MODELING FUNDAMENTALS
IBPSA - USA
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYGENERAL CONCEPTS
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYUSE OF ENERGY MODELING WIZARDS
In what cases are energy modeling wizards most useful?
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
After making edits in main program
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYRULES OF THUMB FOR SIMPLIFICATION
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˚
Simplify
REALITY ENERGY MODEL
• Thermodynamically, only (3) things matter for modeling heat transfer surfaces1. Area2. Orientation3. Tilt
• Total volume matters IF infiltration is specified in ACH
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYRELATIVE PLACEMENT OF SURFACES
What Matters• Area• Orientation• Tilt
Note: With daylighting the building form is important5
Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYRELATIVE PLACEMENT OF SURFACES
Annual Energy by Enduse Annual Energy by Enduse
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USA
SketchUp Plugins• Open Studio for EnergyPlus• IES Virtual Environment
CAD (dwg files)• 2-D CAD plans may be imported
into energy modeling programs• gbXML streamlines the transfer of
building information to and from engineering models
SHELL GEOMETRYGEOMETRY INTERFACES
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYGEOMETRY 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
• Automatic model generation from 3D renderings• Architects/engineers will specify “properties” of materials and
equipment for automatic modelingGoals
• BIM needs work in some segments (i.e. electrical engineering)• Danger of “black box” energy modelingBarriers
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USASHELL GEOMETRYASHRAE 90.1 APPLICATIONS
The number of floors and conditioned floor area shall be identical.
Total gross areas of exterior, opaque surfaces shall be
identical.
The baseline building shall be modeled so that
it does not shade itself.
Vertical fenestration areas for the baseline shall equal the
smaller of:• the proposed design, OR
• 40% of gross above grade wall area
Glazing shall be distributed on each face of the baseline
building in the same proportions in the proposed
design.
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USAEFFECTIVE ZONINGGENERAL 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 multi-
zone systems
Zoning in simulation models can differ from
actual HVAC zoning
# of model zones < # of HVAC zones
Energy model zones are abstract
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USAEFFECTIVE ZONINGCRITERIA 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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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USAEFFECTIVE ZONINGSPACES VERSUS THERMAL ZONES
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.
Thermal Zone = area controlled by a single thermostat
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAEFFECTIVE ZONINGZONE 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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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
Parallel Path Calculations for a Stud Wall
CONSTRUCTIONSOVERVIEW
Types of Constructions
Quick vs. D e l a y e d
Exterior Opaque (walls, roofs,
slabs, underground
walls, etc)
Interior (mass, air, layers, etc) Exterior Glazed
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
Material Properties
• Conductivity• Density• Specific Heat• Thickness
Layers
• Materials are “layered” from outside to inside
• Outside and inside air films
Constructions
• Layers determine U-value
• Surface Roughness
• Solar Reflectivity
CONSTRUCTIONSEXTERIOR (DELAYED) CONSTRUCTIONS - OPAQUE
What about construction assemblies with parallel heat transfer paths?
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
( )
(
) = x +
x 0.09
R-Value of Insulated Section
= + + + +
R-Value (brick)
R-Value (Sheathing)
R-Value (Insulation)
R-Value (Gyp. Board)
R-Value (Inside Air Film)
R-Value of Stud Section = + +
+ +
R-Value (brick)
R-Value (Sheathing)
R-Value (Insulation)
R-Value (Gyp. Board)
R-Value (Inside Air Film)
Overall Weighted R-Value of Wall Assembly 0.91
R-Value of Stud Section
R-Value of Insulated Section
CONSTRUCTIONSPARALLEL PATH CALCS FOR WOOD STUD WALL
ORNL Online Calculator
Typical Stud WallWall Section
ASHRAE 90.1 Appendix A
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USA
Slab Heat Transfer
Underground Surfaces: How to get a better underground heat transfer calculation in DOE-2.1 by Fred Winkelman
CONSTRUCTIONSSLAB HEAT TRANSFER
Do you need to perform outside calculations?
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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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USACONSTRUCTIONSGLAZING 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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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USA
Simplified Library Glazing
Window 6 (LBNL)
3 Options for Modeling Glazing
Includes Spectral Data: varies SHGC and Tvis with solar angles
CONSTRUCTIONSGLAZING CONSTRUCTIONS
SHGC = solar heat gain coefficientTvis = visible light transmission
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USACONSTRUCTIONSWINDOW FRAMING
Include framing effects in glazing construction
• Model large bands of glass, OR
• Model windows individually
Model framing explicitly
• Works well with Window 6 option
• Use window multipliers
2 Options for Modeling Framing
Common Pitfall:Window 6 does not
include framing when you export files
Common Pitfall:Modeling large bands of
glass
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USALIGHTING OCCUPANCY & PLUG LOADSGENERAL CONCEPTS
• Daily/Weekly/Annual Occupancy Schedules• Hourly fractional multiplier for peak values• Daylight Dimming or Occupancy Sensors
• Total watts of all connected power• Peak number of occupants• Can be input with density values
• Assign proportional amounts of heat to space vs. plenum
Peak Power and Occupancy
Fractional Schedules
Fraction of Heat Gain to space
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USALIGHTING OCCUPANCY & PLUG LOADS PEAK POWER AND OCCUPANCY
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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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
1 3 5 7 9 11 13 15 17 19 21 230%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%Lighting
WkSatSun
• Just as important as peak values!
• Unregulated by ASHRAE Std 90.1
LIGHTING OCCUPANCY & PLUG LOADS - SCHEDULES
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USALIGHTING OCCUPANCY & PLUG LOADS - FRACTION OF HEAT GAIN TO SPACE
Radiative (time lag) vs. Convective
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USALIGHTING OCCUPANCY & PLUG LOADS DAYLIGHTING
Direct daylighting within energy model
• Limited daylight simulation engine
• Know the limits on the number of light bounces and interreflectivity
• Carefully specify controls
Daylight Specific Tool
• Generally more accurate, but requires parallel model
• SPOT and Radiance can generate hourly electric lighting reduction schedules for import into energy models
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USALIGHTING OCCUPANCY & PLUG LOADS EXTERIOR LIGHTING
Exterior lighting is modeled separately from interior lighting
Can be controlled via photosensors or with schedules
HID vs LED
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USALIGHTING, OCCUPANCY & PLUG 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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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
Verification
IBPSA-USAMECHANICAL SYSTEMSOVERVIEWGain and Losses: Lights People Internal equipment (e.g. computers) Building envelope (sun, outside temps) Ventilation/infiltrationQ=Σ gains + losses + ventilation load
Equipment SizingQ = (1.08)*cfm*(MAT-SAT)
Q = 500 * ΔT * GPM
air
water
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSCOOLING 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
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSPACKAGED & CENTRAL PLANT SYSTEM DIAGRAMS
Packaged System
compressorsupply
fan
condenser
Central Plant
Air Side
Water Side
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSPACKAGED SYSTEMS
Air-Cooled Condensers• Split DX
systems• Package DX
systems• DX computer
room air conditioners (CRACs)
Water-Cooled Condensers• Dry coolers or
closed-loop cooling towers
• Cooling towers
Evaporatively-Cooled Condensers• Direct evaporative
package units• Indirect/direct
evaporative package units
Heating Systems• Electric
baseboard heaters
• Oil and gas-fired furnaces
Ground-Source• Air heat
pumps• Water heat
pumps
Packaged systems can serve single or multiple zonesEnergy Modeling Tip: Do not double count fan, compressor and condenser power
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSCENTRAL PLANT SYSTEMS
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
Central plant systems typically serve multiple zonesEnergy Modeling Tip: Pay attention to pump power
and part load curves
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSTERMINAL UNITS
Important Inputs• Min. airflow fraction
– Fixed or scheduled
• Thermostat type– Proportional vs.
reverse acting
• Terminal unit fan power
Variable airflow
Standard VAV box with reheat coil
Constant airflow, fan always on
Variable airflow, fan on when reheat needed
Parallel fan-powered VAV box
Series fan-powered VAV box with reheat coil
Reference: Advanced VAV Design Guideline, Appendix 8 How to Model Different VAV Zone Controls in DOE2.2www.energydesignresources.com
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
Source: DOE2.2 Volume 2 Dictionary
MECHANICAL SYSTEMSFAN 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
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 20% 40% 60% 80% 100%
Fan
Po
wer
PLR
Airf low Part Load Ratio
DOE2.2 standardVSD curve
Std 90.1 App G VSD curve
MECHANICAL SYSTEMSFAN CURVES – 90.1 APPENDIX G CURVE
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USA
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 20% 40% 60% 80% 100%
Fan
Po
wer
PLR
Airf low Part Load Ratio
No SP Reset
Good SP Reset
Perfect SP Reset
MECHANICAL SYSTEMSFAN CURVES – STATIC PRESSURE RESET CONTROL
• Static Pressure (SP) Reset
– Continuously adjust pressure to lowest setting that provides adequate zone airflow
– Simulate using fan curve
Reference: Advanced VAV Design Guideline, Appendix 5
Includes fan curve coefficients
www.energydesignresources.com
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSCHILLER CURVES
• Chiller performance model– Capacity = f(temp)– Efficiency = f(temp, part-load ratio)
• Represent chiller types– Centrifugal, rotary, reciprocating…– Variable speed, multi-compressor…
• Default vs. custom coefficients
ReferenceCoolTools Chilled Water Design Guide.
Chiller Bid and Performance Tool, (Excel spreadsheet).
www.energydesignresources.com
IssuesPart load efficiency curve typically includes PLR:
(EIR = energy input ratio = 1/COP)Load Full
Load PartPLRdT)EIRf(PLR,EIREIR
1.0 at full load and
rated temp.
dT)EIRf(PLR, EIRf(T)
CAPf(T) EIR CapElec
Load Full
FullLoadin
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Modeling Fundamentals
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IBPSA-USAMECHANICAL SYSTEMSOUTSIDE 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)
Ventilation Rate Procedure
Indoor Air Quality (IAQ) Procedure Natural Ventilation
ASHRAE 62.1
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Modeling Fundamentals
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IBPSA-USAMECHANICAL SYSTEMSASHRAE 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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Modeling Fundamentals
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IBPSA-USAMECHANICAL SYSTEMSASHRAE 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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Modeling Fundamentals
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IBPSA-USAMECHANICAL SYSTEMSASHRAE 62.1: NATURAL VENTILATION PROCEDURE
62.1-2010 requires mechanical ventilation UNLESS– OA passages are
permanently open, OR – NO heating or cooling
system is installed
Controls required for coordination with mechanical ventilation systems
Prescriptive requirements Ceiling height Openable passages ≥ 4% of
floor area
OR Engineered system with CFD
modeling
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Modeling Fundamentals
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IBPSA-USAMECHANICAL SYSTEMSDEMAND CONTROL VENTILATION (DCV)
• Ventilation airflow resets based on occupancy usingCO2 sensors, timers, occupancy sensors or schedules
• Higher energy savings for buildings with large occupancy swingsMovie theaters, conference rooms
• 10%-30% load reduction and 2-3 yr payback
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IBPSA-USA
Possible to assess within energy models that accurately simulate
radiative heat transfer
Indoor Environment and Personal
Factors
Clothing Insulation
Metabolic Rate
Air Temp
Radiant Temp
Air Speed
Humidity
MECHANICAL SYSTEMSASHRAE STANDARD 55
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Modeling Fundamentals
Performance Rating Method Best Practices Inform Design Measurement &
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IBPSA-USAMECHANICAL SYSTEMSSPECIFIC ENERGY MODELING NOTES
EER: break out fan power and compressor power
Part load curves
Altitude effects
Auto-sizing
Rated vs design conditions
Common Energy Modeling Mistakes
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IBPSA-USAUTILITY RATESTYPES OF CHARGES AND RATE STRUCTURES
• Fixed fee for providing energy servicesMonthly Charge
• Unit cost for total quantity of energy consumedEnergy Charge
• Fee for highest or peak amount of energy usedDemand Charge
• Penalty for lower than optimum power factorPower Factor Charge
• Unit charge based on different blocks of energy use or demandBlock Charge
• Prices change during peak and off-peak timesTime of Use Rate
0–350 kWh $0.06 per kWh
350–700 kWh $0.04 per kWh
700+ kWh $0.02 per kWh
$0.40 per KVAR
$0.06 per kWh
$35 per month
$7.53 per kW
Peak Time $0.24 per kWh
Off Peak Time $0.06 per kWh
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Modeling Fundamentals
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IBPSA-USAUTILITY RATESTYPES OF CHARGES AND RATE STRUCTURES
Summer (June-Sept)
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
Winter (Oct-May)
All days All Hours $0.03 per kWh
Time of Use Rate
Block 1
Block 2
Block 3
Energy Charge Block ChargeDemand Charge
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Modeling Fundamentals
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IBPSA-USAUTILITY RATESENERGY MODELING IMPLICATIONS
• 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
ASHRAE 90.1-2007 Appendix G Applications
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IBPSA-USAUTILITY RATESENERGY 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!
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IBPSA-USA
WEATHER DATA
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IBPSA-USAWEATHER DATAANNUAL 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
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IBPSA-USAWEATHER DATAANNUAL 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
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TEMPERATURES AND DEW POINTS
-20
-10
0
10
20
30
40
1 2 3 4 5 6 7 8 9 10 11 12
MONTH
5
15
25
35
45
55
65
75
WEATHER DATASOURCES FOR WEATHER DATA
• Design Conditions– ASHRAE Handbook of Fundamentals
• Weather Statistics & Observations– National Climactic Data Center (U.S.)– Mesowest (Southwest U.S.)– Weather Bank (International)
• Annual Weather Data– DOE-2 Website (TMY, WYEC, etc)– EnergyPlus Website (EPW, CSV)
• International Weather Data– EnergyPlus Weather Source Data
Wind Direction FrequencyTypical Meteorological Year
0
5
10
15
20
25
30
35
40
45360
345
330
315
300
285
270
255
240
225
210
195180
165
150
135
120
105
90
75
60
45
30
15
`
N
W
S
E
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