Multiple Ground source Heatpump in one alternative

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Good afternoon,

I'm trying to find out a way to model 2 ground source heat pumps in an alternative. One of these systems serves an exhibition spaces, and the other one serves a mechanical room. When I call CDS some representatives say it's possible, others don't think so, making it more confusing. Anybody has experience with this issue?

ccaballero's picture
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Catalina,

This is a complex scenario to model in TRACE 700 and I wanted to shed some light on what is possible and not possible.

The limitations you're finding are related to how system coils are assigned to equipment (plants), and ground heat exchangers. TRACE 700 can only model a single ground heat exchanger per alternative and system coils (e.g. cooling and heating coils) are assigned to a plant, rather than an individual piece of equipment. In your scenario, you may have one or two ground loops but you'll only be able to model a single loop in the alternative. This limitation prevents you from modeling the systems independently.

If you create two ground source heat pump pieces of equipment, you'll still need to assign these to a single plant. Doing so does not allow you to independently operate each plant based upon system coil load because cooling and heating coils are assigned to plants rather than equipment. The sequencing options will not help you in this instance.

To model this system, you'll need to make a ground source heat pump system as shown in the User's Manual. You can make a single cooling plant (water source heat pump equipment type) and modify the energy rate and ground loop information as you desire. You may wish to use a composite energy rate the best represents all heat pumps on the ground loop(s).

I suspect that the information you received from C.D.S. is correct in that you can model two separate heat pump plants, but you cannot model two separate ground loops in a single file.

Regards,
Eric Sturm

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Eric,

I am surprised to hear you say that TRACE will not allow two separate
ground loops in a single file because if you create multiple cooling
plants with ground loops, you are allowed to specify different parameters
for "TLoop Ent Bldg" under Plant Controls, Geothermal Loop. When you run
the model, you even get different energy use results for each cooling
plant (GSHP) when you have two identical rooms with one assigned to each
cooling plant (since the temperature of the ground loop is different based
on TLoop Ent Bldg). This implies that there are two separate loops,
otherwise the one loop would have two different entering temperatures
simultaneously.

Catalina, does that address the question that you asked?

Alex N. Chapin, LEED AP BD+C

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Alex,

The interface will allow you to specify more than one ground heat exchanger, however the results presented for the second loop (as determined by the order of entry) cannot be used. Only one may be simulated per alternative. In your results from an error scan, you should see a fatal error associated with multiple ground loops.

Eric Sturm

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Correct. That's what happened when I tried modeling it. If you have additional comments or suggestions please let me know.

Catalina Caballero, AIA. Assoc., LEED GA.

ccaballero's picture
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It may be easier to split the building into two separate files instead of
trying to model the effect of two separate ground loops in one model when
the software won't allow it. You can use partitions with the adjacent
space temperature method of Prorated to get the interactive effects of
interior walls between spaces with different temperature set points
without including the adjacent space in each of the models (helpful for
where you are splitting the building). Without knowing the specifics of
your model, that's all that I can think of at the moment.

Good luck,

Alex N. Chapin, LEED AP BD+C

AChapin at sriregistrar.com's picture
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Catalina,

We have run into this issue many times. As Alex indicated it is not possible to model individual heat pumps on the same geothermal heat exchanger.

In order to determine a solution we need to know what you are looking for as an end result.

If you are trying to determine potential energy savings with a geothermal system, then determine an equivalent efficiency based on peak loads and/or annual operation.

If you are using the model to create a GT1 file to size the geothermal heat exchanger. I would recommend splitting the model into two separate files with partitions as mentioned. You can then take each GT1 file produced and manipulate it in excel to create a single GT1 file. If you change the .GT1 extension to .csv, the file will open in excel showing the respective load, per month, the well field would experience. Each of these .csv can be combined and used to create an overall GT1 file.

Hope this helps.

Dan Kaler EIT

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We are trying to show LEED compliance under NCv2.2. It needs to show accuracy with design conditions under the LEED Report. Thanks for your help.

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Hi. Sturm is right. Trace only allows one ground loop to reject/take energy to. But here is how I have worked with this before (hybrid Multi-stack heat pump, boiler, DHW heating, and geothermal HP's in barrack rooms)...You will build up as many HP plants as you need, BUT looking at it from Trace's point of view...let's say you have a pitcher of lemonade and three empty glasses. The kids picked their cup from  the cupboard, all different sizes...well, Trace fills the 1st cup created in Trace, then once it's full, starts to fill the next cup, and then the last cup if there's that much demand on that design hour. That said, I had my Multistack 1st in line, then the smaller GSHP's broken out per EER. That said, you will need to take the conservative approach and put the worse EER first, as to not quantify better savings than they are. The LEED reviewers, for me, respected that approach. Does it hit the nail on the head and match exactly, no, but again you're coming forward with a design that is essentially "truthfull" enough to meet LEED criteria. Is there hidden efficiency left on the table not quantified, yes, but does it leave another EAc1 point on the table...that's for you to decide if it's worth to pursue. As long as it is justifiable and can be clearly explained to the LEED reviewer, then it is their decision, ultimately. Ask yourself how much time it will take to go for that extra point, hopefully your LEED team has built in buffer points to strive for the next level of LEED certification.

Be Sustainable -- Never let today use up tomorrow!.

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