Small and crude GSHP for a Michigan garage

[pick1e]

Originally Posted by Daox

BTW, why are you digging now that its cold out?

A more succinct answer: why not?

A nice crisp morning to start digging before heading to work. Here is my newly acquired post hole auger, the first in a long string of tools I'm sure:



Root at 9"! Easiest just to start over.



Started to get yellow clay at about 16"



And dry silt at about 2'.



With pea sized gravel in it, some larger gravel up to maybe 1".



Here are the soil layers (right to left from surface).



This was the biggest rock I ran into so far, about egg sized. The narrow auger seemed to push it into the soil, away from the wall of the hole, as well as steer itself around it (slight bend in the hole). After moving the auger end up and down to remove the soil below the stone seemed to be working its way loose so I just pulled it out before it fell to the bottom. Luckily it was only arm's length down. Hopefully I don't see anything bigger. It was recommended to me by a fence pro that if I hit something larger to abandon the hole and move on, it is just easier.



Time for an extension! This only took about a half hour so that was cool.



Pouring some water in the hole helped bring up the fine, dry silt. It turned to really heavy, thick muck when wet, so I think it is the same material as the upper layer, just dry.

While I was out and about I picked up a piece of pipe for an extension and used it later in the evening to get down to about 5'4". I'll pick up a longer piece tomorrow.

I measured the temps at the bottom of the holes non-scientifically at 35F at 9" and 46F at 36".

[Daox]

Haha, fair enough! Good to see progress being made. How many holes are you going to have to dig?

[pick1e]

Originally Posted by Daox

How many holes are you going to have to dig?

Not sure yet. "The best laid plans of mice and men oft go awry."

I plan on digging the first hole as far as I reasonably can, use that hole to conduct a heat test, then use that to figure out how many more similar holes I need.

[Ryland]

I would say that if you make "U" shaped loops dropping down then you should insulate between the leg going down and the leg coming up unless they are a good distance appart, otherwise you have your coldest water passing right next to what should be your warmest water, so a chunk of ridged foam separating them is a good idea.

[pick1e]

Originally Posted by Ryland

I would say that if you make "U" shaped loops dropping down then you should insulate between the leg going down and the leg coming up unless they are a good distance appart, otherwise you have your coldest water passing right next to what should be your warmest water, so a chunk of ridged foam separating them is a good idea.

Thanks for that suggestion. Any chance you have any references or data on this? It would be interesting.

I was reading something about borehole tube design saying that concentric tubes (small tube inside larger tube) were nearly equal in efficiency as a U tube since the warm water preheats the cold water. Also that a double U tube like W in one hole was more efficient than a single U but only marginally. Unfortunately I don't remember the numbers or where I was reading that.

I was planning on concentric tubes because the preheating makes sense to me, but so does insulating between the U tube legs to maximize the temperature difference like you mentioned. I would be willing to bet that their relative efficiencies depend largely on the desired difference in temperature between water entrance and exit.

It would be cool to test these designs against one another. Maybe I should install a different design in each borehole? I could design them with fittings to be able to test them separately for comparison, but then eventually link them together for the final system...

[Ryland]

My sources for insulating between legs is an engineer who's straw bale house I worked on, he had radiant heat floors with a bunch of different loops and valves so he could test different configurations, but he was the one that pressed on me how important it was to insulate between any supply and return line.
After all you are relying on the thermal conductivity of the soil, if it's not conductive then you want to do everything you can to keep the hot or cold that you put in the soil there, if the soil is a good thermal conductor then you have a greater chance of heat transferring from one leg to the other leg.

One other idea is instead of having a "U" shape is to have an insulated line run straight down then a loose spiral back up, that way you have more surface area.

[pick1e]

Originally Posted by Ryland

My sources for insulating between legs is an engineer who's straw bale house I worked on, he had radiant heat floors with a bunch of different loops and valves so he could test different configurations, but he was the one that pressed on me how important it was to insulate between any supply and return line.

After all you are relying on the thermal conductivity of the soil, if it's not conductive then you want to do everything you can to keep the hot or cold that you put in the soil there, if the soil is a good thermal conductor then you have a greater chance of heat transferring from one leg to the other leg.

Again I appreciate the comments. I went looking for any published data on this. There is much more to read but this one was good, a summary of borehole design research out of Sweden.

He mentions some auto-convection heat pipes, which is cool. I wondered about that. I wonder if I could bury some before pouring a driveway to passively collect heat in the summer and melt the snow in the winter

From what I am reading it sounds like insulating between the lines becomes more important with slower pump rates, which makes sense to me. I guess you don't want to give the heat you gained back to the cold side, although in the back of my mind it still seems like it would even out in the end, less work to warm up the water when it's not as cold. They're saying that it becomes negligible though at high volume rates since the temperature difference between pipes is reduced with higher speed, which also makes sense. If you were pumping the water super fast the water would be about the same temperature.

In my case I want it to Keep It Simple Silly so I don't want to worry about how fast my water is moving, therefore I will want to make sure my supply and return pipes aren't touching as you recommended. This really has me thinking more about U pipe configuration instead of coaxial. If I have to insulate the inner tube then why bother.

From what I'm reading though it sounds like the separation between lines isn't as important as keeping the pipes as close to the outside wall of the borehole as possible. Makes sense to me. The temperature difference between pipes should be far less than the temperature difference between the fluid and the soil.

Here's a figure from a paper by Goran Hellstrom:



I don't think I'd want insulation in the saddle between tubes if the heat flow in that area is still from soil to the pipes, not from pipe to pipe. But this was also with a heat test so I'm sure the flow rate was high as well as temperature difference.

Here's another one from that paper that shows the importance of the separation:



So I'm back to thinking about a U pipe possibly with a spiral as you mentioned.

Originally Posted by Ryland

One other idea is instead of having a "U" shape is to have an insulated line run straight down then a loose spiral back up, that way you have more surface area.

That was actually my first design thought, a spiral wrapped around a PVC pipe core. But now I see the PVC carrier wouldn't be good because it would block the grout material from flowing between the coils if the coils were tight to the borehole wall. But a loose spiral would be cool. Here is a pic of one I made a couple months ago just for kicks.



I just dug it up for your amusement I didn't have a deep enough hole to give it a real test. It is 1/4" copper with a little more total surface area than the same length of 1/2" copper pipe.

[pick1e]

Well the other day it was a nice balmy 20F (-7C) and I had a little time so I went out and dug some more. I got down to 7'4" (2.23m) and was stopped by some rocks. These are a couple that I removed before the current ones that are stopping my auger:



Cool having totally different rocks types, deposited here by glaciers I'm sure. I wonder how long ago?

I don't have a welder so a friend is going to make a chisel bar for me. Hopefully I'll get to try that out this weekend.

[Ron342]

Great Work - But I think you will be limited in your depth if not by the rocks then by hitting water sand which will flow & sift out of your digger ears on the way up! That happened to me at ~ 22 ft Whats your water table at?? You will also quickly find you need a 3 leg tripod with a pulley to pull out the digger as I did. Plan on having large hairy arms!
I did find though that a 8-10 hp pump with 1 or 1 1/2 pipe ending with a nozzle at about 70 - 100 psi will jet down as far as 60-80 feet really quickly if you don't hit roots or rocks bigger that say 1 1/2 in. I hit marine clay there which ended it. Its a quick way to blow down some holes - dig a trench back from the hole to a pit for the return water to go and settle for the pump suction hose - fill it before you start and during as you'll loose some.
I think copper is great if you have it around, 1" copper though will cost at least $25 per 10 ft - many times more that HDPE Copper is certainly easier to solder in a U joint and makes a nice straight drop - but I also think the critics are right that most soil has such low conduction that you will need more hole than you think - unless you can tap into a long strata of water with maybe some movement! I believe the conventional wisdom is 200 ft of hole per ton which I think is 12,000 btu
I have wondered about running the cold refrigerant line of my outdoor heat pump (air) unit through a long trough in which the ground water is pumped to pick up ground heat (Just as a supplement) before it goes into the coil in my outside unit. This wouldnt require going into the refrigerant line - but haven't had the nerve to try it.
Keep up the great work and posts - but damn its cold there.
Ron

[RobertSmalls]

This thread and ACHacker's have gotten me interested in boring. I've always assumed northeastern Buffalo has too shallow of soil for me to dig a borehole, but the USDA has done a soil survey on the area: Web Soil Survey

There's an underpass on an expressway about a mile from here that exposes bedrock (shale) at a depth of a few feet. There's no hope of boring through that, is there?

The floor of my basement is about four feet below ground level, so I'll assume bedrock (and the water table) is deeper than five or six feet, at least.

Originally Posted by USDA

Map Unit Setting

* Mean annual precipitation: 36 to 48 inches
* Mean annual air temperature: 45 to 50 degrees F
* Frost-​free period: 115 to 195 days

Description of Odessa
Setting

* Landform: Lake plains
* Landform position (two-​dimensional): Footslope
* Landform position (three-​dimensional): Tread
* Down-​slope shape: Concave
* Across-​slope shape: Linear
* Parent material: Reddish clayey and silty glaciolacustrine deposits

Properties and qualities

* Slope: 0 to 3 percent
* Depth to restrictive feature: More than 80 inches
* Drainage class: Somewhat poorly drained
* Capacity of the most limiting layer to transmit water (Ksat): Moderately low to moderately high (0.06 to 0.20 in/hr)
* Depth to water table: About 6 to 18 inches
* Frequency of flooding: None
* Frequency of ponding: None
* Calcium carbonate, maximum content: 15 percent
* Available water capacity: Moderate (about 8.6 inches)

Interpretive groups

* Land capability (nonirrigated): 3w

Typical profile

* 0 to 9 inches: Silt loam
* 9 to 22 inches: Silty clay
* 22 to 60 inches: Silty clay