Can my solar heater piping go underground?

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skeeter_ca

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Mar 15, 2013
91
Yucaipa CA
I'm going to be installing solar heater panels for my pool soon. I have been thinking about a few problems i have and can't decide what to do. The best place to put my panels where they won't be shaded half the day, will be about 100' away and about 5' higher from the pump which is an Intelliflo. The pool uses 1 1/2" piping and i am going to use 2" piping to and from the solar panels to reduce friction loss. It goes directly across a driveway but i could close that driveway and reroute around it if need be. If i place the pipe underground would i have much thermal loss and/or how could i insulate it being buried.

skeeter
 
Mine runs underground over about 50 feet and still works fine. If you are worried about it you could insulate the pipes underground.

Posted from my Droid with Tapatalk ... sorry if my response is short ;)
 
I am not sure what to use besides the foam pipe wrap.
I have seen insulating mentioned before .. like between heaters and spas for example ... but not sure what to use.

Even if the foam was compressed it would still help some ... although I am wondering if the losses are really high enough to worry about.
 
I don't think insulation is not going to make that much difference, especially underground. The dominate heat loss component in a solar panel system is radiation back to the sky from the panels. The pipes have a small surface area compared to the panels plus any pipes underground do not suffer from radiation losses and dry dirt is a pretty poor conductor of heat so the underground pipes may be better off than the above ground pipes.
 
If the ground is wet, it could be a lot more conductive than if it is dry.

There is a compound pipe made for outdoor woodstoves that has two pipes each wrapped in about 1/2" foil insulation and then a 4" outer pipe (like corrugated drain pipe) that contains the whole assembly, gives it some dead air space and, importantly, keeps groundwater from touching the inner pipes.

The big drawback is it is only available in 1.25" as the largest size, I'm afraid.

If that size is not a dealbreaker, "google outdoor woodstove pex pipe"

About 4-5 bucks a lineal foot if I remember.
 
The simplest solution would be to use crushed rock or stone. This allows water to drain rapidly and the air gaps between the stones act as an insulator. Plus it is very difficult to compress. But I am still not convinced that it would make a material difference.
 
I don't think there will be enough resolution in the temperature sensor to detect the temperature change. At those flow rates, and typical heat loss through an underground pipe, the temperature difference would be < 1 deg.

However, it is something that can be calculated. I can put together a simple calculation to show you.

What size panels are the panels?

Where is your location?

What is your temperature target?
 
We have a similar situation. Installed last year solar panels a bit over 120' from pool and about 4 or 5' higher (we had a huge flat area there hidden partially by trees from pool and unobstructed southern exposure). Panels are mounted on a slanted rack about 50' by 10' (500Sqft of panels). Trenched 2" piping back and return, no insulation. We have a Pentair SuperFlo 1.5 hp. I was a bit concerned about heat loss, but works beautiful. I'll try to post a few pictures. Typical temperature gain would be 1+ degree higher on the return vs in on a sunny day.
 

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I insulate my above ground pipes because I do get to much loss of heat (I heat my AGP). Search "pipe insulation" on the Home Depot site and you go from 3/8" all the way to 2" thick insulation around your 2" pvc pipe. You will need the 2 1/8" by whatever thickness you want. I would think the cheap 3/8 inch stuff would effectively reduce heat loss that would happen without it to virtually zero for a reasonable amount of money. Since it's the split type, it would be quick and easy for you to install.

Any uninsulated pipe WILL radiate heat, period. I will agree with above that if the pipe was below ground far enough. perhaps heat loss might be acceptable, but for the roughly $200 in 3/8" foam investment compared to what you already have into the solar stuff.....it would be a no brainer for me. Perhaps I am a little more sensitive to pipe heat loss because I use resistive electric heat and small sized solar heaters, any heat loss puts me behind a little bit........generally open and jump in around early to mid April and go until early December, so if it's sunny, I want that heat no matter what the outside temp may be.

Bob E.
 
Keep in mind that the smaller the panel area the longer your pump will need to be run ($$) in order to get heat gain into the pool. You are likely to save more money with more panels even taking into account the extra cost of the panels.

As for the pipe heat loss, I put together a SS based upon the methodology outlined in this paper:

Heat transfer model of above and underground insulated piping systems

Note that the paper shows that the dominate heat loss mechanism for an underground pipe is conduction which makes sense because the dirt is in contact with the pipe so there is no radiation loss and no convection loss because soil does not circulate.

There are five scenarios in the spreadsheet:

Scenario 1: Soil saturated with water - Very unlikely in CA during the summer since we get very little rain if any.

Scenario 2: Most likely scenario with dry soil. Shows a 0.15 degree temperature loss from end to end in the pipe which would be nearly impossible to measure. Keep in mind that this is with a 9.2 GPM flow rate (due to panel size) which is very slow. The faster the flow rate, the lower the temperature difference in the pipe.

Scenario 3: Adds insulation. Cuts the heat loss in half but it is still only about 1% heat gain so probably not worth the effort or cost. This assumes the Home Depot 3/8" foam pipe insulation (R-1.8).

Scenario 4: Same as 2 but with double the panel area. This decreases the impact of the pipe heat loss because the panel is producing twice as much heat plus you only need to run the pump half as long to get the same heat gain so less total heat is lost in the pipe.

Scenario 5: Same as 4 but with double the panel area again.

The most interesting aspect of this analysis was that 3' of dry dirt has an R-12 value which isn't too bad. Depending on the dirt composition, it can be better or worse than this.

You can download the spreadsheet by going to File > Download as...

Let me know if you have any questions.
 
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