4.492 Daylighting in Buildings Class01 - MIT OpenCourseWare...4.492 Daylighting in Buildings Class01...

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Sky models Overcast sky 7’000 (winter) to 20’000 (summer) lux on ground uniform L(θ)=L z 1 1

Transcript of 4.492 Daylighting in Buildings Class01 - MIT OpenCourseWare...4.492 Daylighting in Buildings Class01...

Page 1: 4.492 Daylighting in Buildings Class01 - MIT OpenCourseWare...4.492 Daylighting in Buildings Class01 Author Marilyne Andersen Created Date 4/13/2006 12:42:36 PM ...

Sky models

Overcast sky7’000 (winter) to 20’000 (summer) lux on ground

uniform L(θ)=Lz

1 1

Page 2: 4.492 Daylighting in Buildings Class01 - MIT OpenCourseWare...4.492 Daylighting in Buildings Class01 Author Marilyne Andersen Created Date 4/13/2006 12:42:36 PM ...

Sky models

Overcast sky7’000 (winter) to 20’000 (summer) lux on ground

CIE overcast L(θ)=Lz (1+2sinθ)/3

1 3

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Sky models

Clear sky30’000 (winter) to 100’000 (summer) lux on ground

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Sky models

Clear skyCIE clear sky model

L = fction of zenith luminance and sun position

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Sky models

Clear skyCIE clear sky model

L = fction of zenith luminance and sun position

Perez All Weather sky model (generalization of CIE clear sky)L = fction of zenith luminance, sun position, dew point and 5 coefficients

L fully defined if diffuse and direct irradiance are known

Ex. for overcast: Perez vs. CIE

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W = Area of each window (m²),

τ = Transmittance of each glazing material

θ = Vertical angle of sky as seen from centre of each window

m = Maintenance factor based on angle of glazing and cleanliness (0.5 – 0.9),

A = Total internal surface area of space, including walls, floors, ceilings & windows (m²)

R 2= Area weighted average reflectance of all surfaces making up A

(use 0.1 as reflectance for glass).

Average Daylight Factor calculation

Empirical formula

where

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LEED Green Building Rating System

Daylighting credits§ 8.1 = Daylight 75% of spaces with DF > 2% (1 credit)§ 8.2 = View for 90% of occupied spaces (2 credits)

Estimation using spreadsheet

Chart for ­ Geometry Factor­ Min Tvis

­ Height factor

No information about glare, overheating…

FactorHeightWindowMinT

TActualGeometryWindow

]sf[AreaFloor

]sf[AreaWindow[%]DF

vis

vis ×××=

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Split-flux method for Daylight FactorUK Building Research Establishment (BRE)

D [%] = EP / Eh = sum of:Direct (sky) component: SC

Daylighting protractor lines of sight and correction factors

Internally Reflected

Component

Direct Component

Externally Reflected Component

Eh

Ep

Figure by MIT OCW.

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Split-flux method for Daylight Factor

D [%] = EP / Eh = sum of:Direct (sky) component: SCExternally reflected component: ERC

Consider as sky component withdifferent luminance

Internally Reflected

Component

Direct Component

Externally Reflected Component

Eh

Ep

Figure by MIT OCW.

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Split-flux method for Daylight FactorD [%] = EP / Eh = sum of:

Direct (sky) component: SCExternally reflected component: ERCInternally reflected component: IRC

Use formula

Internally Reflected

Component

Direct Component

Externally Reflected Component

Eh

Ep

Figure by MIT OCW.