whirnot
Basic Member
Posts:186
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| 27 Sep 2012 10:55 AM |
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I read somewhere that the majority of heat transfer between the ground and the ground loop occurs within 6 inches of the ground loop. Since it is common knowledge that any temperature conductor can be made more efficient by adding more surface area, similar to the fins in a radiator, it would seem to me that if a temperture conducting medium such as steel cable were installed serpentined, next to the ground loop it would create a more efficient thermal transfer.
See any reason that would not work? Obviously if you had to buy the cable it would not be cost effective, but if you had thousands of feet available...........? |
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jonr
Senior Member
Posts:5341
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| 27 Sep 2012 11:57 AM |
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Anything with higher thermal conductivity than soil helps and the closer to the pipe, the more difference it makes. A soaker hose does a similar thing. |
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whirnot
Basic Member
Posts:186
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| 27 Sep 2012 01:05 PM |
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That was my thought, using say 10 foot lengths, attaching the center 4 feet to the ground loop with zip ties, and splaying the ends away from the ground loop like a frayed rope.
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joe.ami
Veteran Member
Posts:4377
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| 28 Sep 2012 09:26 AM |
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Posted By jonr on 27 Sep 2012 11:57 AM
Anything with higher thermal conductivity than soil helps and the closer to the pipe, the more difference it makes. A soaker hose does a similar thing. A soaker hose creates a hybrid open loop, with direct extraction of btus from the water trumping extra moisture in soil at first, then temporily enhancing ground TC. Depending on the soil "temporary" may be measured in a few minutes. A soaker hose's true function is to assisst in compaction when loops are first installed. The advantage to improving a loops btu gathering capability is generally employed by shortening loops making net op cost the same. There is a sweet spot in loop design that is generally determined in part by electric rates, but industry standard is to deliver EWTs of 30-90*. We have done countless models here where it has been demonstrated that a significant jump in EWT (for heating dominated climates) is required to even modestly impact op cost....such as a 10* jump in EWT may have a $49.00 annual savings if electric rates are 11 cents/kwh. So lets say the cable raises the EWT 3* saving maybe $10.00/yr but during backfill, at one point the cable came in contact with the ground loop. Years later ground movement or expansion/contraction of HDPE wears a hole somewhere in your 2000' loop field. Was it worth it? |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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docjenser
Veteran Member
Posts:1400
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| 28 Sep 2012 01:27 PM |
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It is important to understand that surrounding ground is also a factor. The conductivity of the HDPE pipe might be very bad, but it still conducts more energy than the ground does, so it does not matter if the ground is the limiting factors. The 6 inch around the pipe are the most important ones, but once that is depleted, new heat has to be conducted to the depleted areas around the pipe.
Also Joe's point is important. You can double the loop field size and increase the average EWT by maybe 5-7 F, which will only have a small impact on the operating costs of the system. It is probably more impactful to exchange the air filter twice instead of once a year to keep the refrigerant temps lower.
Another way to look at this, if you are thinking about increasing loop performance by 10-20% through fins, steel cables etc, you might as well increase the loop field by 20%, probably much cheaper and quicker. |
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| www.buffalogeothermalheating.com |
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geome
Advanced Member
Posts:987
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| 28 Sep 2012 02:55 PM |
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Posted By docjenser on 28 Sep 2012 01:27 PM
Another way to look at this, if you are thinking about increasing loop performance by 10-20% through fins, steel cables etc, you might as well increase the loop field by 20%, probably much cheaper and quicker.
- and warranted. |
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| Homeowner with WF Envision NDV038 (packaged) & NDZ026 (split), one 3000' 4 pipe closed horizontal ground loop, Prestige thermostats, desuperheaters, 85 gal. Marathon. |
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engineer
Veteran Member
Posts:2749
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| 29 Sep 2012 11:14 PM |
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Posted By docjenser on 28 Sep 2012 01:27 PM
It is important to understand that surrounding ground is also a factor. The conductivity of the HDPE pipe might be very bad, but it still conducts more energy than the ground does, so it does not matter if the ground is the limiting factors. The 6 inch around the pipe are the most important ones, but once that is depleted, new heat has to be conducted to the depleted areas around the pipe.
Also Joe's point is important. You can double the loop field size and increase the average EWT by maybe 5-7 F, which will only have a small impact on the operating costs of the system. It is probably more impactful to exchange the air filter twice instead of once a year to keep the refrigerant temps lower.
Another way to look at this, if you are thinking about increasing loop performance by 10-20% through fins, steel cables etc, you might as well increase the loop field by 20%, probably much cheaper and quicker.
It is counterintuitive, but as long as the ground is the limiting factor, the tubes could be copper or, for that matter gold, rather than HPDE, and it would make no difference in system performance. |
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Curt Kinder <br><br>
The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is - Winston Churchill <br><br><a href="http://www.greenersolutionsair.com">www.greenersolutionsair.com</a>
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MikeSolar
Basic Member
Posts:376
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| 30 Sep 2012 07:03 AM |
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One other thing to remember is that the tighter the compressor dT, the less stress is placed on it and the longer it lasts. I would like to see a study of equipment life VS above/at or below optimal loop lengths. Never heard of one tho.... |
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| www.BossSolar.com |
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jonr
Senior Member
Posts:5341
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| 30 Sep 2012 11:03 AM |
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There is never a single limiting factor in the thermal performance of a ground loop. All factors create resistance and decreasing any one of them increases performance. What is true is that one factor can be large enough that even substantial % gains in other factors will have little overall effect. Copper always outperforms HDPE (it's just not worth the price for the small net effect). |
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joe.ami
Veteran Member
Posts:4377
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| 30 Sep 2012 02:14 PM |
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Yes there are often multiple limiting factors, often however one trumps the others. In this case it is the ground TC. DX advocates make a big deal of copper ground loops but fail to mention that the advantage swiftly goes away as soil can't keep up.
So while copper "outperforms HDPE" the end result is BTUs extracted, after the first few minutes, are the same as copper has to wait for soil to catch up.
Larger or deeper are the improvements that most impact loop performance.
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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jonr
Senior Member
Posts:5341
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| 30 Sep 2012 02:36 PM |
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advantage swiftly goes away ... BTUs extracted, after the first few minutes, are the same The advantage never completely goes away and is never the same, its effect just decreases as a percentage of the total btu transfer. It's a case of "diminishing returns" not "limiting factor". |
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geodude
New Member
Posts:58
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| 02 Oct 2012 11:11 AM |
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Posted By jonr on 30 Sep 2012 02:36 PM
advantage swiftly goes away ... BTUs extracted, after the first few minutes, are the same
The advantage never completely goes away and is never the same, its effect just decreases as a percentage of the total btu transfer. It's a case of "diminishing returns" not "limiting factor".
its both
The advantage does go away because it is coupled to a heat pump.
A GSHP would perform better if it had a stable 40deg EWT and ran longer, than if it had 40 down to 35 and ran for slightly less time. |
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docjenser
Veteran Member
Posts:1400
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| 02 Oct 2012 02:12 PM |
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Posted By jonr on 30 Sep 2012 02:36 PM
advantage swiftly goes away ... BTUs extracted, after the first few minutes, are the same
The advantage never completely goes away and is never the same, its effect just decreases as a percentage of the total btu transfer. It's a case of "diminishing returns" not "limiting factor".
The advantage not only goes away, the system will quickly perform worse, since the direct exchange system is connected to lesser ground, so the source will be colder, which will significantly increase the refrigerant pressure, rendering the system much less efficient. If you are lucky, your system will only become less efficient. If you are out of luck, your compressor will give up... |
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jonr
Senior Member
Posts:5341
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| 02 Oct 2012 02:31 PM |
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So running the same compressor more hours is more efficient and the same system using copper (vs HDPE) may cause a compressor to "give up"? Incorrect, but you can wrap any loop with insulation if you actually believe that restricting heat transfer helps efficiency. Use any ground loop modeling program to show that increased thermal resistance is not helpful (and decreased is). A GSHP would perform better if it had a stable 40deg EWT and ran longer, than if it had 40 Increased thermal conductivity between the loop liquid and the soil increases EWT, so 40 and 40 is a bogus comparison. But it is true that an oversized compressor that removes heat quickly and then shuts off will underperform a smaller compressor that extracts heat more slowly. But that's not a loop material issue. |
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docjenser
Veteran Member
Posts:1400
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| 02 Oct 2012 03:29 PM |
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Let me clarify a couple things here. Joe mentioned (very correctly) that the ground loops in direct exchange systems look good at start up (great heat transfer through copper pipe) but quickly loose performance since the ground is not able to keep up with the high conductivity of the copper pipe. So the ground right next to the pipe (which is the ground which matters for the heat exchange) is much colder than beside a HDPE pipe, the increased refrigerant pressure resulting in much lesser COP and much more stress on the compressor, one of the reasons they fail so may times. At the end the volume of soil covered via the direct exchange system is much less compared to water source system, but the BTUs required by the house is the same, thus the ground will be much colder. So the ground conductivity matters much more than the pipe conductivity, which is not the limiting factor.
If the thermal resistance of the ground is so much less than the pipe, it will simply cool the ground right next to the pipe, and reduce the heat transfer unless you drive down the refrigerant temps to continue a sufficient delta T. Again, this means more stress on the compressor and much lesser efficiency. That is why copper pipe in the ground does not make sense, unless the conductivity of the surrounding medium is much higher, like flowing water. |
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jonr
Senior Member
Posts:5341
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| 02 Oct 2012 03:59 PM |
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I think we agree that DX systems use copper (always thermally helpful) and then reduce length and field size (never thermally helpful) and often overdo it on the latter. But this isn't relevant to the OP. |
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docjenser
Veteran Member
Posts:1400
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| 03 Oct 2012 12:14 AM |
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OP??? |
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| www.buffalogeothermalheating.com |
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jonr
Senior Member
Posts:5341
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| 03 Oct 2012 10:07 AM |
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OP = original poster = whirnot, who hasn't expressed any interest in DX or reduced field size. |
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docjenser
Veteran Member
Posts:1400
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| 03 Oct 2012 10:35 AM |
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Silly me, and I thought you forgot the "C" for COP.
Sure it is relevant. While it is true that the most heat exchange occurs right next to the pipe, if that soil next to the pipe does not get enough enough BTUs transfered to by the ground further away, it will cool down, reducing EWT and therefore COP.
In an other example, for pondloops we are using half the pipe lengths than for horizontal ground loops with the same HDPE (since water is a much better conductor than soil), which tells you how important the surrounding media is. Now add flowing water to it, and the pipe could again be cut in half. Now the material of the pipe becomes more of a factor, since the conductivity of the media surrounding the pipe is maxed out. Until HDPE pipe is pretty good for our purposes. |
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joe.ami
Veteran Member
Posts:4377
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| 03 Oct 2012 10:36 AM |
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Lol, jonr mighty champion of O-posts!
The point that over or hyper extraction can actually decrease performance is very relevent to op.
The point that frozen earth can move things around and possibly cause harm (radial cuts) to copper or metal cable chaffing HDPE in OPs scenario.
You are semantically correct jonr when you suggest an advantage doesn't go away but can be mitigated. So if it can be mitigated to a point where it offsets effort or even performs worse than a more traditional install, then it is of no value- now that's on point!
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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