modeling fundamentals

MODELING FUNDAMENTALS

IBPSA - USA

1

Erik Kolderup

Are the simulation files referenced in the notes available?

Modeling Fundamentals

Performance Rating Method Best Practices Inform Design Measurement &

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IBPSA-USASHELL GEOMETRYGENERAL CONCEPTS

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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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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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IBPSA-USASHELL GEOMETRYRELATIVE PLACEMENT OF SURFACES

What Matters• Area• Orientation• Tilt

Note: With daylighting the building form is important5



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IBPSA-USASHELL GEOMETRYRELATIVE PLACEMENT OF SURFACES

Annual Energy by Enduse Annual Energy by Enduse

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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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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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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.

9

Erik Kolderup

this slide could be skipped or moved to PRM section



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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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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)11



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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

12

Erik Kolderup

This slide is a bit confusing. Perhaps delete.



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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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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

14

Erik Kolderup

Add underground surfaces as a fourth construction type.



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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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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

16

Erik Kolderup

Should note that this method works only for wood-framed walls.

http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/Calc.htm

http://www.ornl.gov/sci/roofs+walls/AWT/InteractiveCalculators/NS/Calc.htm



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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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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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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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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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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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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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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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IBPSA-USALIGHTING OCCUPANCY & PLUG LOADS - FRACTION OF HEAT GAIN TO SPACE

Radiative (time lag) vs. Convective

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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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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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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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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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IBPSA-USAMECHANICAL SYSTEMSCOOLING AND HEATING LOADSMechanical 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

k29



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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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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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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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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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http://www.energydesignresources.com/



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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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IBPSA-USA

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Fan

Pow

er P

LR

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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IBPSA-USA

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0% 20% 40% 60% 80% 100%

Fan

Pow

er P

LR

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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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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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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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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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

qualityMass balance

analysisOccupant evaluation

40

Erik Kolderup

delete? doesn't seem relevant to modeling.



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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 areaOR Engineered system with CFD

modeling

41

Erik Kolderup

perhaps delete. also not as relevant to modeling. Or explain how it is relevant.



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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

42

Erik Kolderup

Discuss modeling issues. Use of schedule for min vent rate to meet standard 62 min rate per sf.



Verification

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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IBPSA-USAMECHANICAL SYSTEMSSPECIFIC ENERGY MODELING NOTES

EER: break out fan power and compressor power

Part load curvesAltitude effectsAuto-sizingRated 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 kWh350–700 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 kWhOff Peak Time $0.06 per kWh

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IBPSA-USAUTILITY RATESTYPES OF CHARGES AND RATE STRUCTURES

Summer (June-Sept)

Peak 1pm–6pm (M-F) $0.16 per kWhMid 11am–1 pm and 6pm–8 pm (M-F) $0.06 per kWhOff 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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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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IBPSA-USA

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

345330

315

300

285

270

255

240

225

210195

180165

150

135

120

105

90

75

60

45

3015

`

N

W

S

E

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http://www.ncdc.noaa.gov/oa/ncdc.html

http://mesowest.utah.edu/index.html

http://www.weatherbank.com/archive.html

http://doe2.com/index_Wth.html

http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm

http://apps1.eere.energy.gov/buildings/energyplus/weatherdata_sources.cfm

modeling fundamentals

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