idea for improving the thermal performance of a vertical borehole

I'm an electrical engineer and homeowner. My wife and I are pursuing new construction, and since geothermal has always been a dream of mine, I decided to make it reality for this house (with some kind help from Uncle Sam). Due to the layout of the lot, we are forced to use vertical wells. The house is in Allentown PA, and the system was originally specced with two 300' vertical wells.

Being an engineer, I'm interested in anything which improves efficiency. I was intrigued by the Geo Clips, but was concerned about some of the installation difficulty stories I've read. Also, it has the drawback of being less effective as you increase the thermal conductivity of the grout, since you are increasing heat flow between the hot and cold side of the loop. I've been doing some thinking about the nature of vertical loops and whether there is any gain to be had with a small amount of time and cost, and I'd love to get some feedback on my thinking.

We can think of the vertical loop as a bunch of vertical segments. For each segment, there is some bad heat transfer between the supply and return lines (1), as well as good heat transfer between both lines and the surrounding earth (2). Newton's Law of Cooling tells us that the amount of heat transfer varies proportionally to the temperature difference. At the top of the loop, the temperature difference between the supply and return lines is the greatest, meaning the opportunity for undesired energy transfer between the lines is the highest. Geo Clips attempt to mitigate this by pushing the lines physically apart, since more distance reduces (1) while perhaps very slightly improving (2). But, this can only buy so much in a narrow borehole. The lines are still in physical proximity, and still exchanging energy through the grout. And as better grouts are used, we increase both (1) and (2) - potentially increasing bad (1) more quickly than good (2)!

Instead of trying to push the lines apart, what if we insulated the return line instead to prevent its energy from being able to couple in the first place? This would directly reduce (1). Here's where it gets difficult for me to think this through, because I don't know the final steady-state conditions of a loop. When a loop reaches steady-state, has the supply line achieved its steady-state temperature at the very bottom of the loop, and the coolant simply needs to return to the top? If so, then it seems the theoretically best improvement to the system could be made by thermally isolating the return (upward) portion of the loop, to get it out of the borehole as undisturbed as possible. But, I suspect the loop dynamics are more complicated than that. As the coolant begins its upward journey, it may still be bleeding off energy. However, as it moves upward, the temperature difference between the return line and the earth will decrease, since it is traveling through earth where the supply line has increasingly affected the temperature of the earth. At some point, there is going to be an equilibrium point where the temperature of the return line meets the temperature of the earth. At every point upward from that equilibrium point, the return line coolant will be pulled away from its best reached sink temperature. Of course the effect will be slight right above this balance point and stronger at the surface, due to Newton's Law of Cooling. It would take some proper modeling of the system to determine how far down the return line should be insulated for best practical insulation. Or who knows, maybe this cross-flow effect is so small that I should quit overthinking it!

I first started thinking down this path by contemplating how my horizontal return line from the wells could be insulated from the shallow earth temperatures. I was thinking the low-buck approach would be to run it through some 4" corrugated black ADS drain pipe, and hit it with a glob of canned spray foam at either end of the sleeve. Since the ADS is corrugated, there would be minimal surface contact between the return line and the ADS sleeve. That got me to thinking about the cross-line thermal transfer problem, and then I started thinking about what might happen if insulation were run some distance down the vertical return well too...

4" ADS pipe might be fine for the horizontal return line to the house, but it's too big for stuffing down a narrow borehole. But, let's think about how my 4" ADS idea works:

• a glob of foam at either end of the ADS pipe prevents earth/water from intruding into the pipe, so air provides insulation
• the corrugation of the pipe's interior minimizes physical contact

What if there were a smaller double-walled sleeve tubing which was smooth on the outside and inside, with a corrugation running back and forth between these sleeves? You could slide this right onto the loop pipe with no problems. The sleeve could be marked every 10' to make it easy to know how much to feed down. Since the air pockets in this corrugation are captive, this would provide insulation even when immersed below the water table. Think about how cheap a 100' roll of 4" ADS tubing is - I imagine something like this should be reasonably inexpensive to make as well. Of course, the disadvantage of this idea is that you'd need to cut the end of the return pipe to slide this insulation sleeve down the line. The actual sliding shouldn't be too hard, since both the insides and outsides of this sleeve would be smooth-wall. If cutting the return line is a no-go, an alternative way to do it would be to create a similar sleeve which is slit down the length like wire loom, sealed along the slit edge to preserve the air-tight pockets. You'd just need to slide this onto the return line, wrap it with tape every 5-10' to prevent borehole snags from pulling it off the line, feed it down the hole to the desired length, then cut it. There will still be captive air pockets around most of the line to provide insulation.

So, that pretty much sums up my idea. It would be great if the math and modeling provided some rule of thumb line, "always insulate the top 20% of the return line", and you could just cut a length of this insulation sleeve to length and slide it down the return line. It's quick, easy, and provides increasing benefit with higher thermal conductivity grouts. What are the geo drilling community's thoughts on this idea?

P.S. If any fellow forum members in the Allentown PA area are willing to give me a deal on drilling, shoot me a PM!

 

Two general thoughts

If your idea had merit, a much simpler method would be to reduce flow to laminar on the return through increased pipe sizing.

I'd suspect though the gains would be small and potentially just offset by the cheaper 2% more drilling.

One could hook it all up to some thermoconductivity testing equipment or model through solidworks to get a result.

 

Nope. The brine temperature everywhere in the loop will be lower
than undisturbed ground temperature -- so, net heat flux will always
be from the ground toward both the supply and return pipes.

For example, in my SE PA location, undisturbed ground temperature
is approximately 53°F, whereas EWT (a.k.a. "return line temperature")
is typically in the high 30°'s to mid 40°'s throughout the heating season.

LWT is reliably 4°F lower than EWT in stage1, and 6°F lower in stage2.

Yep. The supply-to-return delta-T (in my very typical system) is usually
4°F and never more than 6°F. The earth-to-loop delta-T is several times
greater -- so insulating any part of the pipe will lower EWT and reduce
efficiency.

 

The transfer of heat or cold is an issue in large commercial systems where a small % makes a large impact. It is overcome in the transfer lines by simply seperating them in the ditch while backfilling. For a residential system the gain or loss is negligible. The surface area of the pipes in the borehole that may actually be touching each other is also miniscule.
While openly admitting that advanced mathematics is not my friend without the use of software, many times I am reminded that...

in the field we measure with a micrometer
Write the results in crayon
and cut the results with a blunt object

Eric

 

Thanks for the clarification! This puts things in perspective. Yep... I guess I was overthinking it. My wife would not be surprised.