In being involved with geothermal over the years, we've learned a lot.

When first starting into it, IGSHPA defined loop spacing at 10'. Later, it was 12'; then 15', 20', even 30'. ???

Questions:
- Is the spacing ever going to be exactly defined?

- Has there been testing to show for absolute sure that the ground temperature is affected up to 10' out from a water loop?
    *Sidenote* EarthLinked has tested out from their DX vertical loops, and past 3', there is no affect from the ground loop.

I suspect that the real problem lies more with the amount of loop in the ground rather than the spacing issue. DX geothermal works with a latent heat transfer in the ground. Water source geothermal uses a sensible heat transfer. In my primitive mind, I can't imagine that a water loop would have more affect on ground temp. at any distance further than 3' if DX loops don't. Rather, because of the type of heat transfer (sensible vs. latent) the water system needs far more loop to do the same amount of work.

In thinking about this, why can't water loops be spaced at 7' just the same as DX copper loops?

One more side question:
- Typically, how many feet per ton does a horizontal water source loop require?

Clark, I'd appreciate if your could elaborate in more detail on the above two comments.  I could use some help understanding what you're saying here.

Many thanks.

Best regards,

Bill

I'm going to try to keep this brief...

Latent heat exchange is the heat exchanged when a medium (refrigerant in geothermal's case) changes state--so from a liquid to gas, or from a gas to a liquid.

Sensible heat exchange is the heat exchanged when a medium changes temperature--so 40 deg. water to 41 deg. water.

When refrigerant (or ANYTHING) changes from a liquid to a gas, it stays the same temperature, but it exchanges a lot of energy. This is the type of heat exchange going on in the ground with a DX loop. It boils the refrigerant in the heating mode, or condenses the refrigerant in the cooling mode.

This same latent heat exchange is happening in the water to refrigerant heat exchanger in a water-source heat pump. However, the exchange made with the ground in a water source system is a sensible heat exchange. There needs to be a decent delta T between ground temp and loop water temp for that exchange to have a usable effect.

Conversely, a DX loop only needs energy (BTUhs) from the ground, not degrees. In the cooling mode, it dumps the heat much more effectively, and also more quickly. This will allow the ground to have more time to recover from dumping heat from an air conditioning mode. In the heating mode, the latent exchange allows for a more efficient heat transfer from the ground to the home....

Anyway, because of the different type of heat exchange, it seems to me that a water loop would just need to be longer, not necessarily spaced further apart... Of course, I warmly welcome a good explanation shooting down my observation. I like this forum for learning all I can about geo.

I don't see a flaw in your thinking, but I've no hard data. Most everyone has crowded loops a time or two (say a bottleneck between trees or buildings) and added a few feet to compensate. Curiously if spacing were key, the most efficient loop design would be 1 pipe. What you do notice in design software is a different length based on loop design but never different spacing.
How many feet/ton for closed loop is a loaded question as you know, but if you're looking at an average, we tend towards 1) 600' slinky/ton in damp soils of mid-MI.
(BTW saw you in the Earthlinked newsletter).
J

Clark, much appreciate the detail on sensible versus latent and how it relates to water vs DX geo.  Very helpful.  Now I understand a key difference in the two technologies (water vs DX).

Many thanks!

Best regards,

Bill

Joe,

I'm well aware that it's a loaded question. After experience and experience has taught someone, they can most likely employ rules of thumb. Your answer is good enough, though, for my curiosity. Thanks

What we've found in the front range of Colorado is that horizontal is roughly 1 ton per 800' of pipe.  We go 200' down and back, and then down and back again fro a total of 800'.  You can also do slinky.  Hope this helps.

Mike Kramer, P.E.
Geothermal Systems of Colorado, LLC
www.geosysco.com

mike

i assume then with a 5 ton system you are talking 4000 ft of pipe is this correct. are you going down 6ft? how many lines are you laying in your diitch and how wide is it if i may ask?

thanks

bob

  It has been my experience that the lithology of the soil dictates the spacing required/utilised for a given vertical loop field.  If the lithology is of dry poor conductivity you would employ the greatest amount of seperation you could get within reason.  If the lithology was a wet saturated sand, you may be able to close the seperation to as little as 5 foot.
  There are many instances where the loop field temprature exceeded the design parameter due to a number of factors, one of them being seperation distance.  In a dominoe pattern the risk is the greatest and the least forgiving.  The temprature change can link from bore to bore, much like a cone of depression for dewatering, and render the loop field useless.
  A college in NJ is the field study for this.  The conductivity testing was flubbed, the bores were to close together, not deep enough.  The field failed.  In an attempt to use the field after the field had declined below design temps, groundwater is being injected into the strata to try and overcome the problem, the jury is still out.

I'm no expert, but it would seem like the greater the soil conductivity, the easier it would be for loops to interfere with one another if close together (maybe it's a wash as it is also easier to conduct heat from other surrounding soil).
J

This is a rather complicated subject based upon the detailed modeling I have reviewed (and I have spent a lot of time studying this). Spacing matters more if:
1) a large unit is involved ( say +20 tons),
2) the heating/cooling requirements are greatly mismatched (Florida requires 4:1 cooling so we could heat the soil), and
3) the medium (dry sand or saturated soil)
4) ok there is a fourth factor and that is far field temperature.

If you use vertical wells and have a shallow aquifer then you can space closer because water conducts heat better than soil plus groundwater tends to move slowly but this still helps carry away the heat.

Yes, phase change (what you call latent) heat transfer coefficients are significantly higher than conduction (sensible in your terminology) coefficients. In fact this difference is why heatpipes have such high heat carrying capacity. However, the rate heat can flow through the ground may be the limiting factor on spacing. Also, if the spacing is too close (say 3 feet) then the entire area heats up and you lose efficiency since heat transfer is a function of delta temp (like Joe mentions).

Heat flow through the ground is glacially slow compared to air, extremely slow compared to moving water, and very slow compared to still water. We did a remediation project and heated the ground to over 150 F in a large area (1/2 acre or more). The monitoring wells are still a few degrees above background and it has been 3 years. Parts of the ground remained over 100F for almost 2 years!!

Joe: You are on the right track but the answer is, "it depends." If the conductivity is high enough then the heat flows away fast enough that it doesn't matter. If too low then the heat builds up right beside the pipe (this is why high conductivity well casing is desired).

Hope this helps. Next weekend I will try to provide a couple of calculations on heat flow rate through soil - family obligations permitting. :)

Alex

That's about the best explanation I've come across. Thanks Alex.

I finally took the time to run some numbers in my commercial loop program.

On a cooling dominated building with

 Bore Hole Spacing             Bore  Length Needed            Delta T of ground after 10 years

25'                                          919'                                                 +.6°
20'                                          935'                                                 +1°
15'                                          969'                                                 +2°
10'                                        1070 '                                                +4.4°


On a heating dominated building with  

 Bore Hole Spacing             Bore  Length Needed            Delta T of ground after 10 years

25'                                      1431'                                                    -.8°
20'                                      1458'                                                    -1.5°
15'                                      1519'                                                    -2.9
10'                                      1697'                                                    -6.3°  



On a building with balanced loads,  there is not any   Delta T of ground after 10 years

How many tons did you use for that example - or is it per ton?

You wrote "bore" so I assume vertical - yes?

What are other variables in the multi-year effect you describe - rainfall, soil composition.

What happens after 25 years?

I do remember reading somewhere, perhaps in a McQuay guide, about multi-year field degradation on large systems.

This was a 50 ton system.  After 25 years,   the delta T of the ground in the cooling example was  +5.4°.

The calculation was done using a soil conductivity of 1.3 BTU/h*ft*°F.  Which would be solid rock.

If the conductivity is changed to .5   ( dry sand or clay ) then the  data looks like this

On a cooling dominated building with

 Bore Hole Spacing             Bore  Length Needed            Delta T of ground after 25 years

25'                                        1614'                                                 +1°
20'                                        1660'                                                 +1.8°
15'                                       1764'                                                 +3.3°
10'                                       2060 '                                                +6.8°

Dewayne,
With the number 1614', that can't be total, is it? If so, that's only 32' of borehole per ton...I must be missing something.

What's the thermal conductivity of damp clay? or sandy mucky(wet) clay mix?

Clark, I screwed up it is a 4 ton system.

Soil types


Cond = Thermal Conductivity

One more clarification.

1614' of borehole length... Is that borehole length or loop length? IOW, is it a total of 400' of drilling per ton, or just 200' per ton?

What you are talking about is called, ‘Convergence of the Thermal Radii of Influence’.

The virtual ‘bible’ on this and other related subjects is called, “Ground-Source Heat Pumps - Design of Geothermal Systems for Commercial and Institutional Buildings” (1997) by Stephen P. Kavanaugh; Kevin Rafferty.

Another noteworthy publication is ‘CAN/CSA-C448.2-02’, ‘Design and Installation of Earth Energy Systems for Residential and Other Small Buildings’ as Canada, through the Canadian Standards Association, presently has the highest world standard for (residential) geothermal system certification.

DX, however, is a different animal…

Having said that and being an IGSHPA accredited geothermal installer and the owner of a DX (Nordic) GSHP in my own home, I personally don’t see where a conventional geothermal system should ever be installed over a (residential) DX system (properly designed & installed of course).

DX is the Apple/Macintoch of the (residential) GSHP world.

IMO

SR