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

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

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

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

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

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

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

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

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 & LinuxSimulation engine onlyInterfaces available from private

software developers

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

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

12

EnergyPlus Concepts (cont’d)

Simple input/output file structuresNo 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

EnergyPlus Structure

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

Integrated Simulation Manager (cont’d)

CTF Calculation

Module

Window Glass Module

Daylighting Module

Shading Module

Sky Model Module

COMIS

Building Systems

Simulation Manager

EnergyPlus Simulation Manager

Integrated Solution Manager

Surface Heat Balance Manager

Air Heat Balance Manager

Air Loop Module

Zone Equip Module

Plant Loop Module

Condenser Loop Module

PV Module

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 DOE 2

An EnergyPlus input file is not intended to be the main interface for typical end-users

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 DOE 2 input to EnergyPlus input Materials and constructions Schedules Building envelope surfaces

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

Basic Input and Output Issues

General PhilosophyInput/Output Files

Overall File StructuresInput Object Structure

Input Data Dictionary (IDD)Weather Files

20

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

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

Ou

tpu

t P

roce

ssor

Input Data Files (IDF)

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

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

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

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

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

IDD File (cont’d)

“\”code Specifications Field descriptions Units Value ranges (minimum, maximum) Defaults Autosizing

28

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

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

Weather Data(epw file)

Weather year for energy use comparisons, similar to other programs

Hourly, can be subhourlyHourly data is linearly interpolatedData include temperature,

humidity, solar, wind, etc.Several included in standard install

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

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

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

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 UAT) and

see if the numbers are “in the ballpark”