Am looking into pond loop slinky design and found that copper is about 100 times more thermally conductive than HDPE. So my question is, why do we not consider copper tubing. I know it cost more however it seems as though you could use much less of it! There is probably a very reasonable answer, can anyone help me understand?

Sure, there is the cost and then durability. With ground loops the ground moves heat more slowly than the plastic (in most cases) so HDPE is no problem. In a pond you might deliver btus more quickly than plastic moves them, but plastic is cheap and less bothered by surrounding conditions and boat anchors.
Joe

Posted By joe.ami on 13 Feb 2011 08:39 PM
With ground loops the ground moves heat more slowly than
the plastic (in most cases) so HDPE is no problem.

'Zactly! Analogously, light travels faster in vacuum than in air,
so your ceiling lights would (appear to) turn on more quickly
if you just reduced the atmospheric pressure in your home.

...a solution looking for a problem,

Looby

I have mulled the same question

Basically the issue is that natural convection in a body of water would greatly exceed the heat transfer through plastic pipe walls. Using copper in a pond should allow for substantially less loop length and some reduction in pumping power.

Copper is much more expensive than HPDE, and Joe's points about durability are well taken.

Another option for use in a pond is a "slim jim" plate heat exchanger, but they are pricey as well.

Posted By engineer on 13 Feb 2011 09:59 PM
Using copper in a pond should allow for substantially less loop length ...

Doubtless less length, but "substantially less" ain't so obvious (to me).

By my calculations, my 3-ton vertical loop system (with about 1000'
of in-ground pipe), should see only a 3°F T-drop across the HDPE
pipe walls when running flat-out in stage2. So, replacing the HDPE
loop with copper (or pure silver!) would raise my EWT by about 3°F.

It would have little effect on the total BTU-delivery capacity of the
loop field over the course of a heating season. OTOH, I won't guess
what effect a 3°F change in EWT might have on a pond loop system.

… or, you can install a DX GSHP with a copper pond loop. No plastics, no liquids , no flow centres.

BTW: Last I heard, the speed of light was a constant.

SR

Posted By fsq4cw on 14 Feb 2011 12:35 AM


… or, you can install a DX GSHP with a copper pond loop. No plastics, no liquids , no flow centres.

BTW: Last I heard, the speed of light was a constant.

SR

Not really, in air at standard temp and pressure the speed of light is 0.02926% slower than in vacuum. It usually decreases with higher density of the medium it travels through.

Posted By fsq4cw on 14 Feb 2011 12:35 AM


… or, you can install a DX GSHP with a copper pond loop. No plastics, no liquids , no flow centres. 


SR

Thus adding hundreds of dollars in lost refrigerant to the anchor scenario.......copper being no more impervious to impact in DX systems.
j

"Thus adding hundreds of dollars in lost refrigerant to the anchor scenario.......copper being no more impervious to impact in DX systems." The anchor scenario is a disaster no matter what you have in the lake. SR

People have used copper pond coils. Evidently it works fine.

Duplicate post -- deleted by Looby

Posted By fsq4cw on 14 Feb 2011 12:35 AM

BTW: Last I heard, the speed of light was a constant.

Maybe ewe shoulda stayed awake in H.S. science.

Google: "refractive index"

In the meantime, you might be wise to refrain from
offering advice on basic technical stuff.

Posted By jonr on 14 Feb 2011 09:49 AM
People have used copper pond coils. Evidently it works fine.

No doubt, though the points of cost and durability remain.

"The anchor scenario is a disaster no matter what you have in the lake. SR"

True, though more likely with copper and costly with DX. Few small boat anchors would harm hdpe, I would not make the same bet on copper. Shall we drop an 5 pound anchor on each from 5 feet and see which fares better?

If you want to push DX, why not put loops safely in the earth in a nice bed of crusher dust and keep them out of harms way? Why in the world would you want your loops in a pond?

Joe

My thinking (not backed by experience or calculation) is that water completely free to move via natural convection should make better use of copper's greater heat conductivity. The reduction in loop length might be enough to drop pump size a notch or two.

I agree that HPDE is tougher than copper. I wouldn't have a loop in a pond I don't control.

Perhaps fsq4cw has seen the light...

I’m just saying DX would work in a pond.

However, I have heard of a case where HDPE was eaten through by beavers. So we can’t control everything.

Would I install DX in a pond for myself? Not likely, but maybe if nothing larger than a canoe or peddle boat were allowed. Otherwise I would opt for HDPE. I would never use copper for a liquid system, even if it is a better conductor; that would just be silly, not to mention expensive.

Actually, truth be told, I only really like DX for either vertical or 30° off-axis drilling.

SR

Actually, if memory serves, we have an example wherein copper was successfully deployed in a "liquid system" in a manner so as to incorporate its high heat transfer property.

Water from a spring run was diverted through a plastic farm fertilizer tank within which a coil of copper was substituted for HPDE. Moving water has very high heat transfer ability, and running it by copper tube makes good use of that.

Actually, actually, IIRC, the copper worked just fine for a
little while, but it perforated in just a few months, and had
to be replaced with hdpe -- which also worked just fine,
and still is, AFAIK.

http://forum.geoexchange.org/geothermal-heat-pump-discussions/4196-installing-new-geothermal-system-2.html#post33356

...BTW, silver is an even better conductor than copper,

Looby

We've had more than one instance of considering defacto open loop systems with talk of employing compact heat exchangers to take advantage of moving water (it was an artesian well by me as well as MC's stream loop). I think those examples are different than copper exposed on the bottom of a pond.
I hadn't caught the rotting of the one loop post. For my guy he elected to ignore local ordinance banning open loop so we lost touch (not a moral issue for me...it's a dumb rule particularly with an artesian well available, but a practical one as a code enforcement official).
At the end of the day many things are better conductors than plastic (we use it as insulation after all) but, little is as impervious to enviromental and external impact......or as cheap.
J

Great advise from all, thanks for the info. Will stay with hdpe. Tell me what you think of this; Middle GA, average ground temp of 62 F 3 acre pond, 10 foot depth, current temp at 10' of 58 F 400 feet one way to pond 1.5 " hdpe to pond, 2' separation, 4' depth of cover, red clay. 4- 500 foot - 3/4" hdpe dr11 slinky, includes 75' feet from shore to 10' depth 12 gpm for 3 ton & separate 8 gpm for 2 ton.

Should work fine - if anything GPM may be higher than necessary resulting in somewhat excessive loop pump power.

Middle GA is potentially within my combat radius if questions arise or an onsite system checkout is needed.