Thermal solar for pool - panel design

[Nate14]

Hey folks,

New to the forum. I did a little searching online, and this looked like the most promising group. I'm work as an energy manager for a large industrial, and have a DIY project I'm considering for my house. Looking for some feedback from the group.

Background - I have a large pool with a gas heater. Last year I built a 16x16 lean to off the back of my house with a black steel roof. Faces south at about the right slope, ideal for solar. I may consider hybrid solar in the future, but for now I just want to experiment with a thermal pool heater setup.

Plan - I'm going to tie into my pool pump loop and put a circulation loop to a thermal panel. I'll be using about 300' of 1" black irrigation tubing to get to the solar location and back. I'll put in a small cheap hot water circulation pump on a timer for the solar panel loop (maybe run it off solar PV one day). I only run my pool pump at night for about 8 hours, plus I only want to run the loop when it hot.

Question is about design of the solar panel... As I said before, my lean to has a big black steel roof, so it's already a heat magnet. I just need to run some water over it.

• Option #1 - Build panel out of black irrigation tubing, simple and cheap. I was thinking of splitting the 1" supply into 3 parallel coils of 3/4" x 100' to keep pressure drop down and give it a little more time on the roof.
• Option #2 - Build it out of copper pipe. Maybe 12 parallel runs of 1/2" x 12'. It would be more expensive to build that's for sure. The only reason why I am considering this is because the thermal conductivity of copper is 1000X times better polyethylene tubing (400 vs 0.34). So even if I go with 100X less coil tubing, I should get 10X more heat.

Thoughts?

 

[tinyt]

Thirty years ago, I installed five of something similar to this. Takes about 3 continuous days of summer sun to raise the temperature to a comfortable level.

 

[wpns]

If you buy, make sure you get pool-water-chemistry-rated panels, chlorine, salt, other chemicals will eat potable-water panels within a year.

 

[Nate14]

Thanks Tinyt and wpns. I was looking at buying, but they are pretty expensive per ft2. I think I'm going to go ahead and build the panel with black poly tubing. It's pretty cheap and has good chemical compatibility and UV resistance. I can get 500 ft of 3/4 coils for under $200. Plus it will blend into my black roof and you won't really notice it.

FYI, I did a little more research that swayed my decision. Thermal conductivity is important, but only if the black roof and copper have good contact to conduct the heat, which it won't really. What is more important is the ability to absorb thermal radiation, this rated by an emissivity coefficient. A 'black body' being a perfect 1 on the scale. Black poly tubing is rated at 0.92, so damn good. Whereas copper pipe is less than 0.05, which is pretty crappy. Since the roof and tubing are not good contact, it's better to use the black poly with good emissivity and take the hit on thermal conductivity.

 

[Backyard]

Don't use copper. As someone who regularly services pools. Copper doesn't last long in a pool environment. Use the black polyethylene tubing

Make sure not to make too many parallel loops as the water may heat up too much to soften the plastic. You want to find that happy medium where you don't have a lot of resistance but also don't get too hot.

This is an interesting thread where someone did pretty much what you are looking to do. He tried series and parallel arrangements to test the heating

https://www.reddit.com/r/DIY/comments/17yfsfd/pool_solar_heater

 

[Nate14]

Don't use copper. As someone who regularly services pools. Copper doesn't last long in a pool environment. Use the black polyethylene tubing

Make sure not to make too many parallel loops as the water may heat up too much to soften the plastic. You want to find that happy medium where you don't have a lot of resistance but also don't get too hot.

This is an interesting thread where someone did pretty much what you are looking to do. He tried series and parallel arrangements to test the heating

https://www.reddit.com/r/DIY/comments/17yfsfd/pool_solar_heater

Thanks for the tips and link. Yeah I had mixed views on the copper from a compatibility side. I'm pretty good at keeping the pool dialed for pH and minimize chorine, but shi* happens. I've heard of people getting crazy with the chemicals and wrecking the pool heater core.

Yeah that pick is basically the idea, but I won't have the coils so tight. They will be spread out over the 250 ft2 rppf like heated flooring. Agree, trying to find that sweet spot for coil design. I did a lot of calcs, designing for high flow and low pressure. I am worried about it getting to soft and splitting or something.

 

[BarracudaBob]

I found putting a solar matt over the pool and having an easy way to take it off is the best option. Use the entire surface for the pool as the heater and pull it off for when you want to use it. Part of the issue is loosing all the warmth during a cold windy day or in the evening.

Now in the summer when it is almost warm enough and you don't' want to mess with the blanket you can floating black air-filled objects. I even used the noodle and irrigation weed mat that could be quickly rolled up. It surpassed any small recirculating system.

Heat Retaining Blanket for Pool​

Pool Cover Reel Set 18 FT Solar Reel

 

[Nate14]

I found putting a solar matt over the pool and having an easy way to take it off is the best option. Use the entire surface for the pool as the heater and pull it off for when you want to use it. Part of the issue is loosing all the warmth during a cold windy day or in the evening.

Now in the summer when it is almost warm enough and you don't' want to mess with the blanket you can floating black air-filled objects. I even used the noodle and irrigation weed mat that could be quickly rolled up. It surpassed any small recirculating system.

Heat Retaining Blanket for Pool​

Pool Cover Reel Set 18 FT Solar Reel

Agree about half the pool heat loss is to evaporation, best thing you can do is a pool cover. I have a roll out setup with a clear cover, use it religiously, especially at night. Pool is in the shade of the trees part of the day which doesn't help. Panel will get full sun all day.

I live in the north, so trying to give it a nudge to make it a little more comfortable. Hoping for maybe 100,000 to 150,000 Btu/hr. I'll test it when I'm done and post results.

 

[sinoba]

Hey folks,

New to the forum. I did a little searching online, and this looked like the most promising group. I'm work as an energy manager for a large industrial, and have a DIY project I'm considering for my house. Looking for some feedback from the group.

Background - I have a large pool with a gas heater. Last year I built a 16x16 lean to off the back of my house with a black steel roof. Faces south at about the right slope, ideal for solar. I may consider hybrid solar in the future, but for now I just want to experiment with a thermal pool heater setup.

Plan - I'm going to tie into my pool pump loop and put a circulation loop to a thermal panel. I'll be using about 300' of 1" black irrigation tubing to get to the solar location and back. I'll put in a small cheap hot water circulation pump on a timer for the solar panel loop (maybe run it off solar PV one day). I only run my pool pump at night for about 8 hours, plus I only want to run the loop when it hot.

Question is about design of the solar panel... As I said before, my lean to has a big black steel roof, so it's already a heat magnet. I just need to run some water over it check out swimmsolutions.com.

• Option #1 - Build panel out of black irrigation tubing, simple and cheap. I was thinking of splitting the 1" supply into 3 parallel coils of 3/4" x 100' to keep pressure drop down and give it a little more time on the roof.
• Option #2 - Build it out of copper pipe. Maybe 12 parallel runs of 1/2" x 12'. It would be more expensive to build that's for sure. The only reason why I am considering this is because the thermal conductivity of copper is 1000X times better polyethylene tubing (400 vs 0.34). So even if I go with 100X less coil tubing, I should get 10X more heat.

Thoughts?

So I've bitten the bullet and am busy building our pool - We tried to place it in our yard where the most sun will be shining during the day so that the temperature will be comfortable. Swambo and little one do not really like cold water. I on the other hand, do want to cool down during the Pretoria hot summer months - so to do a bit of compromise I started to look at solar heating. Will the pool actually get too hot during 34-36 degrees day time temperatures during our Summers or will it be still ok in terms of not too hot. Our pool is 5,5x3m and avg depth is 1,2m. Any of you have experience with solar heating?

 

[BarracudaBob]

People run their solar heater at night time and it'll actually cool the pool off.

 

[Nate14]

Hey Folks, just following up on progress. I'm a scientist by school, so I kind of nerded out on this a bit. I'll share some data, calcs and conclusions.

This is the install. I started with 3 x 100' 3/4" poly loops.
I converted the loop on the far right to copper to test the difference. Ended up being about 60' of 1/2" and 10' of 3/4". I sanded it and treated it with vinegar and salt to oxidize it and increase emissivity. Shinney copper has factor of about 0.05, while tarnished increases up to 0.65, still less than poly with 0.92.



On a partly cloudy spring day, I measured flows and delta T's to determine heat output. Somewhat underwhelming results...
A single plastic poly loop was generating about 7,000 Btu/hr,
The copper loop was generating about 5,000 Btu/hr.
On a perfect day with clear sky and sun at it's peak there is about 1000 W/m2, which is about 90,000 Btu/hr for 1/3 of roof (single loop section). Only about 8% efficiency, it will be interesting to see what it is like on a perfect sunny day.

I thought the thermal conductivity of copper would make a big difference. I went back and did a few more calcs, which I should have done in the first place to answer my original question. You can determine heat transfer (heat flux). The amount of heat transfer is driven largely by delta T, so I decided to work with the assumption of 15C (~28F ).
When you do the math on 100' x 3/4" poly loop it has the potential to transfer 45,000 Btu/hr. So black poly should do the job for the most part.
The copper loop has the potential to transfer 55,000,000 Btu/hr illustrating the conductivity of copper. That is overkill when you dealing with the relatively low energy intensity of sunlight. No worth it, like putting F1 tires on kids power wheels.

I'm going to play around with it this summer and get a few more data points. I'll probably make one modification to the copper to drop each section down so it lays on the metal and can conduct the heat better. The corrugated ridges in the roof are keeping it off the metal roof.

Anyway that's where I'm at with it, maybe some learnings for the group. With 3 poly loops I should get >20,000 Btu/hr. Nothing compared to my 350,000 Btu/hr pool heater, but basically free heat. The little pump I got is only 100w, so COP is >50, I'll take it.
Key message at this point - black poly is cheap and good enough for solar roofs.

 

[wpns]

On a perfect day with clear sky and sun at it's peak there is about 1000 W/m2, which is about 90,000 Btu/hr for 1/3 of roof (single loop section). Only about 8% efficiency,

Hmm, I suspect you are only covering maybe 8% of the roof with your black poly...

 

[Nate14]

Hmm, I suspect you are only covering maybe 8% of the roof with your black poly...

Yeah you're probably right, thanks. Seems obvious to look at haha. I guess i was hoping the black roof would emit the energy it receives and the poly would absorb it. Neat thing about black bodies. They are just as efficient at radiating energy and absorbing it. Changing from solar light energy to long was thermal radiation.

Maybe I'll test the black matt someone recommended earlier. Was trying to do it on the cheap. Or maybe I'll add a more black poly, need to find a cleaner way to lay it out.

 

[onrbikes]

I've worked a lot with gravity fed hot water set ups.
Thermosyphoning.
Coils don't work properly as hot water gets stuck.
The only rule you need to know is hot water rises and will take the shortest route.

The pool overflow should run into the bottom of your piping set up. The water goes into the bottom of the pool and displaces warmer water up.
A continuous cycle.

 

[Backyard]

Can you run your circulation pump on solar so the higher the sunlight the faster it runs and the higher GPM. When the sun goes down the pump stops automatically. Sized right the temperature of the water should be pretty stable regardless of the sun strength

 

[Nate14]

Can you run your circulation pump on solar so the higher the sunlight the faster it runs and the higher GPM. When the sun goes down the pump stops automatically. Sized right the temperature of the water should be pretty stable regardless of the sun strength

Makes sense and I thought about it, but decided to keep it simple for now. I might even upgrade my homemade thermal panels to hybrid solar panels to make electricity and heat the pool water. On the fence about that...

I decided to go with a wifi switch for controls. IoT makes for a pretty easy and cheap setup. I bought an outdoor switch with 2 ports for $20.
The first switch is controlling a ball valve that opens and closes during 'warm hours'. I need this because if the pool pump is on at night, it will circulate water through the panel and actually cool things down.
The second switch is controlling the small recirculation pump (100W) and runs on the same schedule as the valve.

 

[dktool]

When I had a pool I tried the poly tubing approach but in the end a solar cover on a reel was WAY better.
And it keeps a lot of dirt and debris out of the pool.

 

[Stircrazy]

Don't use copper. As someone who regularly services pools. Copper doesn't last long in a pool environment. Use the black polyethylene tubing

Make sure not to make too many parallel loops as the water may heat up too much to soften the plastic. You want to find that happy medium where you don't have a lot of resistance but also don't get too hot.

This is an interesting thread where someone did pretty much what you are looking to do. He tried series and parallel arrangements to test the heating

https://www.reddit.com/r/DIY/comments/17yfsfd/pool_solar_heater

I like the cart idea, mine are 4x4 with a coil in each one.

 

[Stircrazy]

Hey folks,



Question is about design of the solar panel... As I said before, my lean to has a big black steel roof, so it's already a heat magnet. I just need to run some water over it.

• Option #1 - Build panel out of black irrigation tubing, simple and cheap. I was thinking of splitting the 1" supply into 3 parallel coils of 3/4" x 100' to keep pressure drop down and give it a little more time on the roof.
• Option #2 - Build it out of copper pipe. Maybe 12 parallel runs of 1/2" x 12'. It would be more expensive to build that's for sure. The only reason why I am considering this is because the thermal conductivity of copper is 1000X times better polyethylene tubing (400 vs 0.34). So even if I go with 100X less coil tubing, I should get 10X more heat.

Thoughts?

I built these two seasons ago. with the solar blanket on a 10000gal aboveground pool I was getting temps in the mid to high 90's and had to shorten it down to 3 to 4 hours a day to maintain low 80's. last year I left the top off all season and it sat right around low 80's running 12 hours a day. I lust have a little pond pump pushing water through the coils which are in series. the first year I installed them and last year I was swimming in early may, this year we has a cold rainy spring so it is just ready now.

they are 4x4 in size, painted flat vlack, and each one has a full roll of 1/2" black vinal irragation tubing. I was thinking of trying to put clear plexy on them as a glazing to see if it increased the output at all, or I may just make two more and add them inline with a bigger pump. right now on a sunny day I am getting a 20 degree increase at about a 1 gal per min flow (just estimating the flow, might be more).

they are sitting on the ground right now ontop of sheet steel, but I think I am going to build legs and lift them up this year.

 

[Nate14]

I built these two seasons ago. with the solar blanket on a 10000gal aboveground pool I was getting temps in the mid to high 90's and had to shorten it down to 3 to 4 hours a day to maintain low 80's. last year I left the top off all season and it sat right around low 80's running 12 hours a day. I lust have a little pond pump pushing water through the coils which are in series. the first year I installed them and last year I was swimming in early may, this year we has a cold rainy spring so it is just ready now.

they are 4x4 in size, painted flat vlack, and each one has a full roll of 1/2" black vinal irragation tubing. I was thinking of trying to put clear plexy on them as a glazing to see if it increased the output at all, or I may just make two more and add them inline with a bigger pump. right now on a sunny day I am getting a 20 degree increase at about a 1 gal per min flow (just estimating the flow, might be more).

they are sitting on the ground right now ontop of sheet steel, but I think I am going to build legs and lift them up this year.

View attachment 305055

Nice job, a lot cleaner than my mess lol. Thanks for sharing. My pool is a Full L all deep end and about 50,000 gal, so it takes a lot to heat it up. I need to do another test. Last one I did the pool was at 80F and I was getting about 15,000 Btu/hr. Definitely better than nothing, but not as much as I was hoping for.

I'm surprised you could get the coil that tight without it kinking. I was struggling with the 3/4", bend radius is a little larger. Mind you I was dancing around on the roof, would love to prefab in the shop like you did, smart. How man feet of 1/2" did you get into each 4x4 section?

 

[Stircrazy]

Nice job, a lot cleaner than my mess lol. Thanks for sharing. My pool is a Full L all deep end and about 50,000 gal, so it takes a lot to heat it up. I need to do another test. Last one I did the pool was at 80F and I was getting about 15,000 Btu/hr. Definitely better than nothing, but not as much as I was hoping for.

I'm surprised you could get the coil that tight without it kinking. I was struggling with the 3/4", bend radius is a little larger. Mind you I was dancing around on the roof, would love to prefab in the shop like you did, smart. How man feet of 1/2" did you get into each 4x4 section?
View attachment 306961

I can't recall the length right now. for some reason I am thinking it was 100 feet in each one. I coiled it just to the point where it was starting to kink then backed off a smidge. I only have about 12000 gal and this year it is struggling as it is the wettest coldest spring I have ever seen here. so I am going to build two more. I did put a larger pond pump on the setup this year so I only have a 10 degrees F rise but a lot more flow. I will have to do a flow calculation on my next days off to see what kind of BTU I am actually getting.

 

[ksmithaz1]

A simple plastic bubble style solar blanket will work you up 3-5 degrees though they are annoying. My father was in the pool business for 30 years. Others mentioned it above, the problem is evaporation, causing cooling over a large surface area, coupled with the sheer volume of water and the thermals of the ground around the pool sucking the heat out. Every time you move heat you take a loss. You simply cannot pump enough water thru a pipe while maintaining a significant temperature rise in most cases. If you are getting 10 degrees that is actually suprising to me, and extremely aggressive, but the denser the material, the more heat conductive, and the ground around the water is a lousy insulator. Then the evaporation does it's thing and wham. You'll go broke using gas to heat an 18x36 in the winter even here in Phoenix, much less Canada.

How much of a temperature boost are we looking for, and what is the general unheated average for the time of year in question? What is the general ground temp in your area? In ground? Gunnite or Vynil on sand or vermiculite? I'm assuming you are trying to just make the water a bit warmer to extended your summer season. I will say again the #1 loss of heat in a pool is usually from the surface evaporation. It was weird moving here, you have water features like fountains to keep the water cool in the summer, and you would think you could swim year round, but 60-65 degree water is really cold to swim in, and the evaporation here in the winter takes it down even more.

They used to make solar pool water heaters similar to what you are building, they were never very effective. I would think if you could make the heat sink more dense with a tar and some sort of radiator type of heat collector. I'd love to see your targets, actual's and deltas when you get this thing done. Also where you tie into your circulation system, you want to pump the warm water in at your lower returns.

Most importantly, be sure to have some fun playing with it.