[Equest-users] CHW loop process load

[James Hess]

Those are good questions.

Answer to 1st question = yes

Answer to second is the following:

To be practical about this and simplify the modeling, we create a constant efficiency air cooled chiller to simulate the default DESv2.0 plant.

The EIR is entered equal to 0.839 kw/ton, which equates to 4.4 COP + 5% losses.

This represents the energy usage of the plant chillers, plant pumps, and cooling tower.

We further create and apply performance curves that keep the EIR the same versus load and temperatures. In other words, Avg EIR = Max EIR = Min EIR = the EIR we enter into eQuest.

It's easier to do this with an air cooled chiller because then you don't have to worry about a cooling tower and tower water pump in the model.

This approach is simple and it works. I've used it on several LEED projects involving district chilled water systems.

We probably give up some savings with this approach, but it's a lot of work to correctly model the actual performance of an actual CHW system in accordance with DESv2.0. If this isn't in our modeling scope of work, we use the stipulated efficiency route.

Hope this helps.

P.S. The stipulated efficiency approach does not work out well when modeling heating water and steam systems because the default efficiency numbers for those systems are pretty bad. That's not the case with chilled water, at least if you are comparing to a Baseline based on air cooled equipment. If the building was big enough to require chilled water as a baseline, this approach will not work that well, and you would have to pivot to a new approach.  Comparing the stipulated efficiency against a App G Baseline water cooled chiller plant would probably result in little to no savings in the cooling equipment category.

Thanks!

Regards,

JAH

On Apr 5, 2013, at 1:51 PM, "Adam S" <adam.scherba at gmail.com<mailto:adam.scherba at gmail.com>> wrote:

Do you interpret the USGBC guidance for baseline district energy systems to be 4.4 COP + 5% losses averaged over the year? Meaning that the chiller in your model should be modeled with a better EIR so that the average annual COP calculated using the PS-C report is 4.4?

Their use of the term "average efficiency" seems a bit vague.

Thanks,

-Adam


On Fri, Mar 29, 2013 at 3:24 AM, James Hess <JHess at tmecorp.com<mailto:JHess at tmecorp.com>> wrote:
I definitely agree with that suggestion, versus reinventing the wheel. Our experience with the RMI tool is that it works.  There seems to be a quirk regarding a little shifting due to DST, but that’s a very small thing in the grand scheme of things.  I want to say we tested it and the annual average value coming out of eQuest was ~ 99.3% of the average input value.  Close enough!  :)

Also, I don’t know if you are modeling the other buildings because a GBCI reviewer said you had to do that (assuming this is a LEED project) in order to ensure that correct part load ratios on the chillers operating profile are appropriately accounted for.  I got that sort of comment on a recent project and my response was something to the effect of … I could do that, but since we aren’t getting paid by the owner to model buildings outside of the project scope just to make the chiller modeling more accurate, instead I will just use the default efficiency allowed by the DESv2.0 guidelines for water cooled chiller plants --> 4.4 COP + 5% losses = ~ 0.839 kW/ton on average.

That saved a lot of time with that change because if you do that, you don’t have to model any other buildings served by the system.  If you are comparing against air cooled equipment for the Baseline, you still get a decent amount of savings.  I probably left some savings on the table because the plant had high efficiency VFD water cooled chillers, and VFDs on the pumps and cooling tower.  I’m positive that the annual efficiency was much better than 0.839 kW/ton, but (the way I look at this) we have to balance the level of effort on energy modeling versus the fees we are getting for the modeling.

Hope this helps!  :)

Regards,

JAH

James A. Hess, PE, CEM, BEMP
Energy Engineer
TME, Inc.
Little Rock, AR
Mobile: (501) 351-4667<tel:%28501%29%20351-4667>

From: equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org> [mailto:equest-users-bounces at lists.onebuilding.org<mailto:equest-users-bounces at lists.onebuilding.org>] On Behalf Of Javed Iqbal
Sent: Thursday, March 28, 2013 1:55 PM
To: Kimberly Wiebe
Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] CHW loop process load

You should use RMI tool to generate schedule which will eventually gives you schedule in INP format.

Here is the link to download it..

http://www.rmi.org/Content/Files/EMIT1.4.zip
On Thu, Mar 28, 2013 at 1:41 PM, Kimberly Wiebe <Kimberly.Wiebe at burohappold.com<mailto:Kimberly.Wiebe at burohappold.com>> wrote:
Hello,

I am modelling a central cooling plant that serves many buildings on an college campus. For some buildings I am building the geometry and inputting internal gains. For other buildings I have annual 8760 cooling load profiles. If I want to add these annual cooling load profiles to the CHW loop as process loads, would I have to make 365 day schedules (and 52 weeks)? If so I will write some sort of text generator out of an excel file and paste in the .inp, but before I do want to see if there was any other way.

Thank you,
Kim

Kim Wiebe
Direct: +1 310 945 4813<tel:%2B1%20310%20945%204813>
kimberly.wiebe at burohappold.com<mailto:kimberly.wiebe at burohappold.com>


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--


Javed Iqbal, LEED AP, CEA
Energy Analyst






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