Ozark01
New Member
Posts:26
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| 10 Feb 2010 02:53 PM |
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Please forgive me for what most will think are elementary and useless questions but I am trying to understand what makes a ground loop a good ground loop vs a poorly designed ground loop. It seems that the amount of contact the pipe in the loop has is one of the most important parameters for a good system design.
The system I am looking to install calls for 1800 feet of 3/4" pipe in a 1800 feet long single trench. The loop will start and end at the same place. This would give me approximately 353 sq ft of pipe surface in contact with the ground. I am also looking a 2500 gallon poly underground storage tank that has 400 sq ft of contact with the ground.
My question is if I buried the 2500 gallon tank 8' in the ground and put the 1800' of pipe in the tank to make a heat exchanger with the tank full of water, would I get the same heat transfer as I would with the pipe buried directly in the ground?
Any ideas would be appreciated. (I have not taken the pressure drop into consideration for this example) |
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geotek
Basic Member
Posts:154
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| 10 Feb 2010 04:17 PM |
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The system I am looking to install calls for 1800 feet of 3/4" pipe in a 1800 feet long single trench.
The loop will start and end at the same place.
You would have to have 3600 feet of pipe to do that.
But if you did that you would be spreading it out to get the BTU from the earth.
If you confine it in a tank you would not get the same ( or any where close to) the same BTU from the earth.
Also the pressure drop would be off the charts. |
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jonr
Senior Member
Posts:5341
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| 10 Feb 2010 04:30 PM |
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It would work great for a very short amount of time. Then you wouldn't be drawing heat from a large enough volume of earth.
It's not just pipe contact area, it is also how big an area you are drawing from. |
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geome
Advanced Member
Posts:987
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| 10 Feb 2010 04:41 PM |
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Would the storage tank freeze? |
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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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geotek
Basic Member
Posts:154
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| 10 Feb 2010 04:56 PM |
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Would the storage tank freeze?
Only in the winter, come summer you could use it for DHW.  |
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rjdalga
New Member
Posts:32
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| 10 Feb 2010 05:21 PM |
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Actually I think geome just wants to know if the tank would freeze or not at that depth (8ft below) in the winter and the answer is no. However, your idea of snaking all the piping inside the tank and then filling the tank with water wouldn't work very well (as mentioned above). The problem is that the tank has a much smaller surface area (relative to the piping). Do some math and calculate the surface area of the tank and then calculate the surface area of the piping. I'm sure you find that the piping has a much larger total surface area. In theory, if the two were about the same (or better yet the tank had a larger surface area) then it should work. Here's a thought. If you live in a region of the country which gets a fair amount of sunsine you could heat the water in the tank with a set of solar collectors. I know a guy up here in MI that is doing just that (not sure how well it's working though...I should go talk to him???). He buried two VERY large tanks in the ground (1500 gallon/ea) and the tanks were insulated with several inches of closed cell foam. |
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| RJDalga, CRI<br>Home Analysts, Inc.<br>Kalamazoo, MI 49009 |
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rjdalga
New Member
Posts:32
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| 10 Feb 2010 05:38 PM |
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Ozark01, Maybe YOU did do the math. Correct me if I'm wrong but the total surface areas are roughly the same. I calc the piping to have an approx. total sq. ft. of 350. In theory it should work. |
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| RJDalga, CRI<br>Home Analysts, Inc.<br>Kalamazoo, MI 49009 |
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geome
Advanced Member
Posts:987
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| 10 Feb 2010 06:32 PM |
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I'm not an expert. Actually I was wondering if the loop would freeze the water in the tank and degrade system performance? Could an antifreeze solution be used to keep the tank from freezing? Could the loop "dump" the leaving water into the tank and "draw" the entering water back into the geothermal system, kind of like an open pond loop (but with a tank instead of a pond)? "Who invented liquid soap and why?" |
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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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jonr
Senior Member
Posts:5341
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| 10 Feb 2010 06:53 PM |
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Yes, if you keep trying to remove heat from the tank, it will freeze. Without or without antifreeze. |
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rjdalga
New Member
Posts:32
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| 10 Feb 2010 07:04 PM |
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geome, Yes, you could design it that way (probably dump into the top of the tank and draw from the bottom or vise versa). I still don't understand why you think it would freeze though? Jonr, You aren't just removing heat from the tank (and yes it would ultimately freeze). The tank is in contact with the ground which is dumping heat back into the tank (hench geo). |
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| RJDalga, CRI<br>Home Analysts, Inc.<br>Kalamazoo, MI 49009 |
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Ozark01
New Member
Posts:26
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| 10 Feb 2010 07:24 PM |
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For surface contact area I got 353 sq ft for the 1800' of 3/4" pipe and 400 sq ft for the 2500 gallon tank. |
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jonr
Senior Member
Posts:5341
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| 10 Feb 2010 07:41 PM |
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What volume (in cubic feet) of soil is within close thermal proximity (let's approximate with 5') of some part of the long loop? And for the tank? |
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Palace Geothermal
Veteran Member
Posts:1609
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| 10 Feb 2010 09:01 PM |
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Putting the loop in a tank will not work for very long. Even though the surface area might be the same, the amount of earth that the tank has available to extract heat from is far less than the trench loop.
The problem is that heat moves very slowly through the ground. If heat moved quickly through the ground, then it would work. |
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Dewayne Dean
<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system |
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Ozark01
New Member
Posts:26
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| 10 Feb 2010 09:03 PM |
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It looks like the 1800' feet of pipe would have180,000 cf of dirt on all four side of the pipe 5' away. The tank would have 10,084 cf of dirt around it on all six sides 5' away from the tank. |
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Ozark01
New Member
Posts:26
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| 10 Feb 2010 09:24 PM |
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Posted By geodean on 10 Feb 2010 09:01 PM
Putting the loop in a tank will not work for very long. Even though the surface area might be the same, the amount of earth that the tank has available to extract heat from is far less than the trench loop.
The problem is that heat moves very slowly through the ground. If heat moved quickly through the ground, then it would work.
That is the question I have about the Kelix system talked about in another thread. That 300' feet hole would have about 30,000 cubic feet of dirt 5' away from the hole. That is 6 times less than the 1800' of 3/4" pipe. I don't see how the ground can recover quick enough for the Kelix system to work. The tank idea I had was cost prohibitive but didn't take up much space.
Thanks for the replies!
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jonr
Senior Member
Posts:5341
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| 10 Feb 2010 10:58 PM |
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It's one thing to make a system that will work for a short time (like the tank working great for awhile). It's another to make one that will continue to work for the entire heating/cooling season.
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waterpirate
Basic Member
Posts:467
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| 11 Feb 2010 06:50 AM |
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The op said he wanted to "loop" the tank, not use huge burried buffer tank. If it was that easy to bury a tank and put a loop in it to build a GSHX don't you think we would be doing more? The tank will freeze under the winter load. The btu's being pulled out of the water in the tank will be quickly depleted by the loop. The size of the tank required to accomplish this would be HUGE! It would be more expensive and take up more space than a more conventional GSHX.
Geo is full of thrifty/cheap/ people who really enjoy delivering te most for the least. This idea is not thrifty or sound financialy.
Eric Sackett
weberwelldrilling.com |
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| Eric Sackett<br>www.weberwelldrilling.com<br >Visit our Geothermal Resource Center! |
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engineer
Veteran Member
Posts:2749
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| 11 Feb 2010 08:44 AM |
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I like the calculation of dirt available nearby for exchange - that well explains the shortcomings of burying a tank |
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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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jonr
Senior Member
Posts:5341
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| 11 Feb 2010 09:35 AM |
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As a rough sanity check, 180,000 cu ft of soil with a 20F temperature drop will release about 86M btu* over the entire heating season. This is a reasonable number.
* using 120 lb/cf and .2 btu/lb/F
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cnygeo
Basic Member
Posts:170
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| 11 Feb 2010 09:41 AM |
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Don't forget latent heat from freezing - I imagine this depends strongly on the moisture content of the soil. |
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