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One day short course on Green Building Assessment Methods - Daylight Simulation
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One day short course on Green Building Assessment Methods - Daylight Simulation



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  • 1. AgendaEvaluation method for daylighting designSophisticated computational methodStandard skiesComparing of existing assessment methodsShortcomings of existing method and the use of climate-base daylight modeling
  • 2. Evaluation method for daylighting design• Scale-model measurement• Simplified calculation method• Sophisticated computational method
  • 3. Scale-model measurement• Building professionals use scale models as design tools to study various aspects of building design and construction• Widely recognised by building professionals for years• Scale models portray the distribution of daylight within the model room almost as exactly as in a full-size room.
  • 4. Suggested scale of modelScale Application(s)1:200 - 1:500 For preliminary design and concept development To provide a gross sense of the massing of the project To study the shadow created by the future building or from a neighbouring building1:200 - 1:50 To study direct sunlight penetration into a building (e.g. efficiency of solar protection To study diffuse daylight in a very big space (e.g. atrium)1:100 - 1:10 To consider detailed refinement of spatial components To have highly detailed inside view (e.g. video or photos) To study accurately diffuse and direct dayligth penetration1:1-1:10 To integrate critical industrial components To consider daylighting devices that cannot be reduced in scale To proceed to final evaluation of advanced daylighting systems through monitoring and user assessment
  • 5. Real-sky measurement• Overcast Sky – 0.36 < foc < 0.44 (True Value = 0.396) – Any time of a year• Clear Sky – winter solstice (+/- 4 weeks, 1 day) – equinox, either spring or autumn (+/- 4 weeks, 1 day). In locations where there is a significant discrepancy between the spring and the autumn equinox, it is recommended to measure during both equinox. – summer solstice (+/- 4 weeks, 1 day)
  • 6. Artificial sky measurement• Artificial sky• Sky-simulator
  • 7. Artificial Sky
  • 8. Sky simulator
  • 9. Simplified calculation method• Daylight factor• Average daylight factor• Vertical daylight factor
  • 10. Daylight factorWhere, DF = daylight factor (%) Ev = illuminance level of a point (lx) Evd = horizontal diffuse illuminance (lx)
  • 11. Average daylight factorWhere, DFave = average daylight factor (%) τ = transmittance of window (dimensionless) W = window area (m2) θ = angle subtended by the visible sky (rad) A = area of internal surface (m2) R = average reflectance (dimensionless)
  • 12. Vertical daylight factorWhere, VDF = vertical Daylight Factor (%) Ews = illuminance for light reflected onto the window opening from unobstructed sky(lx) Ewr = illuminance for light reflected onto the window opening from surrounding building (lx) ρg = ground reflectance (dimensionless) Evrg = ground illuminance (lx)
  • 13. Sophisticated computational method• Maxwell’s electromagnetic wave• Radiosity• Ray tracing
  • 14. Maxwell’s electromagnetic wave• Treating light as an EM wave• Could model the behaviours (reflection, refraction, diffraction, interference) of light• Uncommon neither in daylighting research or design
  • 15. Radiosity• Originally developed for energy calculation (energy balance for a set of surfaces exchanging radiant energy)• Surfaces must be subdivided into finite elements• All elements are assumed to be perfectly diffuse• View independent• Pre-eminence of major light sources
  • 16. Ray tracing• Determine the visibility of surfaces by tracing imaginary rays of light form viewer’s eye.• Account for every optical phenomenon that can analytically expressed by physical equation• Can consider specular material• Include forward and backward ray tracing approaches
  • 17. Examples of simulation packageAlgorithm PackageRadiosity AGI32 ( Lumen Micro ( – discontinued)Forward ray tracing Photopia ( TracePro ( OptiCAD ( ray tracing RADIANCE ( Desktop Radiance ( – without any update since 2002)
  • 18. Major components in Radiance systemClimate dependent parameter Climate independent parameters Sky Material Geometry Lighting Converter Binary file Calculation Rendering High dynamic ASCII results range image
  • 19. Sky description• A program gensky is included in Radiance. This program can create a sky description file for the following skies: – CIE overcast sky – Uniform sky – CIE clear sky – CIE intermediate sky
  • 20. Sky description Sky brightness, distribution pattern andSun solar positionbrightness Sky brightness multiplier and colour correctionSolarpositionand size Sky vault direction and angular size
  • 21. Material descriptionMajor materials in Radiance:Light-emitting Non light-emitting Virtual materialmaterial materialLight Mirror MistIllum Prism AntimatterGlow PlasticSpotlight Metal Trans Dielectric Glass Interface
  • 22. Material description Incident light Specular reflection Diffuse reflection
  • 23. Material description Surface type Colour of material Roughness Specularity
  • 24. Geometry• The following geometries are available in Radiance: Polygon Ring Cylinder Tube Cone Cup Sphere Bubble
  • 25. Geometry descriptionMaterial Coordinate of verticesType of surface
  • 26. Lighting• Descriptions similar to that for other material• Built-in an ies2rad program to assist designers convert IES file (IESNA) to radiance description file
  • 27. Renderings – simulation model
  • 28. Physically based renderingsDaylight Artificial light False-colour image
  • 29. Daylight factor analysis False-colour image of Contour lines overlay on modeldaylight factor on working plane
  • 30. Glare and sunpath studiesGlare-source identification Sunpath diagram
  • 31. Sky type• CIE overcast sky• CIE clear sky• CIE standard skies
  • 32. Lights coming from sun and sky
  • 33. CIE overcast skyWhere, L = sky luminance in an arbitrary sky element (cd/m2) Lz = sky luminance at the zenith (cd/m2) Z = zenith angle of a sky element (rad)
  • 34. CIE clear skyWhere, f(χ) = indicatrix function (dimensionless) φ(Z) = gradation function (dimensionless) χ = scattering angle (rad) Zs = solar zenith angle (rad)
  • 35. CIE standard skiesWhere, f(χ) = indicatrix function (dimensionless) φ(Z) = gradation function (dimensionless)
  • 36. Daylighting requirements for LEED and BEAM Plus Sky type Time CriteriaLEED (2.1 and 2.2) Clear sky 12:00 on equinox 75% of areaEQ Credit 8.1 achieves 25 fc (269 lx) or moreLEED (3.0) Clear sky 9:00 and 15:00 on 75% of area fallsEQ Credit 8.1 equinox between 25 fc (269 lx) and 500 fc (5381.9 lx)BEAM Plus (1.1) Overcast sky N/A 80% of areaIEQ 15 achieves a daylight factor 2% or more
  • 37. Questions?• Which clear sky should we choose?• Does the CIE overcast sky really represent the worst scenario?• Does the current assessing criteria sufficient?
  • 38. Which clear sky should we choose?• CIE clear or CIE standard skies Sky model Type of sky 11 White – blue sky with a clear solar corona 12 Very clear / unturbid with a clear solar corona 13 Cloudless polluted with a broader solar corona 14 Cloudless turbid with a broader solar corona 15 White – blue sky turbid with a wide solar corona effect
  • 39. Which clear sky should we choose?Sky model Direct sun (lx) Diffuse sky (lx) 11 86,737 28,539 12 102,048 12,408 13 82,164 34,743 14 77,831 34,743 15 86,739 37,225 Radiancebuilt-in CIE 86,024 9,018 clear skyAt solar noon on the equinox
  • 40. Sky luminance distribution CIE Clear Sky 11 Sky 12Relativeluminance(%) Sky 13 Sky 14 Sky 15
  • 41. Which clear sky should we choose?• ANSI/ASHRAE/USGBC/IES Standard 189.1- 2009 “Standard for the Design of High- Performance Green Buildings Except Low- Rise Residential Buildings”• Clause – “Simulation shall be done using either CIE Overcast Sky Model or the CIE Clear Sky Model”
  • 42. Does the CIE overcast sky really represent the worst scenario? North-facing window at the noon on equinox (Both skies are generated CIE Clear Sky CIE Overcast Sky by gensky)
  • 43. Does the current assessing criteria sufficient?• Does the monitoring period long enough?• Is this city dominated by clear or overcast sky?• Does the weather data enough for conducting long-term analysis?
  • 44. Climate-base daylight modeling• Based on measured / modeled outdoor illuminance/luminance distribution data• Algorithm – Daylight coefficient approach• Define a range of useful daylight level (e.g. 300lx – 2,500lx)• Provide annual daylighting performance analysis• Example of a Radiance Based program is DAYSIM (
  • 45. Questions & Answers Session