Solar Geometry

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 -

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6.1 Introduction

Why Solar Geometry?

Understanding solar geometry is essential in order to:

• Do building design (for heating and cooling)

• buildings properly

• in the building and its surroundings

• Design .

passive

Orient

Understand seasonal changes

shading devices

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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

( ).

• The amount and composition of solar

radiation reaching the earth’s surface

vary widely with

-

-

Fig. 6.2a The surface temperature of the sun

determines the type of radiation emitted.

Fig. 6.2b The solar spectrum at the earth’s surface

consists of about , ,

and .

solar constant About 1,370 W/m2

sun angles

the composition of the atmosphere

47% visible 48% short-wave infrared

about 5% ultraviolet radiation

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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.

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6.4 Tilt of the earth’s axis

Fig. 6.4a The seasons are a consequence of the tilt

of the earth’s axis of rotation.

• On June 21, all of the earth north of

the Arctic Circle will have 24 hours of

sunlight.

-

• Six months later on December 21,

all of the earth above the Arctic

Circle experiences 24 hours of

darkness.

-

• 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

known as the . Fig. 6.4b During the summer solstice(June 21), the

sun is directly overhead on the Tropic of Cancer

summer solstice

winter solstice

spring and fall equinox

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6.5 Consequences of the altitude angle

• The vertical angle at which the sun’s rays

strike the earth is called the

and is a function of

-

-

-

• There are two important consequences of

this altitude angle on climate and the seasons.

1. At low angles the sun’s rays pass through

more of the atmosphere.(Fig 6.5b)

2. The second effect of the altitude angle is

illustrated in the diagram of the cosine

law. (Fig 6.5c)

Fig. 6.5a On the equinox, the sun’s altitude(A) at solar noon at any place on earth is equal to 90°minus the latitude(L).

Fig. 6.5b The altitude angle determines how much of the solar radiation will be absorbed by the atmosphere

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

altitude angle

the geographic latitude

time of year

time of day

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6.6 Winter

• The temperature of the air is mainly a result of

absorbed by the land.

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

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

2) The

“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.

the amount of solar radiation

fewer hours

cosine law

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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

for that day.

- The Highest sun path of the year :

- The lowest sun path :

- The middle sun path :

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

sun path

summer solstice

winter solstice

equinox

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6.9 Determining the altitude and azimuth angle

• : the vertical angle from its horizontal projection to the sun ray

• : the horizontal angle, which is measured from a north-south line

Fig. 6.9a Definition of altitude and azimuth angles

Altitude

Azimuth

고도(h, altitude)

방위각(A, azimuth)

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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).

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SUN-PATH DIAGRAMS (태양궤적도)

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

수직면 상에 투영된 천구

수평면 상에 투영된 천구

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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.

< 수평사영 태양궤적도>

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

6.11 Horizontal Sun-Path Diagrams

http://new-learn.info/learn/packages/clear/thermal/climate/sun/sunpath_diagrams.html

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6월22일

12월22일

6.11 Horizontal Sun-Path Diagrams

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< 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.

6.12 Vertical Sun-Path Diagrams

Alain Liebard, Andre De Herde, Traite D’architecture et D’urbanism Bilclimatiques

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6.12 Vertical Sun-Path Diagrams

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6.13 Sun-Path Models

• 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.

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

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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

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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

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

Fig. 6.17b The Sun Emulator is the latest sun machine developed by the author Model included for scale