ASHRAE Journal - October 2021 - 54

ENGINEER'S NOTEBOOK
COLUMN
David Sellers
Modeling Perfection
BY DAVID SELLERS, P.E., MEMBER ASHRAE
I've been thinking about changes since the olden days when I learned predictive
energy modeling. The state of the art then was a blunt instrument compared to
current methods. Yet, challenges still exist correlating the model to a reality that
doesn't exist. This column shares some techniques I use to assess chilled water plants
to inform better plant staging/optimization strategies and/or equipment selection.
Because my career evolved to focus on existing facilities,
my modeling has become forensics-oriented and
spreadsheet-based. The spreadsheets are not intended
to calculate the building's resource consumption; the
building tells us that. Rather, they use utility/trend data
to provide a hypothetical picture of what might be possible
in a somewhat perfect world.
The Existing Building Commissioning (EBCx) Angle
A subtle difference exists between applying models for
design/new construction projects vs. existing building
projects. In the former, the model is informing design
decisions and/or demonstrating compliance with codes
or building certification programs.
In the latter, the model helps you understand why
the facility is not meeting the design target, correct the
issues identified, adapt it to the current operating profile
or program and take things to the next level in terms
of sustainable, efficient operations.
One benefit associated with an existing building is
that you have utility and trend data to model with. But
for many EBCx practitioners, there can be constraints.
1. There may not be a calibrated model of the facility
or it may be constrained by the algorithms contained in
the calculation engine behind it.
2. The budget/timeline may not support modeling.
3. Many practitioners are passionate about building
54
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
efficiency, proficient with spreadsheets, familiar with
HVAC fundamentals, and willing to learn. However,
they may not have a design background and may find
the level of rigor required to develop an energy model
intimidating and/or beyond their expertise.
The good news is that the building knows the answer and
can tell us how well it is performing, how it is using
resources and where its issues are via its trend data.
Armed with this information and spreadsheets, we can
compare the data with what we think should happen in
a perfect world to guide our efforts.
Complementing the analysis are field observations
documenting the equipment operating state/physical
configuration and performance data procured from
documentation and field tests.
The diagnostic tool (spreadsheet) I will use as an
example applies a simplistic relationship between outdoor
air temperature (OAT) and cooling load to identify
deviations from what might be expected given the physics
of the system and facility.
The technique may not meet the rigor associated with
a true investment-grade model. But, in my experience,
the information provided can be used to make investment
decisions for resource efficiency and optimization
projects-decisions based on the physics of the systems
David Sellers, P.E., is senior engineer at Facility Dynamics Engineering's office in
Portland, Ore.
http://ashrae.org

ASHRAE Journal - October 2021

Table of Contents for the Digital Edition of ASHRAE Journal - October 2021

Contents
ASHRAE Journal - October 2021 - Intro
ASHRAE Journal - October 2021 - Cover1
ASHRAE Journal - October 2021 - Cover2
ASHRAE Journal - October 2021 - 1
ASHRAE Journal - October 2021 - Contents
ASHRAE Journal - October 2021 - 3
ASHRAE Journal - October 2021 - 4
ASHRAE Journal - October 2021 - 5
ASHRAE Journal - October 2021 - 6
ASHRAE Journal - October 2021 - 7
ASHRAE Journal - October 2021 - 8
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ASHRAE Journal - October 2021 - 54
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ASHRAE Journal - October 2021 - HR1
ASHRAE Journal - October 2021 - HR2
ASHRAE Journal - October 2021 - HR3
ASHRAE Journal - October 2021 - HR4
ASHRAE Journal - October 2021 - HR5
ASHRAE Journal - October 2021 - HR6
ASHRAE Journal - October 2021 - HR7
ASHRAE Journal - October 2021 - HR8
ASHRAE Journal - October 2021 - HR9
ASHRAE Journal - October 2021 - HR10
ASHRAE Journal - October 2021 - HR11
ASHRAE Journal - October 2021 - HR12
ASHRAE Journal - October 2021 - HR13
ASHRAE Journal - October 2021 - HR14
ASHRAE Journal - October 2021 - HR15
ASHRAE Journal - October 2021 - HR16
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ASHRAE Journal - October 2021 - HR19
ASHRAE Journal - October 2021 - HR20
ASHRAE Journal - October 2021 - HR21
ASHRAE Journal - October 2021 - HR22
ASHRAE Journal - October 2021 - HR23
ASHRAE Journal - October 2021 - HR24
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ASHRAE Journal - October 2021 - Cover3
ASHRAE Journal - October 2021 - Cover4
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