[Bldg-sim] GSHP in hot climate

[Dan Nall]

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?
>
>Thanks,
>
>Ying
>
>Quoting Edward.A.Decker at jci.com:
>
>> For a GSHP, the surface temperature of the earth should not matter...
>> 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
>>
>>
>>
>>
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>>
>> 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
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