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 a rtificial 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
and night.
on the duration of the day
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 ray s 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 ca st by tree on flat ground (B) is longer than the one cast by same sa me 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. 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 c an 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 ca lled 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 o r 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 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 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, 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. Customerspecific 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® THRU® double glazed unit 10% SHADING SYSTEMS External Venetian blind (white)* External fabric canopy* Internal roller blind (white)*
12% 9% 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
