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Old 02-07-2008, 03:16 AM   #1
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Geothermal cooling loop help


I am planning to add an adsorption chiller to my existing solar thermal heating system to use the excess summer heat to drive a cooling cycle. I have identified a unit to use but it has a requirement for a recooling cycle that needs to be able to drop a 15 gpm flow of water from 92 to 80 degrees F. My idea is to use a geothermal cooling loop to achieve this. But I need pointers on how to calculate the necessary loop length and required circulating pump size. Any and all help appreciated. Including this idea is crazy and would never work (as long as you explain why)

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Old 04-26-2008, 10:30 AM   #2
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Geothermal cooling loop help


If you haven't already used the forum search function on this website I'd suggest checking that. In the archives you will come across specific persons that you can ask specific questions directly to ( you will have to make a specific mail to send to their e mail address) and they may respond. I've had good luck doing that. There are a lot of skilled people popping up on this site. Good luck. Cliff

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Old 09-29-2008, 10:18 PM   #3
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Geothermal cooling loop help


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Originally Posted by stuartm View Post
I am planning to add an adsorption chiller to my existing solar thermal heating system to use the excess summer heat to drive a cooling cycle. I have identified a unit to use but it has a requirement for a recooling cycle that needs to be able to drop a 15 gpm flow of water from 92 to 80 degrees F. My idea is to use a geothermal cooling loop to achieve this. But I need pointers on how to calculate the necessary loop length and required circulating pump size. Any and all help appreciated. Including this idea is crazy and would never work (as long as you explain why)
Factors to consider: Size of house (in cubic feet not square feet), ground temperature at the bury level versus the temperature you want to cool to, how much cooling you (hours per day, total weeks and months), size of pipe in the ground, flow rate of the fluid, insulation of the home, wind protection (more wind means more heat and cooling loss), vegitation covering the dwelling, latitude, roofing (dark requires more cooling than lighter roofing), foundation (crawl space versus cement slab versus below ground basemet versus walk out basement), configuration of the cooling field (a whole dug in the ground to 6 feet and pipe shoved in versus a large field with trenches that go back and forth for a hundred feet or so with 10 feet between trenches, and even more factors.

The bottom line is that if in doubt just put in lots of pipe at least 6 feet or more to make sure you are covered. It is alwasys a lot more expensive to add pipe later than to just get more than you need to begin with.

If you use smaller pipe that will allow more cooling area compared to the amount of fluid being cooled. I use 3/8 inch Pex and in the average home I use 1500 feet buried at 6 feet in a moderate climate. The numbers here are a 15,000 cu ft home, ground temperature of 60, daily temperatures of 90 and night temperatures of 60 in the summer. Winter is quite cold so the ground has time to recover from dumping in the summer heat.

Depending on the ground there is the possibility that it could heat up over time. Water (rain or watering a lawn over the pipe) is excellent in keeping the ground cool.

Most of the GPM is in the range of 5-7 and that is sufficient to move the fluid through the exchanger fast enough to keep from heating the fluid up too much. I get input to the house in the range of 60 and back into the ground at 64-65 at that rate.

Another big issue is making sure that there is no voids on the pipe in the ground. I always put in some sand on top of the pipe to make sure there are no voids. The sand closes in aroung the pipe and since it doesn't clump it won't create voids down the road as the ground settles.

Of course your mileage may vary and it is more of an art than hard science (although if you really wanted to spend the money you could calculate the total cooling needed and system required to make that happen if you could factor in all the variables).
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