Application of design and passive technologies for thermal comfort in buildings in hot and tropical climates
Abstract:
The indoor thermal environment of buildings is usually influenced by the outdoor environment (e.g. climate, landscape and urban pattern) and building design (e.g. building materials, form and planning). Controlling thermal interaction through the building envelope is important for maintaining indoor thermal comfort and therefore it requires proper selection of thermal control strategies and building materials. This chapter looks into the impact of climate on human thermal comfort and urban patterns and building form and fabric in tropical regions. It explores thermal comfort criteria derived from laboratory and field experiments. Published data on thermal comfort illustrate how people respond differently to similar environmental conditions due to factors such as adaptation. The chapter then presents a number of examples from traditional and modern architecture to illustrate the integration of passive cooling and heating technologies into buildings and how these are applied using conventional and more advanced modern building materials. Finally mathematical modelling of known passive heating systems such as Trombe-mass wall, water wall and solar chimney is explained with some performance data.
27.1 Thermal comfort in different climates
To be thermally comfortable, means to experience a state of mind which expresses satisfaction with the thermal environment. The following section introduces the importance of this concept when considering different climatic regions that exist now or, with climate change, in the future.
27.1.1 Introduction
Due to biological differences, thermal comfort may not be equally applicable to all building users. This is a particularly important fact to consider when a large proportion of research studies are conducted with respect to climatic regions in Western Europe and the USA, e.g. formal guidance documents from bodies such as the Chartered Institution for Building Services Engineers (UK), or The American Society for Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). In addition, with the changes and predictions brought about by climate change we can expect global temperatures to rise which has implications for future-proof building design strategies and low energy approaches to meeting thermal comfort requirements.
To design for thermal comfort indoors, it is usually recommended to satisfy at least 80% of the building occupants. Within thermal comfort, the core temperature of the body should be maintained within a small range (around 37 °C) regardless of fluctuations in the thermal environment. To maintain thermal balance, heat is usually produced within the body at all times. Heat is also exchanged with the enclosing environment through various modes. Indices have been developed to model and predict such thermal interaction and the thermal response of the human body. Attempts to develop thermal indices which model and predict the thermal interaction between the human body and its surrounding environment are continuously being made, by means of theoretical, as well as subjective investigations. A comfort index presents the combined effect of a number of parameters (personal and environmental) on the thermal sensation and response of the human body to the thermal environment. Thermal comfort is influenced by the following parameters:
In addition, there is also a sub-group of parameters which are related to and influenced by changes in the main parameters. An increase in metabolic rate may result in increase in the rate of sweat secretion through the skin as a measure to maintain the core temperature necessary for thermal comfort and survival. The secondary parameters are:
• clothing wettedness and permeation to moisture
Thermal comfort is achievable under different combinations of these parameters. Thermal indices vary in their assessment of the effect of each parameter on thermal comfort.
27.1.2 Global thermal comfort criteria
One of the well known earlier models for predicting human thermal response was that developed by Fanger in 1970. Fanger recommended an optimal temperature for thermal comfort (25.6 °C) which was the result of subjective experiments conducted on persons living in the Northern Temperate zone (Kansas City and Copenhagen). For some time, Fanger’s comfort theory was considered by many designers to be a universal source of criteria for human thermal comfort. Fanger developed his well-known comfort equation after extensive experiments on human subjects.
With respect to application of the comfort equation to tropical regions, a comparison is given by Fanger against experimental results with European and Asian residents in Singapore (Ellis, 1953). The comparison gives 27 °C as the optimum for the subjects when relative humidity was 80% and air velocity was 0.4 m/s. A second field study, referred to by Fanger and conducted by Webb in 1959, gives 28.5 °C as the optimum temperature for approximately similar conditions and location.
Allowing the indoor temperature to drift along the perimeter of the comfort zone can save energy in both heating and cooling seasons (McNall et al., 1978). Even by controlling the indoor temperature to reach the limits of the comfort zone, a saving of up to 10% is possible (Fleming, 1979; Zmeureanu and Doramajan, 1992).
A house temperature survey was conducted by the Building Research Establishment over the winter and summer seasons of 1991/92. A total of 515 occupants were interviewed in the survey. The reported thermal sensation during the winter survey produced neutral temperatures which were 5 °C lower than those predicted by the ISO 7730 algorithm which is based on Fanger’s Predicted Mean Vote (PMV) model. In summer the discrepancy was 3 °C (Oseland, 1994). At approximately the same time, a different investigation was carried out in five naturally ventilated office rooms at the University of Reading in the United Kingdom (Croome et al., 1992). Results showed differences of 0.1–2.4 K between measured neutral temperatures and predicted temperatures using Fanger’s PMV model. Fanger’s model overpredicted the neutral temperatures. This finding is in agreement with other works by Schiller in 1988 and Brager in 1992. The Reading experiment also illustrated that for the UK weather when the outdoor temperature is 10 °C, an energy saving of 8.5% could be achieved by lowering the indoor temperature by 1.0 K.
People’s tendency and ability to adapt to their environment were investigated by Humphreys in 1970, 1973 and 1975 among schoolchildren in the United Kingdom and office workers abroad. He noted a strong correlation (0.96) between the observed comfortable (neutral) temperature indoors and the mean indoor air or globe temperature. The correlation is based on 36 neutralities which showed a range of 13 °C. One of the reasons given by Humphreys for this range is the expected difference in clothing across the field studies which is estimated to be one unit clo. As the indoor thermal environment is often related to the outdoor climate, Humphreys later introduced the neutrality temperature as a function of the mean outdoor temperature (Humphreys, 1976).
A similar correlation coefficient (0.97) was obtained by Humphreys for the neutrality in the non-air-conditioned building which was better than the neutrality for the air-conditioned building, (0.56). The lower correlation is reasoned to be the artificial thermal environment inside the air-conditioned building being less influenced by the outdoor weather. Humphreys calls these relationships ‘adaptive model’.
Humphreys later supported his theory by further work (Humphreys, 1979, 1981). In 1992, during the Second World Renewable Energy Congress, Humphreys published a new study which highlighted the bias between the actual Mean Vote ‘MV’ obtained from field studies and the Predicted Mean Vote ‘PMV’ obtained from the ISO 7730 algorithm (based on Fanger’s model). The average (bias) was found to be 0.8 on the PMV scale. Considering the scale unit which is equivalent to 3 °C of indoor temperature, it follows that the average discrepancy is about 2–3 °C. When plotted against the mean room temperature the PMV was about 1.5 units lower than the actual PMV at 20 °C and about 1.5 units higher at 33 °C.
In his neutral temperature survey, Humphreys specifies that the data were reports of field studies published by Baillie et al. in 1987, Griffith in 1990, Schiller in 1990 and Dedear and Auliciems in 1985. The work of Dedear and Auliciems in 1985 was carried out in six field studies in three Australian cities with vastly different outdoor climatic conditions.
The Australian experiment presented some differences between observed neutral temperatures for non-air-conditioned buildings at latitudes 37° and 27°. The 10° difference in latitude, together with the geographical variations, resulted in 3.8 °C difference in observed neutral temperatures. The respective difference given by Fanger’s model is only 1.2 °C. Dedear and Auliciems’ model produced a 0.5 °C difference. The model which gave a temperature difference nearest to the observed is Humphreys’ model of 1975. Its differential result is 3.1 °C. Next nearest to this value is the 2.8 °C difference produced by Humphreys, model of 1976. The 1975 model is based on indoor mean temperature while the 1976 model is based on mean monthly outdoor temperature as input.
For latitude 37° (Melbourne City), the discrepancy between Fanger’s model and the observed neutrality for non-air-conditioned buildings is 3.2 °C. In contrast, the discrepancy between the observed neutrality for the same type of buildings and latitude and the neutrality resulting from Humphreys’ model of 1976 is only 0.7 °C. According to its latitude, Melbourne’s climate should be classified as temperate.
Not far from Dedear and Auliciems’ region of study, there was another subjective experiment conducted in India by Sharma and Sharafat in 1986. Their proposed chart for the tropical summer index (TSI) which was plotted on a psychometric chart, indicates that about 70% of the Indian subjects who participated in the experiment felt comfortable at an index value of 27.5 °C.
Within the same geographical zone, another published field study, conducted in Pakistan (Humphreys, 1994), has shown that it is not only Fanger’s comfort model which is not valid for world-wide applications, but also the design temperatures recommended by the well-known ASHRAE Standards. Humphreys’ research work in Mingora City recommended 30 °C as a good summer maximum temperature. This is, according to Humphreys, associated with slowly rotating ceiling fans (0.1 m/s) and moderate humidity. In winter, 15.5 °C is recommended as the comfort temperature which is associated with office workers dressed in typical traditional Pakistani winter clothing. The thermal comfort zone concluded by Humphreys’ work is clearly wider than the comfort limits recommended by ASHRAE Standard 55-81.
Differences in comfort temperatures have also developed over the years. Changes in comfort temperature in the last few decades occurred for a number of reasons. These include changes in clothing, lifestyle and diet and development in buildings’ construction and thermal control systems. A survey of domestic temperatures over the period 1946 to 1978 in the United Kingdom was published by Hunt and Steel in 1979. It represents clear evidence that indoor domestic temperatures have risen by a rate of 1 K per decade.
27.2 Climate impact on urban pattern and building form and fabric
27.2.1 Impact on urban pattern and building form
Prior to the stage of building design, a planning process on a larger scale is usually undertaken. This involves the plotting of streets in the urban areas required for different functions as in residential, industrial, commercial and open public spaces. Major decisions are taken in consultation among various authorities which will ultimately influence and even constrain the architect and the engineer in their attempts to perform thermal design of individual buildings within the pre-set urban fabric. Planning decisions are also governed by the available resources and the requirements such as density and type of traffic, road intersections, road-related services, drainage patterns and view. Unfortunately, renewable energy sources are often ignored or given token consideration. When environmental elements are considered, as they have been in the past, the planning process tends to make the thermal designer’s job much easier and successful. Streets are properly laid, public spaces are adequately planned and green areas are conveniently integrated within the urban grid.
Street planning
Streets are important because of the need for communication between different sectors of the town and the provision of services for inhabitants. For the thermal designer, they also control the spacing between buildings, their orientation and distribution of indoor spaces.
Spacing within buildings will determine availability of ventilation, daylight and solar energy. Proper spacing is usually ensured by consideration of the spacing angle. This is the vertical angle between the horizontal and a line extending from base of a wall up to the top edge of the opposite wall. Typical spacing angles which satisfy heating and daylight requirements in the tropics fall in the range 30–45° (Evans, 1980). Street spacing can be determined not only by heating and daylight needs. There is also the need for summer cooling which may well contradict such values recommended for the spacing angle. An expensive compromise solution is often to increase wall insulation and shading on the street side. Even with this solution, there still remains the problem of urban heat accumulating within the street and enhanced by traffic and solar absorption on the tarmac. The heat absorbed and stored in the street floor during summer days needs to be allowed to radiate to the sky sink by long-wave radiation. A significant parameter in this process is known as the view factor to the sky dome.
Nowadays, streets are becoming more packed with cars which are a disturbing source of pollutants. These pollutants need to be dispersed from pedestrianised areas and near residential and other sensitive public buildings. Dispersal of pollutants is achieved by adequate wind movement within the urban environment. For wind to blow normal to the street axis this can be a problem. Narrower streets and high buildings can create wind shading between buildings. They can also induce turbulence and street eddies which could blow fumes through windows and main entrance doorways. Studies on the effect of street cross-section ratio on pollutant dispersal concludes that the height/width ratio should not exceed 0.65 (Oke, 1988). In hot dry regions, compactness of urban layout has been a dominant feature of town planning in the past and continues to be promoted in modern times.
Building form
The courtyard is one of the well-known building forms that was adopted in different parts of the world, especially in hot dry regions. The courtyard form created or helped to create the compact urban layout in tropical regions known for their hot dry climate. Overheating of indoor spaces in summer can reach intolerable conditions and care is therefore needed to reduce the environmental impact on the building and its occupants. This problem is still facing building designers in the tropics today and often inspires them to reconsider the adoption of traditional strategies such as the courtyard concept in their design process.
In 1979 Mohsen conducted a study which was concerned with investigating the relationships between geometrical parameters of the courtyard form and its thermal performance in relation to solar radiation (Mohsen, 1979). A computer model was developed for the purpose which facilitated the optimisation of the courtyard’s interaction with the sun. A detailed simulation was carried out employing real climatic data for Egypt. Mohsen’s study shows that not every courtyard can be a desirable one. It also illustrates that careful consideration to the courtyard dimensions should be made to achieve a balanced year round performance, that is if the courtyard is used in its usual conventional form. Al-Azzawi (1992) pointed out that incorporating a courtyard in a building does not necessarily indicate that it is intended to alleviate summer conditions.
In contrast to the courtyard is the box-like building form, which was investigated by Olgyay in 1963. He tried to find the building form which gains the lowest amount of heat in summer and loses the highest amount of heat in winter. For the case of building in a hot dry climate, Olgyay found that a rectangle with plan ratio 1:1.3, was the optimum when the long side is oriented in the east–west direction. He also suggested creating an open space within the building.
Roof form
Studies on curved roofs in general highlight their preference over flat roofs which is based on noted lower indoor summer temperatures associated with the curved roofs (Konya, 1980). The indoor thermal environment is a function of the outdoor thermal conditions. A major source of heat in hot regions is the sun. In these regions, during summer the sun is at a high altitude which is nearly normal to the horizontal surface at noon. A flat roof will be completely exposed to the sun while a curved roof, depending on its curvature and orientation, is partially exposed and partially facing away from direct irradiance. The solar radiation is actually ‘diluted’ on a curved roof (Olgyay, 1973) and is spread over a larger area resulting in reduced heat transmission through the roof shell (Fathy, 1973). Due to their climatic benefits curved roofs have been investigated for possible integration in modern buildings (Gadi 2000a,b,c,d,e,f). Their influence on building ventilation performance has also been studied using CFD computer simulations (Asfour and Gadi, 2007a,b,c).
27.3 Climate impact on building fabric
27.3.1 Thermal insulation
An important component in the passive system is thermal insulation. Movable insulation reduces heat loss during winter nights and consequently increases glazing temperature for direct gain solutions. This reduces the mean radiant temperature asymmetry for the occupants and therefore reduces thermal discomfort. Movable insulation could be hinged panels, sliding shutters or rolling shades. It can also be installed externally or internally, with respect to room or even integrated within the glazing panes. Rolling insulation materials have the advantage of occupying a small volume during operation and storage which can be hidden within the building components (Moore, 1993). They may be made from quilt-like fabrics which can resist heat transfer and reflect thermal radiation. For this type of insulation to be effective, it has to be made to allow no or minimum infiltration to reduce heat loss in winter due to wind action and any pressure differences through the building. Airtightness may be provided by magnetic tape on the edges of the roller. Rolling insulation has also been produced in the form of layers of highly reflective flexible plastic films which are separated, but linked to each other by curved plastic strips. These strips flatten when the insulation is rolled up and expand when rolled down. The benefit of the plastic strips, besides strengthening the insulation, is to reduce convective currents between the main insulation layers.
27.3.2 Glazing
There are several glazing types which include clear, grey, heat absorbing, light reflecting, infra-red reflection and infra-red transmitting glazing. Materials used in the glazing could be glass, perspex, plastic or transparent fibres. Clear glass is the most common in building practice which is transparent to visible radiation. Heat absorbing glass is less transparent to visible radiation and thus more absorbent to infra-red radiation. Reflective glass is produced by depositing a light reflective film on transparent glass in order to reflect both the visible and invisible portions of the solar spectrum. Some plastics (polyethylene), are transparent to upper infra-red radiation. This type of cover is useful in passive radiative cooling systems as it permits thermal radiation emitted by roofs to the sky at night.
27.3.3 Thermal storage
There are various forms in which thermal storage components are usually integrated in a passively heated or cooled building. These include Trombe walls, water walls, solariums and roof ponds. However, some of these forms are often used in combination with each other or assisted by a different technique such as an earth-air tunnel and a wind tower to enhance their performance.
Barra (1979) conducted a theoretical study into the free convection within a Trombe-like system. The main difference between the two concepts is that the thermal storage in Barra’s system is located within the room. This strategy is intended to improve the system efficiency by reducing thermal losses during off-sunshine hours and providing better control of the indoor thermal environment.
A conventional Trombe wall system is often employed for both winter heating and summer ventilation. In certain regions, where ventilation is the main requirement for inducing human thermal comfort, a Trombe-like system known as a solar chimney is preferred (Awbi and Gan, 1992). That is because its channel height, which is an effective parameter in producing a higher ventilation rate, can be extended even above the building roof resulting in a larger air density gradient between its vents. Studies on solar chimney performance highlight the influence of channel width to height ratio and the chimney orientation on the mass flow rate and the total heat storage within the chimney structure (Bouchair et al., 1986, 1988). A solar chimney may also be built as part of a roof parapet where its ventilation performance can be enhanced by wind action (Khafaji and Murta, 1989).
Thermal performance of buildings incorporating roof heating and cooling systems such as water ponds has been investigated in many parts of the world. The use of water sealed in PVC bags of 20 cm depth on a roof covered by movable insulation, in California, USA, was found to be adequately capable of heating or cooling a building to acceptable thermal comfort levels (Niles, 1976).
Evaluation of the performance and efficiency of any solution, however, not only depends on its thermal and ventilation characteristics but also on user acceptance and satisfaction (Tsapos, 1992).
27.4 Approaches and lessons learned from traditional hot-climate architecture
27.4.1 Introduction
The traditional strategy of creating an adaptive living environment was applied at various levels, from town planning to construction of dwellings and planting of trees. The whole process was, no doubt, carried out at a local level, i.e. with local labour and materials. Most builders in the past were familiar with the climate in which they were building. They were also aware of the means to benefit from certain climatic elements and overcome others which were less favourable. From a number of tours through North Africa, the present author sadly noticed that many traditional settlements have become ruins and were replaced by ‘modern’ concrete towns. Such change came as a result of factors including a new lifestyle.
27.4.2 Traditional passive cooling and heating strategies
Although often ignored, the abundant examples of traditional architecture undoubtedly represent a dominant part of human civilisation. Traditional dwellings have, naturally, enabled man to survive the extremes of heat and cold, provide security for his family, store food and goods and above all preserve his cultural and social structure. In many examples of traditional architecture the environmental factors which influence building forms are clear and well defined. Culture and climate have combined to produce unique architectural forms and expressions which are, of necessity, direct and effective.
The city of Marrakech, Morocco
The city of Marrakech, located in Morocco at latitude 31.5° North and longitude 10° West, has three climatic zones. The first zone has a cool day with average temperature 20 °C and moderate humidity (55%), and a cold night with average temperature of 9 °C and 50% RH. The second zone has comfortable day-time conditions and cool night with average temperature 13 °C and 80% RH. The last zone is dominated by an overheated period with average temperature reaching 35 °C which is associated with 30% RH. This shows a need for both cooling and heating. These needs can be mostly met by natural means such as controlled solar gain, air movement and proper structure and planning (Baroum, 1983).
The traditional house in Marrakech is introverted. Rooms are arranged in two or three storeys around a central courtyard which is paved with marble and decorated with a fountain and vegetation (orange or lemon trees). The inward looking of the building leaves the outer skin free from major openings. This allows neighbouring buildings to be attached to each other, thus reducing the impact of solar radiation on the walls. It also generates a very compact urban layout which is characteristic of the city of Marrakech, where the streets are pedestrianised and could easily be shaded by local plants and organic materials. The high ceiling increases the overall height of the courtyard walls which consequently shades the courtyard floor against direct sunlight in summer for most of the day. The courtyard ground with its vegetation and fountain acts as a cooling and humidifying system. The day-time cooling potential is improved by the high thermal mass of the building structure.
The mountain dwellings of Morocco
On the rocky parts of this region, where land is as needed for shelter as for planting, dwellings are usually laid separated from each other by steep ravings. Each localised group of households shares a mosque which is their central meeting place where many administrative affairs for the village can be accomplished. This is often known as the mosque community. Another important element of this small settlement is the market, which is located away from the village on a separate site within nearest distance to neighbouring villages. Dwellings are built of 2–4 storeys and rooms are grouped around an open court which sometimes reduces to a central air shaft often incorporating a staircase. This sort of different solution is probably produced by the cold climate which is characteristic of mountain regions, where compactness of the indoor spaces is desired to minimise heat loss and maintain most of the heat generated within the dwelling by the relatively large number of occupants. Family guests are usually received in a special room on the upper floor with its own entrance and staircase. Hosting the guests at the top of the house, which is the coolest part in summer (the season of social ceremonies), is based on the well-known deep-rooted hospitality. Stones, either dry or mortared, are the main building materials. The back walls of the house are usually part of the rocky slope of the mountain. Walls are often built with the help of wooden forms which are filled with stamped clay, mixed with straw or gravel.
The Mzab valley, Algeria
Mzab is an ancient river valley located in rocky surroundings where five towns were built almost 1000 years ago. These towns are known as El Ateuf, Bou Noura, Beni Isguen, Melika and Gardaia which is the largest of the group. It was built 500 m above sea level at latitude 32° 30′ North and longitude 3° 45′ East. Average winter daily air temperature in the area is in the range 10–12 °C with lowest temperature of 1 °C. In summer the average daily temperature range between 33 and 40 °C with a high of 50 °C.
Traditional building materials are sand, adobe, stone, lime, gypsum and palm product. Walls are thick and constructed from stones while roofs are built from palm materials, lime and adobe. Internal wall surfaces are gypsum plastered. Houses are inward looking with relatively large indoor living areas. Due to the intense heat and strong sunlight, the courtyard is not fully open. Part of the courtyard roof is used as a terrace around which one or two rooms are built for night time activities. Some of the courtyards are oriented south, apparently as a response to summer and winter solar positions. The internal surfaces have a light mixed colour which diffuses daylight with minimum glare. Based on the author’s personal experience in this region, it can be confidently stated that the Mzab architecture is a very strong interaction between man and his environment.
The town of El-Oued, Algeria
The town of El-Oued is located in the Souf region which is made up of 19 oases in the southern part of Algeria about 600 km from the capital Algiers. Each house incorporates about 10 to 20 domes which are arranged around a courtyard. Larger public buildings, such as mosques, may employ as many as 80 domes supported on rows of columns. In nearby villages, houses also incorporate vaulted roofs, which present the village as one harmonious community (Fardeheb, 1988). Walking through the streets of El-Oued even on a December afternoon gives the sense of warmth, perhaps due to the sheltering effects from wind provided by the small width of the streets (1.5 m) and the relatively high walls (8 m). This is in addition to the partial cover of the street roof, sometimes by vaults. In summer one could easily imagine the scenario. A properly shaded pedestrianised area, even at noon when the sun is at about 80° from the ground.
Underground dwellings in Libya
Underground dwellings in Libya are mostly found along the hillsides of the city of Garian, 30.5° N and 9.5° E. The average air temperature in July is 30 °C and the average January temperature is 8 °C. Due to high altitude (1000 m), this region experiences high wind speeds in winter, 10 m/s. Once a plot of approximately 15 m2 is dug, then rooms are dug on its sides, each with a small entrance door and an arched roof, basically for structural reasons. All interior surfaces are plastered white to improve daylight levels indoors.
The present author visited the city of Garian, which is about 80 km from Tripoli on two occasions during winter and summer. Having stayed in a nearby modern house, the author was able to take two-interval measurements of air temperature (at 6 and 15 hours) inside and outside the modern as well as one of the underground houses. The modern house was a small ground floor dwelling with a relatively high thermal mass (2000 kg/m2 of floor area). The walls were 35 cm thick and built from sand–limestones while the roof was flat and built from 25 cm concrete slab.
During summer, the outdoor air temperature varied from 25 °C in the morning to 36 °C in the afternoon. These readings coincided with constant indoor temperatures of 25 °C in the old house. In the modern house the indoor temperature varied from 26 °C in the morning to 29 °C in the afternoon. The high thermal mass of the modern house resulted in the small temperature range indoors. The constant indoor temperature in the old house is obviously due to the ground’s high heat capacity. Both houses were comfortable in summer.
In winter, the ambient air temperature in the morning was 1 °C while in the afternoon it was 10 °C. Again the old house experienced a constant temperature of 20 °C. The modern house average temperature was 11 °C.
The city of Ghadames, Libya
This is a unique example of traditional architecture. Although it is built above ground, the town seems from within as if it is built underground. Even streets are covered except for gaps, often small holes, to allow for daylight. Having visited the town twice in 1982 and 1988, the author found it difficult to travel through its streets without a guide from the local people. As a result of its architectural, artistic and social values, it was consequently inscribed by UNESCO, in 1976, in the World Heritage List of Historical Monuments. Ghadames displays traditional craftsmanship, housing organisation reflecting social structure, and urban design appropriate to harsh climatic conditions. All these features are blended beautifully into harmonious traditional architecture. Ghadames is believed to be over 600 years old.
The town of Ghadames is located within a desert landscape at latitude 30° N and longitude 9.6° E, on a joint border location between Libya, Algeria and Tunisia. Mean maximum and minimum ambient temperatures in summer are 40 and 22 °C, respectively. In winter the respective temperatures are 18 and 4 °C. Sunshine is experienced for 12 hours in summer and for 6–8 hours in winter. Relative humidity is generally low varying from 20% in summer to 48% in winter. Measurements of air temperature taken during the author’s summer visit to Ghadames show a clear and impressive picture of the thermal performance of the traditional Ghadamesian house compared with new houses built across the road. On a summer day, while the ambient temperature was 44 °C, the air temperatures inside the old and the new houses were respectively 26 and 38 °C.
There are approximately just over 1000 houses in Ghadames, built interwoven and overlapping with each other and within an area of 30–50 m2. Houses are built on three floors where rooms are arranged around a courtyard. The courtyard is of double height and is closed except for a hole in the middle. All kitchens are built on the roofs for two possible reasons. One reason is thought to be social, as the roof was usually preserved for women while men occupy the ground level. Another reason is possibly related to the heavy use of the kitchen during the main meal which is lunch. Lunchtime coincides with the period of high outdoor temperature and intense solar radiation. Locating the kitchen on the roof is one way of reducing heat input within the house, which is a useful strategy during summer. In winter, however, it also means losing a potential source of heating. Considering the large traditional family size, the amount of heat lost could, at least partially, be compensated by people’s heat output. The roofed streets, on the other hand, contribute to maintaining the street air temperature slightly higher than outdoor ambient temperature in winter and much lower in summer. This is due to wind sheltering in both seasons and also as a result of solar shading in summer. Solar exposure in winter could have increased street temperature, if it was not roofed but such benefit could well be counterbalanced by convective heat loss from walls due to funnelled strong winds in the narrow streets. Walls are constructed from stones joined by mud and plastered by lime or gypsum. In inner parts of the town, walls are constructed from rammed earth. Roofs and floors are built from palm trunks, which are cut into four pieces and shaped to have triangular cross sections. The palm trunks are covered with palm leaves and mud-lime-gypsum mortar, which is strengthened and waterproofed by animal dung.
Traditional architecture in Egypt
Architecture around the river Nile offers many interesting examples of traditional architecture in Egypt. Some of these examples date back to Pharonic Egypt, which were mostly of spiritual nature. They include the temple of Ramses II and the Great Pyramids. They were built from limestone and granite. On a smaller scale, there are also houses which were constructed from mud bricks.
On the Nile one can also see some traces of the Nubian villages, near Aswan, a city famous with its great dam and man-made lake which was created during the 1960s. Nubian houses are compactly built with a mixture of flat, domed and vaulted roofs. The main building material is clay which is dug out of neighbouring hills. This is usually mixed with straw and animal dung to add structural strength and waterproofing. Each house is built with an open courtyard on one side and a number of rooms arranged on the other sides. There are two distinct rooms in the house, one for guests which is covered with a vault and the other is used for sleeping and sitting during summer. The latter is flat roofed with palm leaves. The Nubian villages are planned with a main mosque somewhere in the middle as a focal point. The streets are narrow and winding and naturally lead to the central courtyard of the mosque. The mosque is built from massive walls and thick vaulted roofs, which is a well-known strategy for delaying overheating inside until late at night and at the same time work as an adequate sink for heat released from people attending Friday prayers. Windows are usually high and small. Their location just under the roof naturally induces stack ventilation through the building.
The most outstanding of all traditional examples of architecture in Egypt can be found in the old part of Cairo. The streets of Cairo are narrow and partially shaded from direct sunlight and sheltered against wind. Houses are of the courtyard type. Some courtyards are fully open while others are covered with a clear storey in the middle. Houses also incorporate wind catches which are open to the courtyard in a uniquely combined natural ventilation system. Measurements of air speed and direction, taken in 1973 by researchers from the Institute of Third World Studies in London (Farouk et al., 1973) illustrate the effects of two forces (pressure and suction) acting in the same direction. Outdoor airflow is scooped downward through the wind catch then sucked upward through the clear storey above the middle hall. On its journey through the house, the air is cooled by a water source either along the wind catch or on the house floor. Even under calm wind conditions, the large difference in height along the clear storey creates a temperature gradient which naturally leads to an upward current of air through the space.
The presence of thick walls, roofs and the ground provides a large sink for storing the coolness that is produced by the whole system. Traditional roof construction consists of palm trunks covered with 10 cm of lime mortar and 10 cm of mud, then finally finished with light-coloured tiles. Ground floor walls are much thicker and heavier than upper floors. They are built from 40–120 cm limestone. Upper floor walls are built from 15 cm bricks. Another feature of old traditional houses in Cairo is the design of windows in a way that reduces glare, overheating in summer and at the same time preserves privacy which was and still is considered in some quarters of the city as an essential requirement. Each window is covered on the outside by a wooden lattice with small often circular holes. Some local people claimed that the wood actually absorbs some of the humidity in the air passing through the lattice.
On the urban scale, the old city of Cairo seems to have been planned in response to local climatic constraints. Major streets run north–south from which a number of other narrower streets branch along in an east–west direction. East–west streets are partially shaded by overhanging upper floors at each side. Street intersections are the most exposed to direct sunlight.
Noor (1984) analysed the results of a full-scale thermal study which included traditional as well as modern housing in Cairo. During the period from July to September, roof temperatures at 14:00 hours were 5–6 °C lower in the traditional house. At night, however, a reverse situation was noticed, though with a smaller difference. During the same period, a temperature difference of 11 °C was recorded between the roof and the indoor space of the traditional house.
Planning and design of the old Cairo also show an excellent degree of integration between function and fabric. Vertical piers in the walls form convenient spaces for sitting and built-in cupboards. The relatively large height of the walls within the main hall is broken into small horizontal strips by wooden frieze running around and confining some decorative work (Noor, 1986).
Traditional floor heating in Korea, China and Afghanistan
The traditional floor heating system is known in Afghanistan as Taba kana and in Korea as Ondol. Figure 27.1 presents pictures of a full-scale model of the system, which were taken by the author while attending the 5th International ENERGEX Conference in Seoul, Korea, 1993. The system is based on utilising the heat produced from burning fuel for cooking, such as wood, to heat the living space of the house. An oven is dug in the kitchen floor with three holes. One hole, which is the largest, is used to fire the oven and support the cooking tools. Another hole functions as an inlet for supplying air to the oven. A third hole is connected to a set of ducts running under the floor of the living room, which ends in an exhaust at the other end of the house. Air in the oven which is heated by the burning wood, seeps through the underfloor duct where it is then exhausted to the outside. In the process, the exhausted hot air is replaced by fresher air from the kitchen, which is attached to the other rooms in the house. In this way, the living space is heated by radiation from the floor.
27.5 Applications of design and passive technologies in modern buildings
27.5.1 Introduction
Learning from the past for building the present with the future in mind is a strategy which applies to almost every aspect of life, and architecture is no exception. A review of modern architectural attempts to learn from ‘vernacular’ building techniques and adapt them in today’s urban development has shown an excellent collection of ideas which have been applied worldwide. World collection of modern passive buildings also includes highly inventive technical solutions.
27.5.2 Jeddah International Airport, Jeddah, Saudi Arabia
This is a huge project which was designed and built to accommodate a large number of pilgrims from the Muslim world every year. The airport occupies an area of 105 km2 (Kurdi, 1983). The most dominant part of the airport is the pilgrims’ terminal which consists of two identical complexes, each having an area of 255 000 m2. The terminal is covered with a suspended tent structure made up of 210 tents which are made of fibreglass material coated with Teflon. Each tent measures 45 × 45 m and extends up to 10 storeys high from the ground. The tent material is translucent to light and has a white outer surface which helps reflects direct sunlight. The life expectancy of the material is 40 years. A vent is provided at the centre of each tent, thus incorporating 210 roof vents into the whole pilgrims’ terminal. Jeddah’s climate has a hot and humid summer during which air movement and shade are essential factors for inducing thermal comfort. Due to the heavy use of the building, however, a mechanical cooling system is also installed.
27.5.3 Centre for Energy Studies, New Delhi, India
This is an office building built within the Indian Institute of Technology complex in New Delhi (Muthu et al., 1991). The climate of the site of this project requires both summer cooling (maximum ambient temperature 40.5 °C, minimum winter ambient temperature 7.3 °C) and winter heating. A dominant feature in this project is the use of domed and vaulted roofs and local simple materials such as mud. The lower parts of the building were built from burnt clay bricks as a precaution against dampness. The upper parts, domed structures, were built from soil blocks, stabilised with 6% cement, moulded by mechanical manually operated moulds while the vaulted structures were built from soil blocks which were stabilised with 4% cement moulded by hand employing wooden moulds. The project also incorporates an earth-air tunnel system about 100 m in length, 0.15 m in diameter and buried at a depth of 3.2 m. Mechanical fans are used to assist in drawing air through the tunnel where it is exposed to a ground mean annual temperature of 25 °C. Ventilation through the building is also enhanced by provision of roof vents under the skylights at the top of each dome. Improving the building’s thermal performance is achieved by a number of other measures. These include reducing solar heat gain in summer by applying small surface area to volume ratio and the use of animal dung on the external surfaces.
27.5.4 University guest house, Jodhpur, India
The city of Jodhpur is located at the edge of the great Indian desert. It has hot dry summers and mild winters. Summer can be severe with hot dusty wind. This project was architecturally designed by Vinod Gupta and its energy performance simulation was carried out by Narenda Bansal (Bansal et al., 1994). The project is built on two floors using heavy materials. Most windows are facing north and south with appropriate shading to allow for summer shading and winter direct gain heating. The building also incorporates evaporative cooling towers with wet pads. Roofs are constructed from heavy materials with insulation to reduce solar heat gain in summer.
27.5.5 Patoka Interpretative Nature Centre, Indiana, USA
This is mainly a direct gain building designed by Fuller Moore (Moore, 1993). The building incorporates a south facing double glazing inclined at 60°. The northern side incorporates a reflective ceiling which reflects direct sunlight downwards to indoor thermal mass. The thermal storage is provided by a solid tiled floor and fibreglass water storage tubes. The double glazing on the south face has an airspace 10 cm wide, which in winter nights and summer hot days, is filled with polystyrene beads. The beads are blown in the glazing cavity by vacuum motors linked to tanks on the north slope.
27.5.6 Baer residence, New Mexico, USA
One feature of this residence, designed by Steve Baer, is the use of water as a heat storage medium which is kept in recycled 55 gallon oil drums. It is claimed that water temperature in the drums remained below boiling point and gave about 60–70% efficiency in winter (Anderson and Riordan 1976). The water-filled drums are painted black on the outside and white on the inside to improve daylight and give a pleasant visual effect. They are stacked behind double glazing facing south which is insulated during the night by movable bottom-hinged insulation panels. The panels, each weighing about 75 kg, are operated into vertical or horizontal positions by hand crank and nylon rope. The panels have a reflective aluminium surface for reflecting direct sunlight onto the water wall in winter, thus adding about 50% of extra energy to the water wall (Moore, 1993). Heat absorbed by the drum material from solar radiation is transferred into the water by conduction and natural convection and to the indoor space by convection and radiation.
27.6 Thermal performance of passive solar systems
An important aspect of designing a passive thermal control system is the formulation and simulation of its thermal performance. Defining the variables involved in the heat and airflow transfer through the system would facilitate optimisation of such variables and the efficiency of the system as a whole. Performance of a typical passive system should ideally be assessed as a function of time due to the transient nature of the outdoor thermal conditions. This, however, is a lengthy procedure which involves a considerable number of complex mathematical formulae and it is therefore better performed through specifically written computer programs. There is a wide range of passive thermal control strategies employed worldwide. A simple example of these systems is the mass wall that is usually used as a passive heating strategy which incorporates an opaque wall that is painted black on the outside and covered with glazing (Fig. 27.2). As the dark wall surface is exposed to solar radiation, it is consequently heated. Heat absorbed is then conducted through the wall and transferred to the indoor space by convection and radiation. At steady state, heat delivered into the room during sunshine hours, (Qin, W/ m2) is expressed as (Sodha et al., 1986):
UT = total heat transfer coefficient, W/m2 · K
Tsol = sol-air temperature, °C
S = solar radiation normal to surface, W/m2
α = absorbtivity of the wall surface to solar radiation, dimensionless, 0.9
ho = overall heat transfer coefficient from irradiated wall surface to the ambient (5.8–20.0 W/m2 · K) depending on wind speed and surroundings
hi = heat transfer coefficient from inner wall surface, 0.5 W/m2 · K
Heat loss in the absence of solar gain (Qout, W/m2) is expressed as:
Efficiency of the system (η) with respect to heat delivered to indoor space at room temperature (TR) is given by:
If the opaque wall is replaced by a water wall, the same relationships can still be applied except for the total heat transfer coefficient (UT) which should include two new convective heat transfer coefficients for the water (hw1, hw2) to replace the thermal resistance for the opaque wall (L/k). The total heat transfer coefficient for the water system becomes (Uwa):
hw1 = heat transfer coefficient from collector to water
hw2 = heat transfer coefficient from water to inner wall surface.
In case the system glazing is extended to produce a solarium, the following relationship can be used (Sodha et al., 1986):
UTS = total heat transfer coefficient for the solarium, W/m2 · k
hf = heat transfer coefficient from the outer wall surface to the solarium, 2.8 W/m2 · K
τ = glazing transmittance to solar radiation, dimensionless, 0.8.
Equation 27.8 can, however, be simplified as hm = ho + hf.
If the mass wall has upper and lower vents (Trombe wall), then air heated by solar energy collected at the wall surface would rise and result in a current along the Trombe wall duct and through the room (Fig. 27.3).
A procedure for predicting the rate of heat transfer to the room was proposed by Bansal and Thomas (1991) which starts by introducing the following formula:
According to Bansal and Thomas, steady state heat flow balance implies that the rate of heat absorbed by the wall is equal to the rate of heat convected to the room through the air duct plus the rate of heat conducted through the wall. It is also equal to the rate of heat convected through the air duct plus the rate of heat convected from the inner wall surface:
From Eqs 27.10–27.12 and assuming TB ≈ TR, Bansal and Thomas derived Eq. (27.13).
Vvel = air velocity in the Trombe wall duct, m/s
m = mass flow rate in the Trombe wall duct, kg/s
TA = air temperature at upper vent of the Trombe wall, °C
TB = air temperature at bottom vent of the Trombe wall, °C
Cp = specific heat capacity of air, 1.0 kJ/kg
FR = friction factor due to resistance of air flow caused by geometry of the duct and its vents, dimensionless (≈ 0.5)
h = height of air duct (vertical distance between centres of vents), m
g = acceleration due to gravity, 9.8 m/s2
Tsin = temperature of the inner wall surface (in the room), °C
Tsout = temperature of outer wall surface (collecting surface), °C.
The Trombe wall discussed here is sometimes modified to be used as a summer ventilation system, in which case it functions as a solar chimney (Fig. 27.4). A main difference from its winter use is that the heated air in its duct is not circulated into the room but instead exhausted to the outside. The systems discussed so far can also be incorporated in a pitched roof. if β is the roof angle with the horizontal, then the volume flow rate, Qout, m3/s, through the outlet of the heated air duct can be expressed through the following formulae (Bansal et al., 1993):
Aout = area of air duct outlet, m2
Ain = area of air duct inlet, m2
CD = discharge coefficient, dimensionless, 0.6
hf = heat transfer coefficient, given by Bansal et al. as 15 W/m2 · C
Ub = heat loss coefficient at bottom of air duct, 0.5 W/m2 · C
Ut = heat loss coefficient at top of air duct, 5.0 W/m2 · C
Bansal et al. (1993) used the above formulae to simulate the performance of a solar chimney (0.15 × 1.5 × 1.5 m, β = 30°) integrated in the roof of a room (4 × 4 × 4 m) under ambient temperature (24–36 °C) and solar radiation (0–1100 W/m2). The effect of variation in the ambient temperature on the calculated volume flow rate was only visible at low solar radiation. Assuming 0.6 discharge coefficient, it varied from about 150 to 170 m3/h under 100 W/m2. As the solar radiation increased to 1000 W/m2, the variation in the volume flow rate reduced to about 10 m3/h with the highest flow rate being 350 m3/h. The predicted air change rate varied from 3 to 6 Ac/h. Ventilation requirements of an occupied space, particularly in hot regions, cannot always be met by thermally induced airflow alone. Wind is another driving force for natural ventilation through building vents. The volume flow rate due to wind action (Qwind, m3/s) is expressed as a function of difference in wind pressure coefficient across the vent:
where Vrefvel = velocity measured at a reference point outside the building, m/s.
27.7 Conclusions
Evidence presented throughout this chapter has highlighted a number of facts regarding human thermal comfort in different regions. Seasonal variations in comfort temperatures do exist and are even permitted by the well-known ASHRAE standard. Significant energy saving in terms of reduction in the overheating period was found to be possible by allowing for occupants’ movement within a space in search for comfort.
An interesting conclusion that can be drawn from this chapter is that people in tropical regions and at different times developed various architectural ideas which stemmed from their survival as well as comfort requirements. This actually generates a question. As another generation with obvious differences in both thinking and living standards, should we also develop or add our own ideas? If so, then how much should our ideas be related to the past? If our ideas could meet our survival and comfort needs, then could they be completely different? Before attempting to answer these questions, it would be wise to study some characteristic examples of modern architectural solutions.
From the discussion presented, it could also be concluded that the courtyard, especially with the square plan, is ideally more suited to regions with mainly cooling requirements (hot dry). In regions where both cooling and heating are required, each for a significant period, then application of the courtyard becomes questionable. This is perhaps why some residents of such regions decided to block their existing open courtyards or replace them with completely covered living spaces.
This chapter also presented a useful review and study of modern research and application of passive thermal solutions such as those applied at town planning level and individual building designs and construction. Consideration of environmental elements in town planning tends to make the thermal designer’s job much easier and successful. Proper street planning means allowing for adequate daylight into surrounding buildings. It also requires careful calculation of solar position, especially during summer and winter.
27.8 Sources of further information and advice
Goulding, J., Lewis, J.O., Steemers, T.C.Energy and Architecture. The European Passive Solar Handbook. London: Batsford, 1994.
Hastings, R.Solar Air Systems. Built Examples. London: James & James, 1999.
Hyde, R.Climate Responsive Design. A study of buildings in moderate and hot humid climates. London & New York: E&FN SPON, 2006.
Szokolay, S.Introduction to Architectural Science: The Basis of Sustainable Design. Amsterdam: Elsevier, 2004.
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