Hi all, I have a theoretical question: imagine I have a, say, 50ft rusty steel tube inserted in the ground. The diameter of the tube is just enough to put a vertical loop and some backfilling material, say 3 to 6''. The tube is in extremely close contact with the soil and the inside of the tube is empty. In that tube I insert a vertical loop, and backfill it with bentonite material or similar. How would the steel tube affect the global heat collection of my GSHP? My guess is that steel have a much better heat thermal conductivity than soil, so provided the tube is in very close contact with the soil it would affect positively, or at worst not significantly affect, the heat collection. Am I correct? Thanks and regards. Fabien

The steel would provide a very slight improvement (great conductor but it is thin) and normal bentonite is an insulation and so would provide less heat transfer than most soils.

soil is still the limiting feature in thermal conductivity. Saturated soil would conduct better than steel, but bentonite would not.
Is there a point to the question? i.e. if you had an old 3" well sleeve hundreds of feet deep should you use it......? Sure.
joe

Steel has a thermal conductivty af about 24 BTU/ (ft hr degF) which is much greater than soil or rock. However, soil/rock thermal conductivity also increases significantly with moisture content. This is one of the reasons that IGSHPA recommends minimal borehole depths of 50'-- not just to reach stable temperatures but also to reach stable moisture conditions.

In some subsurface conditions--such as above the water table in a tight silt or clay unit, casing can actually provide for a more stable moisture content. In other conditions--such as when installed in dense rock or below the water table, it would actually reduce heat transfer since it may limit heat transfer occurring due to groundwater advection and conduction. In these cases, it makes sense to use a thermally enhanced grout to ensure a good link from the ground loop out to the outside of the casing that is at least as conductive as the surrouding strata.

-Adam

Keep in mind - as Joe stated - that the soil is the limiting factor here. Having a relatively small piece of metal in the ground that is better at conducting heat does not mean that there is more heat in the ground to collect.

Think of it this way, if water is passing through a sand sand filter and comes out the other side to a pebble filter which isn't as restricting, you will not get more volume of water flow through the pebbles because all of sudden there is no sand to slow it down, the volumetric flow will remain the same because that is just how much water is available.

It doesn't mean you shouldn't use it, but just wanted to point out that your are not going to get more heat (or cooling) because of it.

All resistances add up. Some, under the right circumstances, are small enough that they have little effect. Ie, in the case of the sand and pebbles, you would get more flow if you removed the pebbles, it just wouldn't be much more. The sand does not limit flow to a fixed flow rate and it would be wrong to say "x is the limiting factor, no need to worry about lesser resistances".

In the casing example, if the casing were a solid steel cylinder, it would have a significant positive effect.

Posted By jonr on 25 Mar 2010 09:36 AM
All resistances add up. Some, under the right circumstances, are small enough that they have little effect. Ie, in the case of the sand and pebbles, you would get more flow if you removed the pebbles, it just wouldn't be much more. The sand does not limit flow to a fixed flow rate and it would be wrong to say "x is the limiting factor, no need to worry about lesser resistances".
True but to say the flow would never be greater than that which comes out of the sand is correct.
In the casing example, if the casing were a solid steel cylinder, it would have a significant positive effect.
What does this mean? We are suggesting that the soil limits conduction of heat, are you suggesting that the steel overcomes that?

j

Posted By jonr on 25 Mar 2010 09:36 AM
In the casing example, if the casing were a solid steel cylinder, it would have a significant positive effect.

Ahh! Does any geothermal pro agree with this. I dont.It might not hurt. I dont see how it could have a significant positive effect.

You can run "Ground Loop Design" or similar programs if you want to verify it.

Anything that gets between the loop and the earth is a decreasing factor, with the exception of water.  Grout decreases the thermal exchange, thermal grout was developed to try and improve a neccasary inhibitor.  Any type of casing is just adding another barrier to the exchange proccess.

No, steel improves the heat transfer over no steel (just soil).

Posted By jonr on 25 Mar 2010 05:29 PM
No, steel improves the heat transfer over no steel (just soil).

Got numbers?

If you splice a 2" length of #00 silver wire into a 50' Wal*Mart
awg 14 extension cord, does it materially alter the resistance?

Havin a hard time determining what you are driving at Jonr. Are you suggesting that a steel casing around more common loop materials improves heat flow or do you mean steel pipe vs HDPE?
After all, you mentioned "All resistances add up".
With soil as the limiting factor copper, steel or HDPE matter less.
J

Either one. I find it intuitively clear from a thermodynamics standpoint, but for 6" of steel around a vertical HDPE U-tube vs just soil around the same U-tube - better than 30% reduction in needed loop length (per GLD).

In every soil, dry, damp, saturated? In every loop, horizontal or vertical?
You find it intuitively clear, but you are leaving out details.
If soil is unable to convey heat energy as quickly as the loop, a decrease in loop length is not indicated.
J

I don't buy that. The only way for the steel or any other metal can improve transfer would be if it were configured as a heat fin - a high conductivity elongated surface reaching out into soil so that more soil is in close contact with the heat transferring surface.

A steel well casing that contacts the HDPE u-tube only incidentally isn't going to help much. Resistances are additive in other words, in series, and steel would add very little, but to increase heat transfer you'd need multiple parallel paths for heat transfer, and that's what fins accomplish. Sticking a bunch of fins in the ground is infeasible, so we just go with lengthy unfinned U-tubes

Surprising how many have this wrong. What has lower thermal resistance - steel or soil? Steel by far. So replacing any amount of soil with steel increases heat flow. You could think of it as a heat sink - and while heat sinks typically optimize use of material with fins, fins aren't required for them to have a positive effect - they still increase surface area and heat conduction.

You can't escape the fact that the steel that replaces the soil in this geometry is in SERIES with the surrounding soil layers. It does NOT set up an additional parallel path for increased heat transfer.

If the steel turned out to block otherwise available aquifer advection as has been mentioned, it turns into a distinct liability since it keeps moving water from directly contacting the loop pipe.

Regardless of soil properties, the presence/absence of a metallic casing
should have no appreciable effect on heat flow:

1) Within the bore radius, thermal resistance depends only on the series
resistances of the pipe wall and the grout. (BTW, the grout dominates
because 50-75 mm of grout has more resistance than 2-3 mm of pipe
wall -- regardless of the pipe material or grout thermal enhancements.)

2) The thermal conductivity (BTU/hr-ft-F) of the soil outside of the casing
is similar to that of the grout (within a factor of 2, depending on the soil
conditions and quartzite content of the grout).

3) Thermal conductivity of the casing is very high compared to either
of the above; so, delta-T across the (thin) casing wall is (must be!)
very small -- and thus, the presence/absence of the casing can have
no appreciable effect on the temperature gradient on either side.

...thermodynamics: Not just a good idea, IT'S THE LAW!

Looby

If the steel turned out to block otherwise available aquifer advection as has been mentioned, it turns into a distinct liability since it keeps moving water from directly contacting the loop pipe.

Got numbers?   If such a case,  the solid steel casing conducts the tube heat to a larger fluid contact area and at the very low flow rates involved, increases heat transfer - just like any heat sink does.  

You don't see people removing heat sinks to provide direct contact between the air and the cpu in their computer - you see them adding bigger/thicker heat sinks (a little more resistance but much more area) to increase heat flow.

> the presence/absence of a metallic casing should have no appreciable effect

Depends completely on the thickness involved - very appreciable in the solid steel case being discussed.  As previously stated, not appreciable in the common case.