Dear equest users,
I am currently modeling a 4-pipe fan coil system connected to a ground source heat pump.
I followed this approach to develop the water-side HVAC within equest
1. Create a 'chiller' of the type 'water-to-water heat pump'
2. Create an independent circulation loop of the type 'lake/well'
3. Create a ground loop heat exchanger of the type 'lake/well'
4. Select as the chiller CW loop and GLHX circulation loop the
independent loop you created - they should connect without errors
5. Assign a custom temperature schedule for the lake/well ground loop
in the screen that pops up when you double click the GLHX (read on)
6. Make sure the default head/pump properties for the circulation loop
and GLHX make sense for your system - adjust as necessary. (i.e. if it's a
closed ground loop, there shouldn't be static head)
as explained in this thread "loop to loop heatpump in equest"
http://lists.onebuilding.org/htdig.cgi/equest-users-onebuilding.org/2009-July/001862.html
Instead of using a custom temperature schedule for the lake/well, I am considering a constant temperature all year around (15?C)
Does that assumption has significant impact on the modeling itself ?.
Also, by using a loop-to-loop heat pump, I am no able to change the heating EIR efficiency/curve. See schematic and image attached.
Do you have any idea how to change the heating EIR because it is grey out on my model.
Let me know if I am on the right track
Thanks,
Nicolas Mercier
Hi Nicolas,
This is Nick from the mailing-list discussion you referenced.
I think assuming a constant exiting ground temperature through the year is one way of doing it - this is the way geothermal loop design was first introduced to me (before energy modeling had built the level of momentum it has today). However, recognizing that eQuest is in front of you, I think you'd agree that to not take advantage of eQuest's potential to model ground loop temperature changes through the year (i.e. utilizing the inputs of loop temperature/ground properties in the wizards) would be lamentable.
As with nearly everything in the world of energy modeling, the direct answer to your question of whether these varying temperatures have a "significant impact" for your project would truly be "it depends." In the case of my last project, the exiting loop temperatures were modeled to vary within a range that had significant effects on my heatpumps' operating efficiencies vs. a constant well temperature (where I had started). I think in many cases, establishing a ground temperature schedule is worth the effort to model something that moves towards resembling actual performance and is something I personally plan to continue doing in future projects.
How best to do this however remains something up for debate!
Personally, I feel the method I've put forward to estimate ground temperatures to create a custom lake/well temperature schedule, using the DX system heatpumps which eQuest can model against the ground/loop properties defined in the wizards, while not the most accurate way of going about the task, is the best return I've seen so far with regard to accuracy:time-spent.
You've probably gathered from the mailing list archives: there's some honest and worthwhile debate on how to best come up with an accurate temperature schedule or otherwise account for the realities of varying ground loop temperature (at least until eQuest gets around to a new version supporting other ground source system types). Mr. David Reddy responded in that same linked discussion with an alternative method which is a fundamentally different approach. He did a good job of stating potential flaws to the method I've described.
Something I have not personally explored, but I believe may likely the best approach altogether (something I hope to explore for my next GSHX project), is to use some "true" geothermal loop design software alongside the Quest model to aid in both initial component sizing and to examine/import ground/loop temperature changes over time - not just within a modeled year but over the long-term (say 20 years), allowing the examination of the necessity for additional or future loop heat rejection equipment. From the eQuest help files: "Examples of dedicated ground source heat exchange components sizing programs include: GCHPCalc, GS2000, GLHEPRO, and GLD (Ground Loop Design). " I'd be very interested to hear others' experiences with these and other loop modeling packages, and how their data may be integrated into our eQuest models. Last I checked around, I believe I downloaded GS2000 for a test drive as it's a free package...
To address your other question in a much shorter fashion: When you say the EIR curve is greyed out, did you check the tab labeled "Performance Curves" two tabs over from where you took that screengrab?
NICK CATON, E.I.T.