Performance of Fixed Horizontal Shading Devices in South Facing Residential Buildings in Dhaka

Transcription

1 Global Science and Technology Journal Vol. 1. No. 1. July 2013 Issue. Pp Performance of Fixed Horizontal Shading Devices in South Facing Residential Buildings in Dhaka Saiful Hasan Tariq* and Mahbuba Afroz Jinia** External shading devices have been utilized very extensively in the residential buildings in the tropics to reduce the amount of solar radiation occurring in the buildings. The study focuses on the minimum depth required for the fixed horizontal shading device for optimum shading performance at south façade for commonly used opening sizes in the residential buildings of Dhaka, Bangladesh. Particular emphasis has been given on the opening size, depth of the shading device, vertical shadow angle during the warmest part of the day of warmest seasons and the thermal performance for using different depth of shading devices in various opening sizes. Two residential buildings were fundamentally chosen as topic for this study. The units having two different sizes of opening and shading systems are taken as the study element. The field survey was conducted in a sunny day to measure the temperature difference between indoor and outdoor and the direct solar in both study models. The shadow simulation of the case studies were conducted by the Sketch up v7.0 having location data and based on the dates and times of the warmest months. The simulation is more visual than analytic. This paper is an outcome of the study which encompasses the efficient shading device design strategy to reduce direct solar exposure on the openings at south façade along with the case studies in the context of Dhaka, Bangladesh and its effect on building performance in terms of thermal performance. Key Words: Shading Devices, Vertical Shadow Angle, Thermal Performance, Design Strategy Field of Research: Architecture 1. Introduction: The most significant factor affecting the architectural environment in the tropical region is solar energy. Throughout the year, solar energy impinges on the building which influences either its inside or outside climate. To control the effect of solar energy on the Indoor environment, it is usual to concentrate on the role played by the building skin and fenestration, which act as a filter between the outdoor conditions and those within the building. Focusing on the fenestration, which is the critical point of indoor heat gain, heat transfer can occur by radiation, ventilation (infiltration), conduction, and convection. Fenestration can contribute up to 22% of energy consumption in residential buildings (Al-Mofeez et al, 1991). *Saiful Hasan Tariq, Senior Lecturer, Department of Architecture, Stamford University Bangladesh, Bangladesh. shtariq1757@yahoo.com (Corresponding author) **Mahbuba Afroz Jinia, Senior Lecturer, Department of Architecture, Stamford University Bangladesh, Bangladesh. arch_majinia@yahoo.com 88

2 External shading devices can be utilized to block the solar radiation before it reaches the indoor environment. The greatest source of heat gain can be the solar radiation entering through an opening. This could, in fact, increase the indoor temperature, far above the outdoor temperature even in moderate climates, which is known as the Green House Effect (Koenigsberger et al, 1973). Window glasses are particularly transparent for short wave infra-red radiation by the sun, but almost opaque for long wave radiation emitted by objects in the room. As a result the heat, once it has entered through a window, is trapped inside the building. As the south façade gets maximum exposure to the solar radiation during the warmest part of the day, the openings in this façade requires properly designed shading devices to minimize the solar heat gain. Horizontal shading devices are appropriate to protect the windows from solar heat gain at south orientation. It works efficiently from 10 A.M. to 2 P.M. when the sun is opposite to the window pane and at a high altitude (Anisur Rahman, 2007). The study intends to set a parameter for the minimum required depth of the fixed horizontal shading device at south façade for commonly used opening sizes in the residential buildings in Dhaka city. The study focuses on the minimum depth required for the fixed horizontal shading device for optimum shading performance at south façade for commonly used opening sizes in the residential buildings in Dhaka city. Particular emphasis has been given on the opening size, depth of the shading device, vertical shadow angle during the warmest part of the day of warmest seasons and the thermal performance for using different depth of shading devices in various opening sizes. The objective of the study can be summarized as follows: Practical understanding of the relationship between vertical shadow angle (VSA) and depth of overhang of fixed horizontal shading devices. To find out the parameter for the optimum depth of the fixed horizontal shading device at south façade for commonly used opening sizes. To explore the ideal shading device for South facing residential building with optimum thermal performance. 2. Climate of Bangladesh: Bangladesh has a subtropical monsoon climate characterized by moderately warm temperature, high humidity and wide seasonal variations in rainfall. According to Atkinson s widely used classification it can be categorized as warm-humid (Koenig Berger et al, 1973). Meteorologically Bangladesh has four distinct seasons. Winter, from December to February (mean temperature between12 o C and 28 o C), Pre-monsoon, March to May (20 o C and 35 o C), Monsoon from June to September (25 o C and 32 o C) Post-monsoon covers October and November (17 o C and 31 o C). 89

3 Table 1: Classification of Seasons Bangla Calendar Months Traditional seasons Meteorological seasons Gregorian months Chaitra Bashanta Pre- monsoon (hot-dry) March Baishakh Grisha Pre-monsoon (hot- dry) April Jaishtha Grisha Pre-monsoon (hot-dry) May Ashaar Barsha Monsoon (hot-wet) June Srabon Barsha Monsoon (hot-wet) July Bhadra Sharat Monsoon (hot-wet) August Aswin Sharat Monsoon (hot-wet) September Kartik Hemanta Post-monsoon (hot-wet) October Agrahayon Hemanta Post-monsoon (hot-wet) November Poush Sheet Winter December Magh Sheet Winter January Falgun Bashanta Winter February calendar 3. Weather of Dhaka: Dhaka is located in central Bangladesh at 23 o 42 0 N 90 o E and experiences a hot, wet and humid tropical climate. The city has a distinct monsoonal season, with an annual average temperature of 25 o C and monthly means varying between 18 o C in January and 32 o C in May. Approximately 87% of annual average rainfall of 2,123 millimeters occurs between May and October. Table 2: Climate Data for Dhaka Source: Weather base 4. Literature survey: 4.1 Objectives of shading: Shading the glass affects the quantity of incident radiation and hence modifies both the heat flow to the interior and indoor temperatures. It is useful to set out the purpose of shading in some detail (Steemers et al, 2002). They are as follows, To minimize the total solar energy entering a room and thereby reduce the average temperature of the room 90

4 To prevent sunlight from falling directly onto occupants, resulting in an effective increase of temperature of between 3 o C and 7 o C To prevent the brightly lit outside surface, clouds or sun. 4.2 Calculation for Optimum Shading: It has been found from the investigation that horizontal shading devices were efficient at south elevation only. These shading devices are appropriate to protect windows from solar heat gain at south orientation. It works efficiently from 10 A.M. to 2 P.M. when the sun is opposite to the window pane and at a high altitude. (Anisur Rahman, 2007) The depth of the overhang depends on the opening height and it is independent of the window width. The performance of the horizontal shading device increases with the increase of the depth of the overhang. The important factor is the ratio between the depth of the overhang and the height of the opening. (Anisur Rahman, 2007) For optimum shading, the ratio between depth of overhang and height of the opening is, D = 7/16 x H Where, D = depth of overhang H = height of opening Fig 1: Schematic Diagram Showing Parameters of Horizontal Shading Device sun D= 7 16 H H Source: Anisur Rahman, 2007 The ratio between the side offset from opening edge of overhang and height of the opening is, W = H/2 Where, W = Side offset from opening edge H = height of opening 91

5 Fig 2: Schematic Diagram Showing Parameters of Horizontal Shading Device W=H/2 W=H/2 H Source: Anisur Rahman, 2007 For large height of openings, the shading devices require larger overhangs. Those overhangs transforms into verandahs in most of the cases of fixed overhangs. Another way of minimizing effective height of openings is by using several shorter depth overhangs on the window pan instead of one large overhang. This system is not popular among the residential buildings as it splits the vista through the openings. Optimum shading can also be determined by the ratio between Depth of overhang and opening height, D = H / tanø Where vertical shadow angle = tanø Fig 3: Schematic Diagram Showing Parameters of Horizontal Shading Device sun D VSA = tanø H Source: (Koenigsberger et al, 1973) 92

6 5. Methodology: Two residential buildings are fundamentally chosen as topic for this study considering their long time use by the inhabitants. The units having two different sizes of opening and shading systems are taken as the study element. Difference between maximum outdoor temperature and indoor temperature and the shadow patterns are taken into account during the warmest part of the day to indentify the parameters for existing thermal performance due to shading properties. The field survey was conducted in a sunny day to measure the temperature difference between indoor and outdoor and the direct solar radiation in a selected south oriented room during the warmest part of the day in both study models. The temperature data and vertical shadow angle of the warmest months were taken by calculating the sun path diagram from the ECOTECT v5.20. The shadow simulation of the case studies were conducted by the Sketch up v7.0 having location data and based on the dates and times of the warmest months. The simulation is more visual than analytic. From March to October this region faces pre-monsoon, monsoon and post monsoon seasons, which are the warmer seasons in the local climate.12 P.M. to 2P.M is considered as the hottest part of the day in these months. The dates for simulation have been selected randomly as 21 st march, 21 st June and 22 nd September. The selection of months has been made on the basis of pre-monsoon, monsoon and post monsoon seasons and the selection of dates made randomly. The 3d visual analysis of Sketch up shadow simulation is made on these mentioned dates in three different time segments from 12 P.M. to 2P.M. The outcome of the simulation is then compared with the ideal ratios mentioned in literature review. The thermal performance was measured on the date of 21 st and 22 nd September in similar sky condition because of time constrains. At the End, the ideal condition is picked based on the thermal performance in indoor due to proper shading system by comparing the study models. Diagram 1: Diagram of the Research Process Source: Author 93

7 6. Analysis: Both the study models have 30 wide road at south side (front) and a vacant opposite plot. These study models has been selected for getting optimum solar exposure at south façade without any obstacle during whole day. Having different types of opening and shading systems were another selection criteria. Both of the units used the most common types of shading systems usually practiced in local context. To be more specific in temperature data collection, one south oriented room has been selected as field survey model and simulation model as well. The field survey data has been taken in two back to back days with similar climatic condition and in clear sunny day. 7.1 Study Unit-1: Study unit-1 is the 2 nd floor unit of a Govt. officers colony at Malibagh, Dhaka. A south-east corner bed room has been selected for shadow simulation and field data collection. The overhang at roof slab was not taken into account as it puts no impact on the glass surface. The size of the opening at south façade is 6 x 4.5. The depth of overhang is 2 and the sill level is at 2.5 level from floor. Fig 4: Plan, Section and Image of Study Unit-1 sun 2' 4'-6" 2'-6" N Source: Author At first, the requirement of overhang for an opening height of 4.5 has been checked. D = 7/16 x 4.5 = 1.96 As the existing overhang is 2 ; so, theoretically it should be adequate for optimum shading performance during the warmest part of the day. The minimum requirement has also been checked by the calculation of vertical shadow angle. Minimum vertical shadow angle data has been taken between 12 P.M. to 2 P.M. for the analysis. 94

8 Table 3: Calculating Depth of Overhang from Vertical Shadow Angle (VSA) Data Date Minimum VSA (tanø) from 12 P.M. TO 2 P.M. Opening height H 21 st March o 4.5 or 54 inch 21 st June 82.8 o 4.5 or 54 inch 22 nd o 4.5 or 54 September inch Minimum Depth of overhang D = H / tanø 1.97 < < < 2 The simulation study has been made in the dates of 21 st march, 21 st June and 22 nd September in three different time segment from 12 P.M. to 2 P.M. Simulation study shows the visual outcome of shading performance of the Shading devices. Fig 5: Sketch up Shadow Simulation Study Shows the Shading Performance of the Shading Device Source: Author The simulation study shows that the 2 overhang of shading device is minimum required depth for shading system where the opening height is 4.5. Having this depth of overhang the glass surface gets the protection from direct solar exposure during the warmest part of the day in a clear sky condition. The sectional 95

9 perspectives of the study unit shows the shaded interior space, which means less heat gain due to direct solar radiation and cooler indoor environment than the outdoor. The temperature data was taken on 22 nd September and Outdoor temperature was measured at 33 o C while indoor temperature was measured in constant air change at 30.5 o C during the time segment from 12 P.M. to 2 P.M. 7.2 Study Unit-2: Study unit-2 is the 3 rd floor unit of an apartment building at Uttara, Dhaka. A south- West corner bed room has been selected for shadow simulation and field data collection. The opening at west side in the study unit was not taken into account as it gets no solar exposure from west due to adjacent apartment building. The size of the opening at south façade is 5 x 7. The depth of overhang is 3.33 and the opening is used as an access way to the verandah which is providing support for the shading device. Fig 6: Plan, Section and Image of Study Unit-2 sun 3'-4" 7' Source: Author N At first, the requirement of overhang for an opening height of 7 has been checked. D = 7/16 x 7 = 3.06 As the existing overhang is 3.33 ; so, theoretically it should be adequate for optimum shading performance during the warmest part of the day. The minimum requirement has also been checked by the calculation of vertical shadow angle. Minimum vertical shadow angle data has been taken between 12 P.M. to 2 P.M. for the analysis. 96

10 Date Tariq & Jinia Table 4: Calculating Depth of Overhang from Vertical Shadow Angle (VSA) Data Minimum VSA (tanø) from 12 P.M. TO 2 P.M. Opening height H 21 st March o < st June 82.8 o < nd September o < 3.33 Minimum Depth of overhang D = H / tanø The simulation study has been made in the dates of 21 st march, 21 st June and 22 nd September in three different time segment from 12 P.M. to 2 P.M. Simulation study shows the visual outcome of shading performance of the Shading devices. Fig 7: Sketch up Shadow Simulation Study Shows the Shading Performance of the Shading Device Source: Author The simulation study shows that the 3.33 or 3-4 overhang of shading device is minimum required depth for shading system where the opening height is 7. Having this depth of overhang the glass surface gets the protection from direct solar exposure during the warmest part of the day in a clear sky condition. The sectional perspectives of the study unit shows the shaded interior space, which means less heat gain due to direct solar radiation and cooler indoor environment than the outdoor. 97

11 In the study unit-2, a larger opening at south allows vista, good airflow and optimum daylight. An opening size of 5 wide and 7 height requires larger over hang, which is known form the theoretical perspective. Larger overhang required more structural support from the cantilevered floor. The extension of the floor space beneath the overhang was transformed into verandah and the larger glass opening worked as the access way to the verandah as well. The temperature data was taken on 21 st September and Outdoor temperature was measured at 33.0 o C while indoor temperature was measured in constant air change at 29.5 o C during the time segment from 12 P.M. to 2 P.M. 8. Discussion: Both of the Study units used the most common types of shading systems usually practiced in local context. The Study unit-1 had the fixed horizontal shading device cantilevered from the lintel level of the opening. Both theoretical calculation and visual simulation study shows that 2 over hang is the minimum required shading depth for the opening of 4.5 height. The performance of the shading device is also cross checked by taking the temperature data which shows a 2.5 o C temperature difference from out door to indoor in constant air change situation. In the case of study unit-2 the space beneath the fixed horizontal shading devise has been transformed in to verandah because of large and accessible opening size. The verandah floor slab provided support system for the large overhang. The verandah also works as a heat buffer zone from the outside and keeps the indoor temperature cooler than outdoor. The temperature data shows a 3.5 o C temperature difference from out door to indoor in constant air change situation during the warmest part of the day. The indoor of study unit-2 is also found to be 1 o C cooler than the Study unit-1 while the outdoor temperature was same (33 o C). Reduced indoor temperature signifies reduced energy consumption for thermal comfort. Therefore, for full height openings (up to lintel level) at south façade, it can be recommended to use cantilever verandahs with required depth for shading. Both the study units show optimum shading performance during the warmest times of the warmest months in the year. Either fixed horizontal shading or the cantilever verandah, both of them are suitable for the context of Bangladesh, if properly designed by understanding the relationship between the height of the opening and the depth of the overhang. 9. Conclusion: This study intended to set a parameter for the required depth of the fixed horizontal shading device at south façade for commonly used opening sizes in the residential buildings in Dhaka city. Two study units were taken into account for field survey and simulation study to test the performance of the shading devices. The Thermal performance due to shading properties during the warmest part of the day was found satisfactory in both study units. From the temperature data, it is notable that proper 98

12 use of shading devices may have significant impact on thermal performance as well as reduced energy consumption to achieve the comfort situation in built environment. Further study in this topic may enrich the field of sustainable architecture and will put significant impact on the movement of low energy consumption. References: Al-Mofeez, and Abdul, I., (1991), Insulation in the Opaque Envelope: Effects on Thermal Performance of Residential Building in Hot-arid Climates, Dissertation for degree of Doctor of Philosophy, Texas A&M University, UMI dissertation information service, Michigan. Givoni, B., (1969), Man, Climate and Architecture, Pp , Elsevier Publishing Company. UK. Dubois, M. C., (2000), A Simple Chart To Design Shading Devices Considering The Window Solar Angle Dependent Properties, Proceedings of the Third ISES Europe Solar Congress: Eurosun 2000, June, Copenhagen (Denmark). Koenigsberger, O. H., Ingersoll, T. G., Mayhew, A., and Szokolay, S. V., (1975), Manual of Tropical Housing and Building, Pp , Orient Longman Limited, India Rahman, A., (2007), Performance Evaluation of Shading Devices Used In Tall Office Buildings of Dhaka City, M. Arch. Thesis, (unpublished), Department of Architecture, Bangladesh University of Engineering and Technology, Dhaka. Steemers, K., and Baker. N., (2002), Daylight Design of Buildings, James & James Ltd, London Hien, W. N., and Istiadji, A. D., (2003), Effects Of External Shading Devices On day lighting And Natural Ventilation, proceedings of Eighth International IBPSA Conference, Eindhoven, Netherlands. 99