This document provides an overview of building energy simulation using EnergyPlus. It defines simulation, discusses the goals and importance of building thermal simulation, and reviews key concepts in EnergyPlus. EnergyPlus is an integrated building energy simulation program that can model time-dependent heat transfer, HVAC systems, and plant systems. The document outlines EnergyPlus' input/output file structure and integrated simulation manager. It also addresses common questions about running simulations and validating results.

1. Lecture 1: An Overview of
Simulation and EnergyPlus
Material prepared by GARD Analytics, Inc. and University of Illinois
at Urbana-Champaign under contract to the National Renewable Energy
Laboratory. All material Copyright 2002-2003 U.S.D.O.E. - All rights reserved

2. 2
Purpose of this Lecture
Gain an understanding of
 Simulation as a Concept
 EnergyPlus as a Simulation Tool
Briefly review topics important to your
understanding of building thermal
simulations

3. 3
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)

4. 4
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
building
Examples of building thermal simulation
programs: EnergyPlus, Energy-10,
BLAST, DOE-2, esp-R, TRNSYS, etc.

5. 5
Goals of Building Thermal
Simulation
Load Calculations
 Generally used for determining sizing of
equipment such as fans, chillers, boilers,
etc.
Energy Analysis
 Helps evaluate the energy cost of the
building over longer periods of time

6. 6
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

7. 7
How does Simulation save
Energy?
 Building thermal simulation allows one to
model a building before it is built or before
renovations are started
 Simulation allows various energy alternatives
to be investigated and options compared to
one another
 Simulation can lead to an energy-optimized
building or inform the design process
 Simulation is much less expensive and less
time consuming than experimentation (every
building is different)

8. 8
Quick Review of Important
Background Concepts
 Control Volumes and the Conservation of:
 Mass
 Energy (First Law of Thermodynamics)
 Heat Transfer Mechanisms:
 Conduction—transfer of thermal energy through a
solid
 Convection—exchange of thermal energy between
a solid and a fluid that are in contact
 Radiation—exchange of thermal energy via
electro-magnetic waves between bodies or
surfaces

9. 9
What is EnergyPlus?
Fully integrated building & HVAC
simulation program
Based on best features of BLAST and
DOE-2 plus new capabilities
Windows 95/98/NT/2000/XP & Linux
Simulation engine only
Interfaces available from private
software developers

10. 10
EnergyPlus Concepts
 Time dependent conduction
 Conduction through building surfaces calculated
with conduction transfer functions
 Heat storage and time lags
 Migration between zones
 Approximates air exchange using a nodal model
 Only models what is explicitly described
 Missing wall does not let air in
 Missing roof does not let sun in

11. 11
EnergyPlus Concepts (cont’d)
 Heat balance loads calculation (one of two load
calculation methods recommended by ASHRAE)
 Moisture balance calculation
 Simultaneous building/systems solution
 Sub-hourly time steps
 Modular HVAC system simulation
 WINDOW 5 methodology

12. 12
EnergyPlus Concepts (cont’d)
Simple input/output file structures
No surface, zone or system limits
 Defaults to 50 coils per HVAC loop
 Can be increased
Links to other software
 COMIS, wind-induced airflow
 TRNYSYS, Photovoltaics

13. 13
EnergyPlus Structure

14. 14
Integrated Simulation
Manager
Fully integrated simulation of loads,
systems and plant
 Integrated simulation allows capacity limits
to be modeled more realistically
 Provides tighter coupling between the air-
and water-side of the system and plant

15. 15
Integrated Simulation
Manager (cont’d)

16. 16
Input/Output Data
 EnergyPlus input and output data files
designed for easy maintenance and
expansion
 Will accept simulation input data from other
sources such as CADD programs (AutoCAD,
ArchiCAD, Visio), and preprocessors similar to
those written for BLAST and DOE2
 An EnergyPlus input file is not intended to be
the main interface for typical end-users

17. 17
Input/Output Data
(cont’d)
Most users will use EnergyPlus through
an interface from a third-party developer
Utilities convert portions of BLAST and
DOE2 input to EnergyPlus input
 Materials and constructions
 Schedules
 Building envelope surfaces

18. 18
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.

19. 19
Basic Input and Output Issues
General Philosophy
Input/Output Files
Overall File Structures
Input Object Structure
Input Data Dictionary (IDD)
Weather Files

20. 20
General Philosophy of
Input/Output/Weather
 Simple, free-format text files
 SI units only
 Comma-separated
 Object-based
 Somewhat self-documenting
 Two parts—dictionary and data or simulation
results
 Not user-friendly » Interfaces will help
 Can become large

21. 21
Input–Output Files
Input Data Dictionary
This file is created by
EnergyPlus developers.
Input Data File
This file will be created
by User
Object,data,data,…,data;
Object,data,data,…,data;
Input Data Dictionary
(IDD)
EnergyPlus Program
Main Program
Module
Module
Module
Module
Module
Module
File Types:
Standard Reports
Standard Reports (Detail)
Optional Reports
Optional Reports (Detail)
Initialization
Reports
Overview of File Format:
Header
Data Dictionary
Data
Note: These files will be
created by EnergyPlus.
Output Files
OutputProcessor
Input Data Files (IDF)

22. 22
Input Object Structure
 Begin with object type followed by comma
 A (alpha) and N (numeric) fields in exact order
 Fields separated by commas
 Last field followed by semi-colon
 Commas 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

23. 23
Input Object Structure
(cont’d)
 Alpha fields 60 characters maximum
 “!” exclamation point begins comments
 IDF objects can be in any order
 IDF Editor may rearrange the order
 “!-” IDF Editor automated comments
 IDF 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

24. 24
Input Object Structure
(cont’d)
 Not case-sensitive
 Input 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

25. 25
Input Data Dictionary
(IDD File)
 Energy+.idd
 Located in
EnergyPlus folder
 Conceptually 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

26. 26
IDD File (cont’d)
Lists every available input object
 If it isn’t in the IDD, then it’s not available
 IDD version must be consistent with exe
version
 IDD is the final word (even if other
documentation does not agree)

27. 27
IDD File (cont’d)
“”code Specifications
 Field descriptions
 Units
 Value ranges (minimum, maximum)
 Defaults
 Autosizing

28. 28
IDD File (cont’d)
Get to know the IDD file
Easy way to quickly check object syntax
Refer to Input Output Reference for
detailed explanations of inputs

29. 29
Allowable Ranges
and Defaults
 Allowable ranges
 Some max/min declared in IDD
 Fatal error if outside of range
 Some max/min hidden in source code
 May reset value and issue warning, may be fatal
 Defaults
 Some defaults declared in IDD
 Some defaults hidden in source code
 Some values have no defaults
 Alphas become blank
 Numerics become zero

30. 30
Weather Data
(epw file)
Weather year for energy use
comparisons, similar to other programs
Hourly, can be subhourly
Hourly data is linearly interpolated
Data include temperature, humidity,
solar, wind, etc.
Several included in standard install

31. 31
Output Data Format
Same philosophy as for input;
somewhat human readable output files
EnergyPlus can perform some output
processing to help limit output size
User definable variable level reporting

32. 32
Output Reporting Flexibility
User can select any variables available
for output
User can specify output at time step,
hourly, daily, monthly, or environment
intervals
User can schedule each output variable
User can select various meters by
resource and end-use

33. 33
Questions
How long will my simulation take?
 Depends on size of input file, length of
simulation period (day vs. year), and speed
of computer
 Might 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

34. 34
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 parameter
 Check results output files and see if they are reasonable
 How will I know whether my simulation results are
reasonable or outrageous?
 See previous question
 Consider “Load Check Figures” available from sources such
as ASHRAE
 Compare to other simulations or consult your instructor
 Do some simple hand calculations (such as UADT) and see if
the numbers are “in the ballpark”