AA LONDON - SED - TERM II | RESEARCH PAPER II | Strategies for passive cooling through ventilation

Strategies for passive cooling through ventilation for domestic housing Aly A. Mahmoud

AA SED

ARCHITECTURAL ASSOCIATION GRADUATE SCHOOL

PROGRAMME:

MSc SUSTAINABLE ENVIRONMENTAL DESIGN 2014-15

SUBMISSION

TITLE

RESEARCH PAPER 2

Strategies for passive cooling through ventilation for domestic housing

NUMBER OF WORDS (Excluding footnotes, references and dissertation outline): 2710

STUDENT NAME:

Aly Abdelbaky Mahmoud

DECLARATION: “I certify that the contents of this document are entirely my own work and that any quotation or paraphrase from the published or unpublished work of others is duly acknowledged.”

Signature:

Date: 27/04/2015

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Table of contents Summary……………………………………………………………………………………………………..3 Introduction………………………………………………………………………………………………….5 Weather data…………………………………………………………………………………………………6 Cultural habits & general look …………………………………………………………………………...7 Natural cooling through ventilation……………………………………………………………………...8 1. The stack effect 2. Cross ventilation 3. Wind catchers 4. Courtyard 5. Night ventilation 6. Parapet wall wind-catcher 7. Earth cooling 8. Evaporative cooling Conclusions………………………………………………………………………………………………….12 Bibliography………………………………………………………………………………………………….12 Dissertation outline…………………………………………………………………………………………14

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than 20% of total oil consumption and more than 40% of total dry natural gas consumption in Africa in 2013. Energy subsidies, which cost the government $26 billion in 2012, have contributed to rising energy demand and a high budget deficit.

As shown in figure 1.2, renewable resources are just 1% of the total energy consumption in Egypt which can justify why Egypt generally and Cairo particularly struggle from power shortage, as mentioned in the U.S. Energy Information Administration report (2014), Egypt experiences frequent electricity blackouts because of rising demand, natural gas supply shortages, aging infrastructure, and inadequate generation and transmission capacity. Ongoing political and social unrest in Egypt has slowed the government’s plans to expand power generation capacity by 30 GW by 2020.

Summary: “Today half of the industrialised world’s population lives in urban areas and accounts for half of the world’s total energy consumption. Only 10-20% of the population of the developing world lives in urban areas. It is predicted that within a few decades 60-70% of the total world population will live in urban areas.” (Koch-Nielsen, 2002. P. 12.)

With the high increase of urban areas and with the brutal increase in the use of nonrenewable energy, some countries can afford this use of energy and some cannot, but when it comes to developing countries where energy is limited and economy is at its worst cases, solutions must be proposed in order to cope with the existing problems and the futuristic expected problems.

As shown in figure 1.1, residential sector in Egypt consumes 37.80% of the total energy consumption, and as mentioned by Hipper and Fischer (2009), that as of 2006 slums are estimated to contain more than 65% of the population of the metropolis (10.5 out of 16.2 million inhabitants), this ratio can show how big is the disaster and figure 1.3 shows how much the case is uncontrollable by the government.

Figure 1.2 Primary energy consumption in Egypt, by fuel, 2013 (Source, U.S. Energy Information Administration report (2014)

As a result for these two problems (slums and energy), new environmental strategies should be applied by the government to provide natural resources for buildings, and these strategies should be applied on existing buildings in formal areas (as refurbishment) and new developments, but new developments shouldn’t follow the same strategies of existing urban planning due to its environmental negative impacts, and slums should not be refurbished, because of many disastrous issues like health and the lack of building regulations, but new innovative environmental ideas should be suggested.

Figure 1.1 Total energy consumption in Egypt 2007/2008.

(Source: Rapid Survey of the key sectors in Egypt, 2010)

As mentioned in the U.S. Energy Information Administration report (2014), Egypt is the largest oil producer in Africa outside of the Organization of the Petroleum Exporting Countries (OPEC), and the second-largest natural gas producer on the continent. At the same time, Egypt is the largest oil and natural gas consumer in Africa, accounting for more

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Figure 1.3 Slums development in Cairo (Source: Cairo’s Informal Areas, 2009)

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Introduction:

important to put into consideration the integration between the proportions of ventilation openings inside a building for the purpose of cooling and openings required for occupants’ health, it is also essential to know well where to put openings for the purpose of health and openings for cooling, and when they should be integrated to provide cooling when desired and when they shouldn’t integrate in case of winter when ventilation is only needed for health.

“In buildings that are well protected from solar radiation and that have high insulation of the envelope and high thermal mass, the indoor daytime temperature, in the absence of ventilation, could be well below the outdoor level.” (Givoni, 1994, P. 5).

From the above sentence, one can understand the primary steps to reach thermal comfort without working the opposite way without reaching to an expected result, or reaching to this result by changing the wrong parameters.

So, buildings should be built from outside to inside, the suitable urban form for a wellprotected buildings from solar radiation, then choosing the right materials for the building envelope, then choosing the right materials for the indoor spaces, that could prepare buildings to be at its minimum cooling requirements.

As noted by Baker (), Natural ventilation may be impractical in very noisy or polluted environments. However, it is observed that even in noisy urban environments, people will open windows, trading thermal comfort for traffic noise. And as Cairo is a very noisy city, people are comfortable with a limited level of noise, so this issue will not be classified as a problem when talking about solving slums environmental problems, and as natural ventilation is allowing noise into the building, so it will be suitable to use in Cairo.

Although cooling through ventilation should be the last solution, ventilation is essential for pollutant dispersion and cross ventilation inside the indoor spaces is important for the occupants’ health. So, an integrated solution should be achieved by providing ventilation as well as designing the right urban form, building envelope and thermal mass.

Although “natural ventilation cannot provide such consistent and uniform conditions as mechanical systems” (Baker, p. 5), but Givoni, (1994) argues that people who live in naturally ventilated buildings usually accept a wider range of temperatures (20oC in the morning 28oC in the afternoon ) and air speeds (12m/s) as normal”.

As mentioned in the EMA Air Pollution Report, 2014 that during autumn, Egyptian farmers burn the Rice straw, which causes a dense smoke travelling with the wind from Nile Delta to Cairo. This smoke with the other local pollutants exist in Cairo, because what we called “Black Cloud”. And like most of the hot dry desert climates, Cairo has dusty winds in addition to its increase of pollution and carbon levels, which make it difficult to rely completely on natural ventilation, and when using natural ventilation some improvements should be applied to improve air quality inside the building.

According to that, passive cooling through ventilation for housing in Cairo will be investigated and studied in this paper, in order to provide an integrated design that doesn’t only rely on natural ventilation but on the other parameters that can be studied in the dissertation research.

Givoni (1994) argues that in hot dry climate in order to lower the indoor temperature below the outdoor level during daytime hours, it should be reached by minimizing the heat gain from the outdoor air, and that should be achieved by making a compact building by reducing its surface area of its external envelope to minimize the heat flow into the building, and the ventilation rate should be kept to the minimum required for health (0.5 air change/hr in residential buildings) in order to minimize the heating of the interior by the hotter outdoor air. From here it’s very

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Weather data:

Relative humidity:

Temperature:

As shown in figure 2.3 relative humidity in Cairo is relatively low, with minimum of 40% to a maximum of 80%, with average of 50-60%.

Which classify Cairo as a hot dry climate.

As shown in figure 2.1, it can be observed that between June and August temperature in Cairo reaches to the highest values where it varies between 30oC and 40oC, while between March – May, September and October weather in Cairo is almost within comfort range as it varies between 20oC and 30oC, the rest of the four months are considered as the cold months, though these four months are having sunlight and some periods are relatively warm.

According to KĂśppen-Geiger; the average annual mean temperature in Cairo is 21.4oC, and it is classified as a subtropical desert/lowlatitude arid hot climate. But although there are five months within comfort range, yet people use air conditioners as they provide consistent and uniform conditions, and in the case of slums, people cannot afford the high temperature or air-conditioners.

Figure 2.1 Hourly diurnal averages for Cairo (Source: Meteonorm)

Wind: As shown in figure 2.2, the prevailing wind in Cairo is mainly from North and North East, while in winter it is from South West. Wind in Cairo is varying between 3-4 m/s, and sometimes rises to 5-6 m/s

Figure 2.2 Windrose for Cairo’s four seasons (L-R: Spring, Summer, Autumn & Winter) (Source: Meteonorm).

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Figure 2.3 Relative humidity in Cairo (Source: Meteonorm)

Cultural habits:

General look:

People in Cairo used to wear costumes that provide natural body cooling as shown in figure 2.3, and they still have some daily habits that can be considered as environmental, for example: people in summer tend to scatter water particles on the ground in front of their shops which causes evaporative cooling, and old homes used to have window shutters that let the occupant to control daylight and ventilation levels. That shows that people normally tend to use environmental adaptive solutions before the invention of air conditioning and mechanical cooling or instead of using them because of financial issues.

As the day is long in Cairo, the amount of solar radiation is high, and as sun angle in summer is 83, buildings envelopes and roofs are directly affected by direct solar exposure unless there is a kind of protection outside the buildings and heat loss inside the building.

From here, as Koch-Nielsen (2002) argues that buildings should be close to each other to create this kind of protection as each building will shade the other to reduce the heat gain, and will reduce the spaces between buildings that tend to store and reflect heat, unlike hot humid climate, which needs porous envelopes to allow more wind to get through the building to increase the air flow. And as wind velocity in Cairo is relatively low (4-5m/s), so increasing the air flow inside the buildings without making them porous and without increasing the spaces between buildings is very important and should be achieved by using techniques that can increase air flow inside the building and not to rely on the normal wind velocity.

Passive cooling through ventilation can be provided through different levels (ground, facades, roofs and indoors), so dealing with every parameter is different from the other, dealing with daytime is different from night time, orientation of the urban canyon and the building is affecting the indoor ventilation and air-flow.

Figure 2.3 Evaporative cooling/ventilation in traditional clothing in hot environments, equivalent to 0.4 Clo. (Source: Stay Cool)

By looking at precedents in hot climate, it can be observed that the techniques used for passive cooling through ventilation are horizontal; for example: (cross ventilation, ground cooling, rising the building to allow air flow to access the building from the ground level, as ground level is the warmest level at

Figure 2.4: Egyptian window shutters (Source: www.robinwyatt.org)

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daytime in the hot climate as noted by KochNielsen (2002), courtyards with the addition of vegetation and water features that provide evaporative cooling that integrates with ventilation), and at the same time these features will stabilise dust movements and reduce dust storms as mentioned by KochNielsen (2002), the second type is vertical ventilation; for example: (passive stack, wind towers, double skin), and there are composite techniques that integrate horizontal with vertical techniques, all of the ventilation techniques are dealing with the different air pressures between outdoors and indoors as mentioned by Koch-Nielsen (2002), accordingly facades openings should be put facing wind direction, and the ratios between the intake and outtake openings should be studied regarding the size and placing in order to provide maximum air flow inside the building without missing gaps inside the building.

As mentioned by Koch-Nielsen (2002) the compactness of a building, e.g. with doublebanked rooms, has advantage of reducing solar gains due to low surface area to volume ratio. Accordingly, a compromised solution must be achieved to provide a compact building with internal openings for exhaust air.

This stack effect can be achieved by several solutions, the first could be by adding a duct as shown in figure 3.3, or a light well, the light well at this case will provide daylight and ventilation.

Natural cooling and ventilation: 1. The stack effect:

Figure 3.3: Stack effect (Source: Natural ventilation strategies for refurbishment projects)

The stack effect is caused by the different air pressures, as shown in figure 3.1, and in order to do that, two parameters should be primary applied, the first is by having lower openings in the building which should be cooled as it is coming from a shaded area, and that can be achieved by urban form, orientation, and building positioning without obstructions, and the second parameter is to have another indoor opening/s at a higher level where the hot air can escape, that will circulate the air flow inside the building.

2. Cross ventilation: When there is no availability of having a stack or adding exhaust opening is difficult due to high urban density or because of a double banked rooms, another solution can be applied by adding more openings in the same wall that is facing the prevailing wind, these openings can be positioned horizontally as shown in figure 3.4, or vertically as shown in figure 3.5.

In this technique the same pressure difference must be achieved in order to have cross ventilation, and this can be achieved by adding vegetation or water feature next to an opening and to leave the other to make this temperature difference.

Figure 3.1 Temperature difference between inside and outside creates a pressure difference (Source: Natural ventilation strategies for refurbishment projects)

Figure 3.4 (left): Illustration from studies by B. Givoni on natural (horizontal) air flow in a building or room with two openings with different position and different types of wing walls. Figure 3.5 (right) Vertical air flow in a building or room with two openings (Source: Stay cool) Figure 3.2 Rules of thumb and design aspects for stackdriven ventilation. (Source: Stay cool)

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3. Wind catchers:

most traditional buildings as square or rectangle and its relative height was 1-1.2 H, where H is the height of the space, He also argues 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. While Koch-Nielsen (2002) argues that although the greatest pressure on the inward side of a building is gained when it is perpendicular to the direction of the wind. However, if openings are oriented at 45° to the prevailing wind direction, air velocities increase and the distribution of air improves within a building. From here it can be understood that the maximum angle for wind catcher orientation is ±10° and for the building itself the maximum angle is ±45°. When looking at the cross-section of the shaft itself, Koch-Nielsen (2002) argues that studies of the available literature on buildings with natural ventilation indicate inlet and outlet opening areas for wind towers to be 3-5% of the floor area they serve.

The wind catcher, also known as (Malqaf) in Egypt, as noted by Givoni (1992), 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. Which make it suitable for the case of slums in Cairo, and as mentioned by Attia and De Herde (2009), masses of buildings reduce the wind velocity at street level and screen each other from the wind, the ordinary window is inadequate for ventilation, that is why wind catchers positioned on the top of the building facing the wind direction to get non-obstructed wind with the highest velocity, obstructions on ground level as mentioned could be buildings, trees, urban furniture…etc. As wind catchers are a source for ventilation, it needs exhaust outlet, and this can be achieved by adding openings at high levels inside the rooms as shown in figure 3.6 or by adding stack as shown in figure 3.7. Koch-Nielsen (2002) argues that the air that enters the wind tower will, in the hot dry climate of the Middle East, have a low moisture content. If water -filled, unglazed earthen pots or water-soaked cloth or wet charcoal are placed in the air inlet, the incoming air is cooled as the water evaporates. So if external air is not cooled before getting inside the building, it’s better to exclude it during the midday hours.

4. Courtyards: Courtyards are very well known in the Middle Eastern countries, as it completely isolates people from outdoors, so they can feel free without thinking about privacy issues and at the same time it gives them the feeling of outdoors, as shown in figure 3.8.

Figure 3.6: A traditional Middle Eastern wind tower for wind scoop where wind passes across porous clay jars filled with water. Hot air is released through high openings.

(Source: Stay cool)

Figure 3.7: Wind catchers when integrated with wind stack. (Source: Stay cool)

Figure 3.8: The inner isolated world of the house: view into the courtyard of a traditional 17th/18th century house in Cairo. (Source: Urban form in the Arab world)

Attia and De Herde (PLEA 2009) argue that wind tower shaft’s proportions was observed in

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warmer courtyard. A courtyard pool has a beneficial effect during the day and night. (Source: Stay cool)

“If the floor of the courtyard is shaded for most of the day and open to the sky at night, it will lose more heat by outgoing radiation at night than it will gain during the day. It will also be the coolest part of a building and will absorb the heat from the adjoining rooms.” (KochNielsen, 2002, P. 62)

Figure 3.11 shows how the building is raised above ground level to allow air to come from outdoors for maximum fresh air intake, and to integrate with the courtyard and the stack for a better cross ventilation.

Courtyards create a partially isolated microclimate from the external environment, but in order to reach this type of isolation several parameters should be integrated with the courtyard, these parameters are: 1. Appropriate heights of surrounding walls to provide maximum shading to the courtyard.

2. Water features, for evaporative cooling 3. Vegetation, as plants will act as shading and cause evaporative cooling from soil at the same time.

4. Connectivity between the courtyard, indoor spaces and outdoors (through windows) for cross ventilation.

5. The availability for air to escape through: stack or/and the façade openings, as shown in figure 3.9

Figure 3.11: View from the first floor of Bait As-Suheimi across the courtyard, semi indoor spaces and vegetation in the large courtyard (Source: Urban form in the Arab world)

5. Night ventilation: “Night ventilation can reduce daytime temperatures by as much as 4oK. However, it only works where there is thermal mass available internally, and high rates of night time ventilation” (Baker, 2015, P. 16).

So, night ventilation occurs with the presence of thermal mass inside the building that loses temperature when it is ventilated at night when the external temperature is lower, cool air will decrease the thermal mass’s temperature.

Santamouris (2007) argued that night ventilation can affect internal conditions during the day by creating a time lag between external and internal temperatures. So, that thermal mass can have time to lose the heat that was gained at daytime, in order to have this effect, maximum ventilation should be allowed at night time and minimum ventilation should be allowed at night-time as shown in figure 3.12

Figure 3.9: Cross-section of ground floor and first floor Bait As-Suheimi in Old Cairo (from the 17th/18th Century), where almost all the indoor spaces are connected with the courtyard. The courtyard (in red), fountain (in green), and stack (in blue). (Source: Urban form in the Arab world)

In the previous example, the wind stack will work as a wind catcher at daytime hours as the space below it is completely covered and having water fountain that will help in reducing temperature, and cooled air will flow from the covered courtyard to the opened courtyard, while at night hours the courtyard will lose heat by outgoing radiation in the case of a clear sky and will be the source of cooling, and the wind catcher will act as a stack.

Figure 3.10: A two-courtyard building: air from the shaded courtyard flows over evaporative coolers to the larger and

Figure 3.12: Night ventilation (Source: Natural ventilation 2015)

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It’s obviously clear that night ventilation needs all day and night ventilation and that cannot be acceptable for all kinds of people, and it’s not adaptable with environmental changes, such as windy and dusty days that people will tend to close windows even if they feel warm, it also limits ventilation at daytime while people might need more ventilation at that time and less ventilation at night.

suitable and efficient ways of cooling through ventilation in high urban density, in hot dry climate with dust and pollution, as the source of cooling is below ground surface KochNielsen (2002) argues that a thermal storage reservoir constructed underground and made of materials with thermal storage capacity through which the incoming hot air passes and is cooled on its way to the interior of the building, as shown in figure 3.14, this process helps cleaning the fresh air intake from dust and polluted gases, the only concern is to put the intake shaft in the appropriate place without obstructions and not too far from the building.

6. Parapet wall wind-catcher: In vernacular architecture as people weren’t using insulation materials and the technology of the new materials with low U-values had not been invented yet, people tended to increase walls’ thickness in order to reach high thermal insulation that can keep the building cool in summer and warm in winter. Now with the absence of this kind of architecture and the invention of new materials, buildings in slums in the developing countries as well as buildings supported by the government are built with low quality materials and without insulation. The parapet wall wind catcher can be a substitute for the old thick adobe wall, Koch-Nielsen (2002) argued that the stored heat can be ventilated out of the building by convection and radiation to the ventilation system in the floors and walls. So, the parapet wall wind-catcher can reduce the temperature of the whole building as wind is flowing through the wall itself, but this system needs a very wellconstructed building to have such relations between the façade, the floor and the exhaust shaft as shown in figure 3.13.

Figure 3.14: Harare International School, Zimbabwe (Source: Stay cool)

While Santamouris (2007) argues that when ambient air is directly drawn through buried pipes, the air is cooled in summer and heated in winter. Earth-to-air heat exchangers (EAHXs)) are run horizontally at a ground depth of 3m to 5m. The energy performance of EAHXs is described by the interaction of heat conduction in the soil and the heat transmission from the pipe to the air.

As shown in figure 3.15, the different in temperatures in different earth levels. It shows that there is a remarkable difference between 4m and 8m, as temperature at 8m is more stable during the whole year, although the graph shows that the fluctuation through the year becomes more stable as it goes lower under ground level.

Figure 3.13: Parapet wall wind-catcher (Source: Stay cool)

7. Earth cooling: “In summer the soil temperature, at a depth of a few meters, is always below the average ambient temperature, and is especially below the daytime air temperature. It thus has the potential to serve as a heat sink with either passive or active mechanisms of heat transfer from the building.”(Givoni, 1994, P. 191), Earth cooling can be considered as one of the most

Figure 3.15: Ambient air, surface and ground temperatures.

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Night ventilation is not effective in dusty airpolluted environments as it is directly connected with the outside air.

8. Evaporative cooling:

Earth cooling can be considered as one of the most suitable and efficient ways of cooling through ventilation in high urban density, in hot dry climate with dust and pollution.

Santamouris (2007) argues that evaporative cooling is the process of evaporation results in the conversion of sensible heat to latent heat at a constant wet bulb temperatures; as a result, the air supplied is not only cooler but is also more humid. Evaporating just 1 litre of water would cool about 200 cubic meters of air – comparable to the volume of a modest apartment – by 10oC. As evaporative cooling makes the air more humid and cooler, so, it can be one of the most suitable cooling techniques in hot dry climate, and it should be integrated with other cooling techniques that was investigated in this paper.

As evaporative cooling makes the air more humid and cooler, so, it can be one of the most suitable cooling techniques in hot dry climate, but it should be integrated with the other cooling techniques.

Bibliography: ATTIA, S., DE HERDE, A., Designing the Malqaf for Summer Cooling in Low-Rise Housing, an Experimental Study, PLEA, 2009

Conclusions:

Afify, M., Rapid Survey of the key sectors in Egypt - To facilitate identification of the potential for CDM project activities, 2010

Buildings should be built from outside to inside, from urban form, to the building envelope, then indoor spaces, all of which could prepare buildings to be at its minimum cooling requirements.

Azam, A., Bakry, A., Hassan, H., and Mahmoud, Z., Report about Air pollution Over Egypt, 2014 Baker, N., Natural ventilation strategies for refurbishment projects

Ventilation is essential for pollutant dispersion and cross ventilation inside the indoor spaces, even if other cooling techniques are applied.

Baker, N., Natural ventilation, lecture, 201.

It’s difficult to rely completely on natural ventilation, when the level of pollution is high and there is dusty winds, but some improvements should be applied to improve air quality inside the building.

Bianca, Stefano. Urban Form In The Arab World. London: Thames & Hudson, 2000.

Fischer, F., Kipper, R., Cairo’s Informal Areas Between Urban Challenges and Hidden Potentials, 2009

Ventilation rate should be kept to the minimum required for health (0.5 air change/hr in residential buildings) in order to minimize the heating of the interior by the hotter outdoor air.

Givoni, Baruch. 'Comfort, Climate Analysis And Building Design Guidelines'. Energy and Buildings. (1992).

Givoni, Baruch. Passive And Low Energy Cooling Of Buildings. New York: Van Nostrand Reinhold, 1994.

Passive cooling through ventilation can be provided through different levels (ground, facades, roofs and indoors), so dealing with every parameter is different from the other, dealing with daytime is different from night time, orientation of the urban canyon and the building is affecting the indoor ventilation and air-flow.

Idrissova F., Jochem, E., Reitze F., Riffeser L., Toro F. A., Background Paper - Energy Issues in Egypt - Final Report – 2008.

Koch-Nielsen, Holger. Stay Cool. London, UK: James & James, 2002.

The maximum angle for wind catcher orientation is ±10° and for the building itself the maximum angle is ±45° and and outlet opening areas for wind towers to be 3-5% of the floor area they serve.

Santamouris, M. Advances In Passive Cooling.

London: Earthscan, 2007.

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U.S. Energy Information Administration.

Country Analysis Brief: Egypt, 2014 http://www.cairo.climatemps.com http://www.robinwyatt.org/photography/wpcontent/uploads/2011/04/egypt-fayoum-wadiel-rayan-window-shutters.jpg

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