[Bldg-sim] Simulation Calibration

Bill bkoran at qwest.net
Wed Aug 27 17:27:18 PDT 2008


A few other comments regarding calibration


 

Calibrating to annual utility use is better than nothing, but still not
worth much.  Calibrating to monthly utility bills can be pretty good, but
that is very dependent upon your goals.

 

First, note that DOE-2 and derivatives are (still, I believe) hourly
simulations.  (EnergyPlus can use shorter time steps.)  Utilities usually
bill demand based on a 15-minute period, or less commonly on a 30-minute
period.  Perhaps some bill on an hourly period, but I haven’t run into that.
Depending upon when the peak demand occurs, and what is causing it, the
15-minute and hourly peak demands can be VERY different.

 

Second, it is next-to-impossible to truly calibrate a building in the
absence of zonal energy use information.  VAV box operation and reheat can
be particularly problematic, but there are many other areas of concern.
Building simulation programs generally assume proper or at least decent
operation.  However, this is seldom the case:  economizers don’t work
properly, setpoints are suboptimal, there is simultaneous heating and
cooling due to leaking valves or other problems, etc.

 

A calibrated model MAY improve savings estimates for many retrofits, but
again, it depends upon whether the retrofit will be impacted by such
problems.  One example is a conversion of a system from constant flow to
variable flow.  First, most simulations don’t do a great job of simulating
variable speed pumping or VAV fan operation, although this is improving.
Second, most variable flow systems don’t operate optimally, and proper
operation is what we typically assume in simulation.

 

(For documentation of some of the issues with simulating variable flow
systems, refer to the dictionary for DOE2.2R at 

http://doe2.com/download/DOE-22/DOE22Vol2r-DictionaryRefrigeration.pdf

It states, in part, that 

“Pressure is not considered in the air-side calculations.  VAV fan power is
modeled using a simple curve without considering where the pressure drops
exist in the system, where the static pressure sensor is located, or what
the static setpoint is.”)

 

Put another way, it is pretty easy to simulate a constant flow system, and
the energy use for that system, since it is relatively unlikely that the
system won’t be operating as expected. However, a variable flow system is
likely to not be operating as expected, is not particularly well-modeled by
DOE-2, and hence the savings estimates from even a calibrated simulation
might not be very good.

 

This is not to discourage you from calibration—far from it.  Your model will
certainly be better after calibration, and you will learn a ton in the
process.  However, I encourage you to note the previous resources and
comments.  It certainly is VERY “easy to underestimate the actual time
commitment required:”  In the early 1990s I performed multiple
research-level calibrations.  For one large building, I calibrated a DOE-2
model to hourly end-use AND zonal subsystem data as part of a research
project.  The process was months, not days.  That was a very detailed
calibration, but it demonstrated how a model calibrated to monthly data can
still be far from reality.

 

William E. Koran, P.E. 
Senior Engineer 
Q u E S T 
Quantum Energy Services and Technologies 
Voice: 503-557-7828 
Fax: 510-540-7268 
Web:  <http://www.quest-world.com> www.quest-world.com 

 

 

  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Brandon Nichols
Sent: Wednesday, August 27, 2008 3:17 PM
To: Chris Mullinax; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Simulation Calibration

 

Check out "Computerized Building Energy Simulation Handbook" by James
Waltz... there's a click-through to Amazon.com on my blog
<http://elcca-exchange.blogspot.com/>  for this fundamental energy modeler's
reference manual.  An entire chapter is devoted to model calibration.  In
addition, there are numerous quantitative and anecdotal tips for improving
the quality of and reducing the time required to build energy models.

 

For an example related to model calibration, you may find that only a few or
perhaps no data loggers are required -- setting up trend logs, taking
instantaneous branch circuit amperage readings (for relatively constant load
circuits such as lighting) and accessing existing building schedule
information resident in a modern EMCS may be sufficient.  

 

The goal, as Chris stated, is for your model to be within ± 5% of actual
energy consumption as documented by the utility bills, month-to-month.
Chris is also spot-on about the iterative nature of the process... it's easy
to underestimate the actual time commitment required.

 

Brandon Nichols, PE,  LEED AP
Mechanical
HARGIS ENGINEERS
600 Stewart St
Suite 1000
Seattle, WA 98101
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
www.hargis.biz

 

  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Chris Mullinax
Sent: Wednesday, August 27, 2008 1:26 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Simulation Calibration

I have used equest a number of times to show clients the relative effects of
changing from standard efficiency to higher efficiency equipment or say from
T8 to T5 lighting systems. Generally, pretty basic models. 

 

I now have a client a (a municipality) that is interested in “calibrating”
the models to get more precise information.  

 

I’ve heard the term “calibration” as it relates to models, and I think I
understand the concept, but how is this actually done?  Is it just a matter
of improving the details of the model, or is there testing of walls and
other systems to determine actual installed values?

 

Are there any good articles out there on this topic?

 

All help appreciated.

 

Thanks,

Chris Mullinax, P.E. LEED AP

 

pn: 770-387-1334

fx:  770-387-1383

chris at mullinaxsolutions.com

www.mullinaxsolutions.com

 

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20080827/53b3a484/attachment-0002.htm>


More information about the Bldg-sim mailing list