steps for energy simulation by xiaoyounan


									Energy Analysis Tools
by Richard Paradis

Steven Winter Associates, Inc.

Last updated: 06-10-2010


                                                     Introduction

                                                     Description

                                                     Emerging Issues

                                                     Application

                                                     Relevant Codes and Standards

                                                     Additional Resources


Building thermal performance calculations are made for two primary reasons. They are made to size and
select mechanical equipment or to predict the annual energy consumption of a structure. While these two
tasks are not mutually exclusive, and some programs can handle both tasks, they do tend to be
conducted in isolation from each other.

                                                  Sizing programs are primarily designed to calculate peak
                                                   hourly loads during the heating and cooling seasons.
                                                   Almost all buildings of any complexity have a sizing
                                                   analysis of some kind run by an architect, engineer, or
                                                   mechanical contractor. Most sizing programs are based
                                                   on consensus procedures and algorithms established by
                                                   ASHRAE, but many are proprietary products distributed
                                                   or sold by equipment manufacturers.
                                                  Energy programs are primarily designed to predict the
                                                   annual energy consumed by a structure in terms of BTUs,
                                                   dollars, or pollution avoidance. In the past, few buildings
                                                   benefited from energy analysis. Today energy analysis
                                                   tools are becoming more common and are being applied
                                                   earlier in the design process.

To decide what computer-based, energy analysis tool is best for your project, it is important to have a
basic understanding of how these tools operate.

                                                                                                  BACK TO TOP

A. Calculating Annual Energy Consumption

The flowchart diagrammed in Fig. A indicates the steps that must be followed to fully estimate project
energy costs.

Fig. A. Flowchart to determine energy costs


A "zone" is a segment of a building with similar thermal requirements serviced by the same mechanical
equipment and controls. The number of thermal zones will vary depending on many factors including the
building use, size, and shape. For example, a single family house or free-standing branch bank may have
only one or two zones while a large office building may have over one hundred.


A "load" is the required, hourly rate of heat removal in summer (or heat supply in winter) necessary to
keep a building comfortable. In step two, the annual, peak hourly heating and cooling loads for each zone
must be calculated.


Based on the peak loads calculated in step two, size and select building mechanical equipment. For
comprehensive simulations of multi-zone structures, thermal interactions between zones must be taken
into account (such as the mixing that occurs in water-loop heat pump systems).


Calculate the loads placed on the selected equipment for each hour of a Typical Meteorological Year
(TMY) and determine the amount of energy required by the equipment—based on system efficiencies
and part load curves—to meet these loads.


For the specific building construction site, input energy rate information including electric peak demand

Calculate the cost of the fuel consumed for each hour of the year. Annual performance is calculated by
summing the hourly results for all 8,760 hours of the year.

Some software programs are designed to excel at one or two steps in this process while others tackle the
whole problem comprehensively. Other tools use simplified methods to expedite input requirements or
minimize run time, while still others are more detailed and precise.

For example, HVAC equipment manufacturers have emphasized the development of software that
addresses Steps Two and Three quite well. But these same programs often do not handle the interaction
between strategies (such as daylighting and energy efficient lighting) necessary to accurately model
energy consumption in Step Four.

Boundaries between energy analysis tools are beginning to blur as developers in different industries are
converging to produce software that is more graphic, easier to use, and capable of greater accuracy.
Nevertheless, important distinctions between software programs still remain and will do so for the
foreseeable future. What is unquestionably true is that with today's powerful personal computers it is no
longer necessary to compromise—we can have both speed and accuracy.

B. Match Tool to Task

Not only do energy analysis software programs have varying levels of accuracy; they are also intended to
be used at different phases of the design process; and require very different levels of effort and cost. For
example, a tool such as Energy-10, has been designed to provide immediate feedback to the designer or
project manager during the earliest phases of a project while others such as DOE-2 or BLAST, require
more input time and detail. Consequently, they are generally reserved for later in the design process
when many architectural decisions have already been finalized. EnergyPlus is a newer building energy
simulation program for modeling building heating, cooling, lighting, ventilating, and other energy
flows—building on the most popular features and capabilities of BLAST and DOE-2. Most energy
analysis tools can be classified as being one of four generic types. Note: The software examples listed
are meant to be indicative, not exhaustive.

                                                 Screening Tools for use primarily during budgeting and
                                                  programming of retrofits.
                                                      o    FRESA
                                                      o    FEDS
                                                 Architectural Design Tools for use primarily during
                                                  programming, schematics, and design development of
                                                  new construction and major retrofit.
                                                      o    ENERGY-10
                                                      o    Building Design Advisor
                                                      o    Energy Scheming
                                               Load Calculation and HVAC Sizing Tools for use
                                                primarily during design development and construction
                                                documentation of new construction and major retrofit.
                                                     o    HAP
                                                     o    TRACE
                                                     o    DOE-2
                                                     o    BLAST
                                                     o    VisualDOE
                                                     o    EnergyPlus
                                               Economic Assessment Tools for use throughout the
                                                design process.
                                                     o    BLCC
                                                     o    Quick BLCC

C. Screening Tools

Screening Tools are designed to evaluate project viability during the earliest stages of programming and
often include some economic analysis capability. They also tend to be correlations, rather than full hourly

In a correlation-based program, daily, monthly, or seasonal building performance is computed by
comparing, or correlating, the performance of the building in question against predetermined equations
(or curves) that predict the performance of the building based on key thermal characteristics and climate
information. Correlation programs generally run quickly because they demand a minimum of computation,
but this speed is at the expense of some accuracy. Also because of their relative simplicity, correlation
programs are not able to evaluate the important trade-offs between certain interactive energy strategies
such as daylighting and heating or thermal mass and cooling. The following are some examples of
screening tools.

Fig. 1. FRESA screen capture
FRESA (The Federal Renewable Energy Screening Assistant)
Allows federal energy auditors to evaluate renewable energy opportunities and energy systems options
for possible inclusion in a facility's energy program. The purpose is to focus feasibility study efforts on
those applications most likely to prove cost-effective. FRESA can screen facilities for the following
renewable energy systems:

                                                 Daylighting Controls/Infiltration Control
                                                 Daylighting Apertures
                                                 Active Solar Cooling
                                                 Multiple Glazings
                                                 Window Shading Alternatives
                                                 Utilization of Wind Energy
                                                 Utilization of Water Power
                                                 Solar Thermal Electric
                                                 Solar Swimming Pools
                                                 Ground Coupled Heat Pumps
                                                 Active Solar Space Heating
                                                 Solar Hot Water
                                                 Photovoltaic Applications
                                                 Solar Ventilation Reheat
                                                 Conversion to Biomass
                                                 Conversion to Refuse

Table 1: FRESA Version 2.5 Key Characteristics

Key Strengths                                             Key Weaknesses

        Establishes consistent methodology and                 Provides only first-order screening, to focus
         reporting format for a large number of audits           design; requires more detailed feasibility
         in varying locations and with varying building          analyses on applications most likely to be
         use types                                               cost-effective
        sophisticated analyses of technology                   requires high level of knowledge about
         performance and cost while keeping data                 energy audits and the limitations of the
         requirements to a minimum                               program
                                                                not suitable for general use

FEDS (The Facility Energy Decision System)
Provides a comprehensive method for quickly and objectively identifying energy improvements that offer
maximum savings. It is an easy to use tool for identifying retrofits, selecting minimum life cycle costs,
determining payback, and enabling users to prioritize options. The FEDS system allows data input to
range from minimal to extremely detailed.

Table 2: FEDS Version 5.0.1 Key Characteristics
Key Strengths                                        Key Weaknesses

        Technology and Fuel Independence                    Cannot evaluate interaction of some
        Life Cycle Cost Optimization                         strategies
        Peak Tracking                                       Not a buildings design tool
        Alternate Financing Analysis
        Optimizes retrofit opportunities

D. Architectural Design Tools

Architectural Design Tools are intended to evaluate the relative importance of design decisions such as
building orientation, glazing, and daylighting.


Design tool for small residential or commercial buildings that can be treated as one- or two-zone
increments. Performs whole-building energy analysis for 8,760 hours/year, including dynamic thermal
and daylighting calculations. New features in Version 1.7 include: New window construction dialogue,
two new HVAC system types: VAV DX Cooling and Fixed COP Heat Pump, International weather data,
and 3D bar graph display.

Table 3: Energy-10 Version 1.7 Key Characteristics

Key Strengths                                        Key Weaknesses

        Hour-by-hour simulation                             Not all strategies are active
        AutoBuild defaults building attributes
        Everything is editable
        Excellent educational tool
        Life cycle cost analysis
        Climate Specific
Fig. 2. Energy-10 screen capture

Energy Scheming
A design tool to help the user create an energy-efficient building; provides loads analysis for 24 hours for
each of 4 seasonal evaluation days. Input is graphical and intuitive and is designed to support the earliest
phases of design, where energy considerations can have the most impact.

Table 4: Energy Scheming Key Characteristics

Key Strengths                                         Key Weaknesses

        Graphic input                                         Apple only
        Supports visual thinking                              Uses highly simplified algorithms; single
        Educational tool                                       zone; does not size or calculate HVAC

Building Design Advisor
The BDA is a computer program that supports the concurrent, integrated use of multiple simulation tools
and databases, through a single, object-based representation of building components and systems. BDA
(Building Design Advisor) acts as a data manager and process controller, allowing building designers to
benefit from the capabilities of multiple analysis and visualization tools throughout the building design
process. BDA is implemented as a Windows-based application and is linked to a Schematic Graphic
Editor and two simplified simulation tools, one for daylight and one for energy analyses.

Table 5: Building Design Advisor Version 3.1 Key Characteristics

Key Strengths                                         Key Weaknesses

        Graphic input                                         Limited database of options for building
        Does not require in depth knowledge to use             components and systems
         linked tools for energy and daylighting

E. Engineering Design Tools/Load Calculation & HVAC Sizing Tools

Engineering Design Tools/Load Calculation and HVAC Sizing Tools are designed primarily to size and
help select equipment such as boilers, furnaces, or chillers. Many load calculation and HVAC sizing tools
also include the ability to perform annual energy simulations. Some of the sizing tools are proprietary
software products created and distributed by equipment manufacturers.

TRACE Load 700
Combines the power of the building and load design portions of TRACE 600 with the simplicity of a
Windows-based operating environment. TRACE Load 700 uses ASHRAE-standard algorithms and
enables non-sequential data entry that encourages "what if" analysis. You can edit building construction
details in any order and easily change the building model as the design progresses. The extensive
predefined (but editable) libraries and templates of construction materials and building load information
increase the speed and accuracy of the modeling process. You can export the completed project file to
TRACE 700 for a detailed energy analysis.

Table 6: Trace Load 700 Key Characteristics

Key Strengths                                            Key Weaknesses

        Intuitive Windows interface                           Requires TRACE 700 to perform energy and
        Simplified input methods                               cost analyses
        Models more than 25 types of air distribution

HAP (Hourly Analysis Program)
A system design tool and an energy simulation tool in one package. Uses a Windows-based graphical
user interface and 32-bit software. HAP's design module uses a system-based approach that tailors
sizing procedures and reports to the specific type of system being considered. Central AHUs, packaged
rooftop units, split systems, fan coils, and PTACs can be designed, as can CAV, VAV, single- and
multiple-zone systems. The ASHRAE-endorsed Transfer Function Method is used to calculate building
heat flow.

HAP's energy simulation module performs a true 8,760 hour energy simulation of building heat flow and
equipment performance. It uses TMY weather data and the Transfer Function Method. Many types of air
handling systems, packaged equipment, and plant equipment can be simulated. Costs can be computed
using complex utility rates. Extensive reports and graphs document hourly, daily, monthly, and annual
energy and cost performance.

Table 7: HAP Version 4.2 Key Characteristics

Key Strengths                                            Key Weaknesses

        Bestested to DOE-2                                    Limited ability to calculate interactions
        Compares energy consumption and                        between some strategies
         operating costs of design alternatives

Hourly, whole-building energy analysis program calculating energy performance and life-cycle cost of
operation. Can be used to analyze energy efficiency of given designs or efficiency of new technologies.
Other uses include utility demand-side management and rebate programs, development and
implementation of energy efficiency standards and compliance certification, and training new corps of
energy efficiency conscious building professionals in architecture and engineering schools.

Table 8: DOE-2 Version 2.1E Key Characteristics

Key Strengths                                            Key Weaknesses
        Detailed, hourly, whole-building energy               High level of user knowledge and computer
         analysis of multiple zones in buildings of             literacy required
         complex design
        Widely recognized, the de-facto standard

Windows interface to the DOE-2.1E energy simulation program. Through the graphical interface, users
construct a model of the building's geometry using standard block shapes or using a built-in drawing tool.
Building systems are defined through a point-and-click interface. A library of constructions, fenestrations,
systems, and operating schedules is included, and the user can add custom elements as well. If desired,
the program assigns default values for parameters based on the vintage and size of the building.

VisualDOE is especially useful for studies of envelope and HVAC design alternatives. Up to 20
alternatives can be defined for a single project. Summary reports and graphs may be printed directly from
the program. Hourly reports of building parameters may also be viewed.

A graphical front-end to the DOE-2 building energy analysis software (see below). Includes graphical
editing and scheduling capabilities, and flexible output options. There is online help in addition to a user
manual. Designed for U.S. and international users. Weather data is available for U.S. and some
international locations; custom data may be entered by the user. A free demo is available for download.

Table 9: VisualDOE 2.6 Key Characteristics

Key Strengths                                           Key Weaknesses

        Dramatically reduces the time necessary to            Relatively expensive
         build a DOE-2 model                                   Passive solar features poorly modeled
        Uses DOE-2 as a simulation engine
        Displays a 3-D model to help verify accuracy
        Implements DOE-2's daylighting calculations
        imports CADD data to define thermal zones

BLAST (Building Loads Analysis and System Thermodynamics)
Performs hourly simulations of buildings, air handling systems, and central plant equipment in order to
provide mechanical, energy and architectural engineers with accurate estimates of a building's energy
needs. The zone models of BLAST that are based on the fundamental heat balance method, are the
industry standard for heating and cooling load calculations.

Evaluation of high-potential, cost-effective energy efficiency projects in existing Federal buildings;
calculates results that are within 4-5% of DOE-2 annual energy results; using quick input routines,
permits evaluation of a 10,000 sf. building in about ten minutes. ASEAM (A Simplified Energy Analysis
Method) Version 5.0 automatically creates DOE-2 input files.
Table 10: BLAST Key Characteristics

Key Strengths                                            Key Weaknesses

        Uses detailed heat balance algorithms that            High level of expertise required to operate
         allow for the analysis of thermal comfort and
         other factors that cannot be analyzed in
         programs with less rigorous zone models

is a building energy simulation program that builds on the most popular features and capabilities of
BLAST and DOE-2. EnergyPlus includes innovative simulation capabilities including time steps of less
than an hour, modular systems simulation modules that are integrated with a heat balance-based zone
simulation, and input and output data structures tailored to facilitate third party interface development
(see Table 11). A few of the new features in EnergyPlus Version 1.2.2 include: modeling of ventilated
photovoltaic roof and other cladding systems, natural cross ventilation, simplified definition of HVAC
systems, refrigerated casework, variable speed cooling towers and speed improvements throughout.
Several interfaces and utilities for EnergyPlus are available, including EP-Quick which creates an
EnergyPlus input file based on a broad range of zone templates. For up-to-date information on interfaces
click here.

Table 11: EnergyPlus Version 1.2.2 Key Characteristics

Key Strengths                                            Key Weaknesses

        Accurate, detailed simulation capabilities            Difficult to use without graphical interfaces
         through complex modeling capabilities
        Input is geared to the 'object' model way of
        Successful interfacing using IFC standard
         architectural model available for obtaining
         geometry from CAD programs
        Weather data for more than 550 locations
         worldwide available on the website
Fig. 3. EnergyPlus screen capture

F. Economic Assessment Tools

BLCC (Building Life-Cycle Cost)
Provides comprehensive economic analysis of proposed building capital investments. BLCC is especially
useful for evaluating energy and water conservation projects in buildings. Up to 99 alternative designs
can be evaluated simultaneously to determine which has the lowest life-cycle cost. Economic measures,
including net savings, savings-to-investment ratio, adjusted internal rate of return, and payback period
are calculated for any design alternative relative to the designated base case. It contains modules to
evaluate agency-funded projects according to 10 CFR 436A and projects that are financed through
ESPC or utility contracts as directed by Executive Order 13123. The remaining modules, now in BLCC4
(for DoD military construction projects, OMB projects, and private-sector projects including taxes and
financing) will be programmed into BLCC5 in the next few years. It complies with ASTM International
standards related to building economics and NIST Handbook 135, Life-Cycle Costing Manual for the
Federal Energy Management Program (95 ed.).

Table 12: BLCC Version 5.3-05 Key Characteristics

Key Strengths                                           Key Weaknesses

        Updated annually for discount rates and                Results are not particularly graphic
         energy prices                                          User-requested improvements for alternative
        Performs high quality LCC analysis                      financing are still being incorporated
        User's Guide included as file

QuickBLCC (Quick Building Life-Cycle Cost)
Used to set up multiple project alternatives for life-cycle costing analysis in a single input file. The Quick
BLCC (Quick Building Life-Cycle Cost) program provides a convenient method for solving relatively
simple LCC problems that require finding the lowest LCC design alternative among many mutually
exclusive alternatives for the same project. Input data files are transferable to BLCC for more detailed

Table 13. QuickBLCC Version 2.9-05 Key Characteristics

Key Strengths                                              Key Weaknesses

           Ideal for preliminary economic evaluation of          No private-sector tax analysis included
            multiple design alternatives
           Users guide included as file with program

G. The Limits and Benefits of Energy Analysis Tools

Users of energy analysis tools should be aware that energy calculations, regardless of their
sophistication, cannot precisely predict actual energy consumption. Factors such as construction quality,
occupancy schedules, and maintenance procedures may vary markedly from assumptions contained in
the analysis and skew results. However, this does not mean that energy analyses are not important tools.

It is also important for users of energy analysis tools to understand the interrelationships among all
aspects of building design. Employing an integrated 'whole building' design approach to site selection,
orientation, building envelope and high-performance HVAC system choices, while considering life cycle
cost analysis, is critical to achieving a truly successful building design.

Conducting an energy study of a new building or a major retrofit project is an excellent means by which to
evaluate the relative energy performance of alternative designs. In particular, the effect of low-energy
strategies such as moving windows from one façade to another for passive solar heating or improved
daylighting, optimizing glazing selection or installing dimmable ballasts can be carefully evaluated on a
comparative basis.

H. Other Thermal Simulation Software

In addition to providing energy analysis, programs are available that analyze other building thermal
issues, including:

Mold and Moisture Dynamics in building assemblies such as wall and roofs

                                                    An example of such a program is MOIST available free
                                                     from the National Institute of Standards and Technology
                                                     (NIST) Building Fire Research Laboratory (BFRL).

Window Performance including the effects of frame area on net window performance
                                               An example of such a program is WINDOW developed at
                                                the Lawrence Berkeley National Laboratory (LBNL); it
                                                calculates Uvalues, SHGF, and Tvis of window systems
                                                constructed from glass and frames of known properties.

Natural Ventilation including the effects of complex airflow patterns in atriums and multi-zone spaces

                                               Programs based on the principles of Computational Fluid
                                                Dynamics, (CFD) can calculate three-dimensional airflow

                                                                                              BACK TO TOP


Software developers call the core set of instructions (or algorithms) that determine what calculations are
to be performed, a program's "simulation engine." These engines are usually developed over long
periods of time by experienced building researchers and programmers invariably with the benefit of
government funding. In sophisticated computer models, these engines are written in basic programming
languages such as Fortran or C++ with input and output formats that are not easily understood by the
average professional user.

To address this problem, third party vendors have emerged who have created highly graphic input (called
front ends) and output screens to sandwich around public domain simulation engines. For example,
VisualDOE and DOE 2.2 are proprietary programs developed around DOE-2.

                                                                                              BACK TO TOP


Some Federal agencies may require use of energy analysis tools to determine a project's annual energy
consumption or to verify a project's compliance with agency energy criteria. However, before selecting a
tool, check with your agency's project manager for approved computer-based, energy analysis tools.

                                                                                              BACK TO TOP


                                               Energy Policy Act of 2005 (PDF 1.9 MB)

                                                                                              BACK TO TOP



Atria Systems
Federal Green Construction Guide for Specifiers:

                                             14 20 00 (14200) Elevators
                                             23 70 00 (15700) Central HVAC Equipment
                                             26 50 00 (16500) Lighting
                                             48 14 00 (13600) Solar Energy Electrical Power
                                              Generation Equipment
                                             48 15 00 (13600) Wind Energy Electrical Power
                                              Generation Equipment
                                             48 30 00 (13600) Biomass Energy Electrical Power
                                              Generation Equipment

                                             DOE Office of Building Technology, State & Community
                                              Programs—Building Energy Software Tools Directory
                                             FEMP—Procurement of Architectural and Engineering
                                              Services for Sustainable Buildings: A Guide for Federal
                                              Project Managers
                                             GSA LEED® Applications Guide
                                             GSA LEED® Cost Study
                                             WBDG Tools

To top