[Equest-users] Data center CHW reset negative savings

Nicholas Caton Nicholas.Caton at schneider-electric.com
Fri Jul 27 01:02:46 PDT 2018

Hi Paul,

Since you didn’t provide a PRD, I presume you aren’’t doing this via parametric runs, but that means you’ll need to additionally specify exactly how you’re trying to enact a loop reset sequence so that others can help identify what may be going wrong.  In lieu of that however, I took some liberties to demonstrate one approach.  Attached is a simple PRD to paste adjacent to the other project files (inp/pd2).

I’ve done only 2 things:

  1.  I noticed the entry for MIN-RESET-T was at a default of 40, whereas your loop design temperature is specified at 50.  I’ve adjusted this minimum bounding the reset behavior to matching your CHW loop’s design temp.  Note this input has zero impact in the baseline run, where the setpoint control is simply “fixed” (no reset).  I’d hazard a guess here that if you missed this step, the loop would upon enabling a reset sequence push the chillers to produce colder water – that could be the source of the negative savings you were seeing before.
[cid:image003.png at 01D42553.3C163A70]

  1.  I set up a simple parametric run to only swap the loop’s COOL-SETPT-CTRL from “Fixed” to “Load Reset.”  This will cause the requested supply CHW temp to scale linearly from 50 to 65 in proportion to the relative load on the loop each hour.

[cid:image004.png at 01D42553.8E7BD360]

On simulating both runs, I find the expected positive savings in “Space Cooling” (your chiller compressors aren’t having to work as hard/often), and a minor but negative savings in the heat rejection end use (your dry cooler is trying to reject the same amount of Btu’s over the year with slightly warmer water in the condenser loop --> more cooler fan runtime).

[cid:image005.png at 01D42556.4536EE30]

[cid:image006.png at 01D42556.4536EE30]

Finally, as you made reference to airside fan VFD’s… I don’t think you should expect interaction with the fan energies in the case of using “load reset” (which is supposed to reflect a sequence wherein the BAS is directing the loop setpoint to satisfy the whole buildings per the worst-case coil’s valve position).  Implicitly, the sequence won’t be letting the CHW rise to a temperature that results in an AHU being unable to satisfy it’s duties for lack of airflow/runtime.

Intuitively, I would however caution that if you have or intend to specify variable flow on the associated CHW loop pump(s), that would be something meaningful to input correctly that I’m not seeing at the moment (I observe single speed pumps and 3-way valves at the airside CHW coils).  If you do need to work in variable flow pumping concurrently with a temp reset sequence, you should review/coordinate with the sequence specification to ensure you are correctly picking up on when to let the pump’s turndown lead vs. allowing the temps to reset (else they can end up fighting each other and burning more energy).  This is manipulated in eQuest via the LOOP-FLOW-RESET fraction.

Hope this helps you and others!


[cid:image001.png at 01D42552.8B7C6B80]
Nick Caton, P.E., BEMP
  Senior Energy Engineer
  Regional Energy Engineering Manager
  Energy and Sustainability Services
  Schneider Electric

D  913.564.6361
M  785.410.3317
F  913.564.6380
E  nicholas.caton at schneider-electric.com<mailto:nicholas.caton at schneider-electric.com>

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

[cid:image002.png at 01D42552.8B7C6B80]

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Paul Graves via Equest-users
Sent: Wednesday, July 25, 2018 7:46 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Data center CHW reset negative savings

Good morning all! I was advised to include our model files in case someone could take a quick look at them and tell us the error of our ways. I would still be happy to reimburse anyone for time spent! The problem we encountered was we saw negative chiller savings from resetting the CHW. I expect we made a mistake, but since the fans also have VFDs, it's possible we increase fan energy while saving a little on the chiller, but it still doesn't look right. Any help appreciated. Thank you eQuest users for the help already provided!

Paul Graves
(303) 887 6015

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