Building a loop for solar heater

[LuvNH]

Greetings all.

Last year I had a good deal of success with building a solar heater with 1 inch black tubing coiled (100 ft) and mounted on a plywood platform. I made a separate loop for it that allowed flow in and out (to rejoin return water to the pool itself). I had to force water in to the heater loop by restricting pressure flow after the pump. Basically I have a Y which takes pressure from pump and separates into flow to pool and flow to solar heater. There is a valve that I have partially closed down stream of the Y to induce flow to the heater loop.

I further incorporated a valve in the heater loop that is remotely controlled. I have a sprinkler system that had a spare port. I ran the appropriate wires and had my sprinkler controller send signals to open and close when appropriate.

Now, to avoid stress on my Hayward variable speed pump, I want to keep the shut off valve open and have another small pump in the heater loop to pull water in and push it through instead of using the main pool pump and restricting flow. I have thoughts of using a small 110 vac pump (~10 gal/hr) with standard plug controlled by a relay that I can hook up to the sprinkler system, 12-20 vdc.

Before I get too far in both time and money, any thoughts out there?

 

[mas985]

100 ft of 1" tubing is only about 8 sq-ft of surface area to capture the sun. That will not have much in the way of heat gain for the pool. Rule of thumb is at least 25% of the pool's surface area but I would recommend at least 50%. For that size pool, that is about 250 sq-ft which is 35x what you have now.

Also, for both energy and solar efficiency you want many parallel paths of flow. This way the flow rate through the panels is high and the differential temperature is low to reduce heat loss and increase heat transfer to the pool. Rule of thumb for this is about 0.1 GPM/sq-ft.

Many people on this forum have attempted DIY solar in the same manner that you have. In the end, most realize that it is much more cost effective to just buy manufactured panels. To do it right, the amount of tubing and fittings required to build an efficient DIY system will usually exceed the cost of commercial panels. Plus the efficiency of using larger tubes is lower than using smaller tubes.

 

[LuvNH]

Thanks very much for your reply. I did have reasonable results with this setup, but was thinking about doubling the length of the setup. I am wary of the cost of commercial setups, and the reviews of most are troubling.

I understand the purpose of the parallel design, but that might be too complex. I could make it modular for later simple redesign, though. This really depends on being able to safely, reliably, and efficiently through the heater loop. Therefore, the pump and relay questions remains the big ones.

Your middle paragraph I interpret as meaning the parallel paths will allow for the greatest difference between the incoming water and the heat of the thereby creating the best possible heat transfer. And your formula with the 8 sqft, would that mean a pump that runs 48 gal/hr?

 

[mas985]

Thanks very much for your reply. I did have reasonable results with this setup,

How are you measuring the "results"? If by temperature rise from inlet to outlet, that is an incorrect method of performance assessment. If you have a large temperature rise but low flow rate, that means not much of that heat is getting to the pool and most is getting lost to the environment.

Your middle paragraph I interpret as meaning the parallel paths will allow for the greatest difference between the incoming water and the heat of the thereby creating the best possible heat transfer.

It is counter intuitive but you want just the opposite. Again, a large temperature difference between inlet and outlet results in high heat loss to the environment. With high flow rates, the inlet/outlet temperature differential is at it's lowest, has lower heat loss and higher heat TRANSFER to the pool. Higher panel exit temps do not equate to higher heat transfer. Just the opposite.

And your formula with the 8 sqft, would that mean a pump that runs 48 gal/hr?

Correct. But again, 8 sq-ft will not add much heat to a pool. On average, panels with a full day's sun will produce at most 1500 btu/sq-ft to a pool (average is around 1000 btu/sq-ft). 8 sq-ft is 12,000 BTU. A 27' pool with a depth of 42" is about 15,000 gallons and it takes 1 BTU to raise 1 lbs of water 1 deg F. So 8000 BTU will raise 15,000 g only 0.1 degF. Hardly a rise at all.

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[jimmythegreek]

As mas said in reality you aren't doing much. This has been tried many times and people post YouTube videos on it. The heat gains they see are from the sun naturally as the average sq footage isnt there. You would be better off with a single real solar panel. I did diy solar last year and it's amazing. I have 45% solar to pool ratio and I can still raise my temps over 10 degrees with a solar cover. A solar cover alone will raise your temps a few degrees

 

[LuvNH]

Strange. I’m trying to rationalize the math you give with the results I’ve seen. I admit that all evidence is purely anecdotal, but this last year the pool was warmer than the years past. I do not believe that we had abnormally warmer or sunnier temps.

What sort of solar panel are you talking about? Size, type etc? How did you integrate it into your setup?

 

[wogster]

The biggest problem with a coil, is that by the time water gets half way through the coil, it's already gained most of it's heat. You would be better with a header and footer made up of 2 x 1/2 inch Tees, and then run 1/2 inch parallel pipes between the two. A good idea would be a shinny aluminum plate that is shaped like a series of W's with the pipes in the bottom of the V, so sunlight touches 3 sides of the pipe instead of 1 side.

I think most panels skip the pipes, and simply shape it to allow water to flow along channels in the pipe. What temperature in and out are good for, is determining whether you should be feeding water through the panel at all. For example in temperate out is > temperature in, feed water through the collector; if temperature out < temperature in, bypass the heater.

 

[mas985]

Strange. I’m trying to rationalize the math you give with the results I’ve seen. I admit that all evidence is purely anecdotal, but this last year the pool was warmer than the years past. I do not believe that we had abnormally warmer or sunnier temps.

It is difficult to compare one year to the next because there are so many factors that go into pool temperature that could have changed.

1) Solar cover usage
2) Air temperature
3) Dew point temperature
4) Wind speed
5) Cloud cover

But the physics is sound. You can't add more heat than what is available from the sun. However, heat loss is a big variable and dependent on the above factors which can change quite a bit from year to year.

 

[jimmythegreek]

This was a warmer summer as I saw many pools above average here in NJ. I'm talking about simple solar pool panels meant for this job. The only cost is materials if you can DIY. You are already running your pump so that electric cost is a wash. Many brands out there like vortex and technosolis etc. You simply add a 3way valve and a check valve and a supply up and return down tied into your existing plumbing. I do pool work but never did solar in any form before this. Was super easy best 1800 I've spent on the pool. I had hot tub like water in the mid to upper 90s for weeks this summer was too hot for me but not for wife and kids they loved it