ASHRAE Journal - January 2014 - 65

Column  energy Modeling
Javad Khazaii

Performing Probabilistic
Energy Modeling
By Javad Khazaii, Ph.D., P.E., Member ASHRAE

In my November column, I discussed the necessity of performing a probabilistic energy modeling process instead of a deterministic one for an ASHRAE/IES
Standard 90.1, Appendix G, design building. To be able to run a probabilistic
energy model, the first step is to develop a tolerance margin library for all the
construction material that would be used in constructing a building and for its
associated equipment.
Clarke, et al.,1 have developed a considerably strong
data source in which they have depicted the uncertainty that resides in thermal properties within a
handful of construction materials. This data can be
used to begin generating a comprehensive library of
the uncertainties.
AHRI standards provide manufacturing allowable
test tolerances for most HVAC equipment that can
be used as a source of uncertainty evaluation in the
equipment. For example, ANSI/AHRI Standard 4302009, Standard for Performance Rating of Central Station
Air-Handling Units, allows the tested brake horsepower of the fan to be as high as 107.5% of the nameplate, and AHRI Standard 550/590-2011, Standard for
Performance Rating of Water-Chilling and Heat Pump WaterHeating Packages Using the Vapor Compression Cycle, allows
a tolerance of [0.105 - (0.07× Percent Load)] + [0.15/
(Chilled water temperature difference) × Percent
Load] for centrifugal chillers. This implies that the
actual air-handling unit and chiller power consumption could fall in a margin between the nameplate
(deterministic energy model input) and the maximum allowable test tolerances.
In addition, allowable tolerances for consumed
power by lighting and appliances should be derived
from the proper resources to make sure the majority
of the existing probabilities have been considered.

Another source of uncertainty that is worthy of
evaluating is the effect of accuracy of different sensors throughout the network of the HVAC system.
For example, temperature sensors located inside the
occupied room or at the cooling coil leaving air position usually are set to 75°F or 55°F (24°C or 13°C),
respectively. Generally, each sensor has an allowable
tolerance of 0.5°F (0.28°C). Also, a temperature sensor located outside the building as a measuring reference of the outdoor air temperature usually has a 1°F
(0.55°C) tolerance. These tolerances can contribute
to the total uncertainty in the output of the energy
model as well. This evaluation also can be useful for
validating the use of micro-electro-mechanical systems (MEMS) sensors in HVAC applications due to
their smaller accuracy margins in comparison with
the currently used sensors in the buildings and its
systems.
After gathering a comprehensive library of the
associated uncertainty margins for construction
materials and equipment, the next crucial step is
to assign proper probability distribution to each of
these uncertain input data. This requires collecting
manufacturing data, studying the data and, finally,
Javad Khazaii, Ph.D., P.E., is an associate engineer with Newcomb & Boyd Consulting
Engineering Group in Atlanta.
Jan uary 2014  ashrae.org  A S H R A E J o u r n a l

65



ASHRAE Journal - January 2014

Table of Contents for the Digital Edition of ASHRAE Journal - January 2014

ASHRAE Journal - January 2014
Contents
Commentary
Industry News
Letters
Meetings and Shows
Effect of Heat Rejection Load and Wet Bulb on Cooling Tower Performance
Shaping the Next... Building and Energy
Texas Hospital Central Plant Redesign
Detecting Faults in Hong Kong High-Rise
Engineer's Notebook
HVAC Applications
Refrigeration Applications
Energy Modeling
The Performance Gap
Data Centers
People
Classified Advertising
Advertisers Index
ASHRAE Journal - January 2014 - B1
ASHRAE Journal - January 2014 - B2
ASHRAE Journal - January 2014 - ASHRAE Journal - January 2014
ASHRAE Journal - January 2014 - Cover2
ASHRAE Journal - January 2014 - 1
ASHRAE Journal - January 2014 - 2
ASHRAE Journal - January 2014 - Contents
ASHRAE Journal - January 2014 - Commentary
ASHRAE Journal - January 2014 - 5
ASHRAE Journal - January 2014 - Industry News
ASHRAE Journal - January 2014 - 7
ASHRAE Journal - January 2014 - 8
ASHRAE Journal - January 2014 - 9
ASHRAE Journal - January 2014 - 10
ASHRAE Journal - January 2014 - 11
ASHRAE Journal - January 2014 - 12
ASHRAE Journal - January 2014 - Letters
ASHRAE Journal - January 2014 - Meetings and Shows
ASHRAE Journal - January 2014 - 15
ASHRAE Journal - January 2014 - Effect of Heat Rejection Load and Wet Bulb on Cooling Tower Performance
ASHRAE Journal - January 2014 - 17
ASHRAE Journal - January 2014 - 18
ASHRAE Journal - January 2014 - 19
ASHRAE Journal - January 2014 - 20
ASHRAE Journal - January 2014 - 21
ASHRAE Journal - January 2014 - 22
ASHRAE Journal - January 2014 - 23
ASHRAE Journal - January 2014 - Shaping the Next... Building and Energy
ASHRAE Journal - January 2014 - 25
ASHRAE Journal - January 2014 - 26
ASHRAE Journal - January 2014 - 27
ASHRAE Journal - January 2014 - 28
ASHRAE Journal - January 2014 - 29
ASHRAE Journal - January 2014 - 30
ASHRAE Journal - January 2014 - 31
ASHRAE Journal - January 2014 - 32
ASHRAE Journal - January 2014 - 33
ASHRAE Journal - January 2014 - 34
ASHRAE Journal - January 2014 - 35
ASHRAE Journal - January 2014 - Texas Hospital Central Plant Redesign
ASHRAE Journal - January 2014 - 37
ASHRAE Journal - January 2014 - 38
ASHRAE Journal - January 2014 - 39
ASHRAE Journal - January 2014 - 40
ASHRAE Journal - January 2014 - 41
ASHRAE Journal - January 2014 - 42
ASHRAE Journal - January 2014 - 43
ASHRAE Journal - January 2014 - 44
ASHRAE Journal - January 2014 - 45
ASHRAE Journal - January 2014 - Detecting Faults in Hong Kong High-Rise
ASHRAE Journal - January 2014 - 47
ASHRAE Journal - January 2014 - 48
ASHRAE Journal - January 2014 - 49
ASHRAE Journal - January 2014 - 50
ASHRAE Journal - January 2014 - 51
ASHRAE Journal - January 2014 - Engineer's Notebook
ASHRAE Journal - January 2014 - 53
ASHRAE Journal - January 2014 - 54
ASHRAE Journal - January 2014 - 55
ASHRAE Journal - January 2014 - 56
ASHRAE Journal - January 2014 - 57
ASHRAE Journal - January 2014 - HVAC Applications
ASHRAE Journal - January 2014 - 59
ASHRAE Journal - January 2014 - 60
ASHRAE Journal - January 2014 - 61
ASHRAE Journal - January 2014 - 62
ASHRAE Journal - January 2014 - 63
ASHRAE Journal - January 2014 - Refrigeration Applications
ASHRAE Journal - January 2014 - Energy Modeling
ASHRAE Journal - January 2014 - 66
ASHRAE Journal - January 2014 - 67
ASHRAE Journal - January 2014 - The Performance Gap
ASHRAE Journal - January 2014 - 69
ASHRAE Journal - January 2014 - 70
ASHRAE Journal - January 2014 - 71
ASHRAE Journal - January 2014 - 72
ASHRAE Journal - January 2014 - 73
ASHRAE Journal - January 2014 - Data Centers
ASHRAE Journal - January 2014 - 75
ASHRAE Journal - January 2014 - 76
ASHRAE Journal - January 2014 - People
ASHRAE Journal - January 2014 - Classified Advertising
ASHRAE Journal - January 2014 - Advertisers Index
ASHRAE Journal - January 2014 - 80
ASHRAE Journal - January 2014 - Cover3
ASHRAE Journal - January 2014 - Cover4
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