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