Modelling Analysis of Thermal Performance of Internal Shading Devices for a Commercial Atrium Building in Tropical Climates Kittitach Pichatwatana, and Fan Wang Abstract This paper examines the TAS computer simulation of the application of internal shading devices to provide thermal inside the atrium of a commercial building in Southern China. The modelling study was carried out to investigate the effects of two internal shading forms; the ceiling level shading configuration and the underneath curved roof configuration, within the atrium covering both overcast days and clear day in summer and winter. The system was assessed by the solar gain, cooling load, air temperature and resultant temperature over major occupied area on various levels within the atrium. The simulation results from these studies reveal that the ceiling level shading configuration is generally more effective than the underneath curved roof configuration in term of providing better atrium s thermal performance. Keywords Atrium building, Aluminate reflective shade cloth, retractable shading device and thermal comfort I. INTRODUCTION N modern architecture, an atrium is a large courtyard within Ia building, often several stories high or galleries and having a glazed roof and/or large windows. The development of the construction technology and the innovation in glass manufacturing contributed to a new era for the atrium buildings. This in recent year, the atrium feature has become very popular to employ in many types and forms regardless of cultural and climatic conditions [1]. In Figure 1, there is a significant increase in the use of atrium construction for skyscraper buildings with origin in the Western countries. Skyscrapers are usually designed for commercial use and modern skyscrapers are characterized by large surface areas of glass supported by steel frames and curtain walls. Moreover, most skyscrapers comprised of an atrium linked vertical internal space thereby maximising natural daylight and enhancing aesthetics [2]. The highly gazed large transitional space in tropical climate is likely to become over heated due to high solar attitude and high ambient temperature. The indoor temperature can be above the temperature of external temperature. In additional, the relation high proportion of solar radiation heat absorbed by the atrium envelope will increase the mean radiate temperature of the internal surfaces. Kittitach Pichatwatana is with the school of Built Environment, Heriot-Watt University, EH14 4AS, UK (e-mail: kp126@hw.ac.uk). Fan Wang is with the school of Built Environment, Heriot-Watt University, EH14 4AS UK (e-mail: Fan.Wang@hw.ac.uk). Source: http://www.emporis.com/buildings, 2003 Fig. 1 Gazetteer of Notable Skyscraper Building in Hot Climate and Cold Climate Countries These lead to the increase of the resultant temperature inside the atrium and inflict a great risk of thermal discomfort and overheating problems. Moreover, the air in the atrium generated by solar heat can lead to thermal stratification, which is difficult to control. Therefore, the running costs of the atrium building are generally high resulting from the high energy consumption for the cooling system in order to provide comfort for the occupants [3]. One of the simple way to reduce heat gain through the building roof by evaporative spray cooling which can prevent the overheating of the roof and interior of the building and reduce the thermal stratification at higher level. Another simple effective and in expensive way to reduce overheating is the use of a sun shade system which can not only reduce the solar radiations from the top, but also helped to minimize the differential air temperature between the ground and the top floor [4]. Finally, a way to remove the heat at the top of the atrium can is by the stack effect which can draw hot air from lower part of the building cause it to flow upward through the atrium rooftop openings [5]. This paper presents the results of a computer modelling study using dynamic thermal simulation software TAS to investigate and predict internal thermal environment. The objective of this study was to develop high performance internal shading devices in term of providing better thermal conditions within a large atrium commercial building. 239
II. DESCRIPTION OF BUILDING AND TAS MODEL This study was conducted in a main atrium in Guangzhou International Textile City Building in China. This building is located in Guangzhou city at a latitude of 23.13 o North and a longitude of 113.23 o East. The annual average air temperature of Guangzhou is 22.2 o C, whilst the annual average relative humidity is 77%. The average daily highest temperature is in July and August (29 C) and the coldest is in January (14 C). The weather is generally hot and humid with occasional rain showers. Fig. 2 Guangzhou International Textile City Building This study reports the results of a computer model study using TAS to investigate and predict internal thermal environment. The simulation was done in three configurations: model without internal shading (Basic model), model with internal on ceiling level (Model 1) and model with internal shading underneath curved roof (Model 2). The impact of the internal shading device on the thermal environment has been analysed. According to 2011 meteorological record, a summer clear day and a summer cloudy day has been defined as 17 th July and 21 th July, respectively and a winter clear day and a winter cloudy day has been defined as 28 th January and 27 th January, respectively. The predicted results of the model with internal shading on a ceiling level (Model 1) and with internal shading underneath a curved roof (Model 2) were compared with those of the basic configuration in order to evaluate the impact of the internal shading devices on the thermal environment. Fig. 3 Illustration of the model As shown in Fig.3, the 3-D geometric model of Guangzhou International Textile City Building was created based on architectural drawings and set the same location and north angle of the real building by TAS. The simulated model has the same configuration as the Guangzhou International Textile building (Appendix A). The model was defined zones within each floor for thermal simulation purpose. For internal conditions, atria and corridor were defined as natural ventilation whilst retail shops were confined with airconditioning. It was assumed that the temperature in these spaces have been set to 24.0-26.0 o C from 0800h to 2000h. For simulation purposes, the sensible heat gains from occupants, lighting and equipment were set and conducted by CIBSE Guide A. [6] in each zone. From maximum customer records during business season, it was assumed that there were 4,166 persons around the whole building at any particular hour over the twelve-hour period (0800-2000). The infiltration air rate for all zones was assumed and set at 0.5 ach, whilst the ventilation air rate for atrium and corridor zone was assumed and set to 1 ach (Appendix B). III. PROPOSE SHADING DEVICES Shading system should be design in order to balance daylight requirement and the need to reduce solar gain which could lead to minimization of energy consumption for cooling in perimeter spaces. Shading device type properties and control have a significant impact on building cooling demand especially in atrium space with transparent roof [7]. This paper focused on the effect of various internal textile shading devices by using thermal dynamic simulation model which can be curtains, blinds and retractable shading devices. Solar transmittance and visible lighting transmittance are very important for textile characteristics which have a direct effect on thermal environment and daylight level within the atrium buildings. The fabrics should have the highest lighting transmittance with lowest solar transmittance in order to minimize solar penetration through the atrium. According to fabrics properties provided by SilentGliss Company, the atrium model was simulated by aluminate reflective shade fabric with the light transmittance of 11%, solar reflectance at 50% and the solar transmittance of 0.11%. IV. ASSESSMENT CRITERIA For the aim of this analysis, periods of time were used to examine thermal and daylight performance of Guangzhou International Textile building together with the annual overview: Characteristic day of summer: 17 th and 21 st July 2011 were selected representing a hot clear day and a hot cloudy day for model assessment, respectively; as they had high diurnal ambient temperature between 27.1 o C and 35.3 o C, while 17 th July 2011 had high solar radiation between 12-1200 W/m 2 and 21 st July 2011 had low solar radiation between 2-668 W/m 2 according to typical meteorological weather data by the Meteorological Department for the year 2011. Characteristic days of winter: 27 th and 28 th January 2011 were selected representing cold cloudy day and cold clear day for model assessment, respectively; as they had low diurnal ambient temperature between 7.1 o C and 14.8 o C, whilst 27 th January 2011 had weak solar radiation 40-210 W/m 2 and 28 th January 2011 had strong solar radiation between 45-432 W/m 2 according to typical 240
meteorological weather data by the Meteorological Department for the year 2011. The simulation was done in three conditions: model without internal shading (Basic model), model with internal shading on a ceiling level (Model 1) and model with internal shading underneath curved roof (Model 2). The schematic crosssection of the atrium representative models are shown in Fig. 4. These six scenarios were initially used to investigate the effect of internal thermal behaviour of Guangzhou International Textile City Building. The simulation results by the atrium building without shading devices are defined as a basic model in order to compare with the results from internal shading devices on a ceiling level and internal shading devices underneath atrium curved roofs. To verify the internal shading devices, the internal shading devices have been examined and compared in term of thermal performance in savings for cooling load whilst maintaining thermal comfort and reducing solar heat gain by TAS. Fig. 5 Daily total cooling loads in the retail shops around the main atrium Fig. 5 reveals that the ceiling level shading configuration (Model 1) performs better than the underneath curved roof shading configuration in term of the total cooling load on all retail shops. The energy saving was minimized in range of 6.61-7.24% for the ceiling level shading configuration (Model1) and in range of 3.56-3.73% for the underneath curved roof shading configuration (Model 2). B. Solar heat gain Since large pane of glass roof introduces solar radiation into the indoor space in day time. Therefore, the high solar heat gain normally occurred between 1000h to 1700h especially in the region near the ground of atrium and the highest corridor. Fig.4 Three conditions of internal shading model simulation V. RESULTS AND DISCUSSION The thermal performance of internal shading devices can be verified by estimating the saving amount of the cooling loading requirement, the reducing of solar heat gain and the maintaining of thermal environment comfort such as indoor air temperatures and resultant temperatures. A. Cooling load Indoor cooling load in atrium buildings is influenced by heat transfer through surfaces surrounding the atrium space, the roof surfaces exposed to the external environment and also the side walls of the adjacent zone in each floor [8]. For the purpose of this study, it was assumed that during operating hour (0900h-2000h), all retail shops had a maintained temperature between 24-26 o C as a result of air-conditioning. Thermal modelling software (TAS) predicted the total cooling load for the without shading configuration (Basic model) at 47,557.45kW on a summer clear day, at 46,991.44 kw on a summer cloudy day. In winter period no airconditioning was used. Fig. 6 Comparison of the total solar heat gain within the main atrium Fig. 6 presents the simulation results of total solar heat gain of the without shading configuration (Basic model) compared with the ceiling level shading configuration (Model 1) and the underneath curved roof shading configuration (Model 2). The total daily solar heat gain for the Basic model was 556.59 kw on a summer day without cloud and 448.30 kw on a summer day with cloud. Figure for a winter day without cloud was 193.85 kw and for a winter day with cloud was 107.51 kw. 241
range of 0.1-1.54 o C (average 0.37%) on a clear day and 0.1-1.4 o C (average 0.44%) on a cloudy day, respectively. During winter, the external air temperatures were not so high. The predicted air temperatures within the ground floor atrium were in the range of 7.0-16.1 o C in both clear and cloudy day. It was not necessary to shut the internal shading devices. Fig. 7 Comparison of the total solar heat gain within the corridor around main atrium Fig.7 shows the simulation results of total solar heat gain within the corridor around the main atrium of the without shading configuration (Basic model) compared with the ceiling level shading configuration (Model 1) and the underneath curved roof shading configuration (Model 2). The total daily solar heat gain for the Basic model was 2,622.48 kw on a summer clear day and 2,242.02 kw on a summer cloudy day. Figure for a winter clear day was 1,271.86 kw and for a winter cloudy day was789.03 kw. It can be clearly seen that the Model 1 depicted substantial reduction the total daily solar heat gain in the range of 51.49-69.72% for summer and in the range of 24.93-59.50% for winter, whilst in Model 2 depicted substantial reduction the total daily solar heat gain in the range of 15.11-51.95% for summer and in the range of 13.08-31.97% for winter, respectively. C. Environment comfort parameter The occupants thermal comfort can be affected by the radiant properties of the surrounding, so a satisfactory temperature for achieving thermal comfort needs to take account of both air and radiant temperature. Resultant temperature is a combination of indoor air temperature, air velocity and radiant temperature [9]. In order to investigate the effect of internal blind within the atrium thermal environment comfort, predicted internal air and resultant temperatures were used to evaluate and compare their internal thermal performance in relation to occupants thermal comfort. 1. Air temperature During the hottest hour of the day, the predicted air temperature within the ground floor atrium reached a peak of 36.12 o C on a summer clear day and at 34.8 o C on a summer overcast day in the without shading configuration (Basic model). Fig. 8 shows that during the summer, the predicted air temperature within the ground floor atrium decreased by the ceiling level shading configuration (Model 1) in the range of 0.3-3.66 o C (average 0.95%) on a clear day and 0.3-2.86 o C (average 0.93%) on a cloudy day. There was a reduction in the underneath curved roof shading configuration (Model 2) in the Fig. 8 Comparison of the predicted air temperature within the main atrium 2. Resultant temperature The atrium space is normally used as a transitional space. For the space to be inhabitable, the resultant temperature within the atrium should be kept at level higher than ASHRAE Standard 55, 1992 for natural ventilation in rage of 23-26 o C [10]. Busch [11] has concluded that thermal comfort for naturally ventilated office space in Bangkok with 31 o C, whilst the adaptive thermal comfort for Islamabad, Pakistan was defined by Humphreys in the range of 20-30 o C [12]. On a summer clear day, the highest resultant temperature at 1500h within the ground floor atrium of the without shading configuration (Basic model) was at 39.17 o C, whilst outdoor temperature was at 35.1 o C. Fig. 9 shows that the internal shading devices did help to improve thermal performance within the atrium by reducing resultant temperatures on occupied level particularly on the ground floor for both representative configurations. For the ceiling level shading configuration (Model 1), the predicted resultant temperatures within the ground floor atrium was reduced by 1.75 o C (4.60%) on a clear day and 0.95 o C (2.62%) on a cloudy day. In additional, the predicted resultant temperatures in the typical corridors was reduced in the range of 1.41-1.75 o C (average 4.13%) on a clear day and 0.63-0.76 o C (average 2.00%) on a cloudy day, respectively. For the underneath curved roof shading configuration (Model 2), the predicted resultant temperatures within the ground floor atrium was reduced by 1.21 o C (3.09%) on a clear day and 0.36 o C (0.98%) on a cloudy day. Moreover, the predicted resultant temperatures in the typical corridors were reduced in the range of 0.34-0.93 o C (average 1.43%) on a clear day and 0.25-0.45 o C (average 0.83%) on a cloudy day, respectively. In 242
winter, the highest predicted resultant temperatures within the ground floor atrium were in the range of 13.67-17.9 o C on both clear and cloudy days. Therefore it was not necessary to shut the internal shading devices. surrounding openings, while some hot air was introduced to the lower atrium below. However in winter, internal air temperatures and resultant temperatures within the atrium building were not so high. The internal solar blinds should not be extended. It is therefore recommended that retractable shading devices should be carried out to provide a better internal thermal comfort. APPENDIX A. building Material Fig.9 Comparison of the predicted resultant temperature within the ground floor atrium During the operating hours from 0900h-2000h in summer the resultant temperatures were higher than the comfort range which should not exceed 31 o C for natural ventilation buildings for both representative models. Solar blinds however, helped to improve thermal comfort within the atrium by reducing the temperature of the internal air and the surrounding indoor surface particularly in occupied levels. B. Internal condition VI. CONCLUSION The prediction results showed that the air temperature stratification within the atrium reduce significantly with the internal shading devices installation for both representative configurations. The ceiling level shading configuration (Model 1) was generally more effective than the underneath curved roof shading configuration (Model 2) in term of providing better internal thermal performance. Not only was the solar heat gain considerable lower by 51.49-69.72% in summer and 24.93-59.50% in winter, the cooling load was also minimized by 6.61-7.24%. The predicted results also indicated the ceiling level shading configuration (Model 1) can improve thermal comfort within the atrium in summer by reducing the air temperature in the range of 0.93-0.95% and the resultant temperature in the range of 2.00-4.13% in occupied levels, respectively. These results indicated that the high surface temperature of the glazing by the ceiling level shading configuration (Model 1) led to the rise of the air temperature in the area below the roof which was blocked by internal blinds and escaped through high-level openings as a result of stack effect. On the other hand, the high surface temperature of the glazing and blinds by the underneath curved roof shading configuration (Model 2) contributed radiant energy increasing the air temperature below. The hot air only some escaped through REFERENCES [1] R. Saxon, Atrium Buildings: Development and Design 2 nd Edition, USA: Architectural Press, 1986. [2] Emporis, http://www.emporis.com/buildings, 2003. [Online]. [Accessed 12 December 2012]. [3] O.Gocer, A. Tavil and E. Ozkan, Simulation Model for Energy Performance and User Comfort Evaluation of Atrium Buildings. in Second National IBPSA-USA Conference, Cambridge, MA, 2006 [4] H. Abdulah, Q. Meng, L. Zhao and F. Wang, Field Study on Indoor Thermal Environment in an Atrium in Tropical Climates, Building and Environment, 44(2), 2009, pp. 431-436. 243
[5] A. H. Abdullah and F. Wang, Design and low energy ventilation solutions for atria in the tropics, Sustainable Cities and Society(2), 2012, pp. 18-28 [6] CIBSE, Environment Design CIBSE Guide A, 7th ed., Page Bros. (Norwick) Ltd., 2006, p. 271. [7] A. Tzempelikos and K. A. Athienitis, The Effect of Shading Design and Control on Building Cooling Demand. International Conference, Passive and Low Energy Cooling for Building Environment, Santorini, Greece, 2005. [8] Y. Pan, Y. Li, Z. Huang and G. Wu, Study on Simulation Methods of Atrium Building Cooling Load in Hot and Humid Regions. Energy and Buildings (42), 2010, pp. 1654-1660. [9] http://www.pebblepad.co.uk/personal/download.aspx?oid=114314 &useroid=0&action=view, [Online]. [Accessed 16 December 2012]. [10] ASHRAE, ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy., ASHRAE, 1992, p. 04. [11] J. Brusch., A Tale of Two Populations Thermal Comfort in Air- Conditioning and Naturally Ventilated Offices in Thailand, Energy and Buildings, 1992, pp. 235-249. [12] J. Nicol, Adaptive Thermal Comfort and Sustainable in the Hot Humid Climates, Energy and Buildings 36(7), 2004, pp. 628-637. 244