Role of Computer Simulation in Window Ratings and Design

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Role of Computer Simulation in Window Ratings and
Design

• By Dragan Curcija, Ph.D.
• Center for Energy Efficiency and Renewable Energy
  
• University of Massachusetts
• Amherst, MA
• U.S.A.
• AcknowledgementsThis work was sponsored by the
  Assistant Secretary for Conservation and
  Renewable Energy, Office of Building and
  Community Systems, Building Systems Division of
  the United States Department of Energy

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BRIEF OVERVIEW

• Evaluation of Thermal performance of windows done
  in two ways
• Physical testing
• Computer modeling
• In early days (before energy crisis in 70s) only
  physical testing
• Very limited number of products
• Energy use was not concern, mostly comfort and
  liability issues (early failures)
• First generation of testing standards

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OVERVIEW OF TESTING METHOD

• Physical testing expensive and time consuming
• Early testing not very reliable and repeatable
• In North America and European Union Hot Box
  method
• How is testing used
• in U.S. one specimen per product line. Also all
  specimens that dont validate
• In Canada testing used for challenge
• In Europe mostly testing used

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EARLY COMPUTER MODELS

• Research class computer models and general
  purpose commercial programs
• Better understand energy performance of windows
• Develop algorithms for simplified computer
  programs
• More detailed than testing results
• Not user friendly
• Too costly for certification purposes
• Early research programs later adopted for
  fenestration computer modeling tools

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RESAERCH CLASS COMPUTER MODELS
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DEVELOPMENT OF ALGORITHMS
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PC BASED COMPUTER PROGRAMS

• 20 years ago first computer programs for
  fenestration heat transfer
• 1-D heat transfer and solar optical properties
  for center of glass
• 2-D heat transfer in frames and edge of glass
  (few years later)
• First 2-D programs were FDM based
• Limited functionality
• Inaccurate geometry representation
• Long model preparation time

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ROLE OF EARLY COMPUTER PROGRAMS

• Reduced cost of evaluating thermal performance
  vs. testing
• Greater consistency of results
• Complemented testing in design and research
• Changed common wisdom of the role of computer
  models
• Helped create modern certification bodies
• Accelerated development of stricter code req.
• Helped introduction of performance labels

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MODERN FENESTRATION COMPUTER MODELING TOOLS

• WINDOW program incorporates GUI, database for
  data management
• THERM based on FEM (initially developed for
  military and space research), user friendly
• Allows for accurate geometry representation
• Imports AutoCAD drawings
• OPTICS and RESFEN
• Reduced model preparation time ? decreased cost
  of evaluating fenestration products
• Program(s) are developed through government
  funding and are free for public use

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WINDOW 5.1
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THERM 5.1

• Two-Dimensional heat transfer (conduction and
  radiation) and Glazing cavity convection
  simulation (correlation based)

• First version released in 1994
• Graphic tool to draw the model
• View-factor radiation modeling
• Automatic meshing

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MESH REFINEMENT
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OPTICS 5.1

• Virtual glass laboratory

• Calculate and edit optical properties of glass
  and IGU
• Change glazing thickness
• Switch glazing coating
• Construct glazing with applied films
• Construct glazing with laminate

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RESFEN 3.1

• Calculates Residential Energy Impacts

• Annual energy cost impacts of windows
• Uses DOE2.1E calculation engine
• Uses WINDOW output
• Library of standard houses and HVAC sys.

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COMPUTER TOOLS CERTIFICATION

• Validation studies showed that computer tools can
  be reliably used for certification
• First started in North America (U.S. Canada) 10
  years ago
• One validation test (U.S.) and one quality
  control test (production line testing)
• Validation criteria ?10 (often within 5)
• Limited number of products for which no good
  agreement

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INTERNATIONAL USE OF SIMULATION IN RATINGS

• Computer programs based on best science
• Physics is same everywhere in the world
• Algorithms in WINDOW suite published in North
  American international standards (ASHRAE
  SPC142P, ISO15099)
• Accepted for use in Russia
• Considered for use in Baltic countries, U.K.,
  European Union, etc.

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MAIN PHILOSOPHY

• Complexity is hidden behind ease of use!
• Complex algorithms
• User friendly and convenient users interface
• Premise Accuracy does not have to be sacrificed
  in order to create simple elegant computer tools

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WINDOW5 ENHANCED VISALIZATION

• Virtual Thermograms can provide a full
  temperature map of interior surfaces
• Areas under condensation can be overlaid to show
  critical areas

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INTEGRATED ENVIRONMENT
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ADVANCED FEATURES

• Modeling of projecting products Garden Window

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ADVANCED FEATURES

• Modeling of projecting products Skylights at 20

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CURRENT AREAS OF PROGRAM DEVELOPMENT

• Condensation Resistance for skylights and
  commercial fenestration systems
• 3-D Effects
• Specialty Projecting Products
• Improved accuracy in estimating annual energy
  performance
• Emerging technologies (i.e., Vacuum glazings,
  aero-gels,electrochromics, etc.)
• Improved accuracy and consistency

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FUTURE AREAS OF PROGRAM DEVELOPMENT

• 3-D heat transfer models
• Use of CFD for convection heat transfer
• Multi-dimensional solar heat gain effects
• Thermal-structural interaction analysis
• Acoustics analysis
• Durability analysis
• Contact resistance
• Mass transfer (diffusion, infiltration)

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CONCLUSIONS

• New fenestration computer programs are reliable
  tools for estimating thermal performance
• Computer modeling should become integral part of
  modern ratings procedure
• Computer modeling can effectively be used in the
  design of energy efficient windows
• Further development is necessary to
• improve accuracy for certain class of
  fenestration products
• Further improve ease of use and simplicity of
  users interface

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