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Impact of facade design on indoor air temperature and thermal comfort in residential buildings

Shanshan TongDepartment of Building, School of Design and Environment, National University of Singapore 4 Architecture Drive, Singapore [email protected]

Nyuk Hien WongDepartment of Building, School of Design and Environment, National University of Singapore 4 Architecture Drive, Singapore [email protected]

Erna TanDepartment of Building, School of Design and Environment, National University of Singapore 4 Architecture Drive, Singapore [email protected]

Jianxiu WenDepartment of Building, School of Design and Environment, National University of Singapore 4 Architecture Drive, Singapore [email protected]

Abstract: This study aims to investigate the impact of building façade design on indoor air temperature and thermal comfort in naturally ventilated residential buildings in tropical climate. Firstly, field experiment was carried out in residential buildings in Singapore. In each unit, indoor air temperature, relative humidity and wind speed were measured continuously for one week. The impacts of window-to-wall ratio (WWR) and orientation on the hourly indoor air temperature were analysed. It was observed that the north-facing room could be 3oC hotter than the east-facing room on sunny afternoon in July. Secondly, the thermal comfort inside the measured units were assessed using the Predicted Mean Vote (PMV) model developed for tropical climate. The impact of façade orientation and WWR on indoor thermal comfort was analysed. Thirdly, computer simulation using Energy plus was conducted to investigate the impact of WWR, orientation and length of shading device on indoor air temperature. Based on obtained results, facade design recommendations were provided for better indoor comfort in tropical climate.

Keywords: Thermal comfort; façade design; tropical climate; naturally ventilated building.

1. INTRODUCTION

In the urbanized tropical country of Singapore, people tend to spend substantial of time in buildings. Therefore, the indoor air quality and comfort has been a major concern for public health and quality living. Air-conditioning is extensively used to achieve indoor thermal comfort in Singapore due to its hot and humid climate. In residential buildings, air-conditioner was found to be the largest electricity end-user among various household appliances, accounting for 35-45% of total electricity bill in all the public and private housing types, except for 1-room and 2-room HDB apartments (Xu and Ang, 2014).

However, utilizing natural wind and fans for indoor cooling and ventilation is a much more sustainable strategy, compared with air-conditioning. Moreover, natural ventilation and fan ventilated space is still preferred by some residents, particularly for senior citizens, as it reduces the occurrences of sick building syndrome (e.g. headaches; eye, nose, or throat irritation; dizziness and nausea), which is frequently linked to flaws in heating, ventilation and air conditioning systems.

A lot of research efforts have been spent on the passive design of building envelope to achieve indoor thermal comfort and energy efficiency in residential buildings in Singapore. Field experiments were carried out to investigate the potential benefits of different passive building envelope designs, such as rooftop garden (Wong et al., 2007), solar-reflective roof (Tong et al., 2014) and secondary roof (Tong and Li, 2014). The impacts of floor level, façade orientation and shading device on the thermal performance of building envelop and indoor thermal comfort were also studied (Wong and Li, 2007). In addition, simulation studies were conducted to analyse the impact of other parameters on the thermal performance of building facade, such as thermal insulation, induced natural ventilation flow, window-to-wall ratio (WWR) and shading devices (Wang et al., 2007).

P. Rajagopalan and M.M Andamon (eds.), Engaging Architectural Science: Meeting the Challenges of Higher Density: 52nd International Conference of the Architectural Science Association 2018, pp. 61–69. ©2018, The Architectural Science Association and RMIT University, Australia.

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Currently, more than 80% of population in Singapore live in the public housing flats developed by the Housing Development Board (HDB). The rest live in the flats developed by the private developers such as condominium and landed flats. In this study, field experiment was carried out to investigate the thermal performance of HDB flats and condominium units. The impacts of WWR and orientation on indoor air temperature and thermal comfort were investigated. Furthermore, simulation was performed to study the impacts of WWR, shading device and orientation on the indoor air temperature using validated model. Finally, façade design guidelines were proposed based on measurement and simulation results.

2. FIELD MEASUREMENT

2.1 Experiment setup

Field experiment was carried out in two residential units to measure the thermal performance of façade in public and private housing in Singapore.

In phase 1, a residential unit located on the 8th floor of a 12-story HDB block was selected. The HDB block was built in 1990’s, with the measrued façades facing east in the living room and facing north in the bedroom respectively, as shown in Figure 1 (a). The WWRs of two façades in living room and bedroom were 0.5 and 0.45 respectively, as shown in Figure 1 (b) and (c).

(a) (b) (c)

Figure 1: (a) Floor plan of HDB unit, (b) east-facing living room and (c) north-facing bedroom.

Air temperature, relative humidity and wind speed at 1.1 m high and 30-cm distance from the interior surface of facade were measured. Data was collected at 1-minute interval from 12 July to 20 July in 2017.

In phase 2, a unit located on the 6th floor of a 7-story condominum block was selected. As shown in Figure 2, the façades in living room and bedroom were facing west, with the WWRs being 0.8 and 0.5 respectively. Similarily, air temperature, relative humidity and wind speed at 1.1 m high and 30-cm distance from the interior surface of facade were measured from 23 September to 1 October in 2017.

(a) (b) (c)

Figure 2: (a) Floor plan of west-facing condominium unit, (b) living room and (c) bedroom.

2.2 Experimental results and analysis

The façade performance of the measured facades was analyzed. Firstly, the hourly air temperature near different facades on the hottest sunny day during the measuremen period were analyzed and compared. Secondly, air temperature and indoor thermal comfort during the measured period of 1 week were analyzed and compared.

The hourly outdoor solar radiation and air temperature collected at the meterological station near the HDB and condominium units on the selected days were presented, as shown in Figure 3 (a) and (b) repectively.

S. Tong, N.H. Wong, E. Tan and J. Wen

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(a) (b)

Figure 3: Outdoor weather conditions on selected sunny days: (a) 19 Jul and (b) Sep 25.

2.2.1 Impact of orientation

The two measured facades in the selected HDB unit has similar WWRs (0.5 and 0.45) and different orientations (east and north). The selected sunny day was 19 Jul, 2017, with the peak air temperature of 31.1°C and maximum solar radiation of 851 W/m2 at 13:00. Based on the collected results, the impact of orientaion on indoor air tempereature and thermal comfort was analyzed.

As shown in Figure 4, air temperature at 1.1 m near the north-facing window was much higher than that near the east-facing window in the afternoon, with an average temperature difference of 2.4°C between 13:00 and 18:00 and the maximum temperature difference of 3.1°C at 17:00. The daily maximum air temperature was 34.1°C at 17:00 in north-facing bedroom and 32.1°C at 11:00 in east-facing living room. The daily minimum temperatures were very close at the two locations, being 27.7°C in living room and 28.2°C in bedroom at 08:00 respectively. On average, the north-facing living room is 0.9°C hotter than the west-facing living room.

Figure 4: Comparison of hourly air temperature near the north-facing and east-facing façade.

The external and internal surface temperatures near the two measured facades were also compared, as shown in Figure 5 (a) and (b). The surface temperature of both walls were very close from 1:00 to 8:00. The peak temperature of east-facing wall appeared 4-5 hours earlier than the north-facing wall.

For the external surface, the daily maximum temperature reached 36.0°C at 16:00 in the north-facing wall, and 34.9°C at 12:00 in the east-facing walls. The daily minimum temperatures appeared at 08:00, which were 27.8°C in north-facing wall and 27.5°C in east-facing wall respectively. For the internal surface, the daily maximum temperatures reached 34.0°C at 17:00 in the north-facing wall and 32.8°C at 14:00 in the east-facing walls respectively. The daily minimum temperatures were very close, which were 28.1°C at 09:00 in the north-facing wall and 28.0°C at 9:00 in the east-facing wall. It is observed that, heat was transferred from external to internal wall surface from 9:00 to 18:00, and the situation was reversed during the rest of the day.


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(a) (b)

Figure 5: (a) External and (b) internal surface temperatures of the north-facing and east-facing wall.

The thermal comfort inside the unit was predicted using the equation below (BCA, 2016):

11.8753 0.4232 0.5789PMV T V= − + − (1)

Where:

PMV = Predicted Mean Vote; T = indoor air temperature (°C), and V = indoor wind speed (m/s). This equation was adopted to predict the thermal comfort in naturally ventialted units in Singapore. The PMV index predict the mean response of a large group of people according the ASHRAE thermal scale (ASHRAE, 2004), as listed in Table 1. The indoor environment is assumed to be comfort while PMV falls between -0.5 and +0.5 (BCA, 2016).

Table 1. PMV sensation scale

Sensation Cold Slightly Cool

Neutral Slightly Warm

Warm Hot

Value -3 -1 0 1 2 3

It is observed in Figure 6 that the measured air temperature in north-facaing living room was evidently higher than that in east-facing bedroom. The maximum temperature difference reached 4.7°C on the sunny afternoon. Moreover, according to the comfort criterior (-0.5<PMV<+0.5), only 58% of the time was comfortable in the north-facing bedroom, and 72% of the time was comfortable in the east-facing living room. The north-facing room shows a rather low percentage of thermal comfort in July.

Figure 6: Continuously measured air temperature and PMV in the HDB unit.


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2.2.2 Impact of WWR

The impact of WWR on the thermal performanc of façade was analyzed based on measured results in the west-facing condominium unit. Air temperatures near the facade in living room and bedroom with their respective WWRs of 0.8 and 0.5 were compared. As shown in Figure 7, air temperature in the living room was continously higher than that in bedroom, with the an average difference of 0.6°C. The daily maximum temperature appeared at 14:00, which were 33.4°C in the living room and 32.2°C in the bedroom respectively. The daily minimum temperatures appeared at 07:00, being 29.3°C in the living room and 29.0°C in the bedroom respectively.

Figure 7: Comaprison of air temperatures near windows in living room and bedroom.

The air temperature and calculated PMV in two rooms of the condonimum were presented in Figure 8. The temperature difference between the living room and bedroom reached 7.2oC on sunny days. According to the comfort criterior (-0.5<PMV<+0.5), 49% of the time was comfortable in the living room with WWR of 0.8, and 63% of the time was comfortable in the bedroom with WWR of 0.5.

Figure 8: Continouse air temperature and PMV in Condo unit.

3. COMPUTER SIMULATION

Computer simulation study was conducted to predict the impacts of WWR, orientation and shading device on the indoor air temperature near the façade in the tropical climate of Singapore. Computer simulation was performed using EnergyPlus software, and the computer model was validated by the experimental results.

3.1 Model validation

The experimental data collected in a previous study was used to validate the model. As shown in Figure 9, two west-facing facades located on the same floor with different WWRs were modelled. All the modelled units were naturally ventilated with the air-conditioning system off. In addition, the dimension of this block was measured and the material information was collected to build up the geometric and physical model. Weather data collected at a nearby weather station was used as background weather input, including solar radiation, precipitation, wind speed, air temperature and precipitation.


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(a) (b) (c)

Figure 9: (a) Simulated block and adjacent buildings, (b) and (c) Two selected units for validation.

The exterior wall of simulated block has a U-value of 3.21 W/m2K and it consists of three layers: 5-mm cement plaster, 150-mm reinforced concrete and 15-mm cement plaster. The interior wall was made of 100-mm hollow concrete. The window was made of 12-mm clear glass. The comparison between the measured and simulated air temperatures near the two facades were presented in Figure 10. Two indicative parameters were used to evaluate the accuracy of simulation. One is the mean bias error (MBE), and another one is the cumulative variation of root mean square error (CVRMSE), which are calculated by

(2)

(3)

where Mi and Si are the hourly measured and simulated results respectively. The MBEs in all the predictions were between 2% and 4%, and all the CVRMBEs were less than 7%. According ASHRAE criteria (ASHRAE, 2002), the model is validated if MBE is less than 10% and CVRMSE is below 30%. Satisfactory agreements were obtained between the measured and simulated results.

Figure 10: Comparison between the simulated and measured air temperatures.

3.2 Parametric study

After validation, the geometry model of HDB block and its corresponding material properties were used as a base model in parametric study. The orientation, WWR and shading device of the base model were varied to analyse their impacts on the thermal performance of building façade.

3.2.1 Impact of WWR and orientation

The change in annual air temperature due to the change in WWR was simulated and shown in Figure 11. In general, the indoor air temperatures near the façade increase linearly with the increase in WWR in all the eight orientations. When WWR is 1, the facades with east and west orientation show the highest annual indoor air temperatures of 34.2oC, followed by northeast, southeast, northwest, southwest, north and south orientations. East-facing and west-facing façades are more exposed to solar heat gain compared to the other orientations, and south and north-facing façades have the lowest annual indoor air temperatures.

24

124

1

( )i ii

ii

M SMBE

M=

=

−= ∑

∑224

124

1

(( ) / 24)

/ 24i ii

ii

M SCVRMSE

M=

=

−=∑∑


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Figure 11: Impact of WWR on the annual air temperature.

The impact of orientation on the hourly indoor air temperature during a representative day (Apr 30) (Hong et al., 1999) was investigated as well. It was observed in Figure 12 that air temperature increase evidently with the increase in WWR for all the four orientations. When WWR increases from 0 to 1, peak air temperature increases by 12.6oC, 15.0oC, 13.6oC and 10.0oC near the facades facing east, west, north and south respectively. Moreover, there is an obvious time lag for peak air temperature when the WWR decreases. The peak air temperature near the wall with WWR of 1 occurred 3-5 hours later than that near the full height window with WWR of 0.

Figure 12: Impact of WWR on the hourly air temperature on April 30.

3.2.2 Impact of shading device

The impact of horizontal shading over windows with a fixed WWR of 0.4 and different orientations was simulated and analyzed. As shown in Figure 13, air temperatures near east and west orientations show the highest values, followed by the northeast, southeast, northwest, southwest, north and south orientations. When the length of horizontal shading increases from 0 to 1.0 m, the annual temperature reduced by 0.8-1.1oC for the eight orientations.


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Figure 13: Impact of shading length on the annual air temperature.

When the shading length increases from 0 to 1 m, the peak air temperatures near the facades with east, west, north and south orientations reduce by 1.9oC, 2.3oC, 2.3oC and 1.0oC respectively, as shown in Figure 14. The occurring time of peak temperature is not much affected by the length of shading device. In general, the longer overhang provides larger cooling effect and shading from solar radiation, but the cooling effect becomes negligible when the shading length is beyond 0.8 m.

Figure 14: Impact of shading length on hourly air temperature.

4. CONCLUSIONS

In this work, the impacts of WWR, orientation and shading device on indoor air temperature and thermal comfort in naturally ventilated under tropical climate was analyzed. It is observed from field experiments that the air temperature in north-facing room could be 4.7oC than that in the east-facing living room in July. Moreover, the increase in WWR from 0.5 to 0.8 also reduced the duration with thermal comfort from 63% to 49%.

It is found from the parametric study that the daily peak air temperature could be reduced by 12.6oC, 15.0oC, 13.6oC and 10.0oC respectively, when the WWRs of facades facing east, west, north and south orientations reduces from 1 to 0. When the shading length of 1 m is added on the façade with WWR of 0.4, the peak air temperatures near facades with east, west, north and south orientations reduce by 1.9oC, 2.3oC, 2.3oC and 1.0oC respectively.


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ACKNOWLEDGEMENTS

This project is funded by Building and Construction Authority (BCA) - Green Buildings Innovation Cluster (GBIC), with funding from the National Research Foundation (NRF) Singapore (WBS: R-296-000-169-490).

References

ASHRAE (2002) Measurement of energy and demand savings, ed., ASHRAE, Atlanta, G.A.

ASHRAE (2004) Standard 55-2004. Thermal environment conditions for human occupancy. Atlanta, GA: American Society of Heating, Ventilating and Air-Conditioning Engineers.

BCA (2016) BCA Green Mark for Residential Buildings Criteria GM RB: 2016, in G. M. Department (ed.), Singapore.

Hong, T., Chou, S. K. and Bong, T. Y. (1999) A design day for building load and energy estimation, Building and Environment, 34(4), 469-477.

Tong, S. and Li, H. (2014) An efficient model development and experimental study for the heat transfer in naturally ventilated inclined roofs, Building and Environment, 81, 296-308.

Tong, S., Li, H., Zingre, K. T., Wan, M. P., Chang, V. W. C., Wong, S. K., Toh, W. B. T. and Lee, I. Y. L. (2014) Thermal performance of concrete-based roofs in tropical climate, Energy and Buildings, 76, 392-401.

Wang, L., Wong, N. H. and Li, S. (2007) Facade design optimization for naturally ventilated residential buildings in Singapore, Energy and Buildings, 39(8), 954-961.

Wong, N. H. and Li, S. (2007) A study of the effectiveness of passive climate control in naturally ventilated residential buildings in Singapore, Building and Environment, 42(3), 1395-1405.

Wong, N. H., Tan, P. Y. and Chen, Y. (2007) Study of thermal performance of extensive rooftop greenery systems in the tropical climate, Building and Environment, 42(1), 25-54.

Xu, X. Y. and Ang, B. W. (2014) Analysing residential energy consumption using index decomposition analysis, Applied Energy, 113, 342-351.


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