A landmark research conducted by Ms. Lerdlekha Tanachaikhan, a student
at AIT and Prof. Sivanappan Kumar of AIT’s School of Environment,
Resources and Development (SERD) reveals a huge potential of possible
savings in lighting and air-conditioning costs in tropical countries in
case proper parameters like size, height, placement and glazing are
observed while installing windows.
A research paper jointly authored by the two, which has been published
in the International Journal of Engineering Systems Modelling and
Simulation considers daylighting as an extremely important aspect for
energy conservation in the tropical regions. Daylighting is a technique
used to bring natural light into a building through various openings
like windows, and it is regarded as an efficient way to reduce the
energy consumption for buildings.
“Unlike high latitude countries like the US and UK, where daylighting
in not only helps create a good atmosphere but also helps in increasing
passive heating of the building; the case in tropical countries is very
different”, says Lerdlekha who conducted this research at AIT. Based on
data obtained from tropical countries, she states that buildings in
places like Bangkok should not follow the pattern prescribed in high
latitudes.
“Tropical countries should prefer smaller window area of 10-20 per cent
of the total building façade in case the building has no external
shading while places like Montreal in Canada use a 30 per cent window
to wall ratio,” suggests Lerdlekha. If external shading devices like
projections are used, then the window area can go upto 30-40 per
cent.
External shading like a projection is always better than internal
shading devices like curtains. This is because external shading cools
the heat before it enters the building. Internal shading devices like
curtains and blinds on the other hand tend to trap the heat inside,
again increasing the energy cost of cooling the room.External shading
should be designed keeping the orientation of the building into
consideration. Horizontal shading is better for North/South
orientation, while vertical shading is better for an East/West
orientation.
The research also dispels the fashion of too much glazing as used in
some latest buildings. Some buildings use very dark glazing and they
are unable to benefit from daylight, which means that they use more
electricity for lighting their buildings. In case dark glazing is used
in tropical countries, then the window to wall ratio can be around 50
per cent. However, some buildings tend to supplant windows using it as
a façade, implying that greater heat enters into the building and this
requires more air conditioning.
Apart from size of windows, external shading and glazing, even the
height of windows is an important parameter. Higher windows are better
than wider and shorter windows. “This is because higher windows allow
for more deep lighting inside the room, while wider windows only light
up the area close to the window,” Lerdlekha adds.
Reflectance of the room is another important parameter. If the
reflectance of the room is increased, it not only ensures that the room
is brighter but also ensures more daylighting efficiency. An increase
in 20 per cent reflectance can result in a 20 per cent increase in
daylighting as well.
Says Prof. Kumar, “To address the mitigation of greenhouse gases and to
reduce global warming, daylighting is one important option to reduce
the current lighting energy use.” The other option is to use efficient
lighting systems - compact fluorescent lamps/LED's instead of
incandescent lamps. “Intelligent use of daylighting techniques can also
help in reducing the air conditioning load in commercial buildings,”
Prof. Kumar states.
The share of commercial sector is about 20 per cent and in the
commercial sector, lighting energy can constitute up to 40 per cent in
air conditioned buildings, and upto 80 per cent in case of non air
conditioned buildings, reveals Prof. Kumar. Moreover, lighting in air
conditioned buildings also contribute to increase in cooling load.
“Introducing daylighting techniques in air conditioned buildings in the
commercial and residential sector can result in significant savings and
reductions and Ms. Lerdlekha's work is an attempt in this direction,”
Prof. Kumar adds.
For conducting the study, parameters like solar irradiance, exterior
daylight, luminance, luminance distribution, ambient and sky
temperatures and humidity were measured at the meteorological station
at AIT. Hourly data was measured during work hours (8 am - 5 pm) for a
three year period (2004-2006). A typical office building at Energy Park
at AIT was used as a reference daylit space.
Lastly, “never forget to clean your windows,” Lerdlekha laughs as she
belts out another suggestion to improve daylighting.
The reserach has also been highlighted at Science
Daily
AIT researcher Ms. Lerdlekha
Tanachaikhan in front of the AIT library building which uses windows
for efficient daylighting.