daylighting calculations._kroelinger, mike

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DAYLIGHTING CALCULATIONS Mike Kroelinger ABS 731 Spring 2006

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DAYLIGHTINGCALCULATIONSMike KroelingerABS 731 Spring 2006Methods for Calculation Lumen Method Daylight Factor Method Overcast skyClear sky BRE Protractors Flux transfer MethodLumen Method Provides simple way to predict interiordaylight through skylights & windows. Similar to the zonal cavity method forelectric lighting. Assumes an empty, rectangular room withsimple fenestration & shading devices,including light shelves.Lumen Method Four basic steps:Exterior illuminances at window or skylight aredetermined from daylight availabilitymaterials.Net transmittance is determined.CUs, or ratios of interior to exteriorilluminace, are determined.Interior illuminance is calculated.Lumen Method Available daylightOvercast skyLumen Method Available daylightClear sky - summerLumen Method Available daylightClear sky fall &springLumen Method Available daylightClear sky winterLumen Method Basic equationEi= ExNT CU(8-27, IES Handbook)Where: Ei= interior illuminance in lx Ex= exterior illuminance in lx NT = net transmittance CU = coefficient of utilization Formula for toplighting differsLumen MethodDaylight Factor Low precision procedure for determiningilluminance at any point in an interiorspace. Direct sunlight excluded. Uses sky with known luminancedistribution.Daylight Factor DF is ratio of illuminance at a point on aplane to the illuminance on a horizontalexterior plane produced by a hemisphereof the sky. Has three components that are summed:Sky component (SC)Externally reflected component (ERC)Internally reflected component (IRC)Daylight Factor Sky component (SC)Due to daylight received directly at the pointfrom the sky. Externally reflected component (ERC)Due to daylight received directly at the pointfrom external reflecting surfaces. Internally reflected component (IRC)Due to daylight reaching the point after oneor more interreflections from interiorsurfaces.Daylight Factor ComponentsDaylight Factor DF = SC + ERC + IRC Must determine each factor to determineDF.Daylight FactorDaylight Factor Sky component Internal reflected componentDaylight FactorDaylight Factor Externally reflected componentGenerally small.Usually roughly approximated.See IES Handbook, ninth editionProtractors Used to estimate the sky & internallyreflected components of daylight factor. Used for:Overcast skiesClear skies without direct sun Use during the schematic phase to roughlyestimate performance.Protractors Based on a conceptual model of interiorilluminance being dependant on threevariables:Luminance of the sourceApparent angular size of the sourcePosition of the source relative to the analysispoint.Protractors Most common methods:BRS daylight factor protractors for overcastskiesBryan clear sky protractors The protractors are used to determine thesky component (SC) Combined with other data to determine thetotal daylight factor (DF).BRS Protractors Set includes ten protractors:Five for the uniform skyFive for the C.I.E. overcast sky For apertures that are vertical, 300, 600,horizontal, of without glazing. Two parts to each protractor:For sectionsFor plansBRS Protractors Sections & plans required to useprotractors. Limitations for use:Station point below sill height & at least 1from floor, walls, ceilingBest scale is =1-0 Guidelines for use in Robbins; Moore; orHopkinson, et al.BRS ProtractorsBRS ProtractorsBRS ProtractorsBryan Clear Sky Protractors Developed by Bryan & Carlsberg (1982) Nine protractors based on BRS method For clear skies. Formatted differently than BRS but used in samemanner. IRC determined by table. Cannot be used for horizontal apertures or slantedglazing. Can be used for vertical apertures, impact of overhangs,& vertical fins.Resources IES Handbook, 9thEdition, 2000. Robbins, Claude. Daylighting design &analysis. New York: Van NostrandReinhold Company, 1986. Moore, Fuller. Concepts and practices ofarchitectural daylighting. New York: VanNostrand Reinhold Company, 1985.