[Equest-users] CHW loop process load

fiona.you at cn.bureauveritas.com fiona.you at cn.bureauveritas.com
Sat Apr 6 22:26:03 PDT 2013


Hi James,

Thank you for your sharing. I am doing a district chilled water project at
present, your information is very useful.

I made the Baseline model with chillers and cooling towers as indicated in
Apppendix G. The proposed model is modeled with one air-cooled chiller with
average efficiency+piping loss.

One thing I am not clear is what's the chiller curve like in your model?
Because it is hard to set curves as a constant value. For example, a
chiller's EIR curve should be a quadratic curve, how do you make it into a
constant value?
                                                                                                           
 Best Regards!                                                                                             
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             James Hess                                                    
             <JHess at tmecorp.co                                             
             m>                                                         To 
             Sent by:                  Adam S <adam.scherba at gmail.com>     
             equest-users-boun                                          cc 
             ces at lists.onebuil         "equest-users at lists.onebuilding.org 
             ding.org                  "                                   
                                       <equest-users at lists.onebuilding.org 
                                       >                                   
             2013-04-06 03:36                                      Subject 
                                       Re: [Equest-users] CHW loop process 
                                       load                                
                                                                       Ref 
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




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> 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>
      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!  J





        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 à 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!  J





        Regards,





        JAH





        James A. Hess, PE, CEM, BEMP
        Energy Engineer
        TME, Inc.
        Little Rock, AR


        Mobile: (501) 351-4667





        From: 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
        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> 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


        kimberly.wiebe at burohappold.com






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                             Javed Iqbal, LEED AP, CEA


                                  Energy Analyst
















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