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Lecture 1: An Overview of Simulation and EnergyPlus
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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Purpose of this Lecture
Gain an understanding ofSimulation as a ConceptEnergyPlus as a Simulation Tool
Briefly review topics important to your understanding of building thermal simulations
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What is Simulation?
Definition: “the imitative representation of the functioning of one system or process by means of the functioning of another <a computer simulation of an industrial process>” (Merriam-Webster Dictionary On-Line)
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What is Building Thermal Simulation?
Approximate definition: a computer model of the energy processes within a building that are intended to provide a thermally comfortable environment for the occupants (or contents) of a buildingExamples of building thermal simulation programs: EnergyPlus, Energy-10, BLAST, DOE-2, esp-R, TRNSYS, etc.
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Goals of Building Thermal Simulation
Load CalculationsGenerally used for determining sizing of equipment such as fans, chillers, boilers, etc.
Energy AnalysisHelps evaluate the energy cost of the building over longer periods of time
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Why is Simulation Important?
Buildings consume roughly one-third of all the energy consumed nationally every year
Much of this energy is consumed maintaining the thermal conditions inside the building and lighting
Simulation can and has played a significant role in reducing the energy consumption of buildings
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How does Simulation save Energy?
Building thermal simulation allows one to model a building before it is built or before renovations are startedSimulation allows various energy alternatives to be investigated and options compared to one anotherSimulation can lead to an energy-optimized building or inform the design processSimulation is much less expensive and less time consuming than experimentation (every building is different)
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Quick Review of Important Background Concepts
Control Volumes and the Conservation of:MassEnergy (First Law of Thermodynamics)
Heat Transfer Mechanisms:Conduction—transfer of thermal energy through a solidConvection—exchange of thermal energy between a solid and a fluid that are in contactRadiation—exchange of thermal energy via electro-magnetic waves between bodies or surfaces
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What is EnergyPlus?
Fully integrated building & HVAC simulation programBased on best features of BLAST and DOE-2 plus new capabilitiesWindows 95/98/NT/2000/XP & LinuxSimulation engine onlyInterfaces available from private software developers
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EnergyPlus Concepts
Time dependent conductionConduction through building surfaces calculated with conduction transfer functionsHeat storage and time lags
Migration between zonesApproximates air exchange using a nodal model
Only models what is explicitly describedMissing wall does not let air inMissing roof does not let sun in
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EnergyPlus Concepts (cont’d)
Heat balance loads calculation (one of two load calculation methods recommended by ASHRAE)Moisture balance calculationSimultaneous building/systems solutionSub-hourly time stepsModular HVAC system simulationWINDOW 5 methodology
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EnergyPlus Concepts (cont’d)
Simple input/output file structuresNo surface, zone or system limits
Defaults to 50 coils per HVAC loopCan be increased
Links to other softwareCOMIS, wind-induced airflowTRNYSYS, Photovoltaics
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EnergyPlus Structure
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Integrated Simulation Manager
Fully integrated simulation of loads, systems and plant
Integrated simulation allows capacity limits to be modeled more realisticallyProvides tighter coupling between the air-and water-side of the system and plant
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Integrated Simulation Manager (cont’d)
CTF Calculation
Module
Window Glass Module
Daylighting Module
Shading Module
Sky Model Module
COMIS
BuildingSystems
SimulationManager
EnergyPlus Simulation Manager
Integrated Solution Manager Surface Heat
Balance Manager
Air HeatBalance Manager
Air Loop Module
Zone Equip Module
Plant Loop Module
Condenser Loop Module
PV Module
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Input/Output Data
EnergyPlus input and output data files designed for easy maintenance and expansionWill accept simulation input data from other sources such as CADD programs (AutoCAD, ArchiCAD, Visio), and preprocessors similar to those written for BLAST and DOE-2An EnergyPlus input file is not intended to be the main interface for typical end-users
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Input/Output Data (cont’d)
Most users will use EnergyPlus through an interface from a third-party developerUtilities convert portions of BLAST and DOE-2 input to EnergyPlus input
Materials and constructionsSchedulesBuilding envelope surfaces
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Summary
EnergyPlus builds on the strengths of BLAST and DOE-2 and includes many new simulation capabilities:
Integrated loads, system and plant calculations in same time step. User-configurable HVAC system description. Modular structure to facilitate the addition of new simulation modules. Simple input and output data formats to facilitate graphical front-end development.
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Basic Input and Output Issues
General PhilosophyInput/Output Files
Overall File StructuresInput Object Structure
Input Data Dictionary (IDD)Weather Files
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General Philosophy of Input/Output/Weather
Simple, free-format text filesSI units onlyComma-separatedObject-basedSomewhat self-documentingTwo parts—dictionary and data or simulation resultsNot user-friendly » Interfaces will helpCan become large
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Input–Output Files
Input Data DictionaryThis file is created byEnergyPlus developers.
Input Data FileThis file will be createdby UserObject,data,data,…,data;Object,data,data,…,data;
Input Data Dictionary(IDD)
EnergyPlus ProgramMain Program
Module
Module
Module
Module
Module
Module
File Types:Standard ReportsStandard Reports (Detail)Optional ReportsOptional Reports (Detail)InitializationReports
Overview of File Format:HeaderData DictionaryData
Note: These files will be created by EnergyPlus.
Output Files
Out
put P
roce
ssor
Input Data Files (IDF)
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Input Object Structure
Begin with object type followed by commaA (alpha) and N (numeric) fields in exact orderFields separated by commasLast field followed by semi-colonCommas are necessary placeholders
BASEBOARD HEATER:Water:Convective,
Zone1Baseboard, !- Baseboard Name
FanAndCoilAvailSched, !- Available Schedule
Zone 1 Reheat Water Inlet Node, !- Inlet_Node
Zone 1 Reheat Water Outlet Node, !- Outlet_Node
500., !- UA {W/delK}
0.0013, !- Max Water Flow Rate {m3/s}
0.001; !- Convergence Tolerance
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Input Object Structure (cont’d)
Alpha fields 60 characters maximum“!” exclamation point begins commentsIDF objects can be in any order
IDF Editor may rearrange the order“!-” IDF Editor automated commentsIDF Editor cannot be used with HVAC Templates
BASEBOARD HEATER:Water:Convective,
Zone1Baseboard, !- Baseboard Name
FanAndCoilAvailSched, !- Available Schedule
Zone 1 Reheat Water Inlet Node, !- Inlet_Node
Zone 1 Reheat Water Outlet Node, !- Outlet_Node
500., !- UA {W/delK}
0.0013, !- Max Water Flow Rate {m3/s}
0.001; !- Convergence Tolerance
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Input Object Structure (cont’d)
Not case-sensitiveInput processor checks basic rules, A vs. N, number of fields, valid object type, max/min, etc.IDF objects are generally retrieved by each component simulation module
BASEBOARD HEATER:Water:Convective,
Zone1Baseboard, !- Baseboard Name
FanAndCoilAvailSched, !- Available Schedule
Zone 1 Reheat Water Inlet Node, !- Inlet_Node
Zone 1 Reheat Water Outlet Node, !- Outlet_Node
500., !- UA {W/delK}
0.0013, !- Max Water Flow Rate {m3/s}
0.001; !- Convergence Tolerance
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Input Data Dictionary (IDD File)
Energy+.iddLocated in EnergyPlus folderConceptually simple
A (alpha) or N (Numeric)
BASEBOARD HEATER:Water:Convective,
A1 , \field Baseboard Name
\required-field
A2 , \field Available Schedule
\required-field
\type object-list
\object-list ScheduleNames
. . .
N1 , \field UA
\required-field
\autosizable
\units W/delK
. . .
N3 ; \field Convergence Tolerance
\type real
\Minimum> 0.0
\Default 0.001
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IDD File (cont’d)
Lists every available input objectIf it isn’t in the IDD, then it’s not availableIDD version must be consistent with exe versionIDD is the final word (even if other documentation does not agree)
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IDD File (cont’d)
“\”code SpecificationsField descriptionsUnitsValue ranges (minimum, maximum)DefaultsAutosizing
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IDD File (cont’d)
Get to know the IDD fileEasy way to quickly check object syntaxRefer to Input Output Reference for detailed explanations of inputs
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Allowable Ranges and Defaults
Allowable rangesSome max/min declared in IDD
Fatal error if outside of range
Some max/min hidden in source codeMay reset value and issue warning, may be fatal
DefaultsSome defaults declared in IDDSome defaults hidden in source codeSome values have no defaults
Alphas become blankNumerics become zero
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Weather Data(epw file)
Weather year for energy use comparisons, similar to other programsHourly, can be subhourlyHourly data is linearly interpolatedData include temperature, humidity, solar, wind, etc.Several included in standard install
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Output Data Format
Same philosophy as for input; somewhat human readable output filesEnergyPlus can perform some output processing to help limit output sizeUser definable variable level reporting
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Output Reporting Flexibility
User can select any variables available for outputUser can specify output at time step, hourly, daily, monthly, or environment intervalsUser can schedule each output variableUser can select various meters by resource and end-use
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Questions
How long will my simulation take?Depends on size of input file, length of simulation period (day vs. year), and speed of computerMight range from a few seconds to several minutes (some detailed simulation modules may require even longer)EnergyPlus will display progress in a window on the desktop so that the user knows where it is at
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Questions (cont’d)
How do I know whether the program read my input correctly?
Take a look at the .EIO file (EnergyPlus initialization output)—this may indicate that you have misinterpreted an input parameterCheck results output files and see if they are reasonable
How will I know whether my simulation results are reasonable or outrageous?
See previous questionConsider “Load Check Figures” available from sources such as ASHRAECompare to other simulations or consult your instructorDo some simple hand calculations (such as UAΔT) and see if the numbers are “in the ballpark”
