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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,

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Joined: 2013-04-19
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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

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Joined: 2011-09-30
Reputation: 601

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!
Fiona You

James Hess
To
equest-users-boun cc
ces at lists.onebuil "equest-users at lists.onebuilding.org
ding.org "

2013-04-06 03:36 Subject
Re: [Equest-users] CHW loop process
Ref

Those are good questions.

Answer to 1st question = yes

Answer to second is the following:

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

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Joined: 2012-11-12
Reputation: 0

Ok, here?s how we specifically do this.

On the Chiller Basic Specifications tab, we define an electric screw chiller with efficiency of EIR = 0.2386, which equates to ~ 0.839 kW/ton, or 4.4 COP + 5% markup for losses.

[cid:image009.jpg at 01CE33DC.EEAEAE10]

On the Condenser tab, be sure to define the condenser as air cooled --> just simplifies things since you don?t have to deal with cooling tower and tower water pump.

[cid:image011.jpg at 01CE33DC.EEAEAE10]

On the Performance Curves tab, notice the two curves that we created and assigned --> Chiller Constant EIR and Chiller Constant ERI-fPLR. The 1st curve makes efficiency constant as a function of ambient (condenser) temperature while the 2nd curve makes efficiency constant as a function of part load. We don?t care about the cooling capacity curve.

[cid:image013.jpg at 01CE33DC.EEAEAE10]

These two curves are included in the attached file --> Constant Efficiency eQuest Chiller Curves.inp

You can import these into your file via File, Import File ? then they will be available for you to assign to the chiller.

Do all of that and then run the model, and then check the PSC report to verify.

The image below (i.e. PSC report) shows the annual chiller load to be 2080.8 MBtu, or 173,400 ton-hrs (i.e. converted). The annual electricity usage is 145,469 kWh.

Therefore, the annual average efficiency is 145,469 kWh / 173,400 ton-hrs, or 0.839 kWh/ton-hr.

In my opinion, this meets the DESv2.0 requirements for modeling a chiller plant with the stipulated efficiency number, which includes all plant users (i.e. chillers, pumps, and cooling tower).

Hope this helps! :)

[cid:image014.jpg at 01CE33DC.EEAEAE10]

Regards,

JAH

James A. Hess, PE, CEM, BEMP

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Joined: 2011-09-30
Reputation: 601

Hi James,

I can't have access to the images and .inp files that you have uploaded, and would appreciate if you could send it to me.

Thanks.

Jerome

Jerome Sebbag
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Joined: 2012-06-05
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