MODELING FUNDAMENTALS
IBPSA - USA
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IBPSA-USA
SHELL GEOMETRY
GENERAL CONCEPTS
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SHELL GEOMETRY
USE OF ENERGY MODELING WIZARDS
In what cases are energy modeling wizards
most useful?
After making edits in main program
Initial Model Creation
•Geometry and zoning
•Define all system
types that may be
used
Significant Rezoning or
Major Geometric
Changes
•Copy and paste into
input files to retain
what you have
changed outside of the
wizard
Test or Copy Setups for
Complicated Tasks
•Demand Control
Ventilation
•Skylights with
plenums
•Slab insulation
•Breaking out fan
power
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SHELL GEOMETRY
RULES OF THUMB FOR SIMPLIFICATION
Simplify
REALITY
•
•
ENERGY MODEL
Thermodynamically, only (3)
things matter for modeling heat
transfer surfaces
1. Area
2. Orientation
3. Tilt
Total volume matters IF
infiltration is specified in ACH
ASHRAE 90.1-2007 Appendix G
•
Table G3.1, #5 Building
Envelope, Exceptions (a) and (b)
–
–
Uninsulated assemblies
Exterior surfaces whose
azimuth, orientation, and tilt
differ by < 45˚
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SHELL GEOMETRY
RELATIVE PLACEMENT OF SURFACES
What Matters
•Area
•Orientation
•Tilt
Note: With daylighting the building form is important
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SHELL GEOMETRY
RELATIVE PLACEMENT OF SURFACES
Annual Energy by Enduse
Annual Energy by Enduse
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SHELL GEOMETRY
GEOMETRY INTERFACES
SketchUp Plugins
CAD (dwg files)
• Open Studio for EnergyPlus
• IES Virtual Environment
• 2-D CAD plans may be imported
into energy modeling programs
• gbXML streamlines the transfer of
building information to and from
engineering models
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SHELL GEOMETRY
GEOMETRY INTERFACES
Building Information Modeling
(BIM)
• Generating and managing building
data
• Well developed for architecture, needs
improvement on MEP side
• Early development phase for energy
modeling
Goals
Barriers
• Automatic model generation from 3D renderings
• Architects/engineers will specify “properties” of materials and
equipment for automatic modeling
• BIM needs work in some segments (i.e. electrical engineering)
• Danger of “black box” energy modeling
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SHELL GEOMETRY
ASHRAE 90.1 APPLICATIONS
The number of floors
and conditioned floor
area shall be identical.
Total gross areas of
exterior, opaque
surfaces shall be
identical.
Vertical fenestration areas for
the baseline shall equal the
smaller of:
• the proposed design, OR
• 40% of gross above
grade wall area
The baseline building
shall be modeled so that
it does not shade
itself.
Glazing shall be distributed
on each face of the baseline
building in the same
proportions in the proposed
design.
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EFFECTIVE ZONING
GENERAL CONCEPTS
Number of
zones is
proportional to
complexity of
energy model
Aggregation of
rooms into zones:
significant impact
on energy use
and overheat
prediction
Especially with
large multizone systems
Zoning in
simulation
models can
differ from
actual HVAC
zoning
Energy model
zones are
abstract
# of model
zones < # of
HVAC zones
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EFFECTIVE ZONING
CRITERIA FOR ZONING AN ENERGY MODEL
Usage
• All rooms should have similar internal loads and usage schedules
Temperature Control
• All rooms should have the same Tstat schedules
Solar Gains
• Perimeter zones with windows: Min. one zone for each compass direction
• Unglazed exterior zones can be combined
• Consider shading!
Perimeter or Interior Location
• 12-15’ perimeter zones often require winter heating
• Core spaces can require year round cooling
Distribution System Type
• Combine rooms served by the same type of distribution system (i.e. fan
coil units)
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EFFECTIVE ZONING
SPACES VERSUS THERMAL ZONES
Thermal Zone = area controlled by
a single thermostat
Energy Modeling
– Typical one zone for each space
– Hourly loads are calculated based on an energy balance of the space.
– At the thermal zone level, the loads from the spaces are considered in
conjunction with the temperature set-point and HVAC operating
schedules to determine the zone load.
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EFFECTIVE ZONING
ZONE TYPES WITHIN AN ENERGY MODEL
Conditioned
• Space is heated or cooled
Unconditioned
• Space is neither heated nor cooled
• Examples are false ceiling spaces not used as return air
plenums, attics, crawl spaces and garages
Plenum
• Return air space
• Atrium as return plenum
• Heat transfer within plenums
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CONSTRUCTIONS
OVERVIEW
Types of
Exterior Opaque
(walls, roofs,
Constructions
slabs,
underground
walls, etc)
Quick
Interior (mass,
air, layers, etc)
Exterior Glazed
Parallel Path Calculations for a Stud Wall
vs.
Delayed
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CONSTRUCTIONS
EXTERIOR (DELAYED) CONSTRUCTIONS - OPAQUE
Material
Properties
Layers
• Materials are
“layered” from
outside to inside
• Outside and inside
air films
• Conductivity
• Density
• Specific Heat
• Thickness
Constructions
• Layers determine
U-value
• Surface
Roughness
• Solar Reflectivity
What about construction assemblies
with parallel heat transfer paths?
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CONSTRUCTIONS
PARALLEL PATH CALCS FOR WOOD STUD WALL
ORNL Online
Calculator
Wall Section
R-Value of
Insulated
Section
R-Value of Stud
Section
Typical Stud Wall
=
R-Value
(brick)
=
R-Value
(brick)
Overall Weighted
R-Value of
Wall Assembly =
ASHRAE 90.1
Appendix A
+
+
(
R-Value
(Sheathing)
R-Value
(Sheathing)
0.91 x
+
R-Value
(Insulation)
+
R-Value
(Insulation)
R-Value of Insulated
Section
)
+
R-Value
(Gyp. Board)
+
R-Value
(Gyp. Board)
+
(
0.09 x
+
R-Value
(Inside Air Film)
+
R-Value
(Inside Air Film)
R-Value of
Stud Section
)
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CONSTRUCTIONS
SLAB HEAT TRANSFER
Do you
need to perform outside calculations?
Slab Heat
Transfer
Underground Surfaces: How to get a better underground heat
transfer calculation in DOE-2.1 by Fred Winkelman
1) Choose F-factor from a series of
tables
2) Calculate the exposed perimeter
and area of slab. Use equation
Reffective = A / (F*Pexposed)
3) Set Ueffective = 1/Reffective.
4) Create a material with Reffective
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CONSTRUCTIONS
GLAZING CONSTRUCTIONS
Glazing Properties
• Center of Glass U-value
• Solar Heat Gain Coefficient (SHGC), OR
Shading Coefficient (SC)
• Visible Light Transmission (VLT)
• Light to Solar Heat Gain Ratio (LSG)
Common Pitfall:
Outside Air Films
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CONSTRUCTIONS
GLAZING CONSTRUCTIONS
Includes Spectral Data:
varies SHGC and Tvis with
solar angles
3 Options for
Modeling
Glazing
Simplified
Library
Glazing
Window 6
(LBNL)
SHGC = solar heat gain coefficient
Tvis = visible light transmission
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CONSTRUCTIONS
WINDOW FRAMING
2 Options for Modeling Framing
Include framing effects in
glazing construction
• Model large bands of glass,
OR
• Model windows individually
Common Pitfall:
Window 6 does not
include framing when you
export files
Model framing explicitly
• Works well with Window
6 option
• Use window multipliers
Common Pitfall:
Modeling large bands of
glass
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LIGHTING OCCUPANCY & PLUG LOADS
GENERAL CONCEPTS
Peak Power and
Occupancy
• Total watts of all connected power
• Peak number of occupants
• Can be input with density values
Fractional Schedules
• Daily/Weekly/Annual Occupancy Schedules
• Hourly fractional multiplier for peak values
• Daylight Dimming or Occupancy Sensors
Fraction of Heat Gain
to space
• Assign proportional amounts of heat to
space vs. plenum
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LIGHTING OCCUPANCY & PLUG
LOADS PEAK POWER AND OCCUPANCY
IBPSA-USA
PEAK values include all connected loads
•
•
•
•
Electric Lighting (total fixture wattage)
Emergency Lighting
Plug loads
Kitchen Equipment, Elevators, Servers, etc.
Sources for Estimating Equipment Power Density and Peak
Occupancy
•
•
•
•
•
ASHRAE 90.1 User’s Manual
Title 24 Alternative Calculation Method (ACM) Manual
COMNET (Commercial Energy Services Network)
ASHRAE Handbook of Fundamentals
ASHRAE 62.1 (Occupancy)
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LIGHTING OCCUPANCY &
PLUG LOADS - SCHEDULES
• Just as important
as peak values!
• Unregulated by
ASHRAE Std 90.1
Lighting
100%
90%
80%
70%
60%
Wk
50%
Sat
40%
Sun
30%
20%
10%
0%
1
3
5
7
9
11
13
15
17
19
21
23
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LIGHTING OCCUPANCY & PLUG
LOADS - FRACTION OF HEAT GAIN TO SPACE
Radiative (time lag) vs. Convective
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LIGHTING OCCUPANCY & PLUG
LOADS DAYLIGHTING
Direct daylighting within
energy model
IBPSA-USA
Daylight Specific Tool
• Limited daylight simulation
engine
• Generally more accurate, but
requires parallel model
• Know the limits on the
number of light bounces and
interreflectivity
• SPOT and Radiance can
generate hourly electric
lighting reduction schedules
for import into energy models
• Carefully specify controls
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LIGHTING OCCUPANCY & PLUG
LOADS EXTERIOR LIGHTING
IBPSA-USA
Exterior lighting
is modeled
separately from
interior lighting
Can be controlled
via photosensors
or with schedules
HID vs LED
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LIGHTING, OCCUPANCY & PLUG IBPSA-USA
LOADS OVERESTIMATES OF PEAK EQUIP POWER
Measured
data vs.
typical
values used
in industry
Implications
for
Mechanical
Equipment
Sizing
Name Plate
Ratings vs
Heat Gains
for HVAC
sizing
Energy
Models:
Design Day
Sizing
Feature
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MECHANICAL SYSTEMS
OVERVIEW
Gain and Losses:
 Lights
 People
 Internal equipment (e.g. computers)
 Building envelope (sun, outside temps)
 Ventilation/infiltration
Q=Σ gains + losses + ventilation load
Equipment Sizing
Q = (1.08)*cfm*(MAT-SAT)
air
Q = 500 * ΔT * GPM
water
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MECHANICAL SYSTEMS
COOLING AND HEATING LOADS
Mechanical HVAC systems move energy from one space to another
Cooling systems
Reject heat to the outdoors
via condensers/cooling towers
Heating systems
Deliver heat to the internal
space
k
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MECHANICAL SYSTEMS
PACKAGED & CENTRAL PLANT SYSTEM DIAGRAMS
Central Plant
supply
fan
compressor
Water Side
condenser
Packaged System
Air Side
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MECHANICAL SYSTEMS
PACKAGED SYSTEMS
Energy Modeling
Tip:
Do not
double
count zones
fan,
Packaged
systems can
serve
single
or multiple
compressor and condenser power
Air-Cooled
Condensers
• Split DX
systems
• Package DX
systems
• DX computer
room air
conditioners
(CRACs)
WaterCooled
Condensers
• Dry coolers or
closed-loop
cooling towers
• Cooling towers
Evaporatively
-Cooled
Condensers
• Direct evaporative
package units
• Indirect/direct
evaporative
package units
GroundSource
• Air heat
pumps
• Water heat
pumps
Heating
Systems
• Electric
baseboard
heaters
• Oil and gasfired
furnaces
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MECHANICAL SYSTEMS
CENTRAL PLANT SYSTEMS
Energy Modeling Tip: Pay attention to pump power
Central plant systems
typically
serve multiple zones
and part
load curves
Chilled Water Cooling Systems
• Air-cooled chillers or closed-loop cooling towers serving chillers
• Water-cooled chillers served by open-loop cooling towers
• Evaporatively-cooled chillers
Heating Systems
• Central boiler plant
• Steam boilers
• Hot water boilers
Distribution Systems
• Air handlers with chilled water cooling coils and/or hot water heating coils
• Fan coils
• Radiators
• Chilled beams / radiant panels
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MECHANICAL SYSTEMS
TERMINAL UNITS
Standard VAV box with reheat coil
Variable airflow
Series fan-powered VAV box with reheat coil
Constant airflow,
fan always on
Important Inputs
• Min. airflow fraction
– Fixed or scheduled
• Thermostat type
– Proportional vs.
reverse acting
• Terminal unit fan
power
Parallel fan-powered VAV box
Variable airflow, fan
on when reheat
needed
Reference:
Advanced VAV Design Guideline,
Appendix 8 How to Model Different VAV
Zone Controls in DOE2.2
www.energydesignresources.com
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MECHANICAL SYSTEMS
FAN CURVES
• Fan power =
f(airflow) for VAV
systems
• “Canned” &
custom curves
Fan Curve Issues:
• “Canned” VSD fan curves are often
optimistic
• If creating a custom curve, plot it and
check it, set appropriate minimum
value
• ASHRAE 90.1 Appendix G specifies
the curve to be used for VAV systems
Source: DOE2.2 Volume 2 Dictionary
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MECHANICAL SYSTEMS
FAN CURVES – 90.1 APPENDIX G CURVE
100%
90%
80%
70%
Std 90.1 App G
VSD curve
Fan Power PLR
60%
50%
40%
30%
20%
DOE2.2 standard
VSD curve
10%
0%
0%
20%
40%
60%
80%
100%
Airf low Part Load Ratio
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MECHANICAL SYSTEMS
FAN CURVES – STATIC PRESSURE RESET CONTROL
• Static Pressure (SP)
Reset
90%
No SP Reset
80%
70%
Fan Power PLR
– Continuously adjust
pressure to lowest
setting that provides
adequate zone airflow
– Simulate using fan curve
100%
Good SP Reset
60%
50%
40%
30%
20%
Reference:
Advanced VAV Design Guideline,
Appendix 5
10%
Perfect SP Reset
0%
0%
Includes fan curve coefficients
20%
40%
60%
80%
100%
Airf low Part Load Ratio
www.energydesignresources.com
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MECHANICAL SYSTEMS
CHILLER CURVES
• Chiller performance model
– Capacity = f(temp)
– Efficiency = f(temp, part-load ratio)
Elecin  CapFullLoad
 EIRFull Load
• Represent chiller types
– Centrifugal, rotary, reciprocating…
– Variable speed, multi-compressor…
• Default vs. custom coefficients
Reference
CoolTools Chilled Water Design Guide.
Chiller Bid and Performance Tool, (Excel
spreadsheet).
www.energydesignresources.com
1.0 at full
load and
rated
temp.
 CAPf(T)
 EIRf(T)
 EIRf(PLR,dT)
Issues
Part load efficiency curve typically
includes PLR:
EIRf(PLR,dT)  PLR 
EIRPart Load
EIRFull Load
(EIR = energy input ratio = 1/COP)
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MECHANICAL SYSTEMS
OUTSIDE AIR REQUIREMENTS
• Significant implications for annual energy consumption
• Energy Models: cfm/person OR cfm/sf OR cfm
• PRM: same OA in Proposed and Baseline
– Exception: demand control ventilation
• Healthcare ventilation: Standard 170
• Exhaust requirements mandatory (section 6.5)
ASHRAE 62.1
Ventilation Rate
Procedure
Indoor Air Quality
(IAQ) Procedure
Natural Ventilation
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MECHANICAL SYSTEMS
ASHRAE 62.1: VENTILATION RATE PROCEDURE
Vbz = Rp*Pz + Ra*Az
Vbz = cfm of outside air required in breathing zones
Rp = outdoor airflow rate per person from Table 6-1 [cfm/person]
Pz = the largest number of people expected to occupy the zone
during typical usage [people]
Ra = outdoor airflow rate per unit area from ASHRAE 62.1 Table 6-1
[cfm/sf]
Az = occupied floor area of zone [sf]
 Used to determine design OA for energy models
 Calculating OA for multi-zone VAVs: huge energy implications
 At part-load/occupancy, the minimum OA intake flow ≥ Ra*Az.
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MECHANICAL SYSTEMS
ASHRAE 62.1: INDOOR AIR QUALITY (IAQ) PROCEDURE
Design approach: Allows OA rates to vary if
contaminant levels are below recommended
levels
Contaminant
sources
Contaminant
concentration
Perceived
indoor air
quality
Mass balance
analysis
Occupant
evaluation
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MECHANICAL SYSTEMS
ASHRAE 62.1: NATURAL VENTILATION PROCEDURE
 Prescriptive requirements


Ceiling height
Openable passages ≥ 4% of
floor area
 62.1-2010 requires
mechanical ventilation
UNLESS
– OA passages are
permanently open, OR
– NO heating or cooling
system is installed
OR
 Engineered system with CFD
modeling
 Controls required for
coordination with
mechanical ventilation
systems
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MECHANICAL SYSTEMS
DEMAND CONTROL VENTILATION (DCV)
• Ventilation airflow resets
based on occupancy using
CO2 sensors, timers, occupancy sensors
or schedules
• Higher energy savings for
buildings with large occupancy
swings
Movie theaters, conference rooms
• 10%-30% load reduction and 2-3
yr payback
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MECHANICAL SYSTEMS
ASHRAE STANDARD 55
Clothing
Insulation
Metabolic
Rate
Humidity
Indoor
Environment
and Personal
Factors
Air Temp
Air Speed
Possible to assess within
energy models that
accurately simulate
radiative heat transfer
Radiant
Temp
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MECHANICAL SYSTEMS
SPECIFIC ENERGY MODELING NOTES
Common Energy Modeling Mistakes
 EER: break out fan power and compressor
power
 Part load curves
 Altitude effects
 Auto-sizing
 Rated vs design conditions
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UTILITY RATES
TYPES OF CHARGES AND RATE STRUCTURES
Monthly Charge
•Fixed fee for providing energy
services
$35 per month
Energy Charge
•Unit cost for total quantity of energy
consumed
$0.06 per kWh
Demand Charge
Power Factor Charge
Block Charge
Time of Use Rate
•Fee for highest or peak amount of
energy used
$7.53 per kW
•Penalty for lower than optimum
power factor
$0.40 per KVAR
•Unit charge based on different blocks
of energy use or demand
•Prices change during peak and offpeak times
0–350 kWh
$0.06 per kWh
350–700
kWh
$0.04 per kWh
700+ kWh
$0.02 per kWh
Peak Time
$0.24 per kWh
Off Peak
Time
$0.06 per kWh
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UTILITY RATES
TYPES OF CHARGES AND RATE STRUCTURES
Energy Charge
Demand Charge
Block Charge
Block 3
Block 2
Block 1
Summer
(June-Sept)
Time of Use Rate
Winter
(Oct-May)
Peak
1pm–6pm (M-F)
$0.16 per kWh
Mid
11am–1 pm and 6pm–8 pm (M-F)
$0.06 per kWh
Off Peak
All other hours, and holidays
$0.02 per kWh
All days
All Hours
$0.03 per kWh
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UTILITY RATES
ENERGY MODELING IMPLICATIONS
ASHRAE 90.1-2007 Appendix G Applications
• Same energy rates must be used for Proposed
and Baseline
• Use either actual utility rates or EIA state
averages, except:
– Actual utility rates must be used for purchased hot
water, steam and chilled water
• On-site renewables and site-recovered energy are
NOT included with purchased energy
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UTILITY RATES
ENERGY MODELING IMPLICATIONS
Case Study: Adam Joseph Lewis Center at Oberlin College
•
Project Goals
― Set an example for energy efficiency and
sustainable design
― Net-zero energy building
•
•
The project achieved significant
reductions in total energy use
However, no efforts were made to
lower the peak demand, which
resulted in a much lower energy cost
savings
79%
Total Energy
Savings
35%
Energy Cost
Savings
Utility
Rates Can
Be Crucial!
48
Modeling
Fundamentals
Performance
Rating Method
Best Practices
Inform Design
Measurement &
Verification
IBPSA-USA
WEATHER DATA
49
Modeling
Fundamentals
Performance
Rating Method
Best Practices
Inform Design
Measurement &
Verification
IBPSA-USA
WEATHER DATA
ANNUAL WEATHER FILES
• Necessary for annual energy and economic
analysis
• Useful for developing HVAC design strategies
• Must include 8760 hours
• Generally from sets of averaged data
50
Modeling
Fundamentals
Performance
Rating Method
Best Practices
Inform Design
Measurement &
Verification
IBPSA-USA
WEATHER DATA
ANNUAL WEATHER FILES
TMY = Typical Meteorological Year
• Data sets of hourly weather values for a 1-year period
• Produced from 30 years of data
• Representative of typical, rather than extreme, conditions
(not suitable for sizing systems)
• Intended use is for solar and building computer simulations
51
Modeling
Fundamentals
Performance
Rating Method
Best Practices
Inform Design
Measurement &
Verification
IBPSA-USA
WEATHER DATA
SOURCES FOR WEATHER DATA
TEMPERATURES AND DEW POINTS
• Design Conditions
40
30
– ASHRAE Handbook of Fundamentals
20
CELSIUS
• Weather Statistics & Observations
– National Climactic Data Center (U.S.)
– Mesowest (Southwest U.S.)
– Weather Bank (International)
10
0
-10
-20
1
2
3
4
Wind Direction Frequency
Typical
Meteorological
Year
5
6
7
8
9
10
11
MONTH
N
Average Dry Bulb Temperature C
• Annual Weather Data
Average Dew Point
Average Dry Bulb Temp F
360
15
30
– DOE-2 Website (TMY, WYEC, etc)
– EnergyPlus Website (EPW, CSV)
45
345
330
40
35
45
315
30
25
60
300
20
15
75
10
285
`
5
• International Weather Data
– EnergyPlus Weather Source Data
W
90
0
270
105
255
120
240
135
225
150
210
165
195
180
Modeling
Fundamentals
Performance
Rating Method
S
Best Practices
E
Inform Design
Measurement &
Verification
52
1