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PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Designing the Malqaf for Summer Cooling in Low-Rise Housing,
an Experimental Study
SHADY ATTIA, ANDRÉ DE HERDE
Architecture et Climat, Université Catholique de Louvain, Louvain La Neuve, Belgium(
ABSTRACT: The malqaf or windcatcher is Egyptian vernacular archetypal device that traps the wind into the building.
For centuries, the malqaf has been used as a viable solution to ensure natural ventilation. However, for the last 50 years,
Egyptian practice has failed in combining traditional architectural devices into new techniques that could lead to
sustainable and energy aware buildings. In Egypt, more than half of the urban peak load of energy consumption in the
mean time is used to satisfy air conditioning demands alone. Therefore, the objective of the research is to develop a viable
passive alternative to active cooling by exploring the potentials and design parameters of windcatchers as solution for
passive cooling and natural ventilation during the summer season for low-rise housing. Experimental wind tunnel and
smoke visualisation testing were conducted to compare the air flow in a scale model room with and without windcatcher
on top of the roof with different orientations. The final result shows that the performance of the windcatcher depends
greatly on the position, orientation and size of the inlet and outlet opening in relation to the wall ratio. The study
developed a comparative matrix for examined parameters to support architects with the basic principles for windcatchers
design.
Keywords: malqaf, windcatcher, passive cooling, Egypt
INTRODUCTION
Natural Ventilation in traditional houses Natural
ventilation and passive cooling have been traditionally
two important features in ancient Egyptian Architecture,
the most outstanding example of which are windcatchers
as shown in figure 1. The idea of the malqaf dates back
to very early historical times. It was used by the
Egyptians in the houses of Tal Al-Amarna and is
represented in wall paintings of the tombs of Thebes.
One example, shown in figure 1a, is the Pharonic house
of Neb-Amun depicted on his tomb, which dates from
the nineteenth Dynasty (1300 B.C.). It has two openings,
one facing windward and the other leeward, to evacuate
the air by suction [1]
Fig. 1.a, Malqaf of the Pharonic house of Neb-Amun,
Nineteenth Dynasty (1300 B.C.), fig 1.b. Windcatcher: Sennari
House, Cairo (1798)
In most housing examples, the malqaf was placed
directly over a roof opening and without a shaft to
channel the airflow into the room. Several examples are
found in nineteenth-century Turkish-style houses in
Cairo, illustrated in fig. 1.b.
Egyptian Practice However, for the last 50 years,
Egyptian practice has failed in combining traditional
architectural devices into new techniques that could lead
to much more sustainable and energy aware buildings.
The malqaf is one of those vernacular archetypal devices
that fall outside existing experience. Today in Egypt,
more than half of the urban peak load of energy
consumption in the mean time is used to satisfy air
conditioning demands alone. Consequently, building
cooling using air-conditioners became the single largest
consumer of electricity and it accounts for nearly 60% of
nation’s peak power demand. Therefore, the new
Egyptian Energy Code for residential buildings implies a
minimum natural ventilation rate of 3 L/s-person for
living and sleeping areas and 14 L/s-person for kitchens
and toilets [2]. However, this relatively new code is not
enforced until now. On the other hand, we can read from
figure 2 that the average wind speed in Cairo is
approximately 4 m/s, while the prevailing wind direction
is North, and to a limited extent South as indicated by the
wind rose diagram. According to the Egyptian
Organisation of Meteorology (EOM), there is a strong
seasonal difference with winds in summer blowing
almost exclusively from the north and only two months
in winter the wind direction is reversed and blows almost
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
exclusively from the south. This means that wind can be
used effectively for natural ventilation at times when
outside temperatures have not reached unacceptable
levels.
Fig. 2. Average annual wind speed and direction in percentage
(Egyptian Organisation of Meteorology)
Therefore, the objective of the research is to develop a
viable passive alternative to active cooling by exploring
the potentials and design parameters of windcatchers as
solution for passive cooling and natural ventilation
during the summer season for low-rise housing. In the
present study, the results of the first stage in a series of
studies are presented and discussed. The study analyzed
flow patterns of windcatchers in low-rise buildings
through smoke visualizations in wind tunnel. Relatively,
simple cases were modelled with low air speed. The
parameters involved in this study were position,
orientation, entry cross section, shaft cross section, shaft
height, and outlet opening in relation to the wall ratio.
Function of a Malqaf The malqaf is a shaft rising
high above the building with uni-directional opening
facing the prevailing wind. It traps the wind from high
above the building where it is cooler and stronger, and
channels it down into the interior of the building. The
malqaf thus dispenses with the need for ordinary
windows to ensure ventilation and air-movement in
addition to enhancing indoor environmental quality. The
malqaf is also useful when filters are integrated within
the shaft; in reducing the sand and dust so prevalent
winds of hot arid regions [1].
In fact, the malqaf is an important bioclimatic
archetype in dense cities in hot dry climates, where
thermal comfort depends mostly on air movement,
evaporative cooling and thermal mass [3]. Since masses
of buildings reduce the wind velocity at street level and
screen each other from the wind, the ordinary window is
inadequate for ventilation. This situation can be corrected
by using the malqaf. In this respect it is not only the
inflow of unpolluted fresh air into the area that plays an
important role, but equally the cooling effect on the
buildings during the night. The bottom line is that
windcatchers serve mainly for the following functions
[4]:
1- Provide air circulation and replacement
2- Provide convective cooling
3- Provide evaporative cooling
METHODOLOGY
This study investigated, both theoretically and
experimentally, the interaction between parameters
considered to be important in achieving natural
ventilation, passive cooling and indoor environmental
quality [5]. The research has been carried out in two
parts. The first part consisted of an analysis of existing
malqaf houses, necessary to identify the main design
parameters and traditional design settings. The second
part compromised the flow visualisation in wind tunnel
for a model that resembles the traditional buildings in
Egypt. Each part was started with a literature review on
the specific subject.
Analysis of design parameters In planning for the
malqaf, several geometrical parameters have to be
investigated. In order to realize passive cooling through
natural ventilation during the summer season for low-rise
housing, we must design and position the malqaf to allow
cross ventilation and maximum air speed within the
space [6, 7]. As a result of an extensive analysis of
existing traditional buildings in Egypt the following
parameters were found and identified [4, 8]:
Position Among all examined cases, two settings of
catchers were found as shown in fig.3. In the first setting
(fig 3.a), which was the most common in traditional
buildings, the malqaf was located on top of the roof of
the ventilated space itself facing the on-coming wind.
Next, in the second setting (fig 3.b), the catcher was
located on the windward side in combination with
courtyard.
Fig. 3.a. Malqaf on top of the space, Fig.3.b.Malqaf in the back
in combination with a courtyard
The courtyard was mainly used as a cool reservoir
that redistributes filtered and cooled air to the
surrounding spaces. However, this study is limited to the
first setting.
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Orientation The typical orientation of the malqaf
was found facing the prevailing winds. Given that the
catcher is uni-directional, the entry is sensitive to capture
prevailing wind. Therefore, it is very important to look at
the probability of wind distribution speed and direction
on site for maximum natural circulation. In most
analysed cases, we find that the malqaf was oriented
towards the north-west from 0o to 7o.
Inlet cross-section and shape Traditionally, we find
that the shape of the top end of the malqaf was inclined
with a slope of 30o to 45o. Also, we find that the area of
the entry cross-section identical to the shaft cross-section
area. In fact, the inlet determines the amount of wind
trapped from high above the building and channels it
down into the interior of the building.
Shaft cross-section and height In most traditional
buildings we find that the shaft was built-in the ventilated
space. Generally, the shaft was square or rectangular and
its relative height was 1-1.2 H, where H is the height of
the space. Moreover, it was found that the shaft cross-
sectional area is around 3% of the ventilated space floor
area. The shaft height ranged from 0m to 10m, but the
most common height in Cairene buildings was 6m and
generally the malqaf projected at least one storey above
roof height. However, for this study the examined
prototype shaft height was zero and the shaft cross
section area equalled the entry cross-section area.
Outlet opening We find that the size of the outlet
opening had an important effect on the air flow
acceleration. The ratio of outlet opening to back-wall
(WWR) was similar in most lee side walls of historical
buildings with malqaf. The ratio varied from 0.4 to 0.6.
However, in many cases additional wind escapes such as
shukhshikha (stack-driven roof opening) and
mashrabiyas (screened windows) made it difficult to
precisely calculate this ratio.
Evaporative systems In some of the analysed
buildings, the malqaf was directing the airflow over
water elements in order to achieve evaporative cooling.
However, evaporative cooling is excluded from the
study.
Flow visualisation and test facility Flow
visualizations are the best tool for illustrating air flow
around buildings and they are highly instructive for any
design. Flow visualisations are important for architects
because they make it possible to analyse and propose
suitable solutions for windcatchers design [9]. The
visualization of the airflow clarify the areas of strips,
stagnation, and separated flow and reattachment zones
inside the building [10]. Several practical methods of
flow visualization are available [11]. Such importance
not only in giving clear qualitative images of flow
phenomena, but also in some cases, in yielding
quantitative information [12-14]. In this study, video
image were recorded for the different model settings.
Wind tunnel and smoke visualisation The flow
visualisation tests were conducted in the wind tunnel,
built in the Thermodynamics and Turbo Machines Unit
of the School of Engineering at the Catholic University
of Louvain (UCL). As shown in fig.4, the wind tunnel is
an open circuit type with a test section 1.2x1m and a 30m
long working section. The smoke was exhausted from the
smoke chambers using polyvinyl alcohol. Smoke flow
patterns were recorded on the video tape using a black
background. Smoke flow patterns of fifteen different
settings of the malqaf model were visualized and
recorded.
Fig. 4, Wind tunnel facility, UCL.
Test model Airflow visualizations have been made
for a single floor house prototype that resembles the
traditional buildings in Egypt. The building prototype is
of general form and is not depending on any kind of
mechanical cooling. Simple test model, representing low-
rise residential prototype, was constructed from Plexiglas
sheet 5-mm thickness and has a dimension of 60cm x
25cm x 20cm height as shown in fig.5. It was assumed to
be made to a 1:20 scale in the wind tunnel. The model
had 10 fixed points to place the anemometer and
measure the air velocity. The model was tested in the
wind tunnel as shown in figure 4&5.
Fig.5, Plexiglas model was constructed to allow multiple
settings, scale 1:20.
All the flow visualisations were carried out with the
same wind speed 2m/s using air at atmospheric pressure.
and various wind direction (a = 0°, 10°, 20°, 30°, and
45°). This set of tests was intended to investigate the
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
effect of combinations of openings on flow configuration
inside the model (building space), hence on air flow
patterns therein. The measurements of flow velocity air
flow rates through the test model were carried usinng a
pitot-static tube anemometer and constant injection
tracer-gas. Ten measurement points were fixed taking
measurments.
RESULTS
Figure 6,7 and 8 summarize air flow visualisation and
rates recorded through the different settings of the malqaf
space. The experimental results carried out in the smoke
tunnel visualised the flow patterns inside the test model.
Also we have measured the velocity, not taking in
account internal resistances such as filters. The
measurements were based on influence of design
parameters on the prototypical model. The air velocity
profile at test section was measured and presented. It is
important to mention that the results are not representing
the full scale experience. In fact, the main concern here
was to observe the relative effect of specific parameters
on the resulting natural ventilation performance. At
present, setting 1 was without malqaf and therefore will
be reported later in the comparative matrix (set. 4).
Setting 2 Setting 2 investigated the effect of placing
one malqaf in front in combination with various outlet
openings in the back on the air flow pattern in the model
(fig 6a). The best arrangement for getting maximum air
flow pattern into the space was achieved when the ratio
of outlet opening to wall was 0.6 and when the opening
was placed near the floor. The measurements showed
that movement of air under the ceiling are slow but are
higher in the standard level plane. Setting 2 provided an
increase of 200% in the flow rate.
Setting 3 In the case of setting 3, two malqafs were
used as shown in figure 5.b, one toward the wind and the
other toward the leeward. This case is appropriate, in a
space where it is undesirable to place any window in the
walls, to permit air movement inside the room. More
importantly, the measurements confirmed that this setting
was the most effective and significant settings among all
other design scenarios. This setting increased the airflow
rate and recorded the highest air flow rate. Moreover, this
setting has the potential to encourage cross ventilation in
the space, during the two months in winter, when wind
direction is reversed and blows almost exclusively from
the south. Also a turbulent air motion was observed in
the zone below the inlet opening. This turbulent curved
air movement has also been observed in setting 4.
Finally, setting 3 provided an increase of 280% in the
flow rate.
Setting 4 In setting 4, two malqafs were placed on
the roof both facing the wind. The air flow visualisation
shows that the second malqaf on the left became a wind-
escape due to the suction caused by the airflow pattern
due to the low-pressure zone. The measurements showed
that the movement of air under the ceiling are higher than
the standard plane. Setting 4 provided an increase of
240% in the flow rate (fig. 7a).
Fig.6.a. Setting 2 Fig.6.b. Setting 3
Fig. 7.a. Setting 4 Fig. 7.b. Setting 5
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Setting 5 When the malqaf is placed at the back of
the model facing the wind, but the outlet window covers
the full area of the wall, the air flow direction was from
right to left. Placing the catcher in the back in
combination with various frontal windows showed that
the malqaf became a wind-escape. Therefore this setting
is not successful especially when placed within a dense
urban context (fig.7b).
Setting 6 and 7 The purpose of setting 6 and 7 was to
define the optimal orientation of the malqaf. Flow
visualisations were carried out for a range of wind
direction from 0°, 10°, 20°, 30°, until 45°. Due to the
uni-directional nature of the malqaf, it was found that
orientation is a very important and sensitive parameter.
The performance of the catcher is optimal if the
orientation is kept 0° ±10° towards the north. After this
range, the air escapes from entering the malqaf.
However, it is important to mention that the 45°
orientation (north-east) creates a turbulent flow that
washes the whole model space creating spiral air
circulation (fig. 8).
Fig.8.a. Setting 6 Fig.8.b. Setting 7
Summing up the above mentioned results it was
found that in general the size of the outlet opening plays
a key role in accelerating the air velocity. First of all, the
larger and higher elevated the inlet and outlet opening is
the higher is the airflow rate. Secondly, it is very
important to allow the building occupants to control the
inlet and outlet openings during winter and stormy
weather when outside air is not welcomed. Thirdly, the
malqaf buildings have to be located in sites with
environmental and hygienic qualities away from any
source of pollution. The ancients located their buildings
by hanging pieces of meat in different sites and decided
upon the one where the meat lasted longest [15].
Fourthly, in designing the malqaf, they always kept a
balance between the opening areas and space floor-area
to avoid discomfort caused by high solar radiation
(overheating and glare), cold and dusty wind. Finally and
above all, convective cooling of thermal mass should be
encouraged by maximizing the total accessible volume of
thermal mass materials that has its surface exposed to
cool air.
COMPARATIVE MATRIX
In order to support the designer with solid design
guidelines we have used the measurements to calculate
the air change rates in the various model settings, not
taking in account internal resistances such as filters. The
results of the calculations indicated that by adding the
malqaf(s), one could obtain up to 5.6 air changes per
hour with an opening ratio of 0.6 and the reference
velocity of 2m/s (S3). The results confirm that the best
arrangement for getting maximum air flow pattern into
the space is to make the malqaf inlet and outlet as large
as possible. A comparative matrix was created (fig. 9),
listing air change rates calculated for the different model
settings. In planning for the malqaf, it was observed that
the malqaf position, orientation, inlet shape, shaft height
and outlet opening are some of the main factors in
controlling wind environment. It is found that for
summer cooling in low-rise housing using the malqaf has
the advantages of higher values of air change rates and
higher air velocities over ordinary flat-roof buildings
with windows, which leads directly to keeping spaces
more comfortable.
Fig.9, Air change per hour at 2m/s in a 1:20 model.
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
CONCLUSION
Due to the increasing density of existing and future urban
areas in Egypt there is an increasing electric demand
caused by mechanical cooling in summer. Therefore, it is
essential to take into account the natural ventilation using
the malqaf for summer cooling in low-rise housing.
Indeed, this study proofed that buildings that includes the
malqaf, can significantly alter the direction of the
prevailing winds and keep spaces more comfortable. The
aerodynamic flows of the malqaf induced more inflow
rate through the examined building. Also measurements
and calculations revealed the effectiveness of two
specific settings. Setting 2b proofed that by placing a
malqaf on a 1:20 test model one can obtain up to 4 ACH
with a outlet wall to opening ratio of 0.6 (reference
velocity of 2m/s). Next, setting 3 proofed that one could
obtain up to 5.6 air changes per hour with an opening
ratio of 0.6 and the reference velocity of 2m/s on a 1:20
scale model. The best arrangement for getting maximum
air flow pattern into the space was to make the inlet and
outlet as large and high as possible. This can be achieved
when placing two malqafs one toward the wind and the
other toward the leeward. This paper has investigated the
potential of integrating the malqaf in residential low-rise
housing and proofed that the malqaf can be utilized to
serve the need of sustainability in contemporary Egyptian
practice.
However, this study was conducted on a scale of an
individual single-space. Therefore, it is important to
extend the endeavour to include physical and geometrical
parameters, such as convective cooling of thermal mass,
building layout and urban fabric. Many existing malqaf
buildings succeed only when combined with courtyards.
The urban context including the arrangement of buildings
with the site topography, site environmental and hygienic
qualities, landscape features and climatic characteristics
are basic issues that might natural ventilation in the
malqaf house. Future research should focus on the
aerodynamics of malqaf houses in urban settings. This
might lead to the reduction of electric cooling inside
buildings in addition to the reduction of urban heat island
effect outside the buildings.
FUTURE WORK
In the second phase of the research, the authors will
continue with visualising and measuring the performance
of different building settings including courtyards. Also
CFD simulations will be conducted to analyse and
explore the more complex combination of design
parameters, in order to predict the human comfort within
malqaf spaces for naturally ventilated and cooled spaces.
ACKNOWLEDGEMENTS. The author expresses his
thanks to Prof. Hervé Jeanmart at the Mechanics
Department and Michel at the Thermodynamics Lab for
their assistance in conducting the experimental work.
Also the author extends his gratitude for the valuable
advising of Prof. Dr. Andre De Herde, Elisabeth Gratia
and the research team of Architecture et Climat, at the
Université Catholique de Louvain La Neuve.
REFERENCES
1.Fathy, H., W. Shearer, and A. Sultan, (1986). Natural energy
and vernacular architecture: principles and examples with
reference to hot arid climates, Chicago: Published for the
United Nations University by the University of Chicago Press.
xxiii, 172 p.
2.HBRC, (2005). Egyptian code for energy efficiency
improvement in buildings, ECP306, M.O. Housing, Editor.,
HBRC Cairo.
3.Givoni, B., (1992). Comfort, climate analysis and building
design guidelines. Energy and Buildings,. 18: p. 11-23.
4.Al-Wakil, S. and M. Siraj, (1989) Climate & the architecture
of hot regions [in Arabic]. Third ed., Cairo: Aalam Al-Kutub.
5.Bahadori, M., (1985) An improved design of wind towers for
natural ventilation and passive cooling. Solar Energy, 35(2): p.
119-129.
6.ASHRAE, (2005) ASHRAE handbook. Fundamentals.,
American Society of Heating, Refrigerating, and Air-
Conditioning Engineers: Atlanta, Ga. p. v.
7.Allard, F. and S. Álvarez, (2002) Natural ventilation of
buildings. A design handbook , London: James & James.
8.Asfour, O. and M. Gadi, (2006) .Effect of integrating wind
catchers with curved roofs on natural ventilation performance
in buildings. Architectural Engineering and Design
Management,: p. 289–304
9.Yaghoubi, (1991). M., Air flow patterns around domed roof
buildings. Renewable Energy, 1(3/4): p. 345-350.
10.Hassan, M., et al., (2007). Investigation of effects of
window combinations on ventilation characteristics for thermal
comfort in buildings. Desalination, 209: p. 251-260.
11.Pankhurst, R., (1968). Wind-Tunnel Techniques., London.
12.Bienkiewicz, B. and J. Cermak, (1986) A flow visualization
technique for low-speed wind-tunnel studies. Experiments in
Fluids, p. 5.
13.Lawson, T., (2001). Building Aerodynamics.: Imperial
College Press.
14.Cermak, J. and N. Isyumov, (1998). Wind Tunnel Studies of
buildings and structures.: ASCE.
15.Rabbat, N., (1995). The Citadel of Cairo.: BRILL.
