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
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
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”