Geothermal HVAC, tank vs groundwater?

[sprintcar]

Hi folks.
I'm considering geothermal for the house and the weekend discussion was using a reservoir vs putting in wells. Using the buried coils in this GA dry clay is the last resort. I could dig out my Marks handbook and do the thermal calc, but I figured somebody here probably already has this available.

Georgia will not allow circulation between wells - you can't return water - so it would have to be 1 or 2 new 12" wells with downhole coils. $$ and a lot of mess - my house well is 230ft deep.

The option is one of the large direct burial plastic tanks, I figured about 5000 gallons, at least 4 ft below the surface, surrounded in fine sand for support and contact area. Heat exchanger inlet/outlet would be at opposite ends. Adding coarse sand or clean crushed rock internally would increase the thermal mass.

Assuming 3 ton HVAC unit, outside temp 100F, well insulated house, ground temp 60F, how long to raise the water temp 10F?

PART 2 - If anyone has done this, please share your experience, advice, comments, warnings, etc!!

THANKS!!

It's only a complete day when you Learn something and Teach something!

 

[dcasto]

you'll reject about 20% more heat than you get cooling, so 3 tons = 36,000 BTU/hr, so that about 42,000 BTU/hr.

5000 gallons is 42,000 pounds. so in 1 hour you'll raise the temperature 1 degree F.

one problem I see is how to get the water to give back the heat to the ground. Because the water is stagnet ther will only be natural convection, not forced, so heat transfer rate will be slow.

 

[MintJulep]

I assume your goal it improved efficiency and lower cost.

How about a gray-water spray onto the condenser coil? No holes to dig and probably similar performance improvement.

 

[sprintcar]

This is the feedback that I'm looking for!

I planned to place the inlet at one end of the tank, outlet at the other so there would be slight circulation across the length. I can oversize the circulation pump slightly to get more flow or add one that circs inside the tank at a preset temp. Depending on the thermal transfer of the plastic, the 6 degrees gained during the day (assume 6 hr run, really hot days) should be lost at night.

Mint - I don't generate enough gray water here. What I'm doing on the existing heat pump is adding a misting line, connected to a solenoid that opens when the fan starts - this just runs off house water pressure. The downside would be long term build up of clay or calcium from the well, but this unit will be upgraded within a year so it doesn't matter.

It's only a complete day when you Learn something and Teach something!

 

[MintJulep]

Big tank of warm water in the ground = happy place for Legionella.

 

[sprintcar]

Oh heck yes Mint! I had figured to possibly run the fill water thru a UV purifier and at the least add a few gallons of bleach plus a safe gylcol to keep it all happy... don't need any green growing stuff either which shouldn't be a factor once the sunlite is excluded.

It's only a complete day when you Learn something and Teach something!

 

[idratherbefishing]

If you were to bury a loop in the ground and thus increase the surface area for heat exchange. I realize the ground conditions may not be ideal for it, but you would be forcing the flow. You could then get away with a lesser volume of water that you would have to treat and maintain.

Down side is that you have to increase the size of your circ-pump and that adds $$ to the overall running cost.

 

[ivymike]

how about using the tank as temporary storage for a lawn irrigation system, so that you're regularly replacing the water inside?

 

[Snork]

We have a closed loop heat pump that uses buried coils in a 40 by 60 ft area of our back yard. The coils are about 8 ft deep (it was a really big hole). This works great for us in northern Minnesota with ground water 4 or 5 ft below the surface. We use it for heat in winter; domestic hot water; and air conditioning in summer. When we were looking into these systems it worked out that ground source heat pumps worked well for our climate, but air source systems were better for the south. Have you looked into that option?

 

[carnage1]

My college (OIT) is completely geothermally heated and the cooling system runs ground loops. In Oregon the max depth a trench can be dug without shoring it up is 6 feet so the ground loops are six feet deep. Just cut a trench with a ditchwitch or a backhoe and lay your tube loop in, you don't need to actually dig a big pit. Also have you considered new fancy windows or more insulation or better attic ventilation? Combining some combination of these things may allow you to use a smaller hvac unit saving you money in your install that could pay for some of the other things you are doing as well as reduce your long term costs.