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Agenda
Evaluation method for daylighting design
Sophisticated computational method
Standard skies
Comparing of existing assessment methods
Shortcomings of existing method and the use of climate-base daylight modeling
Evaluation method for daylighting design
• Scale-model measurement
• Simplified calculation method
• Sophisticated computational method
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.
Suggested scale of modelScale Application(s)
1:200 - 1:500 For preliminary design and concept developmentTo provide a gross sense of the massing of the projectTo study the shadow created by the future building or from a neighbouring building
1:200 - 1:50 To study direct sunlight penetration into a building (e.g. efficiency of solar protectionTo study diffuse daylight in a very big space (e.g. atrium)
1:100 - 1:10 To consider detailed refinement of spatial componentsTo have highly detailed inside view (e.g. video or photos)To study accurately diffuse and direct dayligth penetration
1:1-1:10 To integrate critical industrial componentsTo consider daylighting devices that cannot be reduced in scaleTo proceed to final evaluation of advanced daylighting systems through monitoring and user assessment
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)
Artificial sky measurement
• Artificial sky
• Sky-simulator
Artificial Sky
Sky simulator
Simplified calculation method
• Daylight factor
• Average daylight factor
• Vertical daylight factor
Daylight factor
Where,DF = daylight factor (%)Ev = illuminance level of a point (lx)Evd = horizontal diffuse illuminance (lx)
Average daylight factor
Where,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)
Vertical daylight factor
Where,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)
Sophisticated computational method
• Maxwell’s electromagnetic wave
• Radiosity
• Ray tracing
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
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
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
Examples of simulation package
Algorithm Package
Radiosity AGI32 (www.agi32.com)Lumen Micro (www.ltioptics.com –discontinued)
Forward ray tracing Photopia (www.ltioptics.com)TracePro (www.lambdares.com)OptiCAD (www.opticad.com)
Backward ray tracing RADIANCE (radsite.lbl.gov)Desktop Radiance (radsite.lbl.gov – without any update since 2002)
Major components in Radiance system
MaterialSky Geometry Lighting
Converter
Binary file
Calculation
ASCII results
Rendering
High dynamic range image
Climate dependent parameter Climate independent parameters
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
Sky description
Sun brightness
Solar position and size
Sky brightness, distribution pattern and solar position
Sky brightness multiplier and colour correction
Sky vault direction and angular size
Material description
Light-emitting material
Non light-emitting material
Virtual material
Light Mirror Mist
Illum Prism Antimatter
Glow Plastic
Spotlight Metal
Trans
Dielectric
Glass
Interface
Major materials in Radiance:
Material description
Diffuse reflection
Specular reflection
Incident light
Material descriptionSurface type
Colour of material
Roughness
Specularity
Geometry
• The following geometries are available in Radiance:
Polygon Ring Cylinder
Tube Cone Cup
Sphere Bubble
Geometry description
Coordinate of vertices
Material
Type of surface
Lighting
• Descriptions similar to that for other material
• Built-in an ies2rad program to assist designers convert IES file (IESNA) to radiance description file
Renderings – simulation model
Physically based renderings
Daylight Artificial light False-colour image
Daylight factor analysis
Contour lines overlay on modelFalse-colour image of daylight factor on working
plane
Glare and sunpath studies
Glare-source identification Sunpath diagram
Sky type
• CIE overcast sky
• CIE clear sky
• CIE standard skies
Lights coming from sun and sky
CIE overcast sky
Where,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)
CIE clear sky
Where,f(χ) = indicatrix function (dimensionless)φ(Z) = gradation function (dimensionless) χ = scattering angle (rad)Zs = solar zenith angle (rad)
CIE standard skies
Where,f(χ) = indicatrix function (dimensionless)φ(Z) = gradation function (dimensionless)
Daylighting requirements for LEED and BEAM Plus
Sky type Time Criteria
LEED (2.1 and 2.2)EQ Credit 8.1
Clear sky 12:00 on equinox 75% of area achieves 25 fc (269 lx) or more
LEED (3.0)EQ Credit 8.1
Clear sky 9:00 and 15:00 on equinox
75% of area falls between 25 fc (269 lx) and 500 fc (5381.9 lx)
BEAM Plus (1.1)IEQ 15
Overcast sky N/A 80% of area achieves a daylight factor 2% or more
Questions?
• Which clear sky should we choose?• Does the CIE overcast sky really represent
the worst scenario?• Does the current assessing criteria sufficient?
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
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
Radiance built-in CIE clear sky
86,024 9,018
At solar noon on the equinox
Sky luminance distribution
CIE Clear Sky 11 Sky 12
Sky 13 Sky 14 Sky 15
Relative luminance (%)
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 8.5.1.1 – “Simulation shall be done using either CIE
Overcast Sky Model or the CIE Clear Sky Model”
Does the CIE overcast sky really represent the worst scenario?
CIE Clear Sky CIE Overcast Sky
North-facing window at the noon on equinox(Both skies are generated by gensky)
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?
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 (www.daysim.com)
Questions & Answers Session
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