ABSTRACT

Protection of the building from unwanted solar gain is a key cooling strategy which is most readily achieved by blocking the sun’s rays before they reach the building. The aim of this work is to investigate the impact of external solar shading devices on the reduction of solar gains.

In hot climates, it is very important to eliminate the penetration of solar radiation into buildings, easily allowed by glazing.

In Akure, Nigeria, due to its low latitudes, predominantly hot and humid climate and high solar radiation, the use of shading devices is desirable.

This study is to give a good understanding for designer on building design using sun shading devices. Another objective is to provide design method to help them to achieve better solution on the western elevation of buildings in Akure, Nigeria. The study indicates the usefulness of shading devices to control direct solar radiation on the western elevation along with their abilities to admit useable daylight to efficient room depths.

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1.0 INTRODUCTION

There are many different reasons to want to control the amount of sunlight that is admitted into a building. In warm, sunny climates excess solar gain may result in high cooling energy consumption; in cold and temperate climates winter sun entering south-facing windows can positively contribute to passive solar heating; and in nearly all climates controlling and diffusing natural illumination will improve daylighting.

Well-designed sun control and shading devices can dramatically reduce building peak heat gain and cooling requirements and improve the natural lighting quality of building interiors. Depending on the amount and location of fenestration, reductions in annual cooling energy consumption of 5% to 15% have been reported. Sun control and shading devices can also improve user visual comfort by controlling glare and reducing contrast ratios. This often leads to increased satisfaction and productivity. Shading devices offer the opportunity of differentiating one building facade from another. This can provide interest and human scale to an otherwise undistinguished design.

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2.0 THE SUN

The Sun is a massive atomic furnace that works by converting hydrogen into helium. Hydrogen is the lightest and most abundant element in the universe. It has one proton in its nucleus. Temperatures and densities in the centre of the Sun are so great, 1.5 million °C and around 200 billion atmospheres, that colliding hydrogen nuclei sometimes fuse into helium nuclei. The creation of each helium nucleus requires four hydrogen nuclei.

One helium nucleus has 99.3% of the weight of four hydrogen nuclei. This excess 0.7% of hydrogen mass compared with helium mass is converted into energy. In perspective, the Sun converts 600 million tons of hydrogen into 596 million tons of helium every second. The extra 4 million tons is converted into energy - in this case radiation in the form of gamma rays.

The fusion of four hydrogen nuclei to form one nuclei of helium.

You can imagine the enormity of the energy generated when you realise that, given Albert Einstein's famous equation E=MC2, the 4 million ton differential is multiplied by the speed of light, squared. This energy is so great that the Sun gives off 6200 watts of light from every square centimetre of its surface. Compare this to a 60-100 Watt domestic light globe. As far as we know, the Sun has been giving off this light steadily for the last 4.5 billion years, and will continue to do so for several billion more. Only half a billionth of this energy reaches the Earth. The rest is lost in space, so to speak.

The average distance from the Earth to the Sun is 150 million kilometres, which takes sunlight around 8.5 minutes to travel. The diameter of the Sun is about 1.4 million km, 109 times that of the Earth. Its volume is big enough to hold over 1 million Earths.

Inner Workings of the Sun.

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2.1 SOLAR POSITION

The position of the Sun in the sky varies continually during the day and also changes seasonally throughout the year. Sun position is also very location-dependant, so it is critical that you know the latitude and longitude of your development site before you begin any calculations.

Annual Sun Path. Hourly Sun Path.

Despite our fundamental knowledge of solar position (experiencing it every day), calculating it at any specific date and time is not a trivial exercise. The off-axis rotation of the Earth and its elliptical orbit around the Sun add significant complexity to the equations required. For more information on this, see the Seasonal Variation topic.

Whilst an accurate manual calculation method is provided on this site, it is often much simpler and far quicker to read Sun positions directly from a table or a sun-path diagram. Such tables and diagrams are readily available for a range of locations.

On-line Solar Position Calculator can be used to obtain the position at a specific time to as 1/2 hourly values throughout the day.

2.2 POSITIONAL CHARACTERISTICS

The most important characteristic of solar position is its seasonal variation. During dry season in the southern hemisphere, the Sun rises slightly south of east and sets slightly south of west. In the northern hemisphere it rises slightly north of east and sets slightly north of west. In wet season it rises slightly north of east and sets slightly north of west (again, opposite in the northern hemisphere).

In both hemispheres the Sun rises earlier and sets later in dry season than in wet season. The degree of this effect is greater the closer the site is to either pole.

2.3 SOLAR POSITION CALCULATOR

It is an interactive tool for calculating the azimuth and altitude of the Sun at any date and time. The first step in the use of the tool is to set the Latitude and Longitude of the desired location - making sure to first set the Time Zone value as this will make entering the longitude easier. In addition to the

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current Sun position, it also calculates Sunrise / Sunset times, the Declination and Equation of Time as well as the relative shadow length.

2.4 SOLAR POSITION 1/2 HOURLY

The first step in the use of this tool is to set the Latitude and Longitude of the desired location - making sure to first set the Time Zone value as this will make entering the longitude easier. In addition to the 1/2 hourly Sun positions throughout the day, it also calculates Sunrise / Sunset times, the Declination and the Equation of Time. Select the Calculate button to perform the calculation and populate the list.

2.5 SOLAR POSITION VARIATION

The cyclical nature of the Earth's orbit around the Sun and its own axial rotation tilted at 23.45° to this orbital plane are the main reasons we experience seasonal changes in weather conditions. As you move away from the equator and closer to the poles, conditions in December and June vary in terms of both the maximum daily solar altitude of the Sun and the intensity of solar radiation at any point on the Earth's surface.

The Elliptical and off-axis Orbit of the Earth Around the Sun.

The diagram above clearly illustrates this. Whilst variations due to changes in our orbital distance from the Sun (due to the Earth's slightly elliptical path) are small, the main seasonal effects are due to changes in the angle of exposure to solar radiation. In some parts of the orbit, the Northern hemisphere is tilted more towards the Sun than the Southern hemisphere.

From these practices, special dates in the year that correspond to certain solar events have come to have significant meaning, especially the Solstices and Equinoxes. These same special dates are important to modern building designers too.

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2.6 AZIMUTH LINES

Azimuth angles run around the edge of the diagram in 15° increments. A point's azimuth from the reference position is measured in a clockwise direction from True North on the horizontal plane. True North on the stereographic diagram is the positive Y axis (straight up) and is marked with an N.

Azimuth angle indicators.

2.7 ALTITUDE LINES

Altitude angles are represented as concentric circular dotted lines that run from the centre of the diagram out, in 10° increments from 90 to 0. A point's altitude from the reference position is measured from the horizontal plane up.

Lines of equal altitude.

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2.8 DATE LINES

Date lines represent the path of the sun through the sky on one particular day of the year. They start on the eastern side of the graph and run to the western side. There are twelve of these lines shown, for the 1st day of each month. The first six months are shown as solid lines (Jan-Jun) whilst the last six months are shown as dotted (Jul-Dec), to allow a clear distinction even though the path of the Sun is cyclical.

Lines showing daily sun-path on 1st day of each month.

2.9 HOUR LINES

Hour lines represent the position of the sun at a specific hour of the day, throughout the year. They are shown as figure-8 style lines that intersect the date lines. The intersection points between date and hour lines gives the position of the sun. Half of each hour line is shown as dotted, to indicate that this is during the latter six months of the year.

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Hour Lines Showing the Analemma Effect.

This characteristic figure-8 shape results from what is termed the Analemma, an effect resulting from the elliptical orbit of the Earth around the Sun and the slight tilt of the Earth's axis of rotation relative to its orbital plane. This simply means that there is some seasonal variation in the difference between local and solar time.

2.10 SUN POSITIONS

The position of the Sun in the sky at any time of the day on any day of the year can be read directly from a Sun-Path Diagram. The diagram below details the process required to find the position of the Sun at 9:00am on the 1st of April.

2.11 POLAR SUN-PATH DIAGRAMS

Diagram Detailing the Process Required to Read Azimuth and Altitude Values from a Stereographic Diagram.

Follow the steps below to read the Sun position from a stereographic sun-path diagram:

Step 1 - Locate the required hour line on the diagram.

Step 2 - Locate the required date line, remembering that solid are used for Jan-Jun and dotted lines for Jul-Dec.

Step 3 - Find the intersection point of the hour and date lines. Remember to intersect solid with solid and dotted with dotted lines.

Step 4 - Draw a line from the very centre of the diagram, through the intersection point, out to the perimeter of the diagram.

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Step 5 - Read the azimuth as an angle taken clockwise from North. In this case, the value is about 62°.

Step 6 - Trace a concentric circle around from the intersection point to the vertical North axis, on which is displayed the altitude angles.

Step 7 - Interpolate between the concentric circle lines to find the altitude. In this case the intersection point sits exactly on the 30° line.

This gives the position of the sun, fully defined as an azimuth and altitude.

2.12 CARTESIAN SUN-PATH DIAGRAMS

In Cartesian co-ordinates, the azimuth is plotted along the horizontal axis whilst the altitude is plotted vertically. The date and time values are first located in exactly the same way as in the Polar sun-path diagram. Once the date-hour intersection point is found, reading off positions is simply a matter of projecting vertically and then horizontally onto the two axes, as shown in the animation below.

Diagram Detailing the Process Required to Read Azimuth and Altitude Values from A Cylindrical Projection.

Follow the steps below to read the Sun position from a cylindrical sun-path diagram:

Step 1 - Locate the required hour line on the diagram.

Step 2 - Locate the required date line, remembering that solid are used for Jan-Jun and dotted lines for Jul-Dec. In these diagrams, the highest altitude line at noon is always in midsummer (either 1st July or 1st Jan, depending on hemisphere). Each other line represents the 1st of each month, solid Jan-Jun, dotted Jul-Dec.

Step 3 - Find the intersection point of the hour and date lines. Remember to intersect solid with solid and dotted with dotted lines.

Step 4 - The azimuth is given by reading off the horizontal axis. In this case, the value is about 62°.

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Step 5 - The altitude is given by reading off the vertical axis. In this case the intersection point sits almost exactly on the 30° line.

2.13 LOCAL AND SOLAR TIME

In most locations, there will normally be a difference between solar and local time. Solar time is determined by the position of the Sun. At noon it is directly overhead, with sunrise and sunset occurring at symmetrical times either side of noon. Local (or clock) time is determined by the local time zone and is taken at a reference longitude

World time zones coded by colour.

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3.0 INTRODUCTION TO CLIMATE

Environmental conditions at different locations on Earth can vary quite significantly - from freezing cold and snowing at the poles to blisteringly hot and dry in the deserts. Between these two extremes there are both daily and seasonal variations. Winds blow from different directions, bright sunshine erupts from behind clouds and rain cascades from the sky. All these factors create conditions to which different plants and animals have adapted over time. Humans, on the other hand, tend to live pretty much everywhere and often need artificial means to handle the extremes of their environment.

Maps taken from NASA Earth Observatory.

This topic deals mainly with the Earth's climate, as opposed to the weather. The difference between the two is essentially time. The weather for a specific location varies day-to-day, sometimes hour-by-hour. The climate of the location is a statistical encapsulation of all these weather patterns into a description of its annual and seasonal cycles. Thus the average behaviour of a climate tends to be given as values for each calendar month. However, you still need hourly weather data from which to extract the monthly data, so you will find that there is always some overlap between the two in discussions on this site.

3.1 CLIMATE ANALYSIS

The invention of the Carrier air-conditioning system is widely seen as having liberated the architect from the constraints of climate. The capability of mechanical services to produce a controllable and comfortable internal environment within any building is almost unquestioned in modern architecture. This capability is well understood by architects and - together with electric lighting technology - underpins the majority of modern building design.

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The saviour of many a bad design, the domestic window mount air-conditioning unit.

However, this does not mean that the architect can completely ignore the local climate (though many would appear to). Inappropriate building designs add enormously to the utility bills of owners and tenants every year. Whilst it may appeal to some aesthetic concern to leave shading devices off a west-facing glass facade, someone is going to have to pay real money to support that oversized air-conditioning system for the next 40 to 50 years. Ultimately we will all pay with increased greenhouse gasses and higher taxes as such design decisions are just a pure waste.

3.2 IMPORTANT HOURLY DATA

There are a wide range of climatic factors that can be recorded. However, many are interrelated and can often be derived from a combination of other measures. In terms of hourly data, the following is the minimum required for both pre-design site analysis and post-design thermal performance simulation:

Dry-Bulb Temperature (°C or °F)

Relative Humidity (%)

Solar Radiation (W/m²)

Wind Speed (km/h, mph or m/s)

Wind Direction (Deg, 1/4, 1/8 or 1/16ths)

Cloud Cover (%, tenths or octa)

Rainfall (mm or in)

Obtaining the right data for The Weather Tool means obtaining digital hourly data sets from some source or other. If you overcome that hurdle, then the software can import almost any ASCII file format, either fixed or column separated, as well as some binary files. As there are so many different

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'standards' for weather data, you will most likely also need to know the format of the data within the file(s) in case you need to build up an import/export profile, a relatively simple process within the software itself.

3.3 CLIMATE OPTIMUM ORIENTATION

Another important aspect that affects building performance is orientation. An effective passive solar heating design assumes that the building is orientated to receive as much solar radiation as possible in wet season, when heating is required, whilst rejecting as much as possible in dry season when it is not.

To derive the most effective orientation, the Weather Tool calculates the amount of solar radiation incident on a 1m² vertical surface for each 5° of orientation angle. Three values are stored for each angle, the average daily radiation taken over the whole year, over the coldest 3 months and over the warmest 3 months. These three values can then be plotted on a polar graph where the radius of any point from the centre represents the incident radiation value.

Chart showing optimum orientation angles based on solar radiation received in the coldest 3 months (blue), the warmest 3 months (red) and over the entire year (green).

The most favourable orientations occur where the amount of incident radiation in wet season is greater than that incident in dry season, where the blue line extends out beyond the red line.

However, it is also desirable to provide as much protection from the maximum dry season radiation as possible. Hence the optimum orientation provides maximum wet season collection with maximum summer protection which, assuming the building is roughly orthogonal, involves a trade-off between the two based on the relative amounts of heating and cooling stress in the climate. Thus, in the graph above, the compromise angle is not exactly at the point of maximum wet season collection, but slightly to the east in order to 'turn away' slightly from the hot afternoon sun in dry season.

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4.0 LOCATION OF AKURE

Akure is the capital city of Ondo state. Ondo state lies between latitudes 5°45' and 7°52'N and longitudes 4°20' and 6° 05'E. Its land area is about 15,500 square kilometres. Ondo State is bounded on the east by Edo and Delta states, on the west by Ogun and Osun States, on the north by Ekiti and Kogi States and to the south by the Bight of Benin and the Atlantic Ocean.

Map of Nigeria. Map of Akure.

4.1 CLIMATE OF AKURE

Akure has a warm humid climate. This climate is characterised by a low diurnal temperature range, high humidity and generally high temperatures. Comfort is achieved by ventilation and by restricting the flow of heat into the building. To achieve this, a short time lag, low thermal capacity, high insulation and reflective roofs are used.

There is need for permanent provision for ventilation for ten months of the year as a result of the combination of high humidity and hot discomfort in the day. The monthly rainfall never exceeds 200mm. Despite the hot and humid nature of the climate thermal storage is still needed for two months of the year as a result of the combination of low humidity and high diurnal range of more than 10 degrees Celsius. There is no need to provide outdoor living space and protection against cold is not required.

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Climatic data of Ondo State. Source: (BBC weather,2006).

Temperature and Rainfall Chart of Ondo State.

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4.2 WESTERN VIEW OF SOME BUILDINGS IN AKURE, ONDO STATE.

Faloye House, No 42b Oyemekun Road Akure. No 55 Oyemekun Road Akure.

Adaramola House, No 66 Oyemekun Road Akure. Agunloye House, No 63 Oyemekun Road Akure.

5.0 EXTERNAL SHADING DEVICES

Exterior shading devices include vertical fins, vertical slat louvers, shutters, rolling shutters and shades, and solar screens. One of the most important external shading devices is the wall itself. In many instances, the structure and form of the building can be designed such that it protects windows that may normally be exposed to direct summer sun.

AWNINGS

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Maintaining a gap between the top of the awning and the side of the building helps vent accumulated heat from under a solid-surface awning. If in a climate with cold winters, it may be desirable to remove awnings for winter storage, or to install retractable ones to maximise winter heat gains.

The amount of drop (how far down the awning comes) depends on the orientation of the window. An east or west window needs a drop of at least 65% to 75% of the window height. A north-facing window needs only a drop of 30% to 50% for the same amount of shade. A pleasing angle to the eye for mounting an awning is around 45 degrees. It is also important to make sure the awning does not project into the path of foot traffic by keeping the height of its lowest point at least 2 meters above the ground.

One disadvantage of awnings is that they can block the view from inside, particularly on the east and west sides. However, slatted awnings do allow limited viewing through the upper parts of windows.

LOUVRES

Louvres are attractive because their adjustable slats control the level of sunlight entering the building and, depending on the design, can be manually adjusted from inside or outside. The slats can be vertical or horizontal. Vertical slats are required to shade the western elevation. Louvers remain fixed and are attached to the exteriors of window frames. Careful attention to the louvre angle can allow significant wet season sun penetration whilst still excluding all sun in dry season.

SHUTTERS

Shutters are movable wooden or metal coverings that, when closed, keep sunlight out. Shutters are either solid or slatted with fixed or adjustable slats. Besides reducing heat gain, they can also provide privacy and security. Some shutters help insulate windows when it is cold outside.

ROLLER SHUTTERS

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Roller shutters have a series of horizontal slats that run down along a track. These are the most expensive shading options, but they work very well and provide excellent security. Many exterior rolling shutters or shades can be conveniently controlled from the inside. The disadvantage, however, is that they block all light and view when fully extended.

SOLAR SCREENS

Solar screens are effective on the western elevation. These are external fittings to the outside of the frame that are partially transparent. They act to reduce the amount of direct sunlight entering the window and cut down overall glare without fully blocking the view or eliminating air flow. They also provide privacy by restricting the view of the interior from outside the building. Solar screens come in a variety of colours and different materials with weaves that offer varying degrees of sun control. Some screens are also partially reflective, further reducing sun penetration.

6.1 INTERIOR SHADING

Although interior shading is not as effective as exterior shading, it is worthwhile if no external technique is possible. There are a number of different types.

CURTAINS AND DRAPES

Draperies and curtains made of tightly woven, light-coloured, opaque fabrics reflect more of short-wave solar radiation back out the window than they let through. The tighter the curtain is against the

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wall around the window, the better it will prevent heat gain. Two distinct layers improve the effectiveness of the draperies insulation when it is either hot or cold outside.

VENETIAN BLINDS

Blinds, although not as effective as drapes, can be adjusted to let in some light and air while reflecting some of the heat. Some newer blinds are coated with reflective finishes for increase their effectiveness. To be effective, the reflective surfaces must face the outdoor side.

CELLULAR SHADES

Cellular or honeycomb shades are similar to window shades, but are constructed from two layers of material with an air-gap between. This allows them to be raised and lowered easily, and also increases the thermal resistance of the overall window slightly. Some interior cellular shades also come with a reflective mylar coating. When down, these shades block the majority of natural light and restrict air flow.

ROLLER BLINDS

Opaque roller shades are simply a thin sheet of material that unrolls down behind the window to reduce direct sun penetration. They are reasonably effective when fully drawn but also block light and restrict air flow.

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5.2 DESIGN OF SHADING DEVICE

The design of shading devices can be quite complex. Whilst a wide range of computer-based methods to accurately shape shades for very specific purposes are introduced here, in their absence - and with a little understanding of the mechanics ofsolar position and the sun-path diagram - there are also manual methods that can be used.

5.3 DESIGN REQUIREMENTS

The design requirements for a shading device depend entirely on a building's use and local climatic conditions. In a multi-storey open-plan office building in a relatively warm climate, the occupancy and equipment gains may mean that heating is rarely required. In this situation, to avoid unnecessary loads, shading may be designed to completely protect windows all year-round.

In a domestic building or one that is occupied 24 hours, the release of stored heat during cold nights in winter may be important. In this case, the shading might be designed to fully protect windows during the summer months, but to expose them as much as possible to direct sun in winter so that the spaces within have a chance to absorb heat during the day. In climates where summers are also relatively cold, the requirement may even be to allow full solar access all year-round.

5.4 MATERIALS AND METHODS OF CONSTRUCTION

In recent years, there has been a dramatic increase in the variety of shading devices and glazing available for use in buildings. A wide range of adjustable shading products is commercially available from canvas awnings to solar screens, roll-down blinds, shutters, and vertical louvers. While they often perform well, their practicality is limited by the need for manual or mechanical manipulation. Durability and maintenance issues are also a concern.

5.5 DESIGN STEPS

As a preliminary step, you may wish to use a manual method to calculate the size of shading devices you will likely need based on Shadow Angles. To design a horizontal shading device this way, use the following basic steps:

Determine cut-off date:This is the date before which the window is to be completely shaded and after which the window will be only partially shaded.

Determine Start and End Times:These represent the times of day between which full shading is required. Keep in mind that the closer to sunrise and sunset these times are, the exponentially larger the required shade.

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Look up Sun Position:Use solar tables or a sun-path diagram to obtain the azimuth and altitude of the sun at each time on the cut-off date.

Calculate the Shadow Angles:Using the methods detailed in the Shadow Angles topic, calculate the HSA and VSA at each time.

Calculate Required Depth and Width:Once again, using the Shadow Angle methods, calculate the depth and width of the required shade on each side of the window.

Alternatively you can use visual or automatically generated shading profiles to get your basic shading shapes.

5.6 UTILISING SEASONAL VARIATIONS IN THE DESIGN OF SHADING DEVICE

From personal experience, the most important characteristic of solar position is its seasonal variation. At the height of dry season the sun rises much earlier and sets much later and in completely different positions than in wet season. Not only is it visible in the sky much longer, but has a much higher average altitude.

The hourly path of the Sun through the sky in Dry and Wet Seasons.

The aim of good shading design is to utilise these characteristics to maximum advantage - typically to exclude as much solar radiation as possible in dry season whilst letting as much through as possible during wet season.

6.0 EXTERNAL AND INTERNAL SHADES

Both external and internal shades control heat gain. External shades are more effective than internal shades because they block the solar energy before it enters the window. When using an internal shade, such as blinds or a curtain, the short-wave radiation passes through the glass and hits the shade.

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Depending on the colour of the shade, some percentage will be reflected straight back out the window, but the rest will be absorbed by the shade itself, effectively heating it up.

The energy from the hot blind is given off as long-wave radiation, half into the space and half from the other side back towards the window. As discussed in the greenhouse effect topic, window glass is opaque to long-wave radiation so it gets trapped between the window and the blind and ends up heating the air within this space. This heated air will tend to rise, exiting out the top and drawing in cooler air from below. This forms quite an effective thermosyphon that continually draws cool air from the bottom of the space, heats it up and pushes it out the top underneath the ceiling. Over a whole day this can significantly increase internal room temperatures. Also, as the return-air ducts of most air conditioning systems are in the ceiling, this hot air can add significantly to air- conditioning loads.

The effects on internal heat flow of external versus internal shades. If user control is necessary, use blinds encased in a double-glazed unit where excess heat gains are vented to the outside.

Thus, even though internal and external shades seem to be doing the same job (protecting the occupants from solar radiation), their effect on the performance of the building is quite different. Where possible you should always use external shading devices. If you must use internal shading devices, then the best option is to house them inside a double glazed unit with vents at the top and bottom of the external leaf. This isolates the heat from the blind and makes the long-wave opacity of glass work for you not against you. There are many such products commercially available.

The next best option is to use a sealed unit that, when closed, does not allow the vertical circulation of air. This can be as simple as long curtains that extend down to and rest on the floor (retarding the entry of cool air) with a sealed pelmet at the top (retarding the exit of heated air from the top).

6.1 EFFECTS OF SHADING

Being based on specific dates and times, the above methods provide a precisely shaped shade that will provide full protection over the date/time periods selected. However, this does not fully consider how much Sun will get in during wet Season.

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Unfortunately, a shading device does not suddenly stop working after a certain date (unless it is fully retractable). It will usually partially obscure the window year round, more-so in dry and less-so in wet season.

In order to understand the full effect of a shading device, we really need to turn again to the sun-path diagram, Shading Masks and a full assessment of partial shading. It may be that, whilst we want 100% shading throughout most of dry season, we could probably live with only 80-85% shading in order to gain a little extra solar gain in wet season.

Part of this trade-off requires that the designer know what to cut back on or what to add in order to achieve the desired effect. The pattern of shading over the year from even a simple shade can be quite complex. High percentages of shading can occur much later than you expect unless you take significant care with your shading design.

To demonstrate this, an interactive diagram has been prepared below to show a range of different shading configurations and views of their shading masks. You can use the selector beneath it to see the effects of changing certain parameters of a simple shade.

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The four fundamental shading strategies.

The use of appropriate solar controls is very important, especially within air-conditioned buildings. Proper shading greatly reduces what is essentially a needless waste of energy trying to cool a space with large areas of unprotected glazing. The diagram above indicates the four fundamental shading strategies available to you. It is essential that the designer understand the advantages and disadvantages of each in order to select and apply them correctly.

7.0 OVERSHADOWING

Annual patterns of overshadowing can be visualised by plotting the outlines of obstructing buildings and vegetation on to a sun-path diagram. This produces what is known as the solar aperture - being that area through which a particular point can 'see' the sky.

Imagine lying on your back looking through a full 180° fish eye lens pointing straight up towards the zenith of the sky. The image you see would be very close to that shown immediately below. If the path of the Sun through the sky was then overlaid, it would be possible to tell when in the year you would be in direct Sun and when you would be in shade by simply determining when the Sun was obstructed by objects in the surrounding environment.

Diagram showing a photograph of the entire sky dome taken through a 180° fish-eye lens and how the sun-path and solar aperture is defined.

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To construct such a diagram, consider an imaginary hemisphere surrounding some point - usually termed the Focus Point. Areas of Sun blockage are determined by projecting lines out from this point to vertexes on the silhouette of each object and determining where they intersected this imaginary hemisphere. The resulting shapes on this hemisphere can then be transferred to a sun-path diagram. Any area of shadow in the resulting diagram represents an area when the Focus Point is in shade, where an object would block light from the Sun when it is behind it.

Diagram of a simple solar window projected onto an imaginary hemisphere around an object.

From ECOTECT From RADIANCE

Examples output of computerised 3D shadow projection.

The diagram below shows the resulting sun-path diagram when the Sun blockages from above are transferred. When the Sun path for the particular location is overlaid, the designer can quickly determine, from a single diagram, both the times of day and days of the year that the Focus Point would be in direct Sun or in shadow.

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The same solar window projected onto a sun-path diagram.

7.1 OVERSHADOWING PROJECTIONS

The process of projecting shadow blocks onto a sun-path diagram is not particularly complex, just laborious. However, note in the diagram above that all the vertical lines in the model run radially straight from the outer horizon line into the centre at the zenith of the sky. However, the horizontal lines at the top of each object are actually slightly curved.

If you were to trace points running along an infinite horizontal line somewhere above you, their altitude angles would gradually reduce the further away each point got, until they finally disappeared over the horizon - at which point their altitude would be zero. This results in the curvature of all horizontal lines when plotted on a stereographic sun-path diagram. This curvature actually follows lines of constant vertical shadow angle.

7.2 SHADOW ANGLES

When attempting to shade a window, the absolute azimuth and altitude of the Sun are not as important as the horizontal and vertical shadow angles relative to the window plane. These can be calculated for any time if the azimuth and altitude of the Sun are known. Horizontal shadow angles which are a measure of the vertical shading devices are more effective on the western elevation of a building.

7.21 HORIZONTAL SHADOW ANGLE (HSA)

This is the horizontal angle between the normal of the window pane or the wall surface and the current Sun azimuth. The normal to a surface is basically the direction that surface is facing - its orientation.

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The derivation of horizontal shadow angle (HSA).

If the orientation of the surface is known, then HSA is simply given by:

7.3 SPECIAL GLASSES SHADING

Many glass companies promote their solar control glass products as the answer to sun penetration problems. In many cases the use of such glasses can save significant amounts of energy. However, if specified or used incorrectly they can actually add to heat loads in a building. Therefore it is vitally important that designers properly understand how such glasses work and why. As background, you should first read the properties of glass topic - specifically the solar control glass section.

When sunlight hits a pane of glass, it is split into three components - that which is reflected, that which is absorbed and that which is transmitted through. The actual process is a little more complex than this as the three major components comprise a number of sub-components outlined below.

The three major components of sunlight when it hits a pane of glass.

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9.0 SHADING BY EYE

Designing shading devices by eye means simply lining up the shadows from obstructions with certain points in your model - based on the position of the Sun at different times of the year. There are a number of ways you can do this, each detailed in the following sections.

1. Lining up Shadows

The most obvious way is to display the effects of a shadow on a particular surface and then interactively manipulate the shading device until the right shadow length is reached.

Nudging a shade into line.

2. Examining Sun Penetration

In a similar manner, if the aim is to prevent the direct Sun from penetrating into a space, you can also shape a shade by watching for Sun patches on the surface inside the window.

Ensuring no Sun penetration.

3. Viewing from the Sun's Position

By the time the Sun's rays reach the Earth, they are almost completely parallel. Thus, if we view the model in an orthographic projection from exactly the same direction as the Sun, then the objects and

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surfaces we can see will be exactly those exposed to direct Sun at that time. Within such a view, if we choose the right dates and times, we can manipulate the model an simply line things up accurately by eye.

The Sun's view of a shading device.

4. Spraying Solar Rays

Taking the idea of solar rays one step further, ECOTECT allows you to visualise the actual solar rays from the Sun within the model. You can then track them as they bounce off surfaces such as a lightshelf and into the room. This way you can size and angle your lightshelf as accurately as you wish.

Using solar rays to design a shade

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10.0 RECOMMENDED SUN SHADING ON WESTERN ELEVATION IN AKURE

The use of sun control and shading devices is an important aspect of many energy-efficient building design strategies. In particular, buildings that employ passive solar heating or daylighting often depend on well-designed sun control and shading devices.

During cooling seasons, external window shading is an excellent way to prevent unwanted solar heat gain from entering a conditioned space. Shading can be provided by natural landscaping or by building elements such as awnings, overhangs, and trellises. Some shading devices can also function as reflectors, called light shelves, which bounce natural light for daylighting deep into building interiors.

The design of effective shading devices will depend on the solar orientation of a particular building facade. For example, simple fixed overhangs are very effective at shading south-facing windows in the dry season when sun angles are high. However, the same horizontal device is ineffective at blocking low afternoon sun from entering west-facing windows during peak heat gain periods in the dry season.

Exterior shading devices are particularly effective in conjunction with clear glass facades. However, high-performance glazings are now available that have very low shading coefficients (SC). When specified, these new glass products reduce the need for exterior shading devices.

The table below indicates the appropriate types of shading device for use on each orientation of a building. These are intended as guidelines only as there are many variations to these basic shading types and some significant room for innovation in the detailed design of shading.

Table Showing simple shading strategies for different orientation.

ORIENTATION EFFECTIVE SHADING

South Fixed Horizontal Device

East Vertical Device/Louvres (moveable)

North Not required

West Vertical Device/Louvres (moveable)

Thus, solar control and sun shading to the western elevation can be provided by a wide range of building components including:

•Landscape features such as mature trees or hedge rows;

•Exterior elements such as pilasters, vertical louvres and vertical projected fins;

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•Horizontal reflecting surfaces called light shelves;

•Low shading coefficient (SC) glass; and,

•Interior glare control devices such as Venetian blinds or adjustable louvers.

Fixed exterior shading devices such as vertical projected fins are generally most practical for small commercial buildings. The optimal length of an vertical projected fin depends on the size of the window and the relative importance of heating and cooling in the building.

In the dry season, peak sun angles occur at the solstice on June 21, but peak temperature and humidity are more likely to occur in August. Remember that an vertical projected fin sized to fully shade a west-facing window in August will also shade the window in April when some solar heat may be desirable.

To properly design shading devices it is necessary to understand the position of the sun in the sky during the cooling season. The position of the sun is expressed in terms of altitude and azimuth angles.

•The altitude angle is the angle of the sun above the horizon, achieving its maximum on a given day at solar noon.

•The azimuth angle, also known as the bearing angle, is the angle of the sun's projection onto the ground plane relative to south.

Shading devices can have a dramatic impact on building appearance. This impact can be for the better or for the worse. The earlier in the design process that shading devices are considered they more likely they are to be attractive and well-integrated in the overall architecture of a project.

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11.0 CONCLUSIONS

Given the wide variety of building designs, shape and forms, it is difficult to make sweeping generalizations about the design of shading devices. However, the following design recommendations generally hold true:

To the greatest extent possible, limit the amount of east and west glass since it is harder to shade than south glass. Consider the use of landscaping to shade east and west exposures.

Consider shading the roof even if there are no skylights because in the tropics the roof is a major source of transmitted solar gain into the building.

Remember that shading effects daylighting; consider both simultaneously. For example, a light shelf bounces natural light deeply into a room through high windows while shading lower windows.

Do not expect interior shading devices such as Venetian blinds or vertical louvers to reduce cooling loads since the solar gain has already been admitted into the work space. However, these interior devices do offer glare control and can contribute to visual acuity and visual comfort in the work place.

Carefully consider the durability of shading devices. Over time, operable shading devices can require a considerable amount of maintenance and repair.

When relying on landscape elements for shading, be sure to consider the cost of landscape maintenance and upkeep on life-cycle cost.

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REFERENCES

Afshar Farroukh, Allan Cain, John Norton, and M. R. Dardie. Oman: Problems and Potentials of Indigenous Building. Development Workshop Report. London: Architectural Association School of Architecture, 1974.

Alcock, A. E. S., and H. M. Richards. How to Build for Climate. London: Longman, 1960. "Architecture and Energy." Architectural Forum 134, no. 1 (July-August 1973): 1112.

Aronin, A. J. Climate and Architecture. New York: Reinhold Publishing, 1953.

Aslam, Mohammad. Research on Building Materials at Pakistan Council of Scientific and Industrial Research. Karachi: Pakistan Council of Scientific and Industrial Research, 1964.

Atkinson, G. A. "An Introduction to Tropical Building Design." Architectural Design 23 (October 1953): 268.

Atkinson, G. A. Design and Construction in the Tropics. United Nations Housing and Town and Country Bulletin no. 6. New York: United Nations, 1956.

Atkinson, G. A. "Principles of Tropical Design." Architectural Review 128, no. 761 (1960).

Bahadori, Mehdi. "Passive Cooling Systems in Iranian Architecture." Scientific American 238, no. 2 (February 1978): 144.

BBC Weather.org (2006): Climatic data for Nigerian cities.

Bowen, Arthur, Eugene Clark, and Kenneth Labs. Passive Cooling: Proceedings of the International Hybrid Cooling Conference. Newark, N.J.: Dell/ International Solar Energy Society, 1981.

Building Research Board of the National Research Council. Roofing in Developing Countries: Research for New Technologies. Washington, D.C.: National Academy of Sciences-National Research Council, 1974.

Calder, R. Man against the Desert. Mystic, Conn.: Verry, Lawrence, Inc., 1958.

Koenigsberger, O.H., Ingersoll, T.G., Mayhew, A. and Szokolay, S.V. (1974). Manual of Tropical Housing And Building, Part I, Climatic Design.Longman, London.

Markus, T.A. and Morris, E.N. (1980). Buildings, Climate and Energy. Pitman International, London.

Ogunsote B. P. (2006): Reflection of culture and climate in the vernacular and modern architecture of Akure.

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TABLE OF CONTENTS

1.0 Abstract

2.0 Introduction

3.0 The Sun

3.1 Solar Position

3.2 Positional Characteristics

3.3 Solar Position Calculator

3.4 Solar Position 1/2 Hourly

3.5 Solar Position Variation

3.6 Azimuth Lines

3.7 Altitude Lines

3.8 Date Lines

3.9 Hour Lines

3.10 Sun Positions

3.11 Polar Sun-Path Diagrams

3.12 Cartesian Sun-Path Diagrams

3.13 Local and Solar Time Overshadowing

4.0 Introduction to Climate

4.1 Climate Analysis

4.2 Important Hourly Data

4.3 Climate Optimum Orientation

5.0 Location of Akure

5.1 Climate of Akure

5.2 Western Elevation of Some Buildings in Akure.

6.0 External Shading Devices

6.1 Interior Shading

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6.2 Design of Shading Device

6.3 Design Requirements

6.4 Materials and Methods of Construction

6.5 Design Steps

6.6 Utilising Seasonal Variations in the Design of Shading Devices

7.0 External and Internal Shades

7.1 Effects of Shading

8.0 Overshadowing

8.1 Overshadowing Projections

8.2 Shadow Angles

8.21 Horizontal Shadow Angle (HSA)

9.0 Special Glasses Shading

10.0 Shading by Eye

11.0 Recommended Sun Shading on Western Elevation in Akure

12.0 Conclusions

References

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RECOMMENDED SUN SHADING ON WESTERN ELEVATION IN AKURE, ONDO STATE.

By

Adara Rawlings Seun. (Arc/03/1884)

Submitted in Partial Fulfillment of the Requirements for the Award of Masters of Technology in Architecture

Submitted to

The Department of Architecture,School of Postgraduate Studies,Federal University of Technology, Akure.

Applied Climatology (Arc 813)Course Lecturer: Prof. Ogunsote O.O.

August, 2011]

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