HEATING, COOLING, LIGHTING : Design Methods for Architecture

Lecture 08 Solar Geometry

“It is the mission of modern architecture to concern itself with the sun.” - Le Corbusier from a letter to Sert - 6.1 Introduction

 Why Solar Geometry?

Understanding solar geometry is essential in order to: • Do passive building design (for heating and cooling) • Orient buildings properly • Understand seasonal changes in the building and its surroundings • Design shading devices .

1 6.2 The Sun

• The amount and composition of solar radiation reaching the outer edge of the earth’s atmosphere are quite unvarying and is called the solar constant ( About 1,370 W/m 2 ).

Fig. 6.2a The surface temperature of the sun • The amount and composition of solar determines the type of radiation emitted. radiation reaching the earth’s surface vary widely with - sun angles - the composition of the atmosphere

Fig. 6.2b The solar spectrum at the earth’s surface consists of about 47% visible , 48% short - wave infrared , and about 5% ultraviolet radiation .

2 6.4 Elliptical Orbit

• The orbit of the earth is not a circle but an ellipse, so that the distance between the earth and sun varies as the earth revolves around the sun.

• While the earth revolves around the sun, it also spins around its own north- south axis.

• Since this axis is not perpendicular to the orbital plane but is tilted 23.5°off the normal to this plane, and since the orientation in space of this axis of rotation remains fixed as the earth revolves around the sun.

• This tilt of 23.5°is the cause of the seasons and has major implications for solar energy.

Fig. 6.3 The earth’s axis of rotation is tilted to the plane of the elliptical orbit.

3 6.4 Tilt of the earth’s axis

• On June 21, all of the earth north of the Arctic Circle will have 24 hours of sunlight. - summer solstice

• Six months later on December 21, all of the earth above the Arctic Circle experiences 24 hours of Fig. 6.4a The seasons are a consequence of the tilt darkness. of the earth’s axis of rotation. - winter solstice

• Halfway between the longest and shortest day of the year is the day of equal nighttime and daytime hour. This situation occurs twice a year, on March and September 21, and is Fig. 6.4b During the summer solstice(June 21), the sun is directly overhead on the Tropic of Cancer known as the spring and fall equinox .

4 6.5 Consequences of the altitude angle

• The vertical angle at which the sun’s rays strike the earth is called the altitude angle and is a function of - the geographic latitude Fig. 6.5a On the equinox, the sun’s altitude(A) at solar noon - time of year at any place on earth is equal to 90°minus the latitude(L). - time of day

• There are two important consequences of this altitude angle on climate and the seasons. 1. At low angles the sun’s rays pass through Fig. 6.5b The altitude angle determines how much of the more of the atmosphere.(Fig 6.5b) solar radiation will be absorbed by the atmosphere 2. The second effect of the altitude angle is illustrated in the diagram of the cosine law. (Fig 6.5c)

Fig. 6.5c The cosine law states that the amount of radiation received by a surface decreases as the angle with the normal increases.

5 6.6 Winter

• The temperature of the air is mainly a result of the amount of solar radiation absorbed by the land.

• The reasons for less radiation falling on the ground in the winter

1) There are far fewer hours of daylight in the winter.(most important fact)

2) The cosine law “The amount of radiation received by a surface decreases as the angle with the normal increases.”

3) The lower sun angles increase the amount of atmosphere the sun must pass through and, there is again less radiation reaching each square foot of land.

6 6.8 Sky dome

• Sky dome : A large clear plastic hemisphere is placed over the building site.

• Every hour the point at which the sun’s rays penetrate the sky dome is marked. When all the points for one day are connected, we get a line called the sun path for that day. - The Highest sun path of the year : summer solstice - The lowest sun path : winter solstice - The middle sun path : equinox

Fig. 6.8a The sky dome and three sun paths.

7 6.9 Determining the altitude and azimuth angle

• Altitude : the vertical angle from its horizontal projection to the sun ray • Azimuth : the horizontal angle, which is measured from a north-south line

고도(h, altitude)

방위각(A, azimuth)

Fig. 6.9a Definition of altitude and azimuth angles

8 SUN-PATH DIAGRAMS (태양궤적도)

• Just as there are maps of the world that are usually various kinds of projections, so there are vertical or horizontal projections of the sky dome. • The sky dome shown in fig 6.11b has an azimuth grid, an altitude grid, and the sun paths for each month of the year for 32˙N latitude.

Fig. 6.11b A model of the sky dome. The sun paths for the 21st day of each month are shown. Only seven paths are needed for 12 months because of symmetry(i.e. May 21 is the same path as July 21).

9 SUN-PATH DIAGRAMS (태양궤적도)

수평면 상에 투영된 천구

수직면 상에 투영된 천구

Fig. 6.11a Derivation of the horizontal and vertical sun path diagrams.

10 6.11 Horizontal Sun-Path Diagrams

< 수평사영 태양궤적도>

1. Select the chart of the correct Latitude. 2. Select the date line. 3. Select the hour line and mark its intersection with the date line. 4. Read off from the concentric circles the altitude angle. 5. Lay a straight edge from the center of the chart through the marked time point to the perimeter scale and read off the azimuth angle.

Ex. Find the altitude and azimuth angle (Seoul, June 22 and December 22 at 3 p.m.)

http://new-learn.info/learn/packages/clear/thermal/climate/sun/sunpath_diagrams.html 11 6.11 Horizontal Sun-Path Diagrams

6월22일

12월22일

12 6.12 Vertical Sun-Path Diagrams

< 수직사영 태양궤적도>

1. Select the chart of the correct Latitude. 2. Select the date line. 3. Select the hour line and mark its intersection with the date line. 4. Read off the altitude angle from the vertical axis. 5. Read off the azimuth angle from the vertical axis.

Alain Liebard, Andre De Herde, Traite D’architecture et D’urbanism Bilclimatiques 13 6.12 Vertical Sun-Path Diagrams

14 6.13 Sun-Path Models

Fig. 6.13 A comparison of various sun-path models. Note especially the sun paths for the Equator, Tropic, Arctic Circle, and North Pole.

• These models can help a designer better visualize how the sun will relate to a building located at the center of the sun-path model. • The model can be placed on the corner of the designer’s table to be a reminder of where the sun is at different times of the day and year.

15 6.14 Solar site-evaluation tool

• One drawback of the site evaluation tool is that it indicates the solar access only for the spot where the tool is used.

• It cannot easily determine the solar access for the roof of a proposed multistory building.

• Solution : a scale model of the site analyzed with a sun machine is an excellent method of evaluating the site of solar access. Fig. 6.14 The sun-path diagram used as part of a solar site-evaluation tool

16 6.17 Integrating sun machine and sun emulator

Integrating sun machine - simulated by an automatic sequencing of the lights - simulates the instantaneous sun angle - sum up the effect of a whole season

Fig. 6.17a The ‘Integrating Sun Machine’ was developed by the author at Auburn University, Alabama.

The new sun emulator

- maintains the conceptual clarity of the previous integrating sun machine by keeping the model horizontal and making the lights revolve around the model

- the sun emulator is small enough to be manufactured and shipped Fig. 6.17b The Sun Emulator is the latest sun machine developed by the author Model included for scale

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