Lecture 7: Building Modeling Questions
Material prepared by GARD Analytics, Inc. and University of Illinoisat Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights reserved
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Importance of this Lecture to the Simulation of Buildings
Every building is different in many ways: Location/exterior environment Construction/building envelope HVAC system
Building envelope/construction determines how a building will respond to the exterior environment
Thermal simulation requires information about the physical make-up of the building, where various constructions are located and how they are oriented, how the building is subdivided into zones, etc.
Thermal simulation requires information on the building envelope to properly analyze the building from an energy perspective
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Purpose of this Lecture
Gain an understanding of how to specify the building construction Groups of Surfaces (Zones) and
Overall Building Characteristics Walls, Roofs, Ceilings, Floors,
Partitions, etc. Materials and Groups of Materials
(Constructions)
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Potential Questions You Might Have
Is every room a zone? How many zones?
How detailed should the building model be?
How accurate will my results be?Do I need to do a design day run or
an annual run?
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Defining a Building
Getting Started Manual A methodology for using EnergyPlus
Four Step Process Gather information Zone the building Create building model Create input file
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Step 1 - Gather Information
Location and design climateBuilding description
Wall constructions Wall sizes Window, door, overhang details Wall locations (shading)
Building use information Equipment and occupancy information Schedule information
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Step 1 - Gather Information (cont’d)
Building thermostatic controlsHVAC equipment information
Equipment types Operating schedules Control information
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Step 2 – “Zone” the Building
Thermal, not geometric, zonesHeat storage and heat transfer surfacesHeat transfer only when expected to
separate spaces of significantly diff temps Exterior Walls, Roofs, Floors
Heat storage surfaces surfaces separating spaces of same temperature
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Simplifying EnergyPlus Input
Simplify -- Think before typingLayout simple floor planAs few zones as necessaryAs few surfaces as necessarySurfaces, NOT volumesIs shading important?
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As Few Zones As Necessary
Combine similar zonesUse zone multipliers wiselyCombine vertically and horizontally
10 ZONES OR 6 OR 4 OR 2?
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Rules of Thumb Reminder
One zone per major exposure minimum
Separate zones for different usesSeparate zones for different setpointsSeparate zones for different fan
systems (and radiant systems) Do not use “rooms” to determine
zones
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Step 3 - Create Building Model
Heat transfer and heat storage surfaces
Define equivalent surfacesSpecify construction elementsCompile surface and subsurface
infoCompile internal space gain data
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As Few Surfaces As Necessary
Combine similar surfacesCombine small surfaces with larger
surfacesIgnore minor detailsUse internal mass
7 WINDOWS OR 3?
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Step 4 – Create Input File
Materials and ConstructionsBuilding GeometryInternal LoadsSpecial Features
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Case Study
US Army Fort Monmouth education center
Temperate coastal climate, Near New York City
Floor area of over 13,000 sq.Ft.Building height of 10 ft.Total window area in excess of 1,400
sq.Ft.May serve as many as 200 people
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Ft. Monmouth Floor Plan
How many zones should there be?
1 2
3 4
5 6
7 8
9 10 11
12
13 14 15 16
17
18
19
20
21
22
23 24 25
26
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Option 1: One-Zone Model
How accurate is this model?
50 ft
16 ft
113 ft
43.3 ft
34 ft
65 ft
39 ft
65 f
t
20 ft
10 f
t
20 ft
75.3 ft
50 ft
124.
6 ft
(61 ft2)
(42 ft2)
(363 ft2)
101 ft2)
(82 ft2)
(113 ft2)
(62 ft2)
(190
ft2
)
(26 ft2)
(40 ft2)
(209 ft2)
(84 ft2)
(334
ft2
)
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Option 2: Six-Zone Model
Five fan systems or zoning thermally
Expect higher solar on south and west
Zone 1
Zone 5
Zone 4Zone 2 Zone 6
Zone 3
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Modeling Fort Monmouthwith EnergyPlus
With appropriate detail: EnergyPlus can convert a simple model into a
powerful energy analysis Complex interactions modeled for an entire
yearDesigners can then:
Size systems and plants Examine performance of various system and
plant configurations Determine more efficient operational schemes Calculate annual energy consumption
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Six-Zone Model Loads
How does this compare to 1-zone model?
181716151413121110987650
10
20
30
40
50
60
70
80
90
Zone 1
Zone 2
Zone 3
Zone 4
Zone 5
Zone 6
Hours
Co
oli
ng
Loa
d [
kB
tu/H
r]
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Comparison BetweenOne and Six-Zone Models
232119171513119753110
50
100
150
200
250
300
1 Zone Cooling
6 Zone Cooling
Hours
Cooli
ng L
oa
d
[Kb
tu/h
r]
Difference in Total Cooling Load < 10%Difference in Total Heating Load < 1%
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Simple EnergyPlus ModelProduces Incredible
ResultsWhy? EnergyPlus captured the
physics ... Building exterior remains the same Solar load equivalent Internal loads unchanged Internal mass accurately approximated Identical weather conditions
Difference: unconditioned spaces
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Detailed Model Benefits
Improved accuracyBetter resolution of loads for
system sizingIncredible analytical power
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Another Aspect to Consider ...
How much of an effect does the thermal mass of zone surfaces have on zone loads?
Comparison using Ft. Monmouth six zone model Standard EnergyPlus run EnergyPlus run using no thermal mass (R
values)Use output reports from previous run to
change the surface definitions to R values only
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Key Physical Properties
Exterior Walls 4” Dense Face Brick 8” Heavyweight
Concrete Block 6” Mineral Fiber
Insulation 5/8” Gypsum
Slab on Grade Floor 4” Concrete Tile Flooring
Roof 3/4” Roofing 2” Expanded
Polystyrene Insulation Airspace 3/4” Acoustic Tile
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Case 1:Thermal Mass Effects
Cooling loads higher with no mass
Total load off by 14% Peak off by 15%
Larger differences show up in zones 1, 2, and 3
Could result in oversizing of systems and plants
Only thermal mass changed
Other EnergyPlus details a factor
Cooling Loads
No-Mass vs. Mass
Zone Total Peak
1 12% 32%
2 16% 31%
3 16% 40%
5 14% 16%
6 15% 14%
All 14% 15%
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Design Day Calculations
Convenient short time periodEstablished design day conditions
easy to obtainFairly good estimate for system
and plant sizingWill design day results be an
accurate indication of long term trends?
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Case 2: Adding Roof Insulation
What will the effect of doubling the amount of roof insulation be?
Roof 3/4” Roofing 2” Expanded Polystyrene Insulation Airspace 3/4” Acoustic Tile
Will a design day tell the whole story?
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Design DayHeating Load Results
23211917151311975311150
200
250
300
350
400
4in Insulation Roof
2in Insulation Roof
Hours
Hea
tin
g L
oa
d [
Kb
tu/h
r]
Daily Decrease for Heating Loads = 8%
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Design DayCooling Load Results
Daily Decrease for Cooling Loads = 3%
232119171513119753110
100
200
300
4in Insulation Roof
2in Insulation Roof
Hours
Cooli
ng
Load
[K
btu
/hr]
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Annual Run Building Loads
Why are the cooling loads higher with more insulation?Mild summer + High MRT =
High summer heat retention
Overall reduction in loads, but not as expected from design day results
Heating
Load
Cooling
Load
Peak
Heating
Peak
Cooling
Total
Loads
Cheap Roof 468000 147500 588 289 615500
Better Roof 417700 150700 561 282 568400
Annual Diff's 10.75% -2.17% 4.66% 2.42% 7.65%
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Let's Change the Weather. . .
Champaign, IllinoisTemperate inland climate, south of
ChicagoCompare increased roof insulationDesign day heating and cooling loads
both decreasedAnnual building loads also decreased EnergyPlus "changed" the weather for
every hour of the yearEnergyPlus never forgets the physics!
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Summary
Simple models can produce good results
Thermal mass can have a significant effect on loads
Design day calculations can be misleading
Annual runs pick up mild weather effects