Chapter 5. Daylighting

5.1. History of Daylighting

• The history of daylighting and the history of architecture were one.

• The major structural changes in buildings reflected the goal of increasing the amount of light that entered. (until the twentieth century)

- because artificial lighting had been both poor and expensive until then

1) Gothic Architecture

• Gothic architecture was primarily a resultof the quest for maximum window area.

• The roman groin vault replaced the barrelvault partly because it allowed larger windowsin the vaulted spaces.

Romanesque barrel vault(left) and Roman groin vault (right)

Roman Basilica(a rectangular church in Roman, the prototype of Gothic style of church architecture)

2) Renaissance Architecture

• Large and numerous windows were a dominant characteristic of Renaissance architecture.

• ‘E’- and ‘H’-shaped floor plans provided for their ventilation and daylight requirements.

3) 19th Century Architecture

• All glass buildings became possible- because of the increased availability of glass combined with the new ways of using iron for structures.

The Crystal Palace for the Great Exhibition of 1851, London park

4) Modern Architecture

• The masters of twentieth-century architecture have continued to use daylight for both functional and dramatic purposes.

Ronchamp, 1955, Le CorbusierThick walls with splayed windows, colored glass, and light scoops to bring carefully controlled light into the interior

The Guggenheim Museum, New work City, 1959, Frank Lloyd WrightA glass-domed atrium for diffused daylighting

Riola Parish Church, Italy, 1978, Alvar AaltoBent concrete frames to support the roof and to block the glare from the light scoops.

5.3. Daylight(晝光, 주광) – Source of daylight

1) Components of daylight

• Direct sunlight: the visible radiation received on a surface directly from the sun, without reflection by the sky.

• Diffuse skylight: the visible radiation received on a surface from the sky, including background sky brightness, horizon brightness, and circumsolar brightness.

• Reflected light: reflected light from the ground and neighboring structures.

The various sources of daylight

• Reyleigh scattering: scattering by the gas molecules in the atmosphere → blue sky

• Mie scattering: scattering by a floating particles (water vapor, dust) in the atmoshphere → cloudy sky and white cloud

2) Scattering of light rays in the atmosphere – Affects direction of light (direct vs diffuse)

- The sunrise and sunset red glow: The blue and violet ray (short-wavelength) are almost absorbed as a distance transmitting the earth’s atmosphere is lengthening

3) Absorption of light rays in the atmosphere – Affects spectrum and intensity of light (daytime vs sunrise/sunset)

5.4. Sky Models for Daylighting Design

1) CIE Standard Overcast Sky(曇天空 담천공)

• Zenith is three times brighter than the sky at the horizon.

Lθ = Lz (1+2sin θ) / 3

• Horizontal illuminance is about 5000 ~ 20000 lx.

• Under overcast skies, the main challenge for the designer is to maintain QUANTITY of daylight (illuminance).

2) CIE Standard Clear Sky(晴天空 청천공)

• The brightest part of the sky, which is in the direction of the sun, is about ten times brighter than the darkest part of the sky (which is found at about 90 degrees to the sun).

• Horizontal illuminance is about 60,000 - 100,000 lx.

• The direct sunlight is very directional and extremely bright.

• Under clear-sky, the main challenge for the designer is to maintain QUALITY of daylight (glare, shadow)

3) Exterior Horizontal Illuminance by Daylight(sunlight+skylgiht)

• Exterior illuminance is predicted by the luminous efficacy of daylight

• Luminous efficacy(K): the ratio of luminous flux by radiant flux

• Illuminance value is solar radiance times luminous efficacy of daylight- Luminous efficacy of direct sunlight: about 100 lm/W- Luminous efficacy of skylight: about 125 lm/W

]/[680

780

380 Wlmd

dVK

∫∫=

λφ

λφ

λ

λλ

φ(λ) = Radiant flux of wavelength (W/λ)v(λ) = CIE Standard relative spectral sensitivity680 = Maximum spectral luminous efficacy (lm/W)

5.4. Solar Geometry

1) Distance between the Earth and Sun

- The orbit of the earth is an ellipse:

• A small annual variation in the intensity of solar radiation (W/m2)

Sunrise and sunset hoursSolar angles

Solar RadiationExterior IlluminanceShading Design

• Average Distance(RO)= 1 A.U.(Astronomical Unit)= about 150 million km

- then, radiance and illuminance outside the atmosphere : 1353 W/m2, 127.5 klx

Solar diameter

1.39×106 kmEarth

diameter12,700 kmθ=32’

θ

kmRO810496.1 ×=

RMIN = 0.983 A.U.(January 3rd), irradiance 1418 W/m2

RMAX = 1.017 A.U.(July 4th), irradiance 1325 W/m2

• Distance equation between the earth and solar:

km

J: Julian date, 1≤J≤365 (e.g. 2/1=32, 12/31=365)

rR

R O=⎟⎠⎞

⎜⎝⎛+==⎟

⎠⎞

⎜⎝⎛

3652cos033.01

2 JrRRO π

2) Declination Angle (δ, 赤位 적위)

A angle between the equator and a line to/from the earth and Sun: the earth’s axis is tilted 23.5 degrees and varies each day

• Summer solstice(6/21): 23.45°• Winter solstice(12/21): -23.45°• Spring and fall equinoxes(3/21, 9/21): 0°

Sun

Earth

δ

23.45°

earth’s axis of retation

equator

⎥⎦⎤

⎢⎣⎡ +

=365

)284(360sin45.23 Jδ [°]

3) Day Angle (d, 日角 일각)

d

Sun

earthJanuary 1st=0°

365 equal sections

solar

earth

][365

)1(360o

−=

Jd

4) Solar Angles

Zenith angle Altitude angle

Azimuth angle

Profile angleIncident angle

At first, calculate Zenith angle (天頂角 천정각)or Altitude angle (高度角 고도각), and then:

• Azimuth angle(方位角 방위각): used to determine vertical fin design of buildings

• Profile angle(日影角 일영각): used to determine horizontal overhang design of the building

• Incident angle(入射角 입사각): used to calculate exterior irradiance and illuminance.

(1) Zenith angle(Z), Altitude angle(α)

ωϕδϕδα coscoscossinsinsincos +==Z)coscoscossin(sinsin 1 ωϕδϕδα += −

ϕ: latitude (north +, south -) [°]

δ: declination angle

ω: solar hour angle (24 hour=360°, 1 hour = 15°, southing = 0°)

= (12 - T) x 15 [°], T: Solar time

6015eLL

TT ss +

−+=

[ ]60

)(4 eLLTT S

S+−

+=

TS: Local standard time(clock time)LS: Longitude of Standard Meridian

Korean S.M. = Long. 135°EL: Longitude of Sitee: equation of time in minute

Latitude in seoul: 37°34’ ≒ 37.5°NLongitude in seoul: 126°58’ ≒ 127°E

- Equation of Time

• The time difference between solar time and local time in minute, brought on by the elliptical orbit of the earth

[min]4*180 2*180[0.170sin( ( 80)) 0.129sin( ( 8))]*60

373 355e J J= − − −

(2) Azimuth angle(Ø )

αωδφ

cossincossin =

⎟⎠⎞

⎜⎝⎛= −

αωδφ

cossincossin 1

A.M. : (+) P.M.: ( -)

[°]

(3) Profile angle (P)

)costan(tan

costantan 1

wsws

Ppφα

φα −=⇒=

Φws=Wall-Solar azimuth angle=|φw - φ|

where, φw : wall azimuth angle

W

N

S

Φws

E

φwφ

[°]

(4) Incident angle (i)

wsi φα coscoscos =

)cos(coscos 1wsi φα−=

normal line

i

[°]

5) Sunrise Hour Angle, Sunrise Hour, Day Length, Sunset Hour

① Sunrise Hour Angle (ws), Sunrise Hour (St)

when solar altitude angle is 0 (α = 0)

0)coscoscossin(sinsin 1 =+−sωϕδϕδ

)tantan(cos 1 ϕδω −= −s

)tan(tancoscossinsincos ϕδϕδϕδω −=⎟⎟⎠

⎞⎜⎜⎝

⎛ −=S

∴ Sunrise hour (in solar time) St = 12 - ωs /15 [Hr]

[°]

② Day Length: DL

• 12 solar time is the time when solar cross the meridian

• Day and night length is symmetrical about 12 solar time

③ Sunset Time(時刻) (Ss)

• (Solar time) Ss = St + DL [hour]

)tantan(cos152

152 1 ϕδω

−== −sDL [hr]

5.5. Sky Vault (天球)

• An imaginary virtual hemisphere placed over the building site.

• Since the solar radiation is quite weak in the early and late hours of the day, the part of the sky vault through which the most useful of the sun’s rays enter is called the solar window, which is assumed to begin at 9 a.m. and end at 3 p.m. of the year.

Sky Vault Solar Window

East elevation of the sky vault

5.6. Sun Path Diagrams

• Polar projection (Horizontal sun path diagrams)• Mercator projection (Vertical sun path diagrams) - Waldram Sun Path Diagram

1) Horizontal Sun Path Diagram

Example:Solar position in lat. 36°N at 9 am on February 21th:Solar altitude angle: about 27°Solar azimuth angle: about 51°SE

2) Waldram Sun Path Diagram

Example:Solar position in lat. 36°N at 3 pm on March 21th:Solar altitude angle: about 30°Solar azimuth angle: about 60°SW

The Waldram diagram can be used for visualizing and documenting the solar window and any obstacles that might be blocking it.

3) Site analyzing in regard to solar access using the Waldram diagram