[Equest-users] IPLV Curve Fit

[Nick Caton]

Hi Lyle,

 

I think you need to review a little further before proceeding.  Read
through the attached discussion from the bottom up.  This discussion
diverges a bit (as performance curve topics usually do) but I think
you're a bit ahead of the curve so to speak (har har!) in recognizing
what the IPLV numbers really mean.  Plugging in your IPLV values as
coefficients for a cubic equation (where f(x) = Ax^3+Bx^2+Cx+D) is
pretty much wildly different from the correct course of action, but
you're definitely in the ballpark's parking lot ;).

 

Ultimately, you (and others - listen up!) will probably have the best
shot of getting it right on the first go if you visualize what you're
doing in excel.  That's what worked for me when I was climbing this
mountain:

 

*         Assemble your part load percentages and corresponding EER's in
excel

*         Stop and review what the curve in eQuest is looking up, and
from what variable(s).  You can glean this from the title of the default
curve used...  In this case, you want eQuest to determine EIR each hour
as a function of Part load (EIRfPLR). 

*         Knowing that, convert convert your EER values to to EIR

*         Now, normalize each EIR value to the ARI value*.  If the EER
at ARI is 9, and your 50% part load EER is 6, then the normalized vaule
is 6/9 = 0.67.  This normalized number is actually a multiplier of sorts
that eQuest will apply against your specified ARI efficiency in the
chiller input window.  If none of this is making sense, go back further
and review my "recommended reading" list in a thread called "Chiller
curves, oh boy!" from sometime last year in the archives.

*         Using excel, plot these normalized efficiencies (y-axis)
against the part load ratios (x-axis) with a scatter chart.

*         Right click and add a curve-fit to the series.  In the dialog,
try making it third order (quadratic) and see if you get a good fit.
Check the option to display the curve equation.

*         Voila, there are your four coefficients (A,B,C,D - see
eQuest's curve inputs to be sure you know which is which in the
quadratic format) for eQuest!  You've also developed a visual check to
ensure the numbers make sense.

 

*Important:  You MUST work from ARI-condition data points, and normalize
to the ARI condition, and specify your chiller's capacity/efficiency at
ARI conditions, if you are trying to only make PLR curves without doing
a full set of custom curves.  The reasons why are complex, and I've
already nearly written a book on the matter in the archives if you're
itching to know why ^_^.  If you want to specify your chiller around
design conditions, then it's all or nothing (library curves) with regard
to custom curve creation!

 

A general statement/consolation to everyone: We all have to crawl before
we can run!  Make no mistake, coming to a true fundamental understanding
of custom curves in eQuest/DOE2, to the point you can manipulate and
even create them on your own, is no small feat.  They should give out
medals!  The best way to develop this skill set is to accept you will
probably screw it up a few times to start, look hard at your results,
and read up on the literature/advice that's out there in order to learn
what it is that you personally don't know.  Once you come around to
understanding what you do know, and more importantly what you know that
you don't know... (Donald Rumsfeld would make a good energy modeler, lol
<http://en.wikiquote.org/wiki/Donald_Rumsfeld> ),  it's all downhill
from there, and you can begin to ask the right questions of yourself and
others to bridge your personal gaps.  Making mistakes is commendable,
provided you resolve to learn from them!   

 

~Nick

 

 

 

NICK CATON, P.E.

SENIOR ENGINEER

 

Smith & Boucher Engineers

25501 west valley parkway, suite 200

olathe, ks 66061

direct 913.344.0036

fax 913.345.0617

www.smithboucher.com 

 

 



 

NICK CATON, P.E.

SENIOR ENGINEER

 

Smith & Boucher Engineers

25501 west valley parkway, suite 200

olathe, ks 66061

direct 913.344.0036

fax 913.345.0617

www.smithboucher.com 

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Robby
Oylear
Sent: Wednesday, June 15, 2011 2:43 PM
To: Lyle Keck
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] IPLV Curve Fit

 

Lyle,

 

The short answer is no.  IPLV cannot be used to create performance
curves in eQUEST.

 

Part-load curves are a strange animal in eQUEST. You'll need cooperation
from the chiller manufacturer to get the correct data.  You need to have
enough data to describe the way energy varies as a function of
part-load, chilled water supply temp, and outside air temperature.  Take
a look at the attached spreadsheet which can be used to create curves in
eQUEST.  The data in yellow should be entered by the manufacturer or
manufacturer's rep. The data in red should be entered into eQUEST.  

 

If you look into the formulas, you'll note that EIR-FPLR is actually not
an EIR factor, it is based on the power draw of the equipment at
part-load compared to at full-load.  This throws a lot of people off as
it is not described that way in the help documentation.

 

The interesting question that arises when working with custom part-load
curves is how accurate does the eQUEST default performance curve model
the required IPLV for the baseline per ASHRAE 90.1?  In my experience
the default curve is approximately 10% worse than the IPLV requirement.

 

Hope this helps,

 

Robby Oylear, LEED AP BD+C

Mechanical Project Engineer

Energy Analyst

Rushing

 

On Wed, Jun 15, 2011 at 10:59 AM, Lyle Keck <lkeck at aeieng.com> wrote:

All, 

I am trying to create a part load curve for an air-cooled screw chiller.
I have the EER & COP data for 100%, 75%, 50%, and 25% loading.  

 

I have read through the various posts in the archives and I am still
unsure if I should input this data as 'raw data' or 'coefficients'  

 

>From previous posts: 

 

IPLV (or NPLV) = 0.01A+0.42B+0.45C+0.12D

Where:

A = COP or EER @ 100% Load

B = COP or EER @ 75% Load

C = COP or EER @ 50% Load

D = COP or EER @ 25% Load

 

It is my understanding that EIR-PLR curves give the EIR as a function of
part load ratio. 

 

SO, if I create a 'cubic' curve with input type 'curve coefficients',
and input a=(1/COP-100%), b=(1/COP-75%), c=(1/COP-50%), and
d=(1/COP-25%) and specify this curve as the EIR - f(part load ratio)
curve, would that be accurate?  

 

Additionally, this chiller has a condenser water temp of 95 degF and
design conditions that differ from AHRI conditions (design EWT=58 degF
and LWT=44 degF)

Do these values have direct correlation to the curve creation ? Or can
the condenser and CHW temp simply be defined in the 'basic
specifications' tab for the chiller, and the EWT defined via the CHW
loop deltaT?

 

I know this topic has been widely discussed, but I am still uneasy about
the actual inputs for the creation of the part load performance curve.

 

Any input, or reference to a previous post would be appreciated.

 

Thanks in advance,

 

Lyle Keck  
SustainablE Systems Analyst

 

AEI | AFFILIATED ENGINEERS, INC.  
Westlake Center Office Tower, 1601 Fifth Avenue, Suite 1400 | Seattle,
WA  98101

P: 206.256.0800 | F: 206.256.0423  
lkeck at aeieng.com <mailto:lkeck at aeieng.com>   |  www.aeieng.com
<http://www.aeieng.com/>   

 

 


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Subject: Re: [Equest-users] using IPLV in eQuest
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