banner

The following is an example of how a designer might approach a design analysis to optimize daylight in a building.
1415 Views

Optimising Daylight in South Africa: A case study


Light is of decisive importance in experiencing architecture. The same room can be made to give very different spatial impressions by a simple expedient of changing the size and locations of its openings. Moving a window from the middle of a wall to a corner will utterly transform the entire character of the room. To most people a good light means only much light. If we do not see a thing well enough we simply demand more light. And very often we find that it does not help because the quantity of light is not nearly as important as its quality. (Rasmussen, 1964)

At the moment lighting accounts for around 35% of the energy used within non-residential buildings and between 0% and 28%1 in residential buildings. Electricity usage (%) in the residential sector for high/ middle income residences consume typically 5% for fluorescent and 12% for incandescent types of lighting. (UNEP, 2009). Designers are encouraged to use natural daylight in their designs to reduce the energy used (SANS 204-2, 2008).

The use of daylight to supplement or as a substitute for electric light in the window zones of interiors with side windows or over the entire area of spaces with skylights can save lighting energy. This saving should be balanced against the energy required to compensate for heat gains and losses through the daylight openings. During times of low external temperatures more heating and during times of high external temperatures and sunshine more cooling of the interior will be required in order to maintain a constant internal air temperature. The use of daylight therefore will only be energy effective and cost-effective if the savings on lighting exceed the extra expenditure for climate control (SANS 10114-1, 2005)

Uses of Daylight

Screen Shot 2015-02-02 at 5.02.43 PM

Natural daylight is a very important and interesting source of lighting in buildings. Natural daylight can inter alia be used for functional1, decorative2 and artistic3 purposes. In the SANS 204-2 and 10114-1 norms the emphasis is mostly on functional uses. The light levels, power and energy usage for the building is determined in accordance with a lookup table 14 (SANS 204-2, 2008).This table describes the recommended light levels, power and energy for various classes of buildings. The light levels range from 50 lx for entertainment and public assembly to 700 lx for high risk industrial type of spaces.

The developments in electric lighting have not eliminated a widespread preference for daylight in buildings, wherever practicable. The reliance on daylight is greater in homes, offices, schools and patient areas in hospitals than in factories and shops.

The factors listed below will be different for different types of interior, different methods of daylight admission and for different climates (See Table 6.2). Recommendations regarding daylight should inter alia allow for the following factors (SANS 10114-1, 2005): Levels and uniformity. Daylight provides variability and, when it enters through side windows, creates a specific modeling and luminance distribution in the interior. It therefore contributes to visual satisfaction. The quantity of daylight is usually specified by the daylight factor, both with regard to illuminance and uniformity. In interiors with side windows, the available daylight decreases rapidly with distance from the windows. In many cases such as living rooms and small offices this non uniformity is acceptable and even appreciated. In other cases, supplementary electric lighting is required. Roof lights (skylights) can provide ample and highly uniform daylighting, but should be carefully designed to avoid solar overheating and glare.

• External view. Where natural light is used throughout the day for reasons of convenience and economy, an additional advantage is the view of the outside environment. However this is not always possible in large industrial or commercial buildings. The best position, shape and dimensions of the windows will depend on the nature of the outside environment. It also depends on the building design and will take into account architectural, lighting, visual, thermal and acoustic considerations.

Glare from the sun or sky. Daylight can produce sky glare and can adversely affect the comfort in the interior. Direct sunlight is desirable for various types of buildings, such as homes in moderate climates, but should generally be avoided in work areas. Means to avoid direct sun irradiation are appropriate orientation of windows and skylights, the use of various types of curtains or blinds and the use of louvres or screens. The latter are also effective in reducing sky glare and are particularly important on the upper floors of high-rise buildings where large parts of the sky might be visible. Small windows have an effect on the sky glare only to the extent that they prevent parts of bright skies or bright opposite facades or buildings from being seen. When appreciable areas of a bright sky remain in the field of view some glare such as discomfort5 glare or disability6 glare should be expected. Therefore, even with small glass areas, work areas directly facing windows should be avoided. If this is not possible, some means should be provided to reduce possible sky glare. Other techniques to reduce window glare are:

• The use of external or internal devices, such as louvres.
• Deep splayed reveals on the side of the windows, finished with a high reflectance surface and

with the same finish applied to any frames and glazing bars.
• The use of tinted low transmission glazing.
• Arranging for light in the interior to fall on the wall area adjacent to the windows, either from roof lights or from specially located luminaires.

Heat gains and losses. The heat gain through windows might require cooling of the interior during

the warm season, but might reduce heating costs during the cold season. However, heat losses through the window during the cold season can offset the savings and can increase heating costs. The use of daylight as an illuminant can save energy used for electric lighting, but this should be balanced against the energy required to compensate for the heat gains and heat losses through the glazing. Means to avoid excessive solar heat are:

  • Appropriate orientation of glazing.
  • Reduction of areas of glazing.
  • Use of an appropriate daylight system (Table 6.2)
  • Use of heat-reflecting or heat-absorbing glass or coated glass.

The International Energy Agency (IEA, 2000) recognizes a wide range of innovative daylight strategies and systems. Some are rarely used in South Africa. The IEA recognizes two basic types of daylight system i.e. daylighting systems with Shading and daylighting systems without shading. The latter type consists of four subdivisions:

• Diffuse light-guiding systems
• Direct light-guiding systems
• Light-scattering or diffusing Systems • Light transport systems

Gallery below provides some examples of the various types.

Luminance and iLLuminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/ m2). Luminance is often used to characterize emission or reflection from flat diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye’s pupil.

For a perfectly diffusing surface, the luminance can be calculated in accordance with the following formula (SANS 10114-1, 2005):

where

L is the luminance, candelas per square metre; E is the illuminance, in lux;
r is the reflection factor.

For example, if a matt surface that has a reflection factor of 0.5 is exposed to an illuminance of 200 lx, the luminance is

cd/m2

Illuminance is a photometric measure of the total luminous flux incident on a surface per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with the human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

In the SI system these are measured in lux (lx). lluminance was formerly often called brightness, but this leads to confusion with other uses of the word. “Brightness” should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.

4. Daylight factor
The daylight factor is the ratio of internal light level to external light level and is defined as:

Screen Shot 2015-02-02 at 5.13.50 PM

 

 

 

where:

Screen Shot 2015-02-02 at 5.13.57 PM

 

 

 

 

For example, if a matt surface that has a reflection factor of 0.5 is exposed to an illuminance of 200 lx, the luminance is

Screen Shot 2015-02-02 at 5.14.02 PM

 

 

 

 

 

 

 

Illuminance is a photometric measure of the total luminous flux incident on a surface per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with the human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

In the SI system these are measured in lux (lx). lluminance was formerly often called brightness, but this leads to confusion with other uses of the word. “Brightness” should never be used for quantitative description, but only for nonquantitative references to physiological sensations and perceptions of light.

4. Daylight factor
The daylight factor is the ratio of internal light level to external light level and is defined as:

Screen Shot 2015-02-02 at 5.14.08 PM

 

 

 

 

 

where:

Screen Shot 2015-02-02 at 5.14.18 PM

There are basically three paths (daylight factor components) along which light can reach a point inside a room, i.e. through a glazed window, rooflight or aperture as follows:
• The sky component (SC) that is direct light from part of the sky or sun at the point considered.
• The externally reflected component (ERC) that is light reflected from an exterior surface and then

reaching the internal point measured.
• The internally reflected component (IRC) that is light entering through the window but reaching

the point only after reflection from an internal surface.

The sum of the three components gives the illuminance level in lux at the point measured. The daylight factor only gives the proportion of daylight from outside that reaches the interior of the building and does not indicate the absolute level of illumination that will occur.

To calculate daylight factors requires complex repetition of calculations. It is normally undertaken by a software product such as Radiance. This is a suite of tools for performing lighting simulation which includes a renderer as well as other tools for measuring the simulated light levels. It uses ray tracing to perform all lighting calculations. The design day used for daylight factors is based upon the standard Commission Internationale de l’Eclairage (CIE) overcast sky for 21 September at 12h00 and where the ground ambient light level is 11921 lux. Since the CIE standard overcast sky assumes no orientation effects, the estimates of the daylight contribution can be wrong. To correct for this, orientation factors have been derived to be applied to the daylight factors. More recently the CIE has derived a standard based on the spatial distribution of daylight, i.e. the CIE Standard General Sky (CIE, 2002).

Rooms with a DF of 2% are considered daylit. However a room is only considered as well daylit when the DF is above 5%.

Screen Shot 2015-02-03 at 9.42.25 AM

Case study

The following is an example of how a designer might approach a design analysis to optimize daylight in a building. The first step is to determine the solar angles at different times of the year accurately. With the advent of Google Earth it has become much easier to determine these accurately. This is the basis for the calculation of solar angles.

Read the entire article in the Green Building Handbook Volume 4 on pg 114 here. Or sign-up to download the digital version of the handbooks here.


 

Green-Building-Conference

Book your seat here.

Join the discussion here.


 Follow Alive2Green on Social Media

Twitter Facebook LinkedIn Google +

Recently Published

6101415124_d8e25360fe
»

Environment officials talk sustainable tourism in Samoa

Environment officials are in Apia to begin talks starting with a ...

A2g
»

Huge impact of subtle law changes on mining and oil & gas industries

Analysis of proposed change to financial provisioning laws Financial ...

Screen Shot 2017-09-07 at 11.43.12 AM
»

LUX* implements 100% renewable energy production project on Ile des Deux Cocos property

The 20kWp project’s state-of-the-art design and storage solution ...

Screen Shot 2017-09-07 at 11.13.20 AM
»

Langa Residents Turn Trash into Cash with Polyco’s PACKA-CHING Project

In just four days, Langa locals brought 5,2 tonnes of glass,1,9 ...

Screen Shot 2017-09-07 at 10.51.21 AM
»

The State of the Construction Industry on the Table at Building Industry Congress

Economic transformation in the South African construction sector and ...

Screen Shot 2017-09-07 at 10.36.14 AM
»

Tourism month launches with water savings focus

A Western Cape hotel group has reduced its water consumption by more ...

Screen Shot 2017-09-07 at 9.47.48 AM
»

Agri SA to potentially change to Pty Ltd

Agri SA’s intention to change from a civic to a Pty (Ltd) ...

Screen Shot 2017-09-07 at 9.43.22 AM
»

Vehicle, mining industries face “enormous difficulties”

South African Finance Minister Trevor Manuel said on Friday the ...

Screen Shot 2017-09-07 at 9.34.50 AM
»

Private sector makes first move to build electric vehicle infrastructure in SA

JSE-listed information and communication technology (ICT) ...