[Bldg-sim] GSHP in hot climate
XLiu at climatemaster.com
Tue Aug 12 13:43:24 PDT 2008
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
From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org]On Behalf Of Dan Nall
Sent: Saturday, August 09, 2008 3:29 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.
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
1001 Lower Landing Road
Blackwood, NJ 08012
Tel : 610-675-9603
Fax : 856-228-6296
Email : <mailto:edward.a.decker at jci.com> 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.
>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 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
>> yizhao1 at vt.edu
>> Sent by: bldg-sim-bounces at lists.onebuilding.org
>> 08/06/2008 11:50 PM
>> bldg-sim at onebuilding.org
>> [Bldg-sim] GSHP in hot climate
>> We modeled a building with GSHP in a hot climate (zone 1), so it is almost
>> for cooling only. The air-side is PVAVS. water-cooled condenser with GSWL.
>> 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
>> of real data for this?
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