Lecture 11: Air Movement in Buildings

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

Air is critical to sustaining life and also to the thermal environment inside a building (comfort)

While a zone is defined by a common air mass at a particular temperature, air is not static within a building

Unintended flow into/out of the building Purposeful flow into/out of the building Exchange between interior spaces

Air movement may result in energy transfer from one area to another and thus has an energy impact on the zone and/or building

Understanding the energy impact of air movement is critical to our understanding of how much energy is required to maintain thermally comfortable conditions within a space

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Purpose of this Lecture

Gain an understanding of how to: Define air movement between interior

spaces Approximate the effect of ventilating

a building (naturally or mechanically) Perform detailed air movement

calculations using the EnergyPlus link to COMIS

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Keywords Covered in this Lecture

MixingCross MixingVentilationCOMIS Input Syntax

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Mixing

Used to move air from one zone to anotherOnly has an impact on the receiving zone—

user must account for energy impact on source zone through Mixing, Infiltration, etc.

Can be used to set-up multiple air flow paths or a “circular” path between more than two zones

User must specify flow rates and schedule

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Mixing (cont’d)

IDD Description:MIXING,A1 , \field Zone NameA2 , \field SCHEDULE NameN1 , \field Design Level of Air Flow in m3/sA3 , \field Source Zone NameN2 ; \field Delta Temperature in degrees Celsius

“Delta Temperature” controls when mixing air from the source zone is sent to receiving zone; if positive, the temperature of zone from which air is drawn must be T warmer than receiving zone air or no mixing occurs; if negative, the temperature of source zone must be T cooler than receiving zone air or no mixing occurs; if T is zero, mixing occurs regardless of the relative zone temperatures

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Cross Mixing

Used to exchange equal amount of air between two zones

Has an (equal) energy impact on both zones

Only needs to be defined once for one of the two zones

Only one cross mixing statement per zone If mixing occurs with more than one zone,

must use MixingUser must specify flow rates and schedule

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Cross Mixing (cont’d)

IDD Description:CROSS MIXING,A1 , \field Zone NameA2 , \field SCHEDULE NameN1 , \field Design Level of Air Flow in m3/sA3 , \field Source Zone NameN2 ; \field Delta Temperature in degrees Celsius

Delta Temperature controls when mixing air from the source zone is sent to the receiving zone.; if positive, the temperature of zone from which air is being drawn (“source zone”) must be T warmer than the zone air or no mixing occurs; if zero, mixing occurs regardless of the relative air temperatures; negative values for “Delta Temperature” are not permitted

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Mixing and Cross Mixing Examples

Zone 1 Zone 2

Zone 3 Zone 4

mixing

mix

ing

mixingm

ixin

g

Zone 1 Zone 2

Zone 3 Zone 4

crossmixing

crossmixing

Zone 1 Zone 2

Zone 3 Zone 4

mixing

Zone 1 Zone 2

Zone 3

crossmixing

OK OK

OK

Illegal

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Ventilation (Simple)

Intent is to allow simple mechanical ventilation without specifying an HVAC system or natural ventilation

Amount of ventilation determined by user defined design flow rate, schedule, and equation similar to infiltration (allows for variation based on temperature difference and wind speed)

Type and control of ventilation determined by ventilation specific parameters

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Ventilation (Simple, cont’d)

Control Parameters: Minimum Temperature: indoor (zone) air temperature

below which ventilation is shut off Delta Temperature: temperature differential between

inside (zone) and outside air below which ventilation is shut off (negative values allowed)

Advantage: can take effect of natural or simple forced ventilation into account without a lot of input

Disadvantage: user must define the air flow rate (will not figure out how much air flow there will be)

Use COMIS for more serious studies of air movement within the building and between inside and outside

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Ventilation (Simple, cont’d)

Example from an IDF file:VENTILATION, !- Ventilation is specified as a design level which is modified !- by a schedule fraction, temperature difference and wind speed: !- Ventilation = Vdesign * Fschedule * !- (A + B*|Tzone-Todb| + C*WindSpd + D * WindSpd**2) West Wing, !- Zone Name CONSTANT, !- SCHEDULE Name (Fschedule in Equation) 0.12, !- Design Volume Flow Rate in m3/s (Vdesign in Equation) 0.0, !- Delta Temperature in degrees C Natural, !- Ventilation Type (Natural | Intake | Exhaust) 0.0, !- Fan Pressure Rise in Pa 0.0, !- Fan Total Efficiency 1.0, !- Constant Term Coefficient (“A” in Equation) 0.0, !- Temperature Term Coefficient (“B” in Equation) 0.03, !- Velocity Term Coefficient (“C” in Equation) 0.0; !- Velocity Squared Term Coefficient (“D” in Equation)

could be used for stack ventilation

could be used for cross ventilation

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Ventilation (Simple, cont’d)

A few thoughts on how to get the design flow rate for natural ventilation…

ACH: consider how many air changes per hour you might expect for natural ventilation (somewhere between infiltration and fan driven flow, probably closer to infiltration)

Window area and velocity: area times velocity is volumetric flow rate

Determine window opening area (not necessarily the same as window area—depends on window type)

Multiply by some “standard” velocity (you will use the velocity coefficients so consider 1.0m/s)

Reduce this number to account for the fact that the velocity of air at the window will not be the same as the velocity of the air at the weather station and the fact that the air must go through the building

Adjust the temperature and wind speed parameters in the ventilation input to account for potential variations due to stack effect and/or wind effect on cross ventilation (can only estimate this without more detailed simulations)

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COMIS Multizone Airflow

COMIS Conjunction Of Multizone Infiltration Specialists

COMIS model incorporated into EnergyPlusMultizone airflow driven by external wind

and stack effectDoes not model HVAC system impactComputes infiltration and interzone flows

which are passed to the thermal simulationSee Input Output Reference - Airflow

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Overview of the COMIS/EnergyPlus Link

COMIS was developed in 1994 as a stand-alone multizone air flow program with its own input and output processors. In the COMIS/EnergyPlus link, COMIS is called each time step by the EnergyPlus program. Using inside and outside temperatures and the wind pressure distribution at the beginning of a time step, COMIS calculates air flows through cracks and large openings (such as open windows) between outside and inside and from zone to zone. These are then used by the EnergyPlus thermal calculation to determine surface temperatures and zone air temperatures for that time step (which are then used in the next time step to calculate new air flow values, and so on)

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The COMIS Input Objects Airflow Model COMIS Simulation COMIS Zone Data COMIS Surface Data COMIS Standard Conditions for Crack Data COMIS Air Flow:Crack COMIS Air Flow:Opening COMIS Site Wind Conditions COMIS External Node COMIS CP Array COMIS CP Values

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Input Object Description COMIS Simulation defines basic run parameters for the

air flow calculation and specifies whether wind pressure coefficients are input by the user or, for rectangular buildings, calculated by the program (New Feature for Version 1.1.1).

COMIS Zone Data object specifies the ventilation control that applies to all of the openable exterior windows and doors in the corresponding thermal zone.

COMIS Surface Data indicates whether a heat transfer surface (wall, window, etc.) has a crack or opening and references a COMIS Air Flow:Crack or COMIS Air Flow:Opening object that gives the air flow characteristics of that crack or opening. COMIS Surface Data can also be used to specify individual ventilation control for openable exterior windows and doors.

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Input Object Description (Cont.)

COMIS Standard Conditions for Crack Data is used to normalize crack information that is based on measurements of crack air flow.

If wind pressure coefficients are input by the user, COMIS Surface Data also has an associated COMIS External Node, that, via the COMIS Site Wind Conditions, COMIS CP Array and COMIS CP Values objects, gives the wind pressure distribution vs. wind direction for that node and, implicitly, for the cracks and openings in the exterior surfaces associated with that node.

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Relationship Among COMIS Objects

COMIS External Node

COMIS CP Values

COMIS CP Array

Surface:HeatTransfer orSurface:HeatTransfer:Sub

COMIS ObjectsRegular EnergyPlus Objects

COMIS Simulation

COMIS Site Wind Conditions

COMIS Standard Conditions for Crack Data

COMIS Zone Data

COMIS Surface Data

COMIS Air Flow:Crack orCOMIS Air Flow:Opening

Zone

Schedule(of ventingtemperatures)

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What COMIS/EnergyPlus Can Do

Air flow through cracks in exterior or interzone surfaces Air flow through cracks around windows and doors Natural ventilation, i.e., air flow through open (or partially

open) exterior windows and doors Control of natural ventilation based on inside/outside

temperature or enthalpy difference Modulation of natural ventilation to prevent large

temperature swings Interzone air flow, i.e., air flow through open interzone

windows and doors, and through cracks in interzone surfaces Account for how air flow depends on buoyancy effects and

wind pressure Account for how wind pressure depends on wind speed, wind

direction and surface orientation

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What COMIS/EnergyPlus Cannot Do

Account for the effect of supply-air and/or return-air flows in a zone when an HVAC air system is present and is operating. This means that the COMIS air flow simulation will give reliable answers only if there is no HVAC system, the HVAC system is off, or the HVAC system is hydronic. Air flow through cracks around windows and doors.

Air circulation and/or air temperature stratification within a thermal zone. For example, you should not try to divide a high space, such as an atrium, into subzones separated by artificial horizontal surfaces that have cracks or openings with the expectation that COMIS/EnergyPlus will give you a realistic temperature in each subzone and/or a realistic air flow between subzones.

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What COMIS/EnergyPlus Cannot Do (Cont.)

Bi-directional flow through large horizontal openings. See discussion below under COMIS Air Flow:Opening.

Flow through ducts or other elements of an HVAC air system.

Pollutant transport. There are some pollutant-related inputs but they are not used.

Air-flow networks that are not connected. This means you cannot model air flow in two or more separate groups of zones.

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Simple COMIS Air Flow Network

Zone-1

Window-2

Zone-2

Zone-3

Window-1

Window-3

Door-12

Door-23

ExternalNode-1

ExternalNode-2

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Illegal COMIS Air Flow Network

Window-1 Window-2

Window-3Window-4

Door-23

Zone-1 Zone-2

Zone-3

ExternalNode-1

ExternalNode-2

ExternalNode-3

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Correcting the Illegal COMIS Air Flow Network

The previous slide shows an Air-flow network that is illegal in COMIS because there are two separate groups of zones with air flow (one group is Zone-2 plus Zone-3 and the other is Zone-1). To make this legal a link (a crack or opening) between Zone-1 and Zone-2 would have to be added or the zones in one of the groups would have to be “turned off” as COMIS zones.

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New COMIS Feature Version 1.1.1 Release

For rectangular buildings EnergyPlus will automatically calculate surface-averaged Cp values for the walls and roof of the building if, in COMIS Simulation, you specify Wind Pressure Coefficients = SURFACE-AVERAGE CALCULATION. In this case you do not have to enter any COMIS CP Values objects.

If not calculated by program, Cp values can be obtained from wind tunnel measurements, CFD calculations, or from published values for different building shapes.

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COMIS Input

AIRFLOW MODEL, COMIS; !- AirFlowModelValue

COMIS SIMULATION, VENT, !- Ventilation simulation control NO POL, !- Pollution simulation control NO CONC, !- Concentration simulation control 1.00, !- Under-relaxation factor {dimensionless} 1.0E-06, !- Absolute flow tolerance {kg/s} 1.0E-04, !- Relative flow tolerance {dimensionless} 1.0E-04, !- Error estimate for total flow per zone {kg/s} 1, !- Start number of iterations 1.0E-04, !- Limit for laminar flow approximation {Pa} 1, !- Flag for using old pressures 0, !- Flag for pressure initiation 500, !- Maximum number of iterations 10.0, !- Reference height for recorded wind data {m} 0.14, !- Wind velocity profile exponent {dimensionless} Every 30 Degrees; !- COMIS CP ARRAY Name

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COMIS Input (Cont.)COMIS SITE WIND CONDITIONS, 0.0, !- Wind direction {deg} 0.20, !- Plan area density {dimensionless} 0.18, !- Exponent of Wind velocity profile {dimensionless} 0.0; !- Surrounding building height {m}

COMIS SITE WIND CONDITIONS, 180.0, !- Wind direction {deg} 0.20, !- Plan area density {dimensionless} 0.32, !- Exponent of Wind velocity profile {dimensionless} 15.0; !- Surrounding building height {m}

COMIS EXTERNAL NODE, NFacade, !- Name 1.0; !- Outside Pollutant Concentration Factor {dimensionless} . . . COMIS EXTERNAL NODE, WFacade, !- Name 1.0; !- Outside Pollutant Concentration Factor {dimensionless}

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COMIS Input (Cont.)

COMIS STANDARD CONDITIONS FOR CRACK DATA, 20.0, !- Standard temperature for crack data {C} 101.32, !- Standard barometric pressure for crack data {kPa} 5.0; !- Standard humidity ratio for crack data {g/kg}

COMIS AIR FLOW:CRACK, CR-1, !- Name 0.01, !- Air mass flow coefficient {kg/s} 0.667, !- Air mass flow exponent {dimensionless} 1.0, !- Crack length {m} 0.0, !- Pollutant #1 Filter Efficiency {dimensionless} 0.0, !- Pollutant #2 Filter Efficiency {dimensionless} 0.0; !- Pollutant #3 Filter Efficiency {dimensionless}

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COMIS Input (Cont.) COMIS AIR FLOW:OPENING, WiOpen1, !- Name 0.001, !- Air Mass Flow Coefficient When Window or Door Is Closed {kg/s-m} 0.667, !- Air Mass Flow Exponent When Window or Door Is Closed {dimensionless} 1, !- Type of large vertical opening (LVO) 0.0, !- Extra crack length for LVO type 1 with multiple openable pa {m} 2, !- Number of Opening Factor Values 0.0, !- Opening factor #1 {dimensionless} 0.5, !- Discharge coefficient for opening factor #1 {dimensionless} 0.0, !- Width factor for opening factor #1 {dimensionless} 1.0, !- Height factor for opening factor #1 {dimensionless} 0.0, !- Start height factor for opening factor #1 {dimensionless} 1.0, !- Opening factor #2 {dimensionless} 0.6, !- Discharge coefficient for Opening factor #2 {dimensionless} 1.0, !- Width factor for for Opening factor #2 {dimensionless} 1.0, !- Height factor for for Opening factor #2 {dimensionless} 0.0, !- Start height factor for for Opening factor #2 {dimensionless} 0, !- Opening factor #3 {dimensionless} 0, !- Discharge coefficient for for Opening factor #3 {dimensionless} 0, !- Width factor for for for Opening factor #3 {dimensionless} 0, !- Height factor for for for Opening factor #3 {dimensionless} 0, !- Start height factor for for for Opening factor #3 {dimensionless} .

.

.

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COMIS Input (Cont.)COMIS ZONE DATA, WEST_ZONE, !- Name of Associated Thermal Zone WindowVentSched, !- Vent Temperature Schedule Temperature, !- Ventilation Control Mode 0.3, !- Limit Value on Multiplier for Modulating Venting Open Factor {dimensionless} 5.0, !- Lower Value on Inside/Outside Temperature Difference for Mo {deltaC} 10.0, !- Upper Value on Inside/Outside Temperature Difference for Mo {deltaC} 0.0, !- Lower Value on Inside/Outside Enthalpy Difference for Modul {J/kg} 300000.0, !- Upper Value on Inside/Outside Enthalpy Difference for Modul {J/kg} Optional Venting Schedule;

COMIS SURFACE DATA, Surface_1, !- Name of Associated EnergyPlus Surface CR-1, !- Air Flow Crack or Opening Type SFacade, !- External Node Name 1; !- Crack Actual Value or Window Open Factor for Ventilation {dimensionless}

COMIS SURFACE DATA, Window1, !- Name of Associated EnergyPlus Surface WiOpen1, !- Air Flow Crack or Opening Type SFacade, !- External Node Name 0.5; !- Crack Actual Value or Window Open Factor for Ventilation {dimensionless}

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COMIS Input (Cont.)

COMIS Cp ARRAY, Every 30 Degrees, !- Name 10.0, !- Reference height for CP data {m} 0, !- Wind direction #1 {deg} 30, !- Wind direction #2 {deg} . 270, !- Wind direction #10 {deg} 300, !- Wind direction #11 {deg} 330; !- Wind direction #12 {deg}

COMIS Cp VALUES, Every 30 Degrees, !- COMIS CP ARRAY Name NFacade, !- External Node Name 0.60, !- Cp value #1 {dimensionless} 0.48, !- Cp value #2 {dimensionless} 0.04, !- Cp value #3 {dimensionless} -0.56, !- Cp value #4 {dimensionless} -0.56, !- Cp value #5 {dimensionless} -0.42, !- Cp value #6 {dimensionless} -0.37, !- Cp value #7 {dimensionless} -0.42, !- Cp value #8 {dimensionless} -0.56, !- Cp value #9 {dimensionless} -0.56, !- Cp value #10 {dimensionless} 0.04, !- Cp value #11 {dimensionless} 0.48; !- Cp value #12 {dimensionless}

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Ventilation Control Mode Ventilation Control Mode (4 types of natural ventilation control)

Tout = outside air temperatureTzone = previous time step’s zone air temperatureTset = Vent Temperature Schedule valueHzone = specific enthalpy of zone airHout = specific enthalpy of outside air

Temperature: The windows/doors are opened if Tzone > Tout and Tzone > Tset and Venting Schedule allows venting.

Enthalpic: The windows/doors are opened if Hzone > Hout and Tzone > Tset and Venting Schedule allows venting.

Constant: Whenever Venting Schedule allows venting, the windows/doors are open, independent of indoor or outdoor conditions.

NoVent: The windows/doors are closed at all times independent of indoor or outdoor conditions. Venting Schedule is ignored in this case.

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Ventilation Schedules Field: Vent Temperature Schedule

The name of a schedule of zone-air temperature set points that controls opening of a window/door to provide natural ventilation. This schedule consists of weeks and days, with the days containing the ventilation temperature setting in ºC for each hour of the day. This ventilation temperature is the temperature above which the window/door will be opened if the conditions described under the following Ventilation Control Mode are met. [This opening control logic does not exist in the original COMIS program.]

Field: Venting Schedule The name of a schedule that specifies when venting through this

window/door is available. A zero schedule value means venting is not allowed. A value greater than zero means venting can occur if other venting control conditions (specified by Ventilation Control Mode and Vent Temperature Schedule) are satisfied. This schedule should not be confused with Vent Temperature Schedule.

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Summary

Air movement between spaces in EnergyPlus can either rely on user-defined quantities or more detailed calculations

Simple modeling statements: mixing, cross mixing, and ventilation

COMIS link provides more detailed analysis of interzone air flow as well as more sophisticated calculation of infiltration