SOLAR GEOMETRY

Solar Geometry is the consideration of the angular relationship

between the sun, the building and any shading devices and

obstructing bodies. Different approaches exist to analyze the direct

solar beam:

• Graphical plots;

• Manual trigonometric methods;

• Computer based trigonometric methods;

• Scale models examined using a sundial device, and natural

sunlight or an artificial light source; and

• scale models using a heliodor, a device that mechanically

reproduces the geometric movement of the sun.

Effect of the tilt of the earth’s axis on the duration of the day

and night.

Since the tilt of the earth’s axis is fixed, the northern

hemisphere faces the sun in June, and the southern hemisphere faces

the sun in December.

Solstices

On June 21 the sun’s rays will be perpendicular to the earth’s surface

along the tropic of Cancer. This is the longest day in the northern

hemisphere and the shortest in the southern hemisphere. On Dec 21

the situation is reversed.

Effects of the sun on the climate.

The temperature of the air as well as that of the land is mainly

a result of the amount of solar radiation absorbed by the land or the

water, which then heats or cools the air above it.

Winter occurs because:

1. There is a reduced amount of daylight during this time of the year.

The amount of daylight is a function of the latitude.

2. The sun is lower in the sky and its intensity is spread over a larger

surface.

Altitude and Radiation: effect on climate.

The altitude of the sun affects climate in two ways:

1. As the sun is lower in the sky, solar radiation must traverse

through a larger portion of the atmosphere and is weaker when it

reaches the earth.

2. - A given beam of light will illuminate a larger area as the

sunbeam is spread over larger areas.

Topography and Solar Radiation

Area of ground receiving the ray on flat ground (B) is larger than

area on Sun facing slope (A). Thus more energy is received per unit

area on the slope. Shadow cast by tree on flat ground (B) is longer

than the one cast by same tree on slope (A).

Solar Position Angles

The position of the sun in the sky at any given moment can be

determined by two values: the solar altitude and the solar azimuth.

These are plotted in an imaginary celestial sphere or sky dome in

which we are in the centre (loco centric view). We regard the radius

of this sphere to be infinite and we can see only 1/2 of the celestial

sphere at any time. The boundary between the visible and invisible

portions of the celestial sphere is called the horizon. The poles of the

horizon, those points directly overhead and underneath are called the

zenith and the nadir.”

Solar Position Angles: Altitude and Azimuth From Architectural Graphic Standards

The position of the Sun in the sky varies during the day and

during the year and is based on latitude. Complete understanding of

solar positioning is NECCESARY for reasonable solar design and

climatic response. Solar position can be determined using the

previous equations, but nevertheless it is often much simpler and

quicker to read sun positions from a table or off a sun-path diagram.

SUNDIALS

Scale models can be studied outdoors under direct sun or

indoors using a lamp as a simulated sun. To position the model

accurately relative to the sun, place a sundial beside the model and

adjust the model position until the desired time is shown on the

sundial.

a) Build a simple model with accurate geometry. You can study the

whole building or just a portion of the facade.

b) Select the sundial with latitude closest to your site (use 32°).

Mount a copy of the sundial on your model and enlarge for more

accurate positioning. It should be horizontal, oriented properly with

true south on the model, and in a position where it will not be shaded

by the model (flat roof or southern portion of model base are good

places.

Graphical illustration of sun shade:

Controlling shading in various parts of the

building:

Shading the Window

It is possible to shade the windows in three forms:

•External shading devices,

•Internal shading devices or

•With the glass pane itself.

Each of these systems has advantages and disadvantages.

EXTERNAL SHADING DEVICES

An external shading system has the advantage of blocking the

solar radiation before the sun penetrates the building, but has the

disadvantage of exposure to the climatic elements for maintenance.

The size and position of these external shading elements can be

calculated so as to cover the windows on the most problematic hours.

Fixed external shading systems are usually classified as:

Horizontal,

Vertical,

Combined (egg crate).

Operable External Shading Devices

External shading devices can also be operable so that they

can be adjusted with more precision and effectiveness to different

solar positions, during different times of the year.

These respond better to the dynamic nature of weather which

does not always correlate with solar geometry. The position of these

movable devices could be an adjusted as a function of temperature

also instead of only the solar position as the fixed shading devices.

Internal Shading Devices

Even though internal shading devices are not as effective as

external shading devices in blocking energy into the building, they

are interesting for a number of practical reasons.

•They are protected from the outdoor environment and thus do not

have to resist the elements.

•They are often less expensive than external shade systems.

•They are usually very adjustable and movable responding easily to

changing requirements.

•Also, they provide added benefits in the regulation of privacy,

natural light, glare, insulation level of the windows and interior

aesthetics.

•At night they also reduce heat losses through the window.

Internal shading devices can be curtains, roller shades, horizontal

venetian blinds, vertical drapes and shutters.

Glazing as the Shading Element

Glass can regulate the solar gains, usually by tinting, but this

has the disadvantage of also decreasing the amount of light in the

space. Recently special self regulating tinted glass has appeared in

the market. This glass has the advantage of getting darker whenever

conditions require it so it is not continuously dark.

Examples;

Glazing types from the ASI used in some shading:

ASI THRU ASI OPAK White ASI OPAK Creative Line Elegance

Line

ASI OPAK®

ASI THRU: is a semi-transparent module with a see-through effect.

It is available in laminated form or as double glazed units.

ASI OPAK: is the technology for homogeneous facade surfaces,

where no vision is required.

ASI OPAK White; offers a completely uniform appearance.

ASI OPAK Creative Line and Elegance Line offer unique surface

patterns, allowing new architectural design possibilities. Customer-

specific patterns can also be produced. The following designs and

module constructions are available as customer specific solutions:

ASI FADE provides a gradual fade to clear glass

ASI SHADE integrates shading louvers with double glazed units to

provide the ultimate in glare protection.

Anti-reflective coating

All of the above options laminated or double glazed are also

available with AMIRAN anti-reflective glass from SCHOTT.

This significantly reduces the average light reflectivity, enhances the

performance of the solar power and eliminates obtrusive reflections.

Typical Applications

SHGC (G-VALUE)

GLAZING

Single glass pane ~80%

Double glazed with uncoated glass ~80%

Double glazed with solar control coating 30 - 70%

ASI THRU® double glazed unit 10%

SHADING SYSTEMS

External Venetian blind (white)* 12%

External fabric canopy* 9%

Internal roller blind (white)* 40%

Movable louver

REFERENCES

http://btech.lbl.gov/pub/designguide/

btech.lbl.gov/pub/design guide/section5.pdf

More examples of solar control in facades:

http://gaia.lbl.gov/hpbf/casest_r1.htm