[Bldg-sim] GSHP in hot climate

Mark Seibert mseibert at cmtaegrs.com
Thu Aug 14 16:42:38 PDT 2008

A variable that has a major affect on geoexchange well field performance
is the amount of subsurface water movement through the well field;
unfortunately it's not easy (or possible) to determine the actual affect
the subsurface water will have on the field.  Our typical fields consist
of wells that are somewhere between 150' to 400' deep with capacity of
175ft/ton for a grouted well.  We monitor well field supply and return
water temperatures for many sites and with out a doubt, fields with
subsurface water operate much, much butter than dry sites.  If your site
is in a hot/wet climent (i.e. Flordia), I would think a geoexchange is a
valid system, (note, I have not designed a geothermal system in FL).  On
the other hand if you're in a hot dry climate, you may want to rethink
your system.


Mark R. Seibert, PE, LEED AP


From: Dan Nall [mailto:dannall at mindspring.com] 
Sent: Saturday, August 09, 2008 4:29 PM
To: Edward.A.Decker at jci.com
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] GSHP in hot climate


This is a heat balance problem.  While the sol-air temperature cycle
(diurnal or seasonal) may have little impact some distance below the
surface, the annual average surface temperature will have a big impact.
The average temperature underground reflects the balance of heat
transfer between the surface and the very hot depths.  So, the
temperature 30 ft. below grade in Alaska is very much colder than the
temperature 30 ft. below the surface in the Arabian desert.  Lateral
heat transfer (equi-depth) has little effect unless there is a local
heat source or sink, like a geoexchanger.  There is no magic underground
temperature.  It is a product of the local heat balance through a
somewhat conductive continuous medium (the ground), between the surface
and the core of the earth.  Because the core is so far down, it has
little effect until you get very deep, like miles. Recommended minimum
horizontal spacing between vertical wells is on the order of  30 ft.
There is a significant loss of performance when that distance is reduced
to 20 ft.  

In the  case of a hot climate, think about where the heat goes.  It is
being delivered at a continuous, but varying rate over the course of the
year.  There is little or no extraction of heat from the ground by the
heat pump.  The usual assumption is that closed loop wells need about
200 well feet per ton.  In Phoenix, for a residence, you might expect
2000 full load hours for the year.  So, a vastly simplified calculation
would yield that each foot of well has to lose approximately 17 Btu/hr
on average over the year.  Some of that heat will be conducted away and
some will (temporarily, until equilibrium is reached) serve to raise the
temperature of the local earth.  The actual thermal mass of the earth is
large compared with the heat conduction coefficient, so that it takes a
few years to heat up the ground.  Do the calculation and figure out what
the average temperature at the well has to be to drive that much heat
flow, once the system is in equilibrium, using the concentric pipe
insulation formula to calculate the logarithmic mean heat trtansfer
surface area.  I think you will be surprised at how high it is.  

David Schaetzle, a former professor at ASU, has some on-hands experience
with this phenomenon, and first brought it to my attention at the
Cooling Frontiers Workshop organized by the late Jeff Cook in 2001.  

	-----Original Message----- 
	From: Edward.A.Decker at jci.com 
	Sent: Aug 8, 2008 2:08 PM 
	To: Dan Nall 
	Subject: Re: [Bldg-sim] GSHP in hot climate 
	The sol-air temperature stops affecting the ground temperature
at a distance of 5 meters (roughly). If you are installing a vertical
well, that temperature fluctuation should not have a significant effect
on the performance of the well. I also believe that the new equilibrium
temperature that you are referring too is localized to within a meter
(roughly) of the well. Can't this ground temperature stabilization can
be off-set by increasing the spacing of the vertical wells?

Edward A. Decker 
Project Development Engineer 
Building Efficiency 
Johnson Controls 
1001 Lower Landing Road 
Suite 409 
Blackwood, NJ 08012 
Tel : 610-675-9603 
Fax : 856-228-6296 
Email : edward.a.decker at jci.com <mailto:edward.a.decker at jci.com>  
URL : http://www.johnsoncontrols.com <http://www.johnsoncontrols.com/>  









Dan Nall <dannall at mindspring.com> 

08/08/2008 01:26 PM 

Please respond to
Dan Nall <dannall at mindspring.com>


yizhao1 at vt.edu, Edward.A.Decker at jci.com 


bldg-sim at lists.onebuilding.org 


Re: [Bldg-sim] GSHP in hot climate




	The ground temperature is not a constant if it is subjected to
heat fluxes from a local underground source such as a geoexchanger.
think of the ground as a large thermal storage medium, with fluxes at
its extreme bottom and top boundaries.  At the bottom is the hot core of
the earth.  At the top is the fluctating air temperature and radiant
flux at the surface.  In general, the average temperature below ground
is going to be approximately the average sol-air temperature of the
surface.  The deeper you go, the smaller is the variation over time, and
the more delayed is that variation from what is going on at the surface.
At a few meters below the surface, temperature variation is very small.
Deeper still, the temperature will begin to rise. With a geoexchanger,
however, local heat flux from gthe device can cause significant
variations in temperature. If seasonal flux is not balanced, over time,
the ground local to the geoexchanger will conform to a new equilibrium
temperature, sufficiently variant from the "average" subterranean
temperature to disperse that local heat flux into the surrounding earth.
Given that the thermal conducitivity of "earth" is not enormous, that
temperature differential could be quite large.
	Ground source heatpumps were initially very popular in Phoneix.
Within a year or two, they "heat soaked' the ground surrounding their
wells, and the heat pumps ceased operating.  Most of them ahve been
abandoned, or supplemented by evaporative heat rejection devices.
	Think of geoexchangers as annual thermal storage devices, not as
unlimited heat sources or sinks.
	-----Original Message-----
	>From: yizhao1 at vt.edu
	>Sent: Aug 8, 2008 12:45 PM
	>To: Edward.A.Decker at jci.com
	>Cc: bldg-sim at lists.onebuilding.org
	>Subject: Re: [Bldg-sim] GSHP in hot climate
	>According to the source we got,  the ground temperature is 85F,
although the
	>ground temper in a lots of other locations in the world are
about 55F.
	>Do you have some other source for the ground temperature?
	>Quoting Edward.A.Decker at jci.com:
	>> For a GSHP, the surface temperature of the earth should not
	>> isn't the temperature below the surface what makes the GSHP
work? A
	>> constant temperature of ~55 deg F.
	>> Edward A. Decker
	>> Project Development Engineer
	>> Building Efficiency
	>> Johnson Controls
	>> 1001 Lower Landing Road
	>> Suite 409
	>> Blackwood, NJ 08012
	>> Tel : 610-675-9603
	>> Fax : 856-228-6296
	>> Email : edward.a.decker at jci.com
	>> URL : http://www.johnsoncontrols.com
	>> yizhao1 at vt.edu
	>> Sent by: bldg-sim-bounces at lists.onebuilding.org
	>> 08/06/2008 11:50 PM
	>> To
	>> bldg-sim at onebuilding.org
	>> cc
	>> Subject
	>> [Bldg-sim] GSHP in hot climate
	>> Hi-
	>> We modeled a building with GSHP in a hot climate (zone 1), so
it is almost
	>> used
	>> for cooling only. The air-side is PVAVS. water-cooled
condenser with GSWL.
	>> The
	>> cooling COP input is ~5. However, the system performs almost
the same as
	>> ordinary air-cooled chillers.
	>> We think the reason may be the high earth temperature (~85 F
is used due
	>> to the
	>> local
	>> climate).
	>> GSHP does not appear to be a solution for hot climate? Any
one has some
	>> resource
	>> of real data for this?
	>> Thanks,
	>> Ying
	>> _______________________________________________
	>> Bldg-sim mailing list
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