Passive solar architecture can be described as the utilisation of the energy of the sun ... passive solar techniques include minimisation of solar heat gain while ...
PASSIVE SOLAR TECHNIQUES FOR SRI LANKA MTR Jayasinghe, L.C. Sujeewa, K.K.J.S. Fernando, R.A. Wijayapriya Department of Civil Engineering. ABSTRACT Passive solar architecture can be described as the utilisation of the energy of the sun together with the characteristics of the local climate to directly maintain thermally comfortable conditions in buildings while minimising energy consumption. Since Sri Lanka is a tropical country, passive solar techniques include minimisation of solar heat gain while maximising the ventilation and structural cooling. It is shown that passive solar techniques should be included at a very early stage of the design of a passive solar building. The effects of orientation, shading devices, window sizes, roofing materials, insulation, colour of the exterior and interior walls, use of courtyards to maximise natural ventilation, and arrangement of roof to maximise structural cooling is shown with suitable examples. 1. INTRODUCTION Passive solar design means designing a building which conforms to the nature of the site and to the diversities of climate instead of the building controlling the environment by means of mechanical and electrical means. Indigenous architt~cture is a direct result of adoption to resource and environmental constraints and great archit,ecture has always preached and practised these principles. Passive solar design can save energy and prove economical since it uses a minimum amount of energy to provide thermal comfort. The science of passive solar design revolves around the s'tudy of heat flow within a build!ing. In any building, heat flow occurs simultaneously and continuously due to conduction, raditation and convection. The patterns of heat gain or losses due to these are generally predictable. For instance, it is predictable that at night there will be no heat gain, and if is cold outside, much heat is lost. The extent of heat loss depends on the wind, the humidity and the temperature difference between inside and outside. It also depends upon how much heat was gained previously and how much of this remain stored in the building's thermal mass; the capacity of the building to store heat. The thermal mass of the building accounts for the time la1g between heat gain and heat loss and depends upon the interposition of its different building materials which have different thermal properties. For warm humid climates as experienced in Sri Lanka, passive solar techniques should have a different goal where minimising the solar gain and maximisation of the ventilation and structural cooling are important. 2. A BASIS OF PASSIVE SOLAR DESIGN FOR SRI LANKA Throughout the world, people have used passive solrur techniques that have evolved through generations. Traditionally, indigenous people were admirable architects. Their architecture is adoptive, and it evolved by way of a combination of cu)lture and natural selection in response to peculiar environmental conditions. They designed intuitively, and as a result their houses conformed to the site and to diversities of climates (Eber'hlµ·d & O'Ponovan 1990). As described by Silva & Vas (1980), the ancient houses in Sri Lanka were based on organic architecture where the walls had been built with mud which has very good insulating properties.
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The roof structure was made of jungle poles covered with straw, illuk etc. The wide eaves projected beyond the face of the building which helped to keep the sky and glare out thus minimising passive solar gain. The openings were small and maintained to satisfy minimum requirements. They were used only for the passage of people, possessions, air and natural light. The interior was gloomy by western standards, but restful and pleasant and was in contrast to the excessive light available outdoors. Additional openings were placed at roof level to permit the accumulated hot air at the upper levels to escape, reducing the creation of hot pockets. It can be stated that the buildings constructed presently are quite different to these traditional houses. It was reported by Silva & Vas (1980) that in Sri Lanka, the progress has been achieved with western education and neo-colonial attitudes, where the building norms and lifestyles are often adopted from western countries. In this process, an architecture totally unsuitable for Sri Lanka has evolved which has spoilt the urban environment. A similar sentiment has been expressed by Barozzi et al (1992) with regard to the developing countries with hot climates. It is said that in many instances, the indigenous architecture has been superseded by imported modem building design. Compounded by the cost of foreign made materials and components, the increased fuel consumption required to keep these buildings cool has invariably contributed to the financial ruin. An interesting observation reported by Ahmad et a!. (1985) is as follows. When the ambient average temperature was 31°C, a new house had an average indoor temperature of 35°C, while a traditional house built more than hundred years ago in the same city has recorded only 2SoC. On the basis of the above discussion, it could be stated that the passive solar building design for Sri Lanka should consider the effects of orientation, proportion, colour, ventilation, openings, shading devices and lighting with the aim of minimising solar heat gain and maximising the natural ventilation and structural cooling. When using the recommendations, it also necessary to take account of micro-climate factors such as ground topography, height of the building, effect of surrounding buildings since these factors can affect the solar heat gain and wind velocity at a given site. In the design of buildings based on passive solar techniques, it would be necessary to bring in these recommendations at a very early stage of design. As described by Mathens et al. (1992), the foundation for a good thermal design is laid during the sketch design phase. 3. THERMALLY COMFORT CONDITIONS FOR SRI LANKANS Thermal comfort, which is the sensation of complete physical and mental well being, is a subjective quantity which results from internal env ironmental variables such as dry bulb temperature, mean radiactive temperature, humidity and air velocity. It also depends on personal variables such as activity and clothing levels of the occupants. The thermal comfort could be achieved for a number of combinations of the above mentioned environmental and personal parameters. These combinations of parameters form the basis of a comfort zone on the standard psychrometric
chart.
It is shown by Jayasinghe & Attalage (1997) that for a tropical country like Sri Lanka, it would be possible to use a single neutral temperature of zec to obtain the standard comfort zone for any part of the country where the altitude is less than 300 m. Above 300 m up to 900 m, a value of 25°C can be used. It was also shown that it is possible to enlarge the standard comfort zone to suit Sri Lanka by using a higher humidity ratio of 0.015 as the upper boundary. When the
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internal air velocity is greater than 0.25 mIs, it is also possible to modify the standard comfort zone to take account of the physiological effects of cooling. For these modifications, it has been suggested to use a humidity ratio of 0.020 as an upper boundary when used with a neutral temperature of 26°C. The standard comfort zone and the modified zones for different internal air velocities are' given in Figure 1. It can be seen that when the internal air velocity is high. thermal comfort can be achieved at elevated temperatures. The variation of relative humidity of the external environment in four days with different weather conditions in Moratuwa area is given in Figure 2. It can be seen that the highest humidity occurs in the morning around 0600 hrs and it lowers to around 70% around 1400 hrs. The variation of dry bulb temperature for the same days is given in Figure 3. It can be seen that there is a definite pattern where the morning temperatures are generally about 7°C below the maximum temperature. It can be seen that the morning dry bulb temperatures and relative humidities can be within the comfort zones when there is sufficient internal air movement. This discussion shows that it is practically possible to use passive solar techniques in Sri Lanka to obtain thermally comfortable conditions provided that the thermal heat gain during the day time is minimised. 4. A DETAILED STUDY ON ENVIRONMENTAL I'ACTORS Sri Lanka is located at a latitude of SO to 8° N and a longitude of 79° to 82°. Since the latitude is low, the buildings receive intense sunlight throughout the year except when there is thick cloud cover. In order to develop passive solar techniques with proper scientific background for Sri Lanka, a detailed study has been carried out to consider a number of factors with respect to its location. 4.1 The orientation
of the building
The orientation of the building with respect to solar radiation is an important factor in controlling the heat gain. The need for ventilation can also be important in deciding the orientation of the building since it is necessary to improve ventilation conditions during daytime, evening and night. If an example of a rectangular shaped building with the front facing south is considered, the front face will receive sunlight from sunrise to sunset for nearly six months starting from the end of September to end of March. The rear face will receive sunlight from sunrise to sunset for nearly five months starting from mid April to mid September. The wall facing east will receive sunlight until around 1100 hrs. The wall facing west will receive intense sunlight from] 300 hrs until sunset. Of these solar gains, the sunlight falling on the eastern wall is generally considered less offensive since it falls in the morning where the external temperatures are low. When the sun is at a high angle of incidence as around 1000 hrs, it would be possible to use shading devices to cut down the gain of solar radiation. It is shown later that the walls facing north and south can be effectively protected in Sri Lanka with shading devic:es. The wall facing west is the most affected with respect to thermal gain since it receives intense sunlight for about five hours when the external dry bulb temperature is also high. The use of shading devices to reduce the heat
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gain is least effective in walls facing west since the angle of incidence reduces as, the sunsets with little reduction in intensity of solar radiation. " , It should also be noted that the solar radiation penetrated through the windows f~clng west will be absorbed by the internal walls and floors. During the night, this heat will be transmitted within the internal space as radiation and by convection. This discussion clearly shows that for buildings in Sri Lanka, it is advisable to minimise the area of wall facing the east and west which could be achieved by having the longitudinal axis of the building parallel to east-west direction. This should be facilitated when lands are subdivided by ensuring the roads are generally in east-west direction. However, it should be noted that the adoption of this requirement would not necessarily lead to wasting of land since the longer direction of the building and the land need not coincide; the building regulations adopted in Sri Lanka will need sufficient space in the front and the rear of the building on the basis of building lines and lighting planes respectively. 4.2 Effects of shading devices and size of openings The thermal mass of a building can delay the conductive heat gain penetrating through walls. However, it does not reduce the total heat flow moving inwards, unless the accumulated heat in the wall is dissipated back to the cool night air. However, this is unlikely in hot humid areas. There are two ways of dealing with this limitation which are the use of external shading devices and thermal insulation. Use of more insulation .is not very effective in hot and humid areas, especially where there is a chance that. condensation would occur, which greatly degrades the thermal performance of the building environment and also causes the midew problems (Yang & Hwang 1993). On the other hand, shading devices cut part of the solar heat gain so that the total heat flow is actually reduced, not just delayed. Hence, roof construction with adequate eaves could be extremely useful. It should also be noted that shading devices have the added advantage of cutting down the direct sunshine. In order to provide sufficient light and ventilation for the interior, windows are used on the external surfaces. In passive solar houses, the occupants will have to understand the role played by windows and will have to operate them to maximise the passive solar effects. A window in an external surface can be either kept open or closed during the day time. The solar irradiance gained through a window consists of three types: 1. Direct irradiance - the radiation from the sun to the earth on a clear day. 2. Diffused irradiance - some direct irradiance will strike clouds, water, vapour, dust etc. and be diffused in all directions. Some diffused irradiance will strike the earth. 3. Ground reflected irradiance - direct and diffuse irradiance striking the ground and being reflected onto a surface. Generally it is considered that diffused and ground reflected forms are of similar magnitude and they are generally much lower than the direct irradiance. The shading devices are generally used to minimise the direct irradiance through a window. It would be advisable to minimise the' amount of direct solar radiation through open windows since penetrating radiation is absorbed by the indoor surfaces, raising the indoor radiant temperature. During the night, when the wind often subsides, the absorbed radiation is released back to the indoor space and raises its temperature.
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When the glazed windows are closed, colour of the shading affects the indoor lighting condition. White and light coloured shades transmit more visible light to the interior than darker shades. Darker shades are heated while absorbing the radiation, and emit more infra-red
