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 U.S.D.O.E. - All rights reserved

2. Executive Summary Problem Addressed Solution Results
Build a thermal simulation model for building that enables evaluation of energy performance, size HVAC equipment, retrofitting.
Solution
Simulation tools dating back from 1973 energy crisis
Eplus: energy analysis and load simulation program (Crawley, 2001)
Results
Part of the regulatory policy: several building models. Case studies: children’s museum, SDSC.

3. Building Models: HL Inputs
Four parts: Program, Form, Fabric, Equipment
Program: (Architectural Program) describes how the building will be used and the services it must deliver.
Influences inputs related to climage, loads, schedules, ventilation, comfort requirements
Facility Location, Floor Area, Schedules, Lighting Levels, Ventilation needs, Occupancy.
Form: geometry of the building and its elements, how it interacts with the sun and the ambient
#floors, aspect ratio, window fraction, window location, shading, floor height, azimuth

4. Building Models: HL Inputs
Four parts: Program, Form, Fabric, Equipment
Fabric: materials used to construct the building, insulation levels, glazing systems, thermal mass
Exterior walls, Roof, Windows, Interior partition, Internal mass
Equipment: includes HVAC, lighting systems, controls, except for plus and process load equipment selected by the occupants, it includes all the energy consuming equipment as a part of the building
HVAC systems, Component efficiency, Control settings, Lighting fixtures, Lamp types, Daylighting controls.

5. Simulations Core engines
Radiant and convective effects
Combined heat and transfer models
Does not yet have IT related engines
A coordinated simulation framework (ManageSimulation)
Calls four out five main simulation loops
Environment, day, hour, sub-hour
Sets flags for individual simulators
Not a good life-cycle cost analysis models
In fact, costing is a mess given policy interferences.

6. Now let us see what DOE says

7. What is Building Thermal Simulation?
A 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.
Eplus: 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

8. 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

9. 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

10. EnergyPlus Concepts Time dependent conduction Migration between zones
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
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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
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12. EnergyPlus Structure
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13. Integrated Simulation Manager
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14. Input–Output Files EnergyPlus Program Output Files
Input Data Dictionary
(IDD)
Main Program
File Types:
Standard Reports
Standard Reports (Detail)
Input Data Dictionary
Module
Module
Optional Reports
This file is created by
Optional Reports (Detail)
EnergyPlus developers.
Initialization
Module
Module
Reports
Overview of File Format:
Input Data Files (IDF)
Output Processor
Header
Data Dictionary
Input Data File
Module
Module
Data
This file will be created
by User
Object,data,data,…,data;
Note: These files will be
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created by EnergyPlus.
Object,data,data,…,data;

15. 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
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16. 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
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17. Input Data Dictionary (IDD File)
BASEBOARD HEATER:Water:Convective,
A1 , \field Baseboard Name
\required-field
A2 , \field Available Schedule
\type object-list
\object-list ScheduleNames
. . .
N1 , \field UA
\autosizable
\units W/delK
N3 ; \field Convergence Tolerance
\type real
\Minimum> 0.0
\Default 0.001
Energy+.idd
Located in EnergyPlus folder
Conceptually simple
A (alpha) or
N (Numeric)
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18. 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)
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19. 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
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20. 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
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21. Simulation Types Peak Thermal Load Calculation
Simulation run for an extreme (design) day or several design days
Generally used for determining sizing of equipment such as fans, chillers, boilers, etc.
Building Energy Analysis
Simulation run for an extended period of time: a month, season, year, or several years using weather files
Includes the building response to the entire range of conditions expected at a particular site
Helps evaluate the energy cost of the building over longer periods of time

22. Meters Accumulate multiple outputs of same form
Appropriate variables are grouped onto “meters” for reporting purposes
May ease analysis of output
Values are put onto the eplusout.mtr file
Meter component details in eplusout.mtd file
Meter names applicable for the simulation are shown on the Report Data Dictionary file
Meter names are of two forms:
<FuelType>:<MeterType>
<EndUseType>:<FuelType>:<MeterType>

23. Meters – Resource Types
Electricity
Gas
Gasoline
Diesel
Coal
FuelOil#1
FuelOil#2
Propane
Water
Steam
PurchasedCooling
PurchasedHeating
EnergyTransfer (coil & equipment loads)

24. Meters – End Use Types GeneralLights TaskLights ExteriorLights
ZoneSource
ExteriorEquipment
Fans
Pumps
Heating
Cooling
HeatRejection
Humidifier
HeatRecovery
DHW
Cogeneration
Miscellaneous

25. Meters – Meter Types Facility (Master Meters) Submeters:
Zone (lights, plug loads, etc.)
Building (all zones combined plus exterior)
System (air handlers, terminal units)
Plant (chillers, boilers, DHW, etc.)

26. Meters – Meter Types Zone Air loop Plant Loop Gas:HVAC Elec:Plant
Gas:Plant
Lights:Zone
Elec:HVAC
Elec:Zone
Building System

27. Input for Meters Report Meter, Electricity:*, Hourly;
Report Meter, Electricity: Facility, monthly;
Report Meter, Cooling: Electricity, monthly;
All electric meters, for all end uses and all levels
Master electric meter
Cooling equipment

28. Simulation of Buildings
Every building is different in many ways:
Location and exterior thermal environment
Construction
HVAC system
Exterior thermal environment is a driving force that determines how a building will respond
Energy efficient design requires an understanding of and a response to the exterior thermal environment
Thermal simulation requires information on the exterior thermal environment to properly analyze the building from an energy perspective

29. Definitions and Connections
Building:
Entire collection of interior and exterior features of the structure
Buildings may consist of one or more zones
Zones:
Group of surfaces that can interact with each other thermally and have a common air mass at roughly the same temperature
One or more rooms within a building
Zones may consist of one or more surfaces

30. Definitions and Connections (cont’d)
Surfaces:
Walls, Roofs, Ceilings, Floors, Partitions, Windows, Shading Devices
One or more surfaces make up a zone
Surfaces consist of a series of materials called a “construction”
Construction:
Group of homogeneous one-dimensional material layers
Each surface must have a single construction definition
Each construction is made up of one or more materials
Materials:
Define the thermal properties for layers that are used to put together a construction
One or more material layers make a construction

31. Envelope Hierarchy Building Zone Zone Zone … more zones
… more surfaces
Surface
Surface
Surface
Surface
only one construction per surface
Construction
… more materials
Material
Material
Material
Material

32. Defining Thermal Zones by Objective
Objectives of a study can dictate the size and number of thermal zones
Air flow study: sizing fans and ducts
Several rooms per zone
Zone per system type
“Block loads” or central plant study: sizing of heating and cooling producers
Minimize number of zones (maybe only 1)

33. Ft. Monmouth Education Center

34. Defining Thermal Zones by Design Conditions
“DT” test: if there is an air temperature difference between adjacent spaces, separate thermal zones are needed
Might also be seen in different control types
Space usage/internal gains test:
Differences in internal gains may result in different conditioning requirements or distribution
Office vs. gymnasium
Space usage differences may alter the ventilation or exhaust requirements of a space
Office vs. kitchen vs. chemistry laboratory

35. Defining Thermal Zones by Design Conditions (cont’d)
Environmental conditions test: exposure to different thermal surroundings/quantifying the effect
Different space orientations—solar gains
Exposure to the ground
Exposure to the outdoor environment

36. Ft. Monmouth Education Center
“DT” test: loading dock
Space use: kitchen, dining area
Outdoor exposure: west wing solar

37. Loads Features and Capabilities
How does EnergyPlus calculate what it will take to keep a zone at the desired thermal conditions?
EnergyPlus contains the heat balance engine from IBLAST, a research version of BLAST with integrated loads and HVAC calculation.
The major enhancements of the IBLAST heat balance engine include mass transfer and radiant heating and cooling
Heat balance engine models room air as well-stirred with uniform temperature throughout.
Room surfaces are assumed to have:
Uniform surface temperatures
Uniform long and short wave irradiation
Diffuse radiating and reflecting surfaces
Internal heat conduction
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38. EnergyPlus Model For Building Loads
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39. Equipment & People Loads
Sensible and Latent
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40. Loads Features and Capabilities (cont’d)
Three models connected to the main heat balance routine are based on capabilities from DOE­2
Daylighting simulation
Calculates hourly interior daylight luminance, window glare, glare control, electric lighting controls, and calculates electric lighting reduction for the heat balance module
WINDOW 5-based window calculation
Anisotropic sky
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41. Loads Features and Capabilities (cont’d)
Incorporates a simplified moisture model known as Effective Moisture Penetration Depth (EMPD)
Estimates moisture interactions among the space air and interior surfaces and furnishings
Estimates impacts associated with moisture where detailed internal geometry and/or detailed material properties are not readily available
User may also select a more rigorous combined heat and mass transfer model
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42. Solar Distribution Options
Minimal Shadowing
No exterior shadowing except from door and window reveals
All direct beam solar radiation incident on floor
If no floor, direct beam solar distributed to all surfaces
Full Exterior
Exterior shadowing caused by detached shading, wings, overhangs, and door and window reveals
Full Interior and Exterior
Exterior shadowing same as Full Exterior
Direct beam solar radiation falls on all surfaces in the zone in the direct path of the sun’s rays
Solar entering one window can leave through another window
Zone must be convex:
A line passing through the zone intercepts no more than two surfaces
An L-shaped zone is not convex

43. Applying Eplus to reducing energy

44. General Strategies for Reducing Building Heating and Cooling
Non-mechanical system approach
Should always try to minimize heating and cooling requirements first
Mechanical system efficiency important also
Building Envelope: Insulation and/or Isolation
Solar Strategies (Passive Heating)
Alternate Cooling Strategies

45. Building Envelope: Insulation and/or Isolation
Goal: Attempt to minimize the adverse effects of the environment on a building
Note: effect of environment is always changing
Note: in some cases (e.g., temperate/mild climates and high internally loaded buildings), we may want to maximize impact of environment because it is beneficial (Climate Specific Strategies)
Adjust volume to exterior area ratio
Volume/living space desirable (maximize volume)
Minimizing exterior surface area (usually) since it affects conduction, convection, and radiation

46. Building Envelope: Change Wall Construction
Reduce conduction by adding insulation
Conduction (q=ADT/R)increase in R decreases q
Note differences in R-values of various exterior surfaces and their relative areas
Windows vs. walls: windows generally have a lower R-value
Walls vs. roofs: building shape determines where to focus attention
Consider the possibility of movable insulation for various surfaces
Potentially reduce conduction by adding thermal mass
“Interior” internal mass damps various short term effects, reducing or shifting conditioning needs
“Exterior thermal mass delays impact of exterior temperature swings, may send some/much of effect back to exterior side
Thermal mass discussed in more detail later in this lecture

47. Building Envelope: Change Exterior Boundary Conditions
Create a local “micro-climate”
Air vs. ground temperature
Ground can be a thermal mass and insulator
Air temperature changes more extreme (harm or help?)
Modification of air temperature using site water resources (evaporative cooling to reduce local air temperature)
Wind exposure
Use of vegetation as wind breaks in winter (evergreens on north side—location specific)
Allow air movement for cooling?
Note surroundings and impact on air movement around building
Surface properties

48. Solar Radiation: Light and Heat
General concepts
Use solar energy when heating required, avoid it when cooling is required
Sun angles (particularly altitude) can vary with time of yearthis can work to our advantage
Solar adds heat and light, but only during the day
Orientation of openings (windows) critical to the success of the design; in general:
Maximize southern exposure, Minimize east/west exposure
“Passive solar” increasingly important in design
A definition: “a system that collects, stores, and redistributes solar energy without the use of fans, pumps, or complex controllers” (Lechner)
Lower first costs than active solar systems because they are part of the building rather than an additional system

49. Solar Radiation: Light and Heat (cont’d)
Using Direct Solar Gain (Windows)
Utilize the “greenhouse effect” of windows which allow solar radiation to be transmitted but block most thermal radiation
Benefit is maximized with south facing windows
Low winter sun more directly impacts this direction
High summer sun has little effect on south windows, can be easily shaded
Potential for overheating during the day and underheating at night
Thermal mass (interior) helps reduce this effect
Need to exercise caution about thermal mass color and location relative to insulators such as carpet, furniture, etc.
Direct gain easy to provide but there are limitsincreasing windows to increase gain also increases heat loss through windows (at night) or heat gain when undesirable (in summer)

50. Solar Radiation: Systems
Trombe Wall Systems Sunspaces
Transpired Solar Collector
Perforated metal wall covering
Solar energy heats up wall
Fan assists in drawing air through panels
Panels reject heat to air, heating the air before introduction into building

51. Solar Radiation: Shading
Attempt to block solar radiation from impacting the building during cooling season
Devices can be:
Natural or constructed
Fixed or variable (trees of differing types, movable shades, etc.)
Opaque or somewhat transparent
Indoor or outdoor
Categories/characteristics
Overhang—panels or louvers, can be rotated
Fins/wings—panel(s), slanted or rotating
“Eggcrate”—reduced depth combined overhang/fin, slanted or rotating
Roller shades/awnings
Trees/vines—free standing, trellis, “attached”

52. Solar Radiation: General Shading Guidelines
Exterior shading more efficient, but weather can take its toll on mechanized variable systems that are outdoors
South windows
Easiest to shade, overhangs very effective
Fins may be needed for early morning, late afternoon
Trees typically not much help to the south
East/west windows
Difficult to shade due to low altitude angles
Fins (slanted) more effective or eggcrates
Trees best on the east, west, southeast, and southwest (northern hemisphere)
North windows
Little shading required
Desirable and even diffuse daylight
Fins typically enough, if needed at all
Skylights can be problematic
Potential for leaks is greater
Solar/light gain maximized at wrong time of year (summer)
Can be more difficult to shade
Other orientations may require combination solutions

53. Alternate Cooling Techniques: Air Movement
Ventilation
Comfort ventilation: increase comfort by increasing air flow rates with the building
Night purge ventilation: ventilate (naturally or mechanically) at night when the outside air temperature is presumably cooler than inside
Technology
Windows (various types of openings)
Cool Towers (Down Draft Coolers)
Thermal Chimney

54. Alternate Cooling Techniques: Roof Cooling
Basic concept: block solar radiation during the day, then take advantage of radiation to cold sky during the night (clouds will significantly decrease nighttime performance)
Roof Pond
Simply a layer of water contained on a flat roof or containers of water
Daytime operation
Pond is covered with insulation to deflect solar heat and reduce connection to outside environment
Thermal mass of water soaks up heat from the interior space
Nighttime operation
Pond is left uncovered to reject heat from water to outside environment
Heat is rejected via convection to surrounding air and to sky via radiation
Cycle can be reversed in winter to provide a Trombe Wall type roofing system

55. Roof Pond: Drawbacks Added cost of system and extra maintenance
Movable insulation systems are typically not very successful
Concerns about leaks

56. Alternate Cooling Techniques: Roof Radiator
Similar in concept to roof pond, but replaces water and movable insulation with a metal deck that is elevated above the roof
Can use interior movable insulation with a “closed” deck
Can be fan assisted with an “open” deck
Can be used as a heating system in winter if solar energy is trapped between metal deck and roof
Hot air is then circulated to interior spaces
Drawbacks
Added costs of roof deck
Reliability/longevity of movable insulation

57. Roof Radiator: Cooling Operation
Daytime operation
Metal panel reflects a portion of the solar radiation
Insulation blocks heat transfer to building interior
Or ventilation air reduces heat transfer from roof deck to actual roof
Nighttime operation
Roof radiates heat to sky
Roof temperature may be low enough to actually cool outside air even further

58. Earth Coupling: Direct Earth Coupling
Underground or berms
Ground temperatures can be lower than outside air, making this a good heat sink
Concerns about winter may require insulation of ground and/or building surfaces in contact with ground
Potential moisture problems

59. Earth Coupling: Indirect Earth Coupling
Buried supply air tubes
Inlet air diverted through pipes that are buried
Air is cooled by the cooler ground, providing some free cooling
Pipes must be buried significantly deep
Maintenance and moisture issues
Ground “micro-climate” change using evaporation
Cool the ground surrounding a building using evaporation
Ground connected buildings
Elevated buildings

60. EnergyPlus Modeling Capabilities
Thermal Mass
Trombe Wall
Sunspaces
Movable/transparent insulation

61. Thermal Mass/Energy Storage within Buildings: Theory
Storage energy (heat) within building elements (exterior or interior) for use or release at a later time/date (analogy of a sponge or a rechargeable battery)
Building materials store heat as “internal energy”
Thermal mass a function of material properties (specific heat and density) as well as volume of materialhow much thermal mass is “enough”?
Energy stored in a building material will eventually be release—either to the interior or exterior depending on placement of mass, environmental conditions, etc.

62. Thermal Mass: Examples
Traditional Examples
Dense building types with very thick walls
Ice blocks from Lake Michigan
More Modern Examples
“Trombe Walls”
Interior Water Walls or Containers
Phase Change Materials

63. Thermal Mass: Seasonal Effects
Cooling Season
Dampen the effect of outside temperature variations
Shift time of highest cooling loads to the night hours (offices)
Absorb excessive internal gains during daytime hours (usually combined with night ventilation strategies)
Heating Season
Store solar energy absorbed for use during the nighttime hours when temperatures are low and the sun is not visible
Avoid potential overheating problems due to excessive direct solar gains
Note: thermal mass effects will not show up in a winter design day run—must look at an annual simulation with actual weather data

64. Thermal Mass: Key Terms in EnergyPlus IDF
Material:Regular—specification of specific heat and density
Construction—reference to a material layer definition
Surface—reference to a construction definition

65. Trombe Walls: Theory
Primarily a passive heating element used to delay the impact of solar radiation
Intended to cooperate with direct gain through windows to provide heating via solar radiation during all parts of the day and night
Most useful on south, southeast, and southwest facades

66. Trombe Walls: System components
Thermally massive wall (brick, concrete, stone, water) painted a dark color to absorb solar radiation
Air gap
Wall cover (transparent glass) to allow sun light to get through to the thermal mass and to block some of the heat loss to the outside environment

67. Open vs. Closed Trombe Walls
Open System
Similar to a mini-sunspace where the air in the gap between the cover and the wall mass is allowed to circulate to an interior space
More important if no visual link to the outside
These have fallen out of favor (in US) due to difficulty in controlling the amount of exchange between the air gap and the attached space and due to the loss of delay factor (easier to combine wall with windows); also maintenance issues

68. Open vs. Closed Trombe Walls
Closed System
Air gap between the wall mass and the cover is sealed
Heat is trapped and absorbed better into the thermal mass

69. Trombe Walls: Performance
Best for heating when wall mass has both a high storage capacity and a high thermal conductivity
High thermal conductivity increases heat gain/loss of overall wall assembly
Wall cover should be as transparent as possible but also resistive
Solar must be kept out of the Trombe wall in summer through use of:
Shading devices
Specialized transparent insulation materials
Electrochromic or thermochromic glazing

70. Trombe Walls: Examples
Trombe walls are usually but not necessarily restricted to simple flat south-facing walls (compare Zion National Park Visitor’s Center to NREL Visitor’s Center)
Photos courtesy of

71. Sunspaces and Double Skinned Buildings
Sunspaces and double skinned buildings can also be modeled as separate zones
Note that in EnergyPlus solar radiation will pass through one space and into another but that once it gets to the second zone it is assumed to be all “diffuse”
Both sunspaces and double skinned buildings provide an extra buffer from the outside
Sunspaces add potentially usable space
For systems which exchange heat through air transport, definition of a MIXING or CROSS MIXING statement required (not very accurate)