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Old 12-17-18, 10:10 AM   #1
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Default Combination vertical/horizontal geothermal

My idea is this: dig 10 to 20 ten foot wells and use, say 6 or 8 inch pvc pipes (available at the box stores). Put a plug at the bottom. have a reducing T-connector and a PVC plug in the top. Have a 2 inch pipe going through a hole drilled in the top plug that goes down 9 feet. Connect with 2 inch pvc: the effluent comes out of the T-connector and is connected to the center inner pipe of the next large vertical pipe.

Advantages over a large slinky trench system:

1) Pouiseille's Law!!! The slinky field is long and narrow. Resistance to flow is proportional to the length and inversely proportional to the fourth power of the diameter. Pushing water through the long irrigation tubing is no small matter. The higher pressure required can lead to leaks.
2) If you are using a 2 inch pipe in a 6 inch pipe, the cross sectional area of the down flow central pipe is pi (3.14) and the annulus is pi x ( 9 - 1 = 25. This means the upward flow velocity is about 8 times slower. This gives it plenty of time to exchange heat.
3) There are plenty of youtube videos on DIY digging shallow wells.

Many questions, though both technical and mundane. First and foremost, how many vertical wells would be needed? What are the ideal diameters for the inner and outer concentric pipes?

By using the vertical dimension, much less square footage of the property should be needed.

Mundane: The reducing T-connector is easy, but how about the plumbing of the inner pipe through the top plug?

I have a sketch of what I am talking about.

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Old 12-17-18, 10:14 AM   #2
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Default here is the diagram

Easy question for DIY plumbers is how to run a 2 inch pvc pipe down the middle of a 6 or 8 inch pipe?
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File Type: pdf combo vertical and horizontal geothermal.pdf (7.8 KB, 21 views)
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Old 12-24-18, 01:29 PM   #3
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Ok, so there are a few things to consider here.

The first is pressure drop. With your rig, each pipe will have the drop of half of the total length worth of 2 inch pipe. I'm guessing 9 feet worth at 2 inch plus 9 feet at large area equals close to 10 feet at 2 inch.

The second is system balance. Uneven heat distribution means a lot of ground temperature water to extract the first time through. So with a series rig, you have a liquid thermal battery in the ground. As is, the system is good for intermittent operation at high efficiency. Once the loop is depleted, efficiency will depend on how much heat gets pushed each cycle. One can guesstimate a ballpark figure for the ground loop, but the truth will be hard to predict.

I'm sure you could fabricate caps for the big pipes a whole lot faster and cheaper than finding custom caps to make these enormous cold fingers. I'm wondering how much heat will transfer at slow and low flow rates versus turbulent flow. With the long time constant, the loop may not need stellar heat transfer.
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Old 12-26-18, 07:28 PM   #4
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Default Your PVC Scheme...

Your tube-in-tube idea is curious.

I have read your post, and in trying to be the best devil's advocate possible, I came up with this:

* PVC is not usually used for GSHP systems because earth can move and shift and PVC is brittle in comparison to HDPE. HDPE is extremely tough, and it has an added benefit in that can be heat welded. The official heat-welding equipment is expensive, but you can rent it, or you can do what I did, and make your own out of parts from junk stores.

* If you're looking for someone to tell you what the efficiency would be of a tube-in-tube such as you have suggested, existing information would be very difficult to find. There does exist a very large body of information on the characteristics of buried HDPE. It may very well be a good idea that you have there, but I suspect that you would need to develop the data yourself. Construct one full size tube-in-tube unit, and dig and bury it in a hole such as you have mentioned. Then do some testing and look at your measured results. That way you can proceed with knowledge regarding if you need to put in more units than you estimated, or less or try another idea.

* Digging holes can vary from being very easy, to being your worst nightmare. You need to know what the hole-digging conditions are in your specific area. By area, i mean within a few hundred feet. Contact well diggers, they know all about this stuff.

Good luck on your project. Keep us posted. We love to see photos.

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Old 01-02-19, 06:41 AM   #5
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I am not quite sure what you mean about pressure drop. The beauty of (relatively) low flow through large diameter pipes is that the flow is governed by Bernoulli's equation. Now Bernoulli's equation doesn't really care what is in the middle (large pipes, changes in height, splits and joining of the conduit, etc), only the beginning and end. The pressure drop for input 2 inch pipe and output 2 inch pipe (with intervening plumbing as I sketched) would be by Bernoulli's equation be 0. Of course, in the real world, there is some friction and some non-laminar flow but not much.

This contrasts the loopy trench field geothermal technique where you are trying to push fluid through a very long and relatively narrow tube. In that case, friction is very significant.

As to the second part of your comment, again I am confused. Geothermal is all about engaging dirt to exchange heat. By using the vertical nature you bring in a lot of volume of earth. The low flow of the outer pipe would improve the heat exchange. But the really important factor is the surface area difference.
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Old 01-02-19, 07:28 AM   #6
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Thanks for your input. Plastic pipe, both PVC and ABS, is being used for sewage lines. Of course, cast iron is as well and would have better heat transfer characteristics.

As for digging wells, there are of course, places where a 10 foot hole would pose problems but those places would probably pose problems for a slinky trench system. If you go to youtube and search for "shallow well", there are lots of videos (many are very entertaining to see the ingenuity of DIYers).

You are correct about the for a prototype as well. I'm just a geeky guy with little experience in plumbing. My main issue is the question of how to put the center pipe through an end cap so that it doesn't leak. With that question answered, I think that I could make the prototype.

But here is a question for you assuming I can get the plumbing done: Do you know the power requirement for a pump in a slinky trench system? I am assuming that it is significant on a yearly basis. This could be a wrong assumption. The advantage of what I am proposing is less property (surface area) needed - many of us don't have 2 acres or more that seems to be the norm for the slinky systems. But secondly the reduced energy to push the fluid through the pipes. But if this factor isn't that big relative to the power requirement of the heat pump, then the advantage would be less.
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Old 01-10-19, 09:22 AM   #7
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In the wide scope of the entire system, your theory concerning ground loop pumping losses holds up pretty good compared to a pump and dump system. Since the brine solution is going in a circle, all you have to do is move the mass of liquid and the pump motor and some amount of friction. So a relatively long skinny slinky loop will use less power than a not so deep well pump.

How this affects the overall COP depends mainly on the vapor compression side of the system. Usually the refrigeration compressor uses a whole lot more power than the water pump to propel refrigerant through it's loop. That and sizing the heat exchangers properly bears much more on performance than trying to eliminate loop pumping losses.

As for the actual "cold fingers" pipes in the ground, I would probably go for the greenish 6 inch sanitary PVC pipe, due to cheaper. On the bottom, a cap or plug glued on. For the top, I would use a reducing tee and reducing bushing. Tee center connected to outside, small diameter pipe would ideally pass through the bushing to minimize glued joints. To be really snazzy, a bushing could go on the inserted end of the small pipe to keep it centered in the big pipe. Definitely use antifreeze, it would be way bad if ice rose to the top and tried to block the exit.

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