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«CSAAR (7: 2010: Amman) Sustainable Architecture and Urban Development \ Edited by Steffen Lehmann, Husam Al Waer, Jamal AI-Qawasmi. Amman: The Center ...»

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Table 13 above ilIustrates total energy used by the flats before and after adopting the energy efficient features, assuming that the comfort level of the households remaining unchanged. Since the design features reduce the cooling load, 64% was deducted from the cooJing energy of each unit or flat. During the calculation of the reduced energy of the flats, the energy used by electrical appliances has been left unchanged as shown by the va lues in the second row of the table For example, the cooling energy of Unit A2 was 636 kWh before the reduction and it is 229 kWh after a reduction of 64%. The total energy used by Unit A2 was 1762 kWh before the reduction on the cooling energy. After the cooling energy of Unit A2 is reduced to 229 kWh, the consequent reduction in total energy use of Unit A2 is 1355 kWh as compared to the initial value of 1762 kWh. This is a reduction of 23% in the total energy use of Unit A2. The percentage reduction was calculated by subtracting the difference in energy use before and after reduction of 64% and then dividing this value by the total energy use before the reduction.

In a similar way, the percentage reduction on the total energy use of Units A2, A3, 82,84,85, CI and C5 is calculated as 23%, 32%, and 23%, 26%, 24%, 12% and 32% respectively. This is an average reduction of 26% on the total energy use ofthe building. This average reduction was found by adding the total energy use of aB thc units before (8439 k Wh) and after the reduction of 64% in cooJing energy (6262 kWh). The difference of these two values (2177 kWh) is divided by the total energy use before the reduction of64% (8439 kWh) and then multiplied by J00 to give the average percentage reduction of 26% on the total energy use ofthe units surveyed.

The percentage reduction in cooling energy is seen to be more in Units A3 and C5 because these units use 50% ofthe total energy in cooling (shown earlier in Section 5.3.2). Whereas, the percentage reduction in cooling energy of Unit CI i5 only 12% as it U5es only 19% of the total energy for cooling. It was shown in Section 5.3.2 that units A3 and C5 are more dependent on air conditioners as compared to other units in the building and Unit C J does not use air conditioners. The unit without air conditioners (Unit CI) is not as benefitted as those that have air conditioners because the reduction in energy use was directed at those who have air conditioners and are very much dependent on them.

Nevertheless, households of Unit Cl would have a better indoor climate with lower indoor temperatures and with lower energy use for cooJing by fans and lower costs.

Tahmina Ahsana & Orjan Svaneb 200 lt can thus be concluded that a 64% reduction in cooling energy literally implies that more than half of the devices that were used for cooling are no Ionger used. Households would use the fans for a lesser period of time or to enhance cross ventilation or when there is no air f1ow. It would also mean that those who are dependent on air conditioners might use it for a lesser period of time or probably do not need air conditioners.

Further reduction in total energy use is possible if the energy required for Iighting i8 reduced by using energy efficient lights. However, discussions on reducing energy use for lighting are outside the scope of this study as mentioned in Section 1.3.2.

6 Conclusion and Recommendation This study has identified the following energy efficient building features for the

context of Dhaka:

• Doubling the thickness of external walls with 280 mm brick walls including an air cavity of 50 mm on east and west.

• The use of hollow clay tiles (HCT) in place of weathering course for roofs.

• Horizontal overhang ratio of 1.3 for east orientations, 1.2 for west orientations, I for north orientations and 1 for south orientations respectively.

All the features that were analysed in this study for adoption in the case study building reduce the energy use for cooling. The study shows that it is possible to reduce the cooling load of the flats studied by 60-70% and hence reduce the total energy use of the f1ats surveyed by 26-30%.

Dhaka City Building Construction Act need to develop building codes to promote and inf1uence energy efficiency in buildings. The focus of the codes should be to incorporate energy efficient design features right from the design stage.

Considering the significant amount of energy used by the residential buildings in general and the prevailing energy crisis in Dhaka, it is important to adopt the reasonably simple energy efficient design features highlighted in this study. These features can reduce the total energy use of the f1ats in the case study building by a factor of one fourth and also provide increased comfort to the households. Energy efficient design features not only improve the energy efficiency of residential buildings, but can also provide reduced energy costs to users and playa role in improving the overall energy situation of the country Sustainable Architecture and Urban Development 201

References

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BBS. (2008). Statistical Pocket Book ofBangladesh 2007.Bangladesh Bureau of Statistics Planning Division: Ministry ofPlanning.

Chowdhury, A.A., Rasul, M.G. & Khan, M.M.K. (2006). Towards Energy Efficient Building Assets. A Review on Sub-Tropical Climate.





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202 Svaneb Tahmina Ahsana & Ossen, D.R., Ahmad, M.H. & Madros, N.H. (2005). Optimum Overhang Geometry for Building Energy Saving in Tropical Climates. Journal of Asian Architecture and Building Engineering, 4(2), 563-570.

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Sustainable Architecture and Urban Development 203 Experimental Studies of a Passive Cooling Roof in Hot Arid Areas Hamida Ben Cheikh l 1 Departement d'architecture, Universite ' de Laghouat, Laghouat, Algeria Abstract An experimental study of passive cooling roof was carried out for a typical summer day of June for Laghouat in Algeria. The proposed roof design is composed of a metal plate ceiling over which lies a bed of rocks in a water pool.

Over this bed is an air gap separated from the extemal environment by an aluminium plate. The upper surface of this plate is painted with a white titanium­ based pigment to increase the radiation ret1ection process during daytime.

Several passive modifications have been introduced to the roof in order to reduce indoor air temperature in hot climates. An experimental investigation, employing passive procedure, has been carried out to study the possibility of reducing air temperature in buildings. The results show that the air temperature can decrease with a range from 6 to lOOK. This decrease can further be lowered by 2 to 3°C if night natural ventilation ofbuildings is allowed.

Keywords: Evaporative cooling; Evapo-rejlective, Roof Hot dry climate; Night ventilation.

204 Hamida Ben Cheikh 1 Introduction In regions with arid climates such as Laghouat in southern Aigeria, excessive heat is the major problem that causes human thermal discomfort, as concluded Bencheikh & Bouchair (2004). Cooling is then is the basic requirement of building occupants. In modem buildings, this can be provided by mechanical and electrical instruments.

Traditional architecture in hot climate had many passive aspects which contributed to thermal comfort in dwellings, such as, compact urban heavy building structure, white painted external surfaces, blind facades, open courtyards, etc. The presence of trees, vegetation and water around the building in modifying the thermal microc1imate was weB appreciated. With the advent of energy crisis there has been a renewed interest in those aspects of architecture which contributed to thermal comfort in a building without or with minimum energy consumption, Bouchair (1989,2003,2004). Unlike rural areas, where the building is most exposed to external environment via its facades and roof, in urban area the most exposed part of the building to radiation and winds is the roof as shown in Fig. 1 In many studies such as Runsheng, Etzion, and ErelJ (2003), Jain (2006) and Amer (2006) show that the heat gain through the roofpresent 50% ofthe total heat gain in buildings.



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