Long horizontal runs from wells to GSHP

Looking at a residence with several hundred feet between the 12 wells and the building.

I'm an EE not the HVAC person, but was asked anyhow. [Client thinks engineers know everything...]

It seems to me that either the inlet and outlet to the field manifolds should be as isolated as possible. The pipe installers were talking vertical separation, i.e. inbound at 5ft depth, backfill to 3ft and lay outbound. I wondered about using say 8-10 thick sheets of styrofoam from wall to wall of the trench atop the lower for better separation.

We could use separate ditches instead, with ~5 ft. separation between them.

Any writeups on either approach I should read?

 

the several hundred feet of pump[ing will affect performance, since much pumping power will be needed, unless you use large pipe diameter. Vertical pipe separation? Not so much. Why using styrofoam? What do you expect the impact of thermal pipe separation to be?

 

We're aware of the piping losses issue.

Well, if the two runs, hot & cold/out & back, are directly adjacent, there's no getting around the heat transfer between them. So you want insulation inbetween. (After all, the client just spent $75K on the wells to cool the water in the summer/heat it in the winter. Why waste the delta-T?)

That insulation could be via physical separation via separate ditches, OR separation between them in the same ditch with insulation in-between them. That insulation would be dirt or (I ask about..) styrofoam.

Are you saying the industry does not routinely separate the two runs?

 

Normally the runs are not several hundred feet, you usually avoid this. if you have 2 ft separation and your delta T is usually 5 degrees F, you will not have significant heat exchange between the 2 pipes. Think about the borehole, the pipes are much closer together throughout the borehole for many hundred feet.
Thus the water inside the pipe is much more influenced by the temperature of the surrounding soil, than the 5 F degree colder water 2 ft above. I would argue that the temperature of the soil at 3 ft compared to 5ft is probably 5 degree F colder in the winter anyway.

The return pipe will keep the ground around it at a certain temperature, and so will the supply pipe. The heat exchange between soil and pipe will mainly happen within 1-2" around the pipe. The ground temp towards the other pipe is only minimally affected by the other pipe.

 

Thanks for that delta T figure; I'd wondered about that but never measured it on-site.

The proximity of the borehole pipes is a different case than the horizontal run. They are in close contact like it or not, so rather self-defeating. I've speculated about a coaxial well pipe, with the center the down and the outer up. You could make the downward pipe walls thicker reduce heat transfer. The outer pipe will be surrounded on all sides; so it gets the maximum transfer.

With the horizontal piping, you have the luxury of insulating them better, especially from each other.

 

And:

We all wish we could, but:

1. House is on the edge of a ravine.
2. Wells placed on flat area avoiding 100 ft trees.
3. One feed snakes around to back of house and an open "stoop space" under dining room.

 

Hi and welcome!
When doing commercial scale geothermal, and runs are to building are looooong. Specs typically called for 3' of separation between supply and return in the same ditch. Typically horizontal, no vertical.
Eric

 

I would advise against it. This configuration would essentially remove 1/2 of your verticle loop from ground contact. The efficiency of any heat exchanger is most directly related to its surface area in contact with the heat source. In this case the ground.

Likewise horizontal runs at a reasonable depth to your bore field will be in contact with the ground and will contribute to the heat exchange process. This can be accounted for in modeling software by reducing the average depth of the loopfield in calculations.

Doc is correct, the relatively small DeltaT of a closed loop system makes insulation almost inconsequential at typical depths with separation of horizontal mains in the trench of 2-3 feet.

 

Thanks. We're looking at 2 ditches, 4-6 ft apart.

In one section, there's no room for that, and they must be vertically stacked.
There I was pondering 6-8" thick styrofoam vs dirt.

 

Yes, but there's a tradeoff, well several. The opposite pipe is negating the earth contact to some extent. If the coaxial pipe scheme was used, the entire outer surface of the outer pipe would be in contact with the earth. It would have more surface area. Say the outer pipe is 6" in dia, the inner 3" then the outer area is 3.5 X the inner.

Pondering more, if the outer pipe had greater volume (after subtracting the inner) then the water would be moving slower, and I'd think that was more effective in getting heat from the earth. (I'm an EE, and it's been decades since Thermo w/ Prof. Domholtz. so that's an outright guess..)

Lots of modeling would be needed. Of course this is mere speculation, as no one makes such pipe.

Not sure what their models said, but I'll ask.

OK.

 

The surrounding soil in the horizontal trenches contributes to heat up the fluid in both the return and the supply line in an inconsequential way.

You can follow the illusion of precision we all sometimes get caught up in, and add a piece of styrofoam in the section you mention above, or you can stack it horizontally.

Sometimes engineers specify all kinds of "interesting" add ons which only add costs and do not add performance, but they usually also miss the point in understanding where performance is lost or wasted in a geo system.

Think about added thermal separation inside the borehole, it will not diminish or loose your energy you picked up in the borehole, it will simply not allow you to pick up as much. To the point that the added cost and effort are usually not worth it, and you can compensate for it by simply drilling a few feet deeper at a much lower cost than putting spacing clips in it, if you feel that needs to be done.

We usually completely disregard the effect of the horizontal trench in the design of the loop field, because the impact is so small. Thus we for sure do not worry about a section of the horizontal trench.

Another example would be the return tail of a slinky, which touches the slinky loop every 18" (see attached). Imagine we would add 6-8" of styrofoam every 18" to allow for better thermal separation. One could add 1 ft more slinky pipe to it to compensate. Or you could simply not bother with it and move on, and be done with the job 30% quicker....

You have to develop a holistic approach. Can you increase performance here and there measurable in micro units? Yes.But at what costs?
The art is really to build a well performing system overall, at a cost the customer can afford, and allows for the wide spread dissemination of geo system. Lean but mean design.

Your pumping solution and pressure drop will have much more performance impact than you thermal separation for a section between supply and return line.

You worry a lot about an inconsequential detail of a geo system, what is you pressure drop, pumping solution and your overall design look like?

 

Not sure if you ever tried to put in a 6 inch HDPE pipe into a borehole. Every driller I know would be running away.
Yes, water would be moving slower, but with less turbulence, now reducing the heat exchange. Slower moving water might warm up more (delta T), but would have letter total BTU extracted.
Heat extracted = Flow x delta Temp x a constant (485 for water with antifreeze).

There is not a single week going by where I have some inventors or sales people on the phone who are introducing a "new and better way" to extract heat from the ground.

Thus far in term of bang for the buck, nothing in the last 20 years beat a simple u-bend vertical pipe configuration.

 

Well, we have a new problem err opportunity. The longer run was going around the back of the house. (Basement is exposed on that side facing ravine.) Hand digging discovered large undocumented buttresses from the foundation. No way to get through them.

So two options, neither great, come to mind.

A) Dig trench under basement floor level. Install a casing of some ilk and run HDPE through that. (Has to be below the PEX being installed to heat that basement.) Not easy to do.

B) Run it 30 ft horizontally, above ground, along wall under a deck, and into crawl space at the end. This is maybe 10-15% shorter than A). For the exposed length, use split foam pipe insulation <http://a.co/d/1fzmi6k> over the HDPE, run both inside Sch. 40 PVC. Then perhaps inject expanding foam to fill any airgaps.

Comments on these approaches?

 

Option C) Core drill through buttresses?

How thick are those buttressess? I've core drilled (4.5" diameter) through an 8" thick concrete wall in minutes using a rented core drill machine and bit (~$150).

 

I'm sure they'll reject that approach; we'd discussed it on another issue. But thanks.

The above-ground approach is easiest but I've no way to SWAG the efficiency hit, if any.

 

No worries.
Your system will require 3 gpm per ton to operate. Any above grade or even inches below grade piping that can be insulated, will net very little diminished performance in the big scheme of things for your system. Ensure that the anti freeze is correct in the loop and move forward with which ever plan works for your site.
Eric

 

No worries.
Your system will require 3 gpm per ton to operate. Any above grade or even inches below grade piping that can be insulated, will net very little diminished performance in the big scheme of things for your system. Ensure that the anti freeze is correct in the loop and move forward with which ever plan works for your site.
Eric

 

It's probably close to a wash. If water outbound to loop field picks up 0.03 degrees from inbound water, that means it enters the field warmer band therefore leaves the field warmer, so any loss would have been from a higher leaving loop temperature.

The initial suggestion to have deep pipe, styrofoam then shallpw pipe could be a problem. You'd effectively disconnect shallow pipe from deeper heat source and create more likely scenario for frost heave on the shallow and also negate some of that heat transfer function that, while minimal, still occurs on the transit pipe.

 

You certainly want the pipe insulated if you run it above ground. You want some limited heat transfer between the pipes and zero degree outside temperature. Also in case your circulation pumps fails, you don't want the stagnant water protected to 15F not being exposed to outdoor sub zero temps without insulation.

I always bury the pipes no matter what the effort is.

If you have to run it above ground, Eric is correct, the impact is minimal if you run an insulated pipe for 30 ft above ground.

What is the diameter of the header pipe? How long are the total runs? How do you connect them together? What is your total system flow?

 

I'd rather bury it all but, "major project" is a good description of that route. I think our scheme will provide sufficent insulation.

The "some" confuses me. Are you saying too good an insulation is not recommended?

I'm confused re: mtrentw's remarks about inter-pipe insulation. To me it seems you spend many dollars on well fields for creating Delta-T between the in & out lines. You want to preserve it, by spacing the lines with dirt insulation between them. (The best being separate trenches.) I wonder about using sheet styrofoam vs. just dirt with one ditch.

On other questions:
The loop in question is the 5 ton unit and has 1.25" pipe to/from the heat pump to the manifold. There's a "parallel reverse return" manifold roughly centered in the well field. Each of the six wells has 1.0" pipe. The runs are about 300 ft from the manifold to the compressor. I'll have to ask the flow rate.

The loop does have antifreeze; I'll get the "protected to" target temperature. This is on an island near Seattle, not an area known for hard freezes. (If the open water ever froze over solidly, the Navy would be unhappy; everything from attack submarines to carriers goes by periodically.)