GSHP in hot climate

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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

GSHP does not appear to be a solution for hot climate? Any one has some resource
of real data for this?


yizhao1 at's picture
Joined: 2011-09-30
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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?



yizhao1 at's picture
Joined: 2011-09-30
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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.

Dan Nall's picture
Joined: 2011-10-02
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Ying -
If your building is in the U.S., you can find the average ground
temperature in your area with the map in ASHRAE HVAC Applications,
Chapter 32. For moderate climates like most of the continental U.S. the
average ground temperature is just a few degrees above average annual
air temperature. In Zone 1, you could have an average ground
temperature near 80.

If you do a GSHP system, you'll likely want a hybrid system that uses a
cooling tower or fluid cooler to balance your system, allowing the
ground loop to be smaller. This may make it a reasonable system choice.

Scott Hackel

Scott Hackel's picture
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Dan Nall's picture
Joined: 2011-10-02
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yizhao1 at a ?crit :
for ground temparature estimation, book PASSIVE COOLING OF BUILDINGS,
edited buy M. Santamouris and D. Asimakopoulos provide a whole chapter
on Ground cooling. This ref. includes an equation estimating time
evolution of ground temp. at a certain deep using various parameters as
soil thermal diffusivity,
average annual soil temp, time elapsed from the begining of the year, ...
This could be usefull.

Harry BOYER's picture
Joined: 2011-10-02
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I just wanted to throw in a comment from a "sort of" hot climate. Is this discussion referring to deep vertical wells or horizontal field lines? The field line types are highly dependent on correct depth and incorrect depth is a reason for many system performance problems. The well types are not foolproof by any means but tend to produce dependable results, even in Florida (where there are numerous installations).


Jeffrey G. Ross-Bain, PE, LEED

Jeff Ross-Bain's picture
Joined: 2011-09-30
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A couple of clarifications and my opinions regarding GSHP in hot climate.

1. "200 well feet per ton"

The "200 well feet per ton" is not a universal rule for sizing ground heat exchanger (GHX). According to a veteran of GSHP industry, "200 well feet per ton" was originally developed based on calculation/experience for single family house at Stillwater, Oklahoma, which has near-balanced heat rejection and extraction loads and thus there is no concern of heat built up in long term (in the scale of multi years). In addition, the undisturbed ground temperature is around 63F. For other buildings in different locations, GHX has to be sized based on both peak and cumulative heating and cooling loads, geology information, heat pump performance, layout of borehole field, and etc. In addition, long-tern heat built-up needs to be accounted for if unbalanced ground heat rejection and extraction exists. I'm not sure whether "200 well feet per ton" was used for sizing GHX of the GSHP systems in Phoenix mentioned in Dan's earlier e-mail. If it was the case, I will not be surprised about the failed systems. A simple eQUEST simulation can approve this.

2. Ground (loop) temperature

I agree with Dan's analysis of heat transfer in the ground and the resulting variation of fluid temperature in the GHX. Only one addition: due to the fluctuation of the building loads, the ground may be able to "have some rest" to recover from the increased/decreased temperature when the loads is reduced from its peak (i.e. the surrounding ground temperature of a GHX will go down in the summer night, or during summer break if the building is a school). These factors should also be accounted for when assessing the feasibility of GSHP and/or sizing the GHX.

3. Energy efficiency of GSHP in hot climate

While climate zone 1 may be the extreme for GSHP systems, many GSHP systems have been working well and energy efficiently in regions with hot summer and cold winter, such as Oklahoma City and Dallas. For a properly designed GSHP system, the leaving fluid temperature will rarely exceed 90F, but the daytime ambient air temperature could be above 90F for couple of months in summer. It makes the GSHP system more energy efficient than air-cooled chiller when cooling loads are peaked. To further improving energy efficiency of GSHP system in cooling mode, following practices are usually taken:
- hybrid GHX with supplemental heat rejection
- decentralize the borehole field (to reduce the effect of thermal coupling among boreholes) and the pumping system (in lieu of variable speed pumping)
- utilize energy recovery ventilation


Xiaobing Liu2's picture
Joined: 2011-10-01
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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

Mark Seibert's picture
Joined: 2011-09-30
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