«CSAAR (7: 2010: Amman) Sustainable Architecture and Urban Development \ Edited by Steffen Lehmann, Husam Al Waer, Jamal AI-Qawasmi. Amman: The Center ...»
Their main objective was to assess and compare the impact of horizontal shading devices in reducing the unwanted solar heat gain and the amount of natural light penetration into office buildings in Malaysia.. The base-case model developed for the study was a single unit office room with dimensions of 6 metres for length and depth and a height of2.8 metres. The size ofthe window was taken to be 4.4 metres in length and 1.82 metres height (from sill to ceiling line). The window area was assumed to be 50% of the net external wall area.The corresponding window to floor area ratio was 22%.The depth of the overhang (external horizontal shading device) was the main variable in this study. A range of overhang depths was investigated to determine the optimum shading for reducing the maximum solar heat gain from form direct solar radiation. Table 1 outlines the various overhang depth studied and the relative overhang ratio (OHR).
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Figure. 2. The Case Study 8uilding Figure. 3. Location of different units The case study building (Fig. 2) is a typical six-storied multi-unit residential building with three flats on each floor and fifteen households. The three different flats/units in the building are: Type A, Type 8 and Type C (Fig. 3). The sizes of flat Type A, 8 and C are 120, 122 and 120 square metres respectively. Type Ais surrounded by a road on the southern side and by a residential building on the eastern side. Two roads, one on the western side and the other on the south, surround Type B. Type C is surrounded by a road on the west side and a residential apartment on the north.
Seven out of fifteen households were surveyed. These households are A2, A3, 82, 84, 85, Cl and C5. The residents of CI are tenants; all other flats surveyed are owned by the households.
4.2 Design features of the case study building
4.2.1 Building envelope 184.108.40.206 Externat wall All external walls are of 125 mm solid brick. The owner of flat 84 who was the owner of the land now regrets the limited thickness of external walls. DUTing the interview, she complained that the heat gain on the western side of the building is profuse and unbearable. She cIaimed that the developers had suggested 125 mm wall thickness to reduce the construction costs of the building. She now feels that the heat gain on the western side would have Sustainable Architecture and Urban Development 191 been less if the external walls were 250 mm. She has admitted that the extra costs of using 250 mm wall thickness and the reduced indoor floor area as a result ofthe increased external wall would have been worthwhile.
80th external and internal walls have a cement pIaster over the brick and white wall finishes. Some exterior walls that face the roadside are clad with light coloured facing bricks.
220.127.116.11 Roof The roof is flat, about 100 mm thick. It is made of reinforced concrete slab with weathering course, a course laid on the top surface of RCC roof slab to protect it against weather elements like rain, heat etc and neat cement finish. The roof has one big room that functions as a community room.
The roof is also used by the residents for hanging laundry and as a community space.
18.104.22.168 Windows Shading devices Shading devices are needed in Dhaka to ensure protection from the rain and solar heat gain. The depth of the shading device for different orientations of windows in all rooms ofthe different unit types (A, 8 and C) was calculated.
The analysis of shading devices for windows (Tables 4- 6) demonstrate that shading devices are either absent or their sizes are much less than the recommended value. This analysis together with simple observation on shading devices represents the general scenario of shading devices in typical residential buildings of Dhaka.
4.3 Energy usage of the case study Dats 4.3.1 Total energy usage The energy use of the case study flats depend on the household size, occupancy pattern, appliances used, the power rating of the appliances and the duration for wh ich they are used. The study does not take into account the energy efficiency of the different appliances and energy efficiency related to every day habits that cannot be influenced by design because the study focuses on the energy efficiency aspects that can be addressed through planning and design. The energy use of the households has been calculated for a typical summer month, when the maximum temperature can be as high as 34° C. The monthly total energy use for a typical summer month for all the units studied is given in Table 7. A break up ofthe energy use pattern (in percentage) ofthe case study flats for a typical summer month has been outlined in Table 8. Analysis of energy use in Table 8 shows that the energy required for cooling and lighting takes up the largest share of the energy used by a flat. Unit Cl has very low energy use compared to the other units because the household size is small and it does not use excessive fixtures for Iighting or air conditioners for cooling. The average cooling and lighting energy used by all the units of the case study in a typical summer month has been calculated as 40% and 39% respectively ofthe total energy use.
4.3.2 Energy usage for cooling Using the data in Table 9, the average cooling energy as a percentage ofthe total energy used by all the units of the case study has been calculated as 40 % and the average energy use tor air conditioners as a percentage of the total cooling energy alone is 24%. The percentage oftotal cooling energy for Units A3 and C5 is 50% because they are more dependent on air conditioners as compared to other units in the building; Unit A3 uses 36% of cooling energy for air conditioners and Unit C5 uses 37%. On the other hand, the percentage oftotal cooling for Unit CI is extremely low when compared to other units because this household does not have air conditioners.
Cooling (A.C) in kWh Cooling (A.C) in % Cooling (Fan) in kWh Cooling (Fan) in % Total for cooling (kWh) Total in kWh Total for cooling in % The energy used by air conditioners depends on their capacity, type, power rating, and usage and setpoint temperatures. Table 10 shows the energy use of air conditioners based on their capacity, type and power rating. According to Tham (1993), a rise of one degree Celsius in setpoint represents a saving 01' 6% in energy required for cooling.
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energy. This highlights the necessity in paying attention to architectural characteristics and trends that can address the thermal comfort demands of the households without increasing the dependency of air conditioners. Although fans are also used for cooling, the energy used by them is not as significant as the energy use of air conditioners (Table 9).
It must be emphasized that the case study building is representative of upper middle-income households who have a minimum of one air conditioner. The value for the share of energy utilized for cooling by air conditioners would be much more for lower upper and upper upper- income groups who live in four bed-roomed Hats and have air conditioners in all their rooms.
4.3.3 Energy usage ror lighting There are substantial variations in energy used for lighting residential buildings.
In the United States, lighting uses 12% of the energy used by a residential building (UNEP, 2007) and 9% of energy used in residential buildings of lndia contributes to Iighting (UNEP, 2007). Residential buildings of Taiwan, on the other hand use 40% of the total residential sector electricity use for lighting (Yang and Hwang, 1993). Using the data in Table 9, the average lighting energy used by all the units ofthe case study has been calculated as 39%.
The energy use for lighting in the households is seen to vary with type of Iighting, the number of lights, power rating and the usage of lights. Table 11 shows the different types of lights, the total number of lights and the share of energy use for lighting. Analysis of the different types of lights and the energy use of each type of light is illustrates that incandescent lights use more energy than Huorescent lights. The analysis also shows that energy saving lights use less energy. Nagarajan (2006) states that compact Huorescent light (CFL) or energy saving light as it i8 commonly called is energy efficient and consumes 80% less electricity when compared to incandescent light.
Sustainable Architecture and Urban Development 197 Developers in Bangladesh generally provide a minimum of two average quality wall mountable lighting fixtures, but not the lights. The households studied in this building did not use the lighting fixtures provided by the developers. lnstead, they purchased a multitude of lighting fixtures and lights of their own choice or as suggested by the interior designers who were responsib1e for the interior design ofthe flats. The nurnber ofiights in aH flats except unit Cl and C5 are much more than what is needed for strictly practical reasons. Units Cl and C5 are good examples to show that use of excessive lights are not a necessity. It is possible to use fewer lights and have good indoor artificial lighting conditions. The superfluous lights in the remaining households are for aesthetic purposes and to some extent, to signify the status of the households.
Gut and Ackerknecht (1993) have advised against the use of unnecessary lighting as it adds up to internal heat gain.
Even though Unit Cl has the least number of Iights compared to the other units, energy used for lighting in unit CI is more than 50% because this household does not use air conditioners and other major energy consuming applianees. Lighting alone contributes to more than 50% of the share of energy used by this household. It is thus seen that percentages are not relevant on their own, but only in relation to a total.
5 Discussion5.1 Energy efficient design features
The theoretical framework in this study identified energy effident design features that can meet the purpose of this study and can be applied in the context of Dhaka. The features that have been selected pertain only to the building envelope, reduce heat gain by the buildings, and they mainly reduce the energy use for cooling. It needs to be strictly emphasized that the chosen features reduce only the cooling energy; the features do not influence the energy used for electrical appliances. The research front has been surnrnarized in Table 12 to formulate the energy efficient design features that can be applied in the context ofDhaka.
Assuming that the energy savings estimated by the authors above are roughly correct, then addition of the lower range energy saving values of all the features above gives a total energy savings of 64% (7%;- 18% + 39%) for cooling. Given the concrete features of the case study building. the energy efficient design features Iisted above can be recommended for the context of Dhaka and lies within the field of influence 01' the architect.
5.2 Encrgy usc of the flats in tbe casc study on adoption of thc energy emdent features The energy use of each flat in the case study building has been delineated in section 4.3. Out of all the energy use that the households use, only the cooling energy of each tlat has been reduced because the design measures are not connected to energy use for lighting and other appliances. lfthe building were to adopt the energy efficient features discussed above, then the cooling energy use of the surveyed flats in the case study building would be reduced by 64%. As explained above, the 64% reduction is a summation of the lower range energy saving values of all the energy efficient features.