daylight - wtgzik.pairserver.comwtgzik.pairserver.com/courses/273s18/273-daylight.pdf · evans:...

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Ball State Architecture | ENVIRONMENTAL SYSTEMS 1 | Grondzik 1

DAYLIGHT

image by NASA – use this powerful resource with skill and caution


Daylight and Design

“From the dawn of time until the widespreadintroduction of the fluorescent lamp during the post-World War II period, the sun was the predominant source of daytime light in buildings. It was far more available and effective than other sources, and it provided mankind with a continuing relationship with nature and the outdoors which were close to his origins. The introduction of daylight into buildings shaped structural concepts, stimulated product development, and, in general, influenced the form of architecture for some of our greatest buildings.”

Benjamin Evans: Daylight in Architecture

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Solar and Terrestrial Radiation

terrestrial radiation

Lechner: Heating, Cooling, Lighting

file this info away for upcoming“greenhouse effect” discussion

high temperature objects produce shortwave radiation;low temperature objects produce longwave radiation

solar radiationincludes:UV visibleIR


Solar Radiation Components

direct = “ray”diffuse = “spray”reflected = more spray than ray

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Blocking Solar Radiation

direct radiation is more easily blocked than diffuse or reflected; file this idea away for upcoming shading device discussion


Solar Radiation vis-a-vis Daylight

daylight =visible

solar =visible+ UV+ IR

Lechner: Heating, Cooling, Lighting

solar at Earth’s surface

solar radiation

daylight

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Daylight vis-a-vis Glazing

glazing for daylightingwould ideally admit only the visible portion of the spectrum and block radiation in the grey shaded (IR and UV) regions of the graph – through a process called selective transmission

only this part of thesolar spectrum allows

us to see

fade

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scontributes heat


Daylighting versus Sunlighting

energy-efficient, low-glare daylighting uses ONLY diffuse solar

direct solar radiation is used for passive heating or sparkle (or to find truth in a forest)

daylighting sunlighting

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Some Defining Characteristicsof Daylight

• Daylight is variable on a daily cycle– availability (quantity) generally goes from 0 at midnight to some

maximum around noon and then back to 0 in a 24-hour period

• Daylight is variable on an annual cycle– maximum daily availability goes from “a” to “z” then back to “a” in

365 days

• Daylight is variable geographically– is more readily available in some locales (primarily as a function

of site latitude, but also due to climate) than in other locales

• Daylight is variable relative to surface position– vertical, tilted, and horizontal surfaces receive different daylight

illuminances under like sky conditions


Some Defining Characteristicsof Daylight

• Daylight can vary with orientation– depending upon sky type, N, S, E, W orientations

receive different illuminances under like conditions

• Daylight is variable randomly and quickly as a result of weather

• The color of daylight is variable with sky type, orientation, and time of day

Daylight is variable!Its magnitude at any time is statistically predictable

… but not absolutely definable

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The Color of Daylight

daylight varies in wavelength (light-source color) –but is generally balanced (it has a broad spectrum)


Predicting Daylight Availability

• Daylight is statistically predictable– broadly speaking, daylight quantity can be fairly

accurately predicted over long-term periods (say 5 to 10 years) – such patterns are a function of climate

– historical-data-based predictions provide acceptable accuracy for energy analyses and simulations (similar to the accuracy of temperature, humidity, or rainfall projections)

– easily usable daylight data, however, are not readily available (even from Climate Consultant)

• Daylight is specifically unpredictable– it is generally impossible to predict how much daylight

will be available at noon next week – such information is a function of the weather (and how accurate are weather forecasts?)

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it is quite variable and hard to predict …

So Why Use Daylight? • It is a renewable energy resource• It is a use-it-or-lose-it resource (it cannot be stored)• People like it• It requires an architectural design response• It conserves non-renewable resources for potentially

better uses (medicines, transportation, the future)• It can contribute to an energy-efficient design solution• It can contribute to a green design solution• It can contribute to a net-zero-energy design solution• It can contribute to a carbon-neutral design solution• It can contribute to a sustainable design solution

opinion: if this list sounds like a compelling argument for using daylighting …that’s good—because it should


So Why Use Daylight?

a well-designed daylighting system—without direct solar radiation (compare yellow arrows)—can be VERY energy efficient

what’s with the Wattsfor daylight?

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So Why is Daylight not Used More?

• It is variable

• It is not always well understood by designers

• Resource data are often not readily available (not a trivial concern)

• Analysis techniques (including simulations) can be daunting

• It requires an architectural design response

– Thus, its design cannot be farmed out to a consultant (as electric lighting often is)

– Thus, it may require more creative compromises

– Thus, blah, blahopinion: if this sounds more like a list of excuses than justifications …that’s good—because it should


Daylight Sources

• The sky vault (sky dome or hemisphere)– A viable source of daylight for building design

– Three primary “types” of skies are considered for design purposes

• Overcast (fully cloudy)

• Clear (no clouds)

• Partly cloudy (some percentage cloud coverage)

• Direct solar radiation– Almost NEVER used as a light source in buildings …

due to unnecessary heat gain and glare potential– Rethink what you may have thought about this !!!

– Good daylighting does not depend upon access to direct sun !!!

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(Fully) Overcast Sky spring; San Antonio, Texas


Overcast Sky Luminance Pattern

highest luminance (L) at zenith

orientation is not relevant

horizontal skylights work very well under overcast skies:they provide more light per square foot of opening than a vertical window

aperture

aper

ture

aperture = daylight opening

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Clear Sky spring; Australian outback


Clear Sky Luminance Pattern

highest luminance (> L) is toward sun, but that brings problems

luminance at horizon (L) is greater than at zenith

orientation is relevant

windows work well under clear skies: they provide more light per square footof opening than a horizontal skylight … and are easier to shade

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Partly Cloudy Sky

can range from almost clear to almost fully overcast; variability and uncertainty make this a difficult “design” sky (even if a common occurrence)

spring; central Wyoming


Which Design Sky?

A designer must decide which sky condition to use for building design and system analysis:

– In an arid climate (Phoenix), it is logical to design for a clear sky condition … and live with overcast conditions now and then

– In a maritime climate (Seattle), it is logical to design for an overcast sky … and live with clear conditions now and then

– In a temperate climate (Muncie), it is logical to design with both skies in mind and try to optimize performance using computer simulations

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A Cautionary Note

Much of the information written and published about daylighting comes from the UK (or similar dreary climates). An

often unstated assumption is that an overcast sky condition predominates. Consider this

when using daylighting design recommendations and tools. Sky

type makes a difference to design decisions.


Daylight Illuminance DataOvercast vs. Clear Sky

note vertical scale change on graphsand lack of orientation info on overcast graphEvans: Daylight in Architecture,

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Daylight Illuminance DataClear Sky – Summer vs. Winter

Evans: Daylight in Architecture

note vertical scales are identical and wintercurves are depressed versus summer curves


Daylight Illuminance Data Clear Sky vs. Direct Sun

Evans: Daylight in Architecture

note vertical scale change on graphsand patterns of illuminance values

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Handling Daylight Variability

If the only way to present information on daylighting system performance was via

illuminance values, the discussion might go like this

The illuminance is 250 lux, no wait it’s 265, no make that 240, no wait it has changed again. And if you’ll wait 8-10

hours the illuminance will be 0.

Daylight factor (DF) provides a means of handling variable daylight illuminance

One can say: the DF at point “x” is 1.5% (without hedging or the need for detailed footnotes).


Daylight Factor (DF)

DF is the ratio of interior daylightilluminance (E) on a defined point to exterior daylight illuminance (on a

horizontal reference point)

DF = E interior (daylight, at point “x”)

E exterior (daylight, reference)

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• DF normalizes for variances in external daylight illuminances (as the numerator in the ratio increases, the denominator also increases)

• DF is reasonably constant across time, for a given daylighting approach/sky; thus DF provides a more consistent performance indicator than illuminance

• The exterior reference daylight illuminance is normally an ambient, unshaded, horizontal-surface daylight illuminance that excludes direct solar radiation

• The concept of DF was developed for overcast sky conditions; it must be used with caution if applied to clear sky conditions

• DF can be conceptually viewed as the efficiency of a building design in delivering daylight to a desired location in the building (a 2% DF is a 2% efficiency)


Daylight Factor (DF)• DF is a point phenomenon

– There is no such thing as the daylight factor of a room– Daylight illuminance distribution across a room is usually

highly variable (there are many values of DF within a room)– Minimum daylight illuminance is usually more important to

design than maximum (minimum should be used to determine whether design criteria have been met; maximum may be of concern relative to glare)

• DF is often used as a design criterion, usually as a threshold to be met – For example, LEED (NC-2.1—an older version) required a

minimum 2% DF (excluding direct sun) in approximately 75% of building spaces to obtain a daylighting credit

– As a side note: direct exterior views in 90% of spaces could earn another credit, an expression of the anticipated effect of visual accommodation on comfort

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Conceptually, daylight factor (DF) is a measure of the effectiveness

of a building design in admitting and distributing a site’s daylight resource

to a specific point within a building. Many design decisions (aperture size and location,

glazing materials, surface reflectances, shading devices, etc.) will affect DF.

A 2% DF literally means that only 2 of every 100 lumens of exterior light reaches a given interior point … yet this

is often more than enough to make a difference.


Daylight Factor (DF) Contours

DF in %(note the “low” values)

iso-DF contours are often used to represent measured or predicted daylight distribution in a space (note the many DF values for both spaces)

<< hand-plotted

^^ Ecotect analysis

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