[Equest-users] Geothermal Well Field Modeling

[Demba Ndiaye]

eQuest does not pretend to be able to size geothermal well fields. As the Help says:



Important: unlike other HVAC components, the program cannot currently design a ground loop heat-exchanger system based on the predicted heating and cooling loads. Except for the design flow, you must input all necessary design parameters, such as the number and depth of vertical wells, horizontal trenches, etc. Under-specifying these parameters can result in extreme temperature swings in the outlet temperature of the well field. This may cause the WLHP loop to shut down (if the outlet temperature exceeds CIRCULATION-LOOP:MAX-ALARM-T or MIN-ALARM-T), or may cause the heat-pump performance curves to extrapolate to unreasonable values,  yielding results badly in error or possibly fatal to the simulation. You should always specify an hourly report for this component and review the predicted outlet temperature for both winter and summer intervals.



The ground loop heat-exchanger model is considered a test capability at this time (except LAKE/WELL, which is well-tested). We suggest it be used for production work only with very careful examination of the results. You should apply careful engineering judgment to be sure all predictions are reasonable since this feature has not yet been validated against field measurements.





Now, one important consideration is the ground temperature change over time due to the field. eQuest does not necessarily account for that; but normally sizing programs account for it (I don't know if Gaia does).



_____________

Demba NDIAYE



-----Original Message-----
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Edward Allen
Sent: Thursday, April 29, 2010 1:05 PM
To: Dan Russell; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Geothermal Well Field Modeling



Dan,



I recently (as in this week) had cause to do a deep dive into eQuest's geothermal modeling capabilities.  Based on your description and my brief experience I have the following suggestions:



1) Is Gaia Geothermal proprietary?  It may have financial incentives to "suggest" larger well fields than needed.

2) What are the allowable temperature ranges/limits and setpoints for your ground loops?  I found that  matching these with those of an actual HP made a difference in whether or not a field of a particular size would meet the load.  The equest defaults were no where near optimal for my model.  This is particularly important if your well field becomes saturated with heat (cold) and the model shuts down the HP to maintain the ground temperature limits.



Edward M. Allen, CEM

Senior Energy Engineer

LONG Energy Solutions





________________________________

From: equest-users-bounces at lists.onebuilding.org [equest-users-bounces at lists.onebuilding.org] on behalf of Dan Russell [danr at engineeringinc.com]

Sent: Thursday, April 29, 2010 10:34 AM

To: equest-users at lists.onebuilding.org

Subject: [Equest-users] Geothermal Well Field Modeling



Hi All - I am wondering if anyone has much experience with the geothermal well field capabilities of eQUEST, in particular with vertical well fields.  I am concerned about the accuracy of the calculation methodology for the vertical well fields, primarily due to a large discrepancy we are getting between the eQUEST results and the results of a separate geothermal well field software produced by Gaia Geothermal.  Here are the parameters:



Vertical well field

Ground conditions & well field dimensions are identical in both eQUEST and Gaia simulations.

Equest model has four 4x5 rectangular well fields (80 bore holes total) with 200 ft bore depth.  Plant report PS-C shows that the fourth well field has no load, thus using at most 60 of the 200 ft wells.

Gaia software inputs load data from the DOE2 report SS-I for each zone along with the matching heat pump size (specific to manufacturer) selected for the project for the zone.  This software uses its knowledge of the heat pump performance to generate the required well-field size and depth.  The result is 120 well fields of 250 ft bore depth or 80 well fields of 350 ft bore depth.



I would expect some discrepancy between different software, but in this case the eQUEST simulation is using only about 50% of the required well-field size calculated by the Gaia software.  The Gaia calculation is coming out much larger than what the owner and design-build contractor had planned for.



How reliable is the eQUEST simulation on vertical well-fields?



I believe the primary difference is that Gaia software uses "real" manufacturer performance data for heap pump models specified and eQUEST uses PVVT system with default GSHP performance curves.  Could this difference really make such huge impact on the results?



Thanks in advance for any insight.



Dan Russell

[EI Signature]


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