Solar Geometry and Its Importance in Climate Responsive Building Design

CLIMATE AND BUILT FORM II Semester – 4

Asst. Prof. Jesna Mathew College of Architecture Trivandrum

CLIMATE

AND

BUILT

FORM

II

Topic : Sun path, solar geometry: Altitude angle, azimuth angle. Sun path diagram for various latitudes, Solar charts, importance of solar shading, solar heat gain and sunshade design.

SOLAR GEOMETRY Sun” is one of the most predominant factors to be considered while designing climate responsive buildings. In a tropical country like ours radiation entering through the fenestrations can cause severe overheating. Measures to control the same are a crucial aspect of climate responsiveness. “

Sun is the basic source of Energy for Earth. • Solar Energy is available in the form of Electromagnetic Radiations. • Sun is a large sphere of very hot gases, heat being generated by the various fusion reactions in it. • Diameter of Sun = 1.39 x 106 km. • Diameter of Earth = 1.27 x 104 km. The amount and composition of solar radiation reaching the outer edge of the earth’s atmosphere are quite unvarying and are called the solar constant (1.395 W/sq.m). However, it vary widely with sun angles, elevation, and the composition of the atmosphere.

Earth Sun Relationship • Heliocentric view – • The earth is almost spherical in shape, some 12 700 km in diameter and it revolves around the sun in a slightly elliptical (almost circular) orbit. • The earth - sun distance is approximately 150 million km, varying between 152 million km (at aphelion, on July 1) and 147 million km (at perihelion, on January 1) . • The full revolution takes 365.24 days (365 days 5 h 48’ 46” to be precise). • his would mean 0.25 days per year, which is too much. The excess 0.01 day a year is compensated by a one day adjustment per century. • The plane of the earth's revolution is referred to as the ecliptic.

• While the earth revolves around the sun, it also spins around its own north– south axis. • Because this axis is not perpendicular to the orbital plane but is tilted 23.45° off the normal to this plane, and the orientation in space of this axis of rotation remains fixed as the earth revolves around the sun, the angle at which the sun’s rays hit the earth continuously changes throughout the year. • This tilt of 23.45° is the cause of the seasons and has major implications for solar design. • 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,). • Because 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.

• • • •

Points having the same latitude form the latitude circle. The latitude of the equator is LAT = 0⸰, the north pole is +90o and the south pole -90⸰. By the convention adopted southern latitudes are taken as negative. The extreme latitudes where the sun reaches the zenith at mid-summer are the 'tropics' : LAT = +23.45⸰ is the tropic of Cancer and LAT = -23.45⸰ is the tropic of Capricorn.

• • The arctic circles (at LAT = 66.5⸰) mark the extreme positions, where at mid-summer the sun is above the horizon all day and at mid-winter the sun does not rise at all.

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

Summer Solstice When the North Pole of the Earth is tilted toward the Sun, we in the northern hemisphere receive more sunlight and it's summer. The day that the Earth's North Pole is tilted closest to the sun is called the summer solstice. -On 21st June areas along latitude 23.5 N are normal to the sun rays and experience a zenith path of the sun, longest day is experienced. This is the longest day (most daylight hours) of the year for people living in the northern hemisphere. It is also the day that the Sun reaches its highest point in the sky.

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

Winter Solstice –As the Earth moves in its orbit, the tilt of the North Pole changes (see diagram). When it is tilted away from the Sun, it is winter in the northern hemisphere. The day that the Earth's North Pole is tilted farthest from the sun is called the summer solstice. On 21st December areas along latitude 23.5 S are normal to the sun rays and experience a zenith path of the sun, longest day is experienced . The winter solstice, or the shortest day of the year, happens when the Earth's North Pole is tilted farthest from the Sun.

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

Spring and Autumn Equinox In between, there are two times when the tilt of the Earth is zero, meaning that the tilt is neither away from the Sun nor toward the Sun. These are the vernal equinox — the first day of spring — and the autumnal equinox – the first day of fall. Equinoxes – On March 21st and September 23rd areas along equator experience maximum radiation and equal duration of day and night. Equinox means "equal." During these times, the hours of daylight and night are equal. Both are 12 hours long

22st or 23 rd September

21st or 22 March REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

REFERENCE: https://www.youtube.com/watch?v=WLRA87TKXLM

Loco centric view: For practical purposes we consider our point of location on the earth's surface as the centre of the universe. The sky can be imagined as a hemispherical dome with the point of reference as the centre. The sun moves across this sky dome. The points where these sunrays penetrate the sky dome every hour are marked. When all the points for one day are connected, we get a line on the sky dome called the sun path for that day.

Sky dome, assumed with the reference point as the centre (Source: Heating Cooling and Lighting: Sus. Methods for Architects)

Highest and Lowest Sun paths for an year for a particular latitude

Since the solar radiation is quite weak in the early and late hours of the day, the part of the sky dome through which the most powerful sunrays enter is called the solar window .

By far the easiest way to work with the compound angle of sunrays is to use component angles. The most useful components are the altitude angle, which is measured in a vertical plane, and the azimuth angle, which is measured in a horizontal plane. Altitude angle (ALT) - Measured in the vertical plane, between the sun's direction and the horizontal; in some texts this is referred to as 'elevation' or 'profile angle’. It is the vertical angle at the point of observation between the horizon plane and the line connecting the sun with observer. Azimuth (AZI) - The direction of the sun measured in the horizontal plane from north in a clockwise direction. It is the angle at the point of observation measured on a horizontal plane between the northerly direction and a point on the horizon circle, where it is intersected by the arc of vertical circle going through the zenith and sun position (thus east = 90 degree, south = 180 degree and west = 270 degree, whilst north can be 0 or 360 degree); Altitude and Azimuth angles (Source: astronavigationdemystifieddotcom.files.w ordpress.com

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

Angle of Incidence : Angle of incidence can be calculated using the azimuth and altitude angles. The horizontal component of the angle of incidence will be the difference between solar azimuth and wall azimuth. The vertical component will be the altitude angle itself.

Angle of incidence can be defined as the angle between a line perpendicular to the wall and the sun’s direction. The angle of incidence will be required for selecting appropriate solar heat gain actor for windows and for calculating the incident radiation on opaque surfaces. The spherical cosine equation can be used to find out the angle of incidence Cosine (angle of incidence) = cosine (solar azimuth- wall azimuth) X cosine (altitude angle)

GRAPHIC REPRESENTATION

SUN-PATH DIAGRAMS

Horizontal plane projection Equidistant chart

Orthographic projection

Stereographic projection

REFERENCE: Szokolay, S. V. (1996). Solar Geometry. Passive and Low Energy Architecture International

Vertical Projection Cylindrical Projection

REFERENCE: Szokolay, S. V. (1996). Solar Geometry. Passive and Low Energy Architecture International

Vertical Projection Cylindrical Projection

REFERENCE: Szokolay, S. V. (1996). Solar Geometry. Passive and Low Energy Architecture International

Vertical Projection Cylindrical Projection

REFERENCE: Szokolay, S. V. (1996). Solar Geometry. Passive and Low Energy Architecture International

Vertical Projection Gnomonic projections

REFERENCE: Szokolay, S. V. (1996). Solar Geometry. Passive and Low Energy Architecture International

SUN SHADE DESIGN

Solar radiation incident on a window consists of three components: 1. Beam(direct-) radiation 2. Diffuse-(sky-) 3. Reflected radiation. External shading devices can eliminate the beam component (which is normally the largest) and reduce the diffuse component. The design of such shading devices employs two shadow angles: HSA and

SHADOW ANGLES Shadow angles express the sun's position in relation to a building face of given orientation and can be used either to describe the performance of (i.e. the shadow produced by) a given device or to specify a device.

VSA

Horizontal Shadow Angle (HSA) ➢ Horizontal shadow angle (HSA) is the difference in azimuth between the sun's position and the orientation of the building face considered, when the edge of the shadow falls on the point considered. ➢ : HSA = AZI - ORI

➢ By convention, this is positive when the sun is clockwise from the orientation (when AZI > ORI) and negative when the sun is anticlockwise (when AZI < ORI). ➢ When the HSA is between +/- 90o and 270o, then the sun

is behind the facade, the facade is in shade, there is no HSA. ➢ The horizontal shadow angle describes the performance of a vertical shading device. ➢ Figure shows that many combinations of vertical elements can give the same shading performance.

REFERENCE : Koenigsberger, O. H., Ingersoll, T. G., Mayhew, A., & Szokolay, S. V. (1973). Manual of Tropical Housing and Building. Hyderabad: Universities Press (India) Private Limited

The Vertical Shadow Angle (VSA) ➢ The vertical shadow angle (VSA) (or 'profile angle' for some authors) is measured on a plane perpendicular to the building face. ➢ VSA can exist only when the HSA is between -90o and +90o, ➢ i.e. when the sun reaches the building face considered. When the sun is directly opposite, i.e. when AZI = ORI (HSA = 0o), the VSA is the same as the solar altitude angle (VSA = ALT). ➢ When the sun is sideways, its altitude angle will be

projected, parallel with the building face, onto the perpendicular plane and the VSA will be larger than the ALT (Fig.31). ➢ Alternatively, VSA can be considered as the angle between two planes meeting along a horizontal line on the building face and which contains the point considered, ie. between the horizontal plane and a tilted plane which contains the sun or the edge of the a shading device.

REFERENCE: Szokolay, S. V. (1996). Solar Geometry. Passive and Low Energy Architecture International

REFERENCE : Koenigsberger, O. H., Ingersoll, T. G., Mayhew, A., & Szokolay, S. V. (1973). Manual of Tropical Housing and Building. Hyderabad: Universities Press (India) Private Limited

Finding HAS and VSA Using Shadow angle Protractor

• This is a semi-circular protractor, showing two sets of lines • radial lines, marked 0 at the centre, to -90o to the left and +90o to the right, to give readings of the HSA • arcual lines, which coincide with the altitude circles along the centreline, but then deviate and converge at the two corners of the protractor; these will give readings of the VSA.

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THE DESIGN PROCESS By Using Winter and solistice and Equinox The task of shading design can be divided into three steps:

1. Determine the Azimuth and altitude angle (Position of sun) for solstice and equinox. 2. By using the appropriate sun-path diagram and the protractor establish the necessary horizontal or vertical shadow angles (or a combination of the two), as performance specification for the device to be designed for each date

3. Determine the critical date and time (the date and time with lowest HSA and VSA) 4.

Using that HSA and VSA, design the actual device to satisfy these performance specifications.

THE DESIGN PROCESS By using Over Heated Period The task of shading design can be divided into three steps: 1. Determine the overheated period, i.e. the dates and times when shading should be provided. This can be taken as the time when the monthly mean temperature is higher than the lower comfort limit. The daily temperature profile should be looked at to ascertain the hours when shading is necessary. (A more precise definition of this overheated period should take into account also the type of building, the amount of internal heat gain and even the relationship of solar gain to the building mass available for heat storage. This is beyond the scope of the present Note.) 2. By using the appropriate sun-path diagram and the protractor establish the necessary horizontal or vertical

shadow angles (or a combination of the two), as performance specification for the device to be designed. 3. Design the actual device to satisfy these performance specifications.

C

Comfort region Overheated period