, I will reduce the flow rate, or just isolate the coils completely, opening them once every few days just to keep the pipes clean.

[daveob]

Hi Wynn

The first tee will need less restriction than the last but every time you adjust further down the line you will have to readjust the first units until you have the right combination.

After I had the 4 ball valves assembled and connected to the feed pipe, and before connecting the coils, I opened the master valve from the pump, and found that the flow from each of the 4 valves was almost identical - visually measured by comparing the height that each 'fountain' rose from the valve. This is most likely due to the fact that the combined volume that can pass through 4 x 20mm pipes is still less than the volume of the water in the single 50mm feed pipe.

also bear in mind that if you have a 'baracuda' that will affect the feed to the pump.

Definitely agree with this. Connected the kreepy this morning, and the flow rate from the coils reduced to almost zero. I then slightly closed the master valve for the normal return inlet ( restricted the path of least resistance ), and this restored the pressure and flow via the coils.

[Pap_sak]

Hmmm, you say running costs: zero. But (unless you running a solar pump, my mistake if you are) surely your pump is chomping a bit, usually 0.75kw/hr? I like your idea and have thought of doing it (putting it on my soonish to be built lapa....) - but running a pump for a few hours a day grinds me a bit. At the moment I run my pump once a week when I clean +/- 1 hour. Is there a decent way to make use of 12v solar pumps or is it just too costly to implement?

[daveob]

Hi Pap_sak

Yes, my pump does run to push the water via the coils, but then it would be running anyway. I just set the pump to run from 7am ( about an hour after first sunlight hits the coils ) till 3pm, with 2 half hour breaks in the middle.

Not having had to tend to a pool before we bought this house, I went with the recommendations of the local experts in terms of water volume, and how often it needed to be turned over through the filter.

My neighbour says he saves a lot by only running the pump for 1 hour a day - has been doing that all winter. While I was working on the roof with this project, it struck me just how much his pool resembled a bowl of very dark pea soup.

Solar pump : yes, I have read of cases where a pump, powered by a solar panel, is used to circulate the water via the coils. The best I think was a 12v Bilge Pump. I suppose that I could investigate using this, and adding it to the system should be very simple. However, I am running the existing pump anyway, so might as well use the power from that.

Adding more components into the system just adds to the number of things that could go wrong. I think for this season I will just keep it simple, and add another 2 or 3 coils to the roof (over the Xmas holidays) to extend the end of the swimming season.

For next year, I will most likely look into fitting a water blanket to help better retain the generated heat overnight. I already have a few ideas about automating the on / off operation of the blanket and safety cover / net. Having done the net manually almost every day last summer was a pain - a necessity, but still a pain. I want to push a button - open - swim - push button - close. Don't want any strenuous work before and after my exercise

[wynn]

When I was a kid growing up in Keeptoon our gang were too poor to afford wetsuits and we used to get excited when the water was 16 or higher because that meant we could surf and skindive/snorkle without wetsuits.

25 seems a little wussy?

[Pap_sak]

Hi Dave

My pool water is pretty much spot on pH7.2 (with no added acid)- which helps. But my father in JHB(uses quite a bit of acid) also only runs his motor when cleaning and I think once during the week for an hour - both pools are clear 24/7. More so his than mine, in fact I cannot remember his pool even being slightly green. Basically his idea was to check the ph weekly/ chlorine level. Seemed to work fine, but as you say - now at least you are doing two things at once, nothing wrong with that!

[daveob]

Well, after 3 more mostly overcast days, the temp has still reached 25.5c Had the first swim of the season this afternoon - FANtastic. Will be watching the surrounding pools to see when they get to a similar temperature ( +25c ) so that I can calc the extra swim time that the solar coils have added to our pool. Will be doing the same at the end of the season. Last year we stopped when it dropped to 24c ( Yes, Wynn, we're wussies ). Am looking forward to seeing how much longer we can continue after the surrounding pools get to 24.

Also, my log so far is telling me that we gain 2c per (reasonable) day, but lose 1c every night. On an overcast day we might not gain anything, so the 1c loss overnight means that we go back a step.

Am seriously considering the solar blanket now, as it should reduce the overnight loss considerably, making the cumulative daily gains much higher.

[AndyD]

I think even a cheap pool blanket would be a good investment for you. The industrial strength bubble wrap type don't cost a fortune and last 5 or 6 years if you treat them well.

[daveob]

Project Update :

After enjoying the benefit of the first 4 coils and keeping a spreadsheet of the morning and afternoon temperatures for the last 2 months, we're quite happy with the 28 degrees temperature. We had 13 sunny and hot days in a row, and during this time, the temperature never got higher than 28 celcius. I think that this is the point where the heat going in equals the heat lost to the air and ground.

Anyway, after 2 cool and rainy days, I eventually added the 3 extra coils I wanted on New Years eve. This was for 2 reasons. Firstly, I already had the additional piping for the project, and secondly, we got 24 hours notice of visitors arriving from inland, and I knew that without the additional heat that the 3 extra coils would produce, we wouldn't be swimming for most of New Years Day.

This has increased the coil area to 41% of the pool surface.

Temperature increases since then have been as expected - on sunny days, depending on the amount of cloud about, water temp increases between 3,5 and 5,5 degrees celcius.

Also, since summer is in full swing, we're not losing as much heat overnight.

The pool is uncovered, and we live on a sea facing windy hill, so I think that if I get any higher than the current max of 31 celcius ( this afternoon ), then the heat loss ( to air and ground ) will probably start to cap the limit that the coils can heat to.

Yes, I know our new average of 29 celcius seems a bit warm, but the swim school that my kids go to is 31, and until this last week, my 3 year old daughter refused to get in our 'cold' water. Now I have to argue with her to get her out the water. That alone has made it all worthwhile.

My rough calculations show the energy being added to the pool is around 242,000 BTU per day.

[Dave A]

How are you going for water loss?

[daveob]

Hi Dave A

I've not noticed any change in the loss due to evaporation.

Not had to put the hose in the pool for over a year now.

I have made an extension to a downpipe that ends about 1 meter from the pool. When we get rain, I wait for enough water flow to wash the dust off the roof, then I add on the (push on) extension to divert the water to the pool. Have often topped-up 10-15 cm in one day. The extension also has a plate blocking the bottom half of the (almost horizontal) outlet, so any additional dust or debris stays in the pipe.

On the few occasions where I have filled the pool to the brim, I use it to do an extra long backwash ( 4 - 5 minutes ) which seems to help the sand filter.