Lecture 5: Building Envelope Description (Part I)

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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Keywords Covered in this and next Lecture

BuildingZoneSurfaceGeometrySurface (all types)ConstructionMaterial:RegularMaterial:Regular-RMaterial:Air

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Definitions and Connections

Building: Entire collection of interior and exterior

features of the structure Buildings may consist of one or more zones

Zones: Group of surfaces that can interact with

each other thermally and have a common air mass at roughly the same temperature

One or more rooms within a building Zones may consist of one or more surfaces

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Definitions and Connections (cont’d)

Surfaces: Walls, Roofs, Ceilings, Floors, Partitions, Windows,

Shading Devices One or more surfaces make up a zone Surfaces consist of a series of materials called a

“construction” Construction:

Group of homogeneous one-dimensional material layers

Each surface must have a single construction definition

Each construction is made up of one or more materials

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Definitions and Connections (continued)

Materials: Define the thermal properties for

layers that are used to put together a construction

One or more material layers make a construction

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Envelope Hierarchy

Building

Zone ZoneZone … more zones

Surface SurfaceSurface Surface … more surfaces

Construction

MaterialMaterial Material Material … more materials

only one construction per surface

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More on Zones

Thermal zone definition very generic and does not answer the following questions: How many surfaces to a zone? How many zones should be defined

for a particular building? Should each room be a zone? Can the entire building be a zone?

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Defining Thermal Zones by Objective

Objectives of a study can dictate the size and number of thermal zones Air flow study: sizing fans and ducts

Several rooms per zone Zone per system type

“Block loads” or central plant study: sizing of heating and cooling producers Minimize number of zones (maybe only 1)

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Ft. Monmouth Education Center

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Defining Thermal Zones by Design Conditions

“T” test: if there is an air temperature difference between adjacent spaces, separate thermal zones are needed Might also be seen in different control

types

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Defining Thermal Zones by Design Conditions (cont’d)Space usage/internal gains test:

Differences in internal gains may result in different conditioning requirements or distribution Office vs. gymnasium

Space usage differences may alter the ventilation or exhaust requirements of a space Office vs. kitchen vs. chemistry laboratory

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Defining Thermal Zones by Design Conditions (cont’d)Environmental conditions test:

exposure to different thermal surroundings/quantifying the effect Different space orientations—solar

gains Exposure to the ground Exposure to the outdoor environment

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Ft. Monmouth Education Center

“T” test: loading dock

Space use: kitchen, dining area

Outdoor exposure: west wing solar

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Loads Features and Capabilities

How does EnergyPlus calculate what it will take to keep a zone at the desired thermal conditions? EnergyPlus contains the heat balance engine

from IBLAST, a research version of BLAST with integrated loads and HVAC calculation. The major enhancements of the IBLAST heat balance

engine include mass transfer and radiant heating and cooling

Essentially identical in functionality to the Loads Toolkit developed under ASHRAE Research Project (RP-987)

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Loads Features and Capabilities (cont’d)

Heat balance engine models room air as well-stirred with uniform temperature throughout.

Room surfaces are assumed to have: Uniform surface temperatures Uniform long and short wave irradiation Diffuse radiating and reflecting surfaces Internal heat conduction

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EnergyPlus Model For Building Loads

Heat Transfer (Diffusion and Storage)

Internal Radiation

Tair

Solar Beam

Infiltration (Sensible & Latent)

Diffuse Solar

Reflected Solar

Internal Radiation Convection

Conditioned Air

Return Air

Heat & Moisture

Source (People & Equipment)

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Equipment & People Loads

EquipmentOccupant

ConvectionRadiation

Sensible and Latent

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Loads Features and Capabilities (cont’d)

Three models connected to the main heat balance routine are based on capabilities from DOE 2 Daylighting simulation

Calculates hourly interior daylight illuminance, window glare, glare control, electric lighting controls, and calculates electric lighting reduction for the heat balance module

WINDOW 5-based window calculation Anisotropic sky

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Loads Features and Capabilities (cont’d)

Several other modules have been reengineered for inclusion in EnergyPlus: Solar shading from BLAST Conduction transfer function

calculations from IBLAST

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Loads Features and Capabilities (cont’d)

Incorporates a simplified moisture model known as Effective Moisture Penetration Depth (EMPD) Estimates moisture interactions among the

space air and interior surfaces and furnishings Estimates impacts associated with moisture

where detailed internal geometry and/or detailed material properties are not readily available

User may also select a more rigorous combined heat and mass transfer model

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Loads Features and Capabilities (cont’d)

Loads and systems portions more tightly coupled than in BLAST or DOE-2.

Loads calculated on a time step basis and passed directly to the HVAC portion.

Loads not met result in zone temperature and humidity changes for the next time step.

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Keyword: Building

IDD Description (shortened)

Purpose: to control basic information about the building location, its orientation, its surroundings, and some simulation parameters

BUILDING, A1 , \field Building Name N1 , \field North Axis A2 , \field Terrain N3 , \field Loads Convergence Tolerance Value N4 , \field Temperature Convergence Tolerance Value A3 ; \field Solar Distribution

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Keyword: Building

IDD Description (detailed) BUILDING, \unique-object \required-object \min-fields 6 A1 , \field Building Name \required-field \default NONE N1 , \field North Axis \note degrees from true North \units deg \type real \default 0.0

Keyword

User defined building name

Allows rotation of the entire building for the convenience of the user

North Axis Interpretation:

True NorthBuilding North

Angle is North Axis (+45 in this case)

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Keyword: Building

IDD Description (detailed, continued)

A2 , \field Terrain \note Country=FlatOpenCountry \note Suburbs=RoughWoodedCountryTownsSuburbs \note City=CityCenter \type choice \key Country \key Suburbs \key City \default Suburbs N3 , \field Loads Convergence Tolerance Value \units W \type real \minimum> 0.0 \default .04

Allows specification of immediate surroundings of the building

Options and their approximate descriptions

Advanced user feature that should be left as the default in most cases

Note: Terrain mainly affects exterior convection correlations

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Keyword: Building

IDD Description (detailed, continued) N4 , \field Temperature Convergence Tolerance Value

\units deltaC \type real \minimum> 0.0 \default .4 A3 ; \field Solar Distribution \note MinimalShadowing | FullExterior \note FullInteriorAndExterior \type choice \key MinimalShadowing \key FullExterior \key FullInteriorAndExterior \default FullExterior

See next two slides for descriptions

Advanced user feature that should be left as the default in most cases

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Solar Distribution Options

Minimal Shadowing No exterior shadowing except from door and

window reveals All direct beam solar radiation incident on floor If no floor, direct beam solar distributed to all

surfacesFull Exterior

Exterior shadowing caused by detached shading, wings, overhangs, and door and window reveals

All direct beam solar radiation incident on floor

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Solar Distribution Options (cont’d)

Full Interior and Exterior Exterior shadowing same as Full Exterior Direct beam solar radiation falls on all

surfaces in the zone in the direct path of the sun’s rays

Solar entering one window can leave through another window

Zone must be convex: A line passing through the zone intercepts no more

than two surfaces An L-shaped zone is not convex

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Convex Zones

Convex zones Non-Convex zones

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Keyword Example: Building

IDF Example

or BUILDING, NONE, 0.0, Suburbs, 0.4, 0.4, FullExterior;

BUILDING, NONE, !- Building Name 0.0, !- North Axis {deg} Suburbs, !- Terrain 0.4, !- Loads Convergence Tolerance Value {W} 0.4, !- Temperature Convergence Tolerance Value {C} FullExterior; !- Solar Distribution

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Summary

EnergyPlus input files contain a hierarchy of envelope input that includes the Building, Zone, Surface, and Construction definitions

Simulation of the building envelope based on a heat balance applied to a thermal zone

Buildings consist of one or more thermal zones—number of zones based on various factors including space usage, environmental conditions, etc.

EnergyPlus provides access to more detailed simulation of daylighting, windows, moisture, etc.