Posted By engineer on 24 Jun 2010 11:32 PM
... The shoulder seasons (very light loads) provide weeks for loops to revert to local ground temps going into each summer or winter.

This is exactly what my data shows.  During the Spring and Fall time period when there's little heating or cooling, my EWT hoovers around 68°F, which matches up to reports of what the 'down deep' undisturbed soil temperature is for my locale (Dallas).



Best regards,

Bill

For the low usage month of Oct 2009, it looks like 70F. For the low spring usage of mid Mar to mid Apr, it looks like 65F. Why the difference?

There is lots of well documented research for anyone interested in long term (or annual) ground temperature changes. Or a brief summary from the McQuay Design Manual:

Long Term Effect. This is the change in the ground temperature over many years. If the
building has a net heat gain or a net heat loss, the ground temperature will change. The more
densely placed the boreholes are, the larger the effect. Ground water moving through the bore
hole field can help remove energy and limit the long-term temperature change. For commercial
applications, the ground temperature generally climbs.

An example of long term effect would be a 6°F average ground temperature rise over 10 years
due to the heat added to the borehole field. The penalty will not be present during the first year,
but the heat build up will change the system performance over time.

Posted By jonr on 04 Jul 2010 11:01 PM
For the low usage month of Oct 2009, it looks like 70F. For the low spring usage of mid Mar to mid Apr, it looks like 65F. Why the difference?

There is lots of well documented research for anyone interested in long term (or annual) ground temperature changes. Or a brief summary from the McQuay Design Manual:

Long Term Effect. This is the change in the ground temperature over many years. If the
building has a net heat gain or a net heat loss, the ground temperature will change. The more
densely placed the boreholes are, the larger the effect. Ground water moving through the bore
hole field can help remove energy and limit the long-term temperature change. For commercial
applications, the ground temperature generally climbs.

An example of long term effect would be a 6°F average ground temperature rise over 10 years
due to the heat added to the borehole field. The penalty will not be present during the first year,
but the heat build up will change the system performance over time.

For my Dallas climate, the 'shoulder points' don't occur until Nov. and Apr., as shown below (avg monthly power just for HVAC).  These shoulder points coorespond, generally, to about 68 degrees EWT for me.

With respect to long term borehold field heating, I too am concerned about this.  I'll know factually if this is indeed a problem for me in about 8 more years.  So far, on a limited amount of data (2 years), borehole field heating is not yet showing up for me.

Best regards,

Bill

We had this effect in Germany where pretty much all geothermal systems are w-w heatpumps. They don't even have forced air systems there for residential applications.
There you compensate for the expected drop in temperature over 5-10 years with a larger loopfield (about 40-50% larger), since there we never put heat back in the ground.

Horizontal loopfields recharge better due to the higher effect of solar radiation at 6 ft than at at vertical loopfield depth.

I believe the McQuay writing quoted above concerns very large borefields serving hundreds of tons of heat pump capacity.

DJ - Europeans often live in more densely populated areas than our typical suburbs / rural areas. Did your observations in Germany correspond to systems in close proximity to one another, sucn as 5-10 dwellings per acres? Residential systems spaced so closely might begin to imitate large commercial borefields of which McQuay describes long term, multiyear degradation.

For now I stand by my hypothesis that widely spaced (1 house per half acre or more) geo systems should see no multiyear degradation in performance arising from heat saturation, though I'm open to data indicating otherwise.

You can reduce the numbers but can't change the principles. Of course at small numbers, the degradation isn't anything to worry about.

As the ground around the loop cools from year to year, the radius of the unnaturally "cooled" ground increases.  With each increase in radius, there is also an increase in "surface area" (for lack of a bette word) of the affected area.  Eventually, that surface are will be large enough (enough of it in contact to the natually warmer ground), that the cooling effect will be insignifcant as it will have enough "surface area" to draw from.  So yes, I can see a drop in ground temperatures for a number of years, but eventually an equilibrium will be reached.  My understanding was that the 10 year mark was a close estimation of when that happens.

The chart below is my amount of heat being rejected into the earth or extracted from the eartch, each day of the year.  Looking at this it's reasonable to conclude that I'm dumping more heat into the earth than I am extracting from the earth, over the course of a year.

What's not known to me yet is the effect of borehole spacing on overall longterm borehole field heating (if any).  I.e., how far apart do the boreholes have to be to avoid long term borehole field heating (or cooling)?  My boreholes are at least 20' away from closest neighboring holes.  In another 8 years I'll be able to report if indeed at least 20' spacing is enough (i.e., no overall borehole field heating).

This is a Dallas climate example. 




Best regards,

Bill

Posted By engineer on 05 Jul 2010 01:28 AM
For now I stand by my hypothesis ... though I'm open to data indicating otherwise.

The McQuay manual (cited above to support the Henny-Penny theory of loop design)
in fact presents data indicating that it's a total non-issue for residential systems.

For example, a (100-bore!) 10 x 10 grid with 25' spacing supporting a 100-ton
heating-dominated load sees long term decrease of 1.2°F. And, for a 1 x 10 array
with a 10-ton load, the worst case effect is less than 0.4°F, total, over ten years.

"These values are worst case scenarios. Ground water will most likely reduce the impact."
-- McQuay Geothermal Heat Pump Design Manual

http://www.mcquay.com/mcquaybiz/literature/lit_systems/AppGuide/AG_31-008_Geothermal_021607b.pdf

McQuay's tables don't go any lower than that -- but extrapolating to a typical
2-4 ton residential system with 1-4 bores, any long term temperature effects
would be totally masked by random variations in temperature measurement.

I am 8 holes, at 20 feet spacing, at 300 feet deep each, with 1" HDPE pipe and standard grout, in clay soil, in a Dallas climate, supporting the above heat of rejection and/or extraction chart.

Best regards,

Bill

I think we are mixing and mismatching two separate issues. Improperly closely spaced individual bores, even in the case of a small home system will likely result in borefield thermal degradation much earlier than year by year...reduced to absurdity consider a case where all boreholes are a foot apart - they would all quickly steal capacity from each other.

OTOH a big commercial field wherein many bores are completely surrounded by other bores could be expected to suffer from long term degradation as the interior bores eventually stop "seeing" undisturbed ground conditions - the perimeter bores around a big borefield could become the only ones seeing undisturbed ground conditions

Most importantly, does temperature shift occur between years, seasons and months (yes, yes and yes, at any bore hole spacing). Does it occur to an extent that there should be any concern about it (usually no, but as waterpirate (above) or even this shows, sometimes).

Many things to the nth degree can cause bad performance. a properly designed geo system will meet or exceed expectations. mechanical devices show a change in performance after years of use.
If one wishes to hold geo's feet to the fire, they should consider performance loss of the competition....say a 90+ furnace. Studies will be a tad harder to find as folks trust furnaces even though their COP is less than 1.
J