Geothermal-ish cooling water temp control

[JimBob88]

Hey all,

So the decision has been made that we will be installing artificial terf in our small north facing back yard. This means I will be skimming off about 6” of soil off the top of the yard to set my drainage and hard layer. This got me thinking however about what would happen if I dig a 12” trench down before (I live in the PNW so usually only the first 12-15” of ground ever freezes in winter) and laid in water line (as thin as I could find that wouldn’t collapse) and plumbed it into my rain barrel I currently use as my cooling reservoir. When I am stripping hard and fast I replace half the water in the drum at least twice during the run and am looking for a more economic and less wasteful way to maintain a lower cooling water temp without the noise and space of a radiator cooler.

I figure I can get around 120’ of tubing in the ground not including the turn radius’s. I figure there would be enough cooling effect with that length to at least keep the water decently cool.

Any thoughts? Success or failure stories? Let me know your thoughts, thanks!

[Salt Must Flow]

That should work great in the colder months, but I'd be curious what the temp of the surrounding soil is during the warmer months at that depth. In my area we have to trench at least 4' deep which is approx 55F year round.

If there's any chance of it freezing, I'd work it out so I could blow out as much water as possible using an air compressor.

[EricTheRed]

If you are going to dig anyway, why not go a bit deeper to get to cooler ground?

[Matfam]

That’s the way a water source heat pump works. 58F in the PNW @ 4’ year around. You’ll need more like 350’ lineal per 12000 Btuh per hour. Doesn’t matter how you run it as each pass is at least 12” from any other. I wouldn’t go less than 4’ deep.

[still_stirrin]

Something to consider, certainly.

But keep this in mind too: dirt, soil, or earth is a good insulator. Even though it may be cool (temperature-wise), the material is an insulator, so it will not transfer heat from a buried water pipe network very well. According to physics, heat (energy) will travel from a warmer medium to a cooler medium proportional to the heat transfer coefficient. And the coefficient for soil is very low, ie - low propensity to transfer heat (a good insulator).

Now, this doesn’t mean that your plan won’t work … simply that it isn’t very efficient. And to help the heat transfer process, using piping with a good heat transfer coefficient (copper) will help significantly. However, the cost of the “cost savings” system will be dramatically increased. You may be envisioning using plastic pipe for the cooling network, but plastic is also a good insulator and as such, will inhibit the transfer of heat from the water to the soil.

And here’s another thing to consider: the cost of pumping the cooling water through the network. Years ago, I installed a ground water thermal cooling system into a medical office building. Water was pumped from a water well through a series of heat exchange (radiators) in the building, with the intent to cool the building in the summer and warm the building in the winter. The water was returned to the ground through a return well in the ground. So, the system was intended to move the heat in order to normalize the ambient temperature inside the building.

The capital cost, ie - the 1st cost of the installation, was more expensive than conventional systems because of the pipes and pumps when compared to gas piping for natural gas fired heaters and conventional air conditioning systems. But what is worse is that the cost of continuously pumping the water in the loop far exceeded the savings expected from the heat pump system. After 10 years of operation, the medical building changed back to conventional heating and cooling systems (major remodel effort too).

So, the proposed solution may sound viable as an opportunity. But consider all of the costs, known and unknown, so you won’t be surprised in the end when you are questioned by your “significant other” at the cost vs savings disparity in your “bright idea”.

Here’s a suggestion: put in a swimming pool in your yard and pipe (a closed loop system) some heat exchanger coils in it. It will help warm the pool water and cool the cooling water pumped through you still condensers. Twenty to twenty feet of 3/4” soft copper would be perfect for the coil with minimal friction flow pressure drop (pump energy required) and provide plenty of dwell time to transfer the heat into the pool water.

Sorry to be such a “downer” here. It’s just that I am a mechanical design engineer (oil patch and aerospace) with a plumbing background (residential and commercial construction), so I have a little experience with heat transfer system design and build.
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[squigglefunk]

Any thoughts? Success or failure stories? Let me know your thoughts, thanks!

my thoughts are right away you might need a more efficient condenser and proper stacking of the hot water and one 55 gal rain barrel should be plenty for a stripping run. I can do a 15 gallon stripping run on a 30 gallon barrel.

now doing multiple runs gets harder, esp during warmer months when the barrel of water takes longer to cool back off, I have two and am usually able to use one for one run the first night, use the other for a run the next night and then by the third day the first is pretty much cooled back down to normal.