Irrigation tubing solar heater

[johnm_10]

I know this is heavily debated around here with the usual consensus that professional solar panels give you more bang for the buck but that isn't what I'm seeing now that prices are skyrocketing.

Cheapest panels I see on ebay are $130 for 2'x20'. There is only one company offering that price, most I see are twice that price so I'm a bit hesitant as to quality, etc. PVC and connectors are going to easily add an additional $100-$150. And in my town we need a permit to install solar panels. so no idea of those fees ($100+?).

So I bought three 500' lengths of 1/2 inch tubing on amazon for $40 each and a submersible pump for $45. PVC connectors for the pump were another $20. So $185 total for about 60 square feet of solar surface area.

First I tried just running 500' of hose around on the grass. I got about 5000 btu at noon on a sunny day.

Then I tried 500' in one of those hot boxes they make on youtube and got about 7000 btu at noon on a sunny day. Better, but still no buano.

Then I split the hose in half (2x250') and ran that through the hot box and got 8500 btu.

Then I split another spool and ran both halves back and forth across the roof and got 17k btu!! Why you ask? My roof and attic are very hot during the day so it's getting a ton of conduction heat in addition to the direct solar energy. That said I decided to run the other spools on the roof as well.

So for $185 (and a bunch of time and effort) I'm getting around 50k btu/h into my little 4000 gallon AGP (at noon on a sunny day). My pool was averaging around 71 degrees (here in Florida), now it's almost 90. Love it.


If anyone else wants to try, here's some things I learned:

• Don't bother with the hot box idea if you have a hot roof.
• The hot box cost $50+ plus time and effort to build. For that cost it would have been better to simply buy more tubing and run it across the grass.
• Cut the 500' rolls in half (250'), to get twice the gpm and more overall btu. (Yes I have six 250' tubes running from the pump, across my roof, and back into the pool).
• Don't spiral up the hoses. This causes slower water flow. Water moves fastest in a straight line.
• Put gaps between the hoses on the roof and don't let them overlap. Most of the heat is coming from the hose touching the roof. If hoses are placed too tightly together, it will make a cold spot in the roof and you won't get as much benefit from the heat of the roof.
• They make a pvc fitting called a manifold (normally used for spa jets). I connected it to the pump and used slip compression inserts to connect the tubing.
• Don't wear running shoes on a hot roof, the sole glue melts after a bit.
• Have two ladders at different ends of the house in case you knock your ladder down while pulling all that tubing up to the roof (or take your phone with you so you can call someone to set your ladder back up lol)

For all the irrigation tube "nay-sayers" here's what I think you are overlooking with the tubing. Many of you guys are simply calculating potential btu based on direct solar energy hitting the surface of the tube. But you guys seem to be ignoring the conductive heat the hose picks up from whatever surface it's touching. Those "hot boxes" (I call them) are even more efficient because they use the greenhouse effect and also block wind so less heat escapes into the air. (They get hot inside like your car does parked outside on a sunny day). That said the cheapest crappiest 3/8" plywood I could find was like $30 so it's more cost effective (and easier) to just add more pipe. I guess a box could be made with old pallets or something but that's too much effort.

As for the math: My return tubes fill a 5 gallon bucket in 19.5 seconds so I'm getting about 15 gpm and a 7f degree rise from intake to outflow.

btu/h = 15 x 7 x 8.2 x 60

• gpm=15
• delta T = 7
• weight / gallon 8.2
• minutes / hour = 60

Anyway, it was a fun little project that got me outside. I wanted to share what I found because alot of this I had to figure out on my own.

 

[ajw22]

Impressive.

Care to show us some pics of your setup? Specifically the manifold and how your roof looks?

 

[johnm_10]

Impressive.

Care to show us some pics of your setup? Specifically the manifold and how your roof looks?

I still have a bunch of cosmetic work to do, I threw this together a bunch of different ways to find what worked the best so keep that in mind. I plan to make the hoses all nice and symmetrical. Luckily the back of my house faces south and you can't really see my roof from the ground. This pic was taken from a ladder.


Yeah I should also prolly vacuum my pool too. Not sure if I'm allowed to link to the manifold but you can find it if you google: Waterway 1.5 1/2 10 Ports with 4 Plugs Manifold

 

[mas985]

btu/h = 15 x 7 x 8.2 x 60

• gpm=15
• delta T = 7
• weight / gallon 8.2
• minutes / hour = 60

Were these measurements taken after thermal equilibrium? Initial temperature rise is going to be much higher than after thermal equilibrium. The tubing and roof will cool down with time.

1/2" tubing has a O.D. of 0.7" so the physical capture area of 1500' of tubing is 87.5 sq-ft. Solar insolation is about 300 BTU/sq-ft/hr with normal incidence so maximum for direct radiation would be 26,250 BTU. So if you are getting 52k BTU/hr that is double direct radiation.

The contact area of the tubing on the roof is very small (i.e. cylinder on a plane) and the thermal conductivity of polyethylene is not all that great (1/1000th of aluminum). If you assume the contact width of the tubing is 0.1", which is being generous, and that the thermal conductivity of the polyethylene tubing is 3.82 btu/sqft/hr/dF, then the maximum heat transfer from the roof to the water is 47.75 BTU/hr/dF with a tubing to roof contact area of 12.5 sq-ft. In order for 26k BTU/hr, the temperature difference of the roof to the water would need to be 544F which is not possible.

Even if you take into account radiation from the roof to the tubing, I still don't think you get there. So I suspect your measurement may be off somewhere. Perhaps, the panels had not reached thermal equilibrium which is critical for this type of measurement. Also, did you take a temperature measurement of the exposed roof?

About best you could possibly hope for is about 25k btu/hr total. The roof does not add that much in terms of heat gain and if you used tubing to fill in the gaps between the tubes instead of leaving the roof exposed, the heat gain would be considerable more. Here are the calculations:



Note this assumes that the water temperature and air temperature are the same so there is no convective heat transfer.

Solids, Liquids and Gases - Thermal Conductivities

Thermal conductivity coefficients for insulation materials, aluminum, asphalt, brass, copper, steel, gases and more.

www.engineeringtoolbox.com

 

[johnm_10]

Were these measurements taken after thermal equilibrium? Initial temperature rise is going to be much higher than after thermal equilibrium. The tubing and roof will cool down with time.

Yes, it had been running since 9am and I took the measurements around noon.

1/2" tubing has a O.D. of 0.7" so the physical capture area of 1500' of tubing is 87.5 sq-ft. Solar insolation is about 300 BTU/sq-ft/hr with normal incidence so maximum for direct radiation would be 26,250 BTU. So if you are getting 52k BTU/hr that is double direct radiation.

The contact area of the tubing on the roof is very small..........

Not to be rude but apparently you have no idea how hot a roof is in Florida on a sunny 80 degree day. I can't be up there more then 15 or 20 minutes without getting slightly dizzy and it melted the soles off my running shoes.

If you climbed up there I feel like you would completely understand why the water gets so warm.

You may have a point, it's not all conduction. It's probably quite a bit of convection.

I'll throw a thermometer up on the roof and in the attic tomorrow. I bet the attic is 140+ and the roof is hotter.

Also the O.D. of the tubing is .63 which means the tubing may be thinner than you expect. I can't find what this kind is made out of but it's not the same as the polyethylene tubing I use for my sprinklers.

The gaps help because the sun heats up the roof between the gaps which then transfers more heat to the hose. It essentially makes 60' of surface area into twice that. If the hose is touching then one part of the tube will cast shade on the other and there will also be more shade in one area of the roof.

 

[Newdude]

Hey John !!!!

Now first and foremost, you built something, and its working. Hats off all around.

The overwhelming bulk of the 'naysayers' you've seen have much larger pools. The best i can tell is you have a 12 ft round pool. That would be about 53% of your pools surface area matched with solar panel and enough to see gains in perfect situations. For example, heating 4k gallons in FL.

My PB said they aim for minimum 100% match for inground pools in NY. (800 sq ft for me). Its a whole different beast. Errr. 13X the beast.

 

[mas985]

Not to be rude but apparently you have no idea how hot a roof is in Florida on a sunny 80 degree day. I can't be up there more then 15 or 20 minutes without getting slightly dizzy and it melted the soles off my running shoes.

Your right, I don't know what the roof temperature was which is why I asked what is was in my post.

Also, the contact area of the tubing on the roof is much smaller than I had originally assumed because the roof is not flat. The tubing will rest on the bottom edge of each shingle so total contact area of tubing to roof is very small and conduction is likely to be insignificant. My main point for the post was that I do not believe that conduction could explain the additional heat gain.

However, radiation is still present and likely the dominant component for both heat gain and heat loss. But working the problem backwards, in order to get 50k output of the tubing including heat loss, the roof shingle temperature would need to be around 235F. This happens to be the melting point of polyethylene so if the tubing is LDPE, then turning off the pump could be problematic.

Sky temperature also plays a role in heat loss. So if you have an IR thermometer, take the temperature of both the roof surface (between tubing with pump running) as well as the sky temperature straight up but not toward the sun. My primary interest is in understanding and explaining the observations.

 

[denwood]

@johnm_10 ,your calculations do indeed work out to 53K. The submersible pump looks to be rated at 31 gallons per minute, 1,860 gallons per hour at zero feet, so GPH at half that with the manifold setup and parallel flow makes sense too. Those pumps are normally not rated for continuous duty, so please comment here if you see any issues! That said, it is cooled in a pool so overheating should not be an issue! Loving the manifold...and that you figured out that paralell flow is key.

How did you measure your water temp delta? How cool was the pool to begin with? Measurements with cold input water skew the efficiency numbers (much higher) vs heated pool water. I'm with @mas985 on the questions with regard to performance, however I have observed to some degree that roof effect you are speaking of. My system will continue to pump out 25-30K BTU with no sun on it (at about 5:30pm when the array becomes shaded) for 30-45 minutes which I know is radiated attic heat. My roof temps, far, far north of you have peaked at about 125F, on the shingle surface itself so capturing heat from it would favour tube spacing vs close contact. In Florida, your roof surface temp must be higher yet at midday, Google suggests as high as 200F at the surface. That's a lot of heat that could very well be changing the physics of your system with conduction/radiation from the roof adding a lot of BTUs to the equation.

A roof covered in pool solar heaters would actually be cooled by pool water, and never see those temps, so radiated heat from the attic would be less of an effect. Conversely, your roof is almost certainly getting to 160-170F due to the low coverage. I've seen pretty much zero experimentation in this area and figure it would be quite interesting to pursue. Another question I have is how much does pool heating (say with a mostly covered Florida roof) decrease AC cooling cost in the house? Again, very little actual data to be seen on the interwebs on that question.

This study done in Florida demonstrated shingle temps (black) in that 170F range at 2pm: FSEC-PF-336-98

@mas985 , how do the numbers look if you factor 170F instead of 125F ?