analyzing the sun

7/30/2019 Analyzing the Sun

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SHADOW STUDY

Sun as a design element

• In the climatic design of buildings the sun is one of the most important influences.

Solar radiation entering through windows gives a desirable heating effect in winter,

but it can cause severe overheating in summer.

• The assessment of its availability and its control are very important parts of

architectural design.

• By analysing the impact of the sun on a site, as well as the building’s location, the

spatial arrangement, orientation, window placement, daylight access and other

design features, the designer can take full advantage of passive solar designfeatures and increase the energy efficiency and comfort of the building.

When assessing a site for sun

• CONSIDER

• the time during the day that the site receives sunlight

• the sun's path at different times of the day and year

• how the site’s shape, slope and orientation affect solar access

• how obstructions such as adjacent buildings, trees and

landforms will impact on the site and the potential design

• The earth is almost spherical in shape, and it revolves around the sun in a slightly

elliptical orbit.

• The plane of the earth's revolution is referred to as the ecliptic. The earth's axis of

rotation is tilted 23.45o from the normal to the plane of the ecliptic. The angle

between the plane of the earth's equator and the ecliptic (or the earth - sun line)

is the declination (DEC) and it varies between +23.45o on June 22 (northern

solstice) and -23.45o on December 22 (southern solstice)

• On equinox days (approximately March 22 and Sept.21) the earth - sun

• line is within the plane of the equator, thus DEC = 0

Latitude circle

• Points having the same latitude

form the latitude circle

• The latitude of the equator is

LAT = 0o, the north pole is +90o

and the south pole -90o.

• By the convention adopted

southern latitudes are taken as

negative.

• LAT = +23.45o is the tropic of

Cancer and • LAT = -23.45o is the tropic of

Capricorn.

Solar Position angles

• The sun's apparent position on this 'sky

vault' can be defined in terms of two

angles

• Altitude (ALT) - measured in the vertical

plane, between the sun's direction and

the horizontal• Azimuth (AZI) - the direction of the sun

measured in the horizontal plane from

north in a clockwise direction (thus east

= 90o, south = 180o and west = 270o,

whilst north can be 0 or 360o)

• The zenith angle (ZEN) is measured

between the sun's direction and the

vertical and it is the supplementary

angle of altitude:

• ZEN = 90o - ALT

Declination

Declination is the angular distance of the sun north or south of the earth's

equator.

The earth's equator is tilted 23.45 degrees with respect to the plane of the

earth's orbit around the sun, so at various times during the year, as theearth orbits the sun, declination varies from 23.45 degrees north to 23.45

degrees south.

Declination is calculated with the following formula:

d = 23.45 * sin [360 / 365 * (284 + N)]

Where:d = declination

N = day number, January 1 = day 1

• Around December 21, the northern hemisphere of the earth is tilted 23.45

degrees away from the sun, which is the winter solstice for the northern

hemisphere and the summer solstice for the southern hemisphere. Around June

21, the southern hemisphere is tilted 23.45 degrees away from the sun, which is

the summer solstice for the northern hemisphere and winter solstice for the

southern hemisphere. On March 21 and September 21 are the fall and springequinoxes when the sun is passing directly over the equator. Note that the tropics

of cancer and capricorn mark the maximum declination of the sun in each

hemisphere.

Solar Angles

Azimuth angle is measured clockwise from North towards East.

North Direction : (α ) = 0˚ or 360˚

East Direction : (α ) = 90˚

South Direction : (α ) = 180˚

West Direction : (α ) = 270˚

Solar time

• In solar work usually solar time is used. This is measured from the solar

noon, i.e. the time when the sun appears to cross the local meridian. This

will be the same as the local (clock-) time only at the reference longitude

of the local time zone.

• The time adjustment is normally one hour for each 15o longitude from

Greenwich, but the boundaries of the local time zone are subject to social

agreement.

Hour angle (HRA)

• The hour angle (HRA) expresses the time of day with respect to the solar noon: it is the angular distance, measured within the plane of the sun's

apparent path between the sun's position at the time considered and itsposition at noon i.e. the solar meridian.

• As the hourly rotation of the earth is 360o/24h = 15o/h, HRA is 15o for eachhour from solar noon:

• HRA = 15 * (h - 12)

• where h = the hour considered (24-h clock) so HRA is

• negative for the morning and positive for the afternoon• hours,

• e.g: for 9 am: HRA = 15 * (9 - 12) = -45o

• but for 2 pm: HRA = 15 * (14 - 12) = 30o.

Sun Dial

Sun path diagrams

Reading a sun path diagram

The sun's position angles can be read directly from the chart for any given time of the year:

Find the chart corresponding to the latitude of your location

Locate the desired date (sun-path) line

Locate the desired time point

Mark the intersection of the two lines: that point indicates the sun's position at that time

Project a radius line from the centre through point, to the perimeter

Circle and read the azimuth (AZI) angle (in this example: 32o)

Read the altitude (ALT)

Shading Strategies (General design guide)

• OBJECTIVE: Control intense direct sunlight to ensure a comfortable workspace.

• KEY IDEAS

• Exterior Devices

• Use exterior shading , either a device attached to the building skin or an extension of the skin

itself, to keep out unwanted solar heat. Exterior systems are typically more effective thaninterior s

• Design the building to shade itself.

• If shading attachments are not aesthetically acceptable, use the building form itself for

• exterior shading. Set the window back in a deeper wall section or extend elements of the skin

to visually blend with envelope structural features.

Shadow Angles

• When designing shading devices for windows, the required horizontal and vertical

shadow angles need to be established. They are dependent on the sun position

and on the orientation of the window plane.

• The horizontal shadow angle (HSA) is relevant for vertical shading devices such as

fins. It is easy to determine: It's the angle between the normal of the window pane

and the azimuth of the sun.• HSA = azimuth - orientation

• The vertical shadow angle (VSA) is a little bit more difficult. If we imagine a virtual

plane between the bottom left-hand and right-hand corners of the window and

the sun, then the VSA is the angle this plane formes with the ground plane. TheVSA is required when designing horizontal shading devices such as overhangs.

• VSA = arctan( tan(altitude) / cos(HSA) ) VSA = arctan (tan ALT / COS HSA)

analyzing the sun

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