th
The 5 International Conference of the International Forum on Urbanism (IFoU) 2011 National University of Singapore, Department of Architecture Global Visions: Risks and Opportunities for the Urban Planet
INFLUENCE OF BUILT FORM ON THE THERMAL COMFORT OF OUTDOOR URBAN SPACES Lilly Rose* & Ebin Horrison** & Lavanya Jothi Venkatachalam*** *Sathyabama University, Sholinganllur, Chennai, India, Email:
[email protected] ** Sathyabama University, Sholinganllur, Chennai, India, Email:
[email protected] *** Sathyabama University, Sholinganllur, Chennai, India, Email:
[email protected]
ABSTRACT: Urban planners and designers often consider the comfort conditions indoors while neglecting the same in the outdoor urban spaces. In particular, the outdoor comfort conditions in the developing countries are generally unnoticed, and the most affected are the urban poor who spend most of their time outdoors. Therefore, this paper aims at the enhancement of outdoor thermal comfort conditions, through the analysis of urban built form and climate parameters in Chennai Metropolitan Area, India. The study examines the thermal comfort conditions of six urban built forms (dense compact mid-rise urban form and dispersed low-rise urban form) in relation to urban geometry (the density of buildings, height to width ratio), sky view factor (SVF) and green cover (vegetation), using RayMan - a computer model used to model the radiation fluxes in simple and complex urban environments. The comfort conditions of the outdoor urban spaces are analyzed in terms of air temperature and Physiologically Equivalent Temperature (PET). The study revealed that by manipulating the built up density and the street geometry, it is possible to achieve better thermal comfort conditions in outdoor urban spaces. The differential heating that occurred due to the different aspect ratios of the street canyons resulted in different climatic conditions in the six urban built forms. Such climatic differences in urban areas are of value to planners and designers in designing thermally comfortable cities. KEYWORDS: urban built form, aspect ratio, PET, outdoor thermal comfort. NOMENCLATURE
*
PET H W MRT SVF OUT_SET SET*
Physiologically Equivalent Temperature (oC) Height of building (m) Distance between buildings in a street canyon (m) Mean Radiant Temperature (oC) Sky View Factor Outdoor Standard Effective Temperature (oC) Standard Effective Temperature (oC)
1 INTRODUCTION Rapid urbanization has brought many significant changes to mankind, society and the environment it lives in. The environment of an urban area is significantly affected by urbanization resulting in distinct climatic conditions. “Urban climate means the ensemble of values of the various weather elements as they are observed in an urban area” (Lowry and Lowry 2001). Studies on climate change due to urbanization have gained momentum, and have become the main focus of research in the recent past. The inadvertent climatic change of urban areas is due to the human modifications of the surface and the atmospheric properties along with the urban development. The dense urban construction materials stores the heat and waterproofs the surface, the building geometry increases the trapping of radiation and the stagnation of air, the increased air pollution results in the formation of cloud droplets and the heat released by the human activities supplements the natural sources of heat in the urban system (Oke 1987). Chennai, a tropical city characterized by high temperatures and humidities, suffers extensively due to the climatic impact of urbanization and the outdoor thermal comfort conditions are affected significantly. Urban planners and designers often consider the comfort conditions indoors while neglecting the same in the outdoor urban spaces. In developing countries, the outdoor
comfort conditions are unnoticed, and the most affected are the urban poor who spend much of their time outdoors. Also, the houses of the urban poor are not sufficiently adapted to the climate and are sensitive to urban warming (Correa 1989). Therefore, this paper aims at the enhancement of outdoor thermal comfort conditions, through the analysis of urban built form and climate parameters in Chennai Metropolitan Area, India. 2 BACKGROUND LITERATURE Oke (1981) states that the rate of cooling of urban areas at the micro level depends on two parameters of the urban structure: the Height – width ratio (H/W, street geometry) – the ratio of typical height of the buildings to typical width of the neighbouring streets and the sky view factor - the fraction of the sky hemisphere visible from a location at the street level in an infinitely long urban street canyon. Barring et al (1985) found the significant influence of the sky view factor on the surface temperatures. Arnfield (1990) compared the effects of urban geometry and thermal properties of the urban fabric and found that the canyon geometry is the predominant factor that causes variation in nocturnal cooling. The thermal properties of the materials used in the urban fabric enhance the differences in the cooling rates generated by the different urban geometries. Saito et al (1990) found that even small green areas can reduce the urban temperatures by 3oC, when compared to the built up surfaces in the city of Kumamoto, and Akbari et al (1992) identified that large number of trees and urban parks can reduce local air temperatures by 0.5oC to 5.1oC. Ahmed (1994) found that the maximum air temperatures decreased by 4.5K with an increase in the H/W ratio from 0.3 to 2.8, in the hot humid city of Dhaka, Bangladesh, in summer. Unger (1999, 2001) found that there exists a strong relationship between urban thermal excess and land use features and built up density in Szeged, Hungary. Shashua-Bar (2006) indicated that the thermal effects of built form, vegetation and colonnades, in streets and in courtyards depend on the envelope ratio (i.e.,), the overall geometry factor. Human thermal comfort conditions cannot be evaluated with a single climate parameter; instead, it is necessary to include all the thermal components of the environment. The study of the human energy balance is essential in assessing the impact of various climatic parameters on human comfort conditions. The thermal comfort of the human body is affected by the sensible heat, convection, radiation and evaporative heat loss. The heat generated by human metabolism is continuously released to the surrounding environment through sensible heat flow from the skin, evaporation from the skin and respiration, thereby maintaining the thermal equilibrium. Human thermal comfort depends on four environmental parameters and two personal parameters. The environmental parameters include the air temperature, air movement, humidity and radiation, and the personal parameters include clothing and physical activity. However, individual factors such as adaptation or acclimatization, food and drink, age and sex, body shape, subcutaneous fat and the state of health, have also been found to affect the human thermal sensation. The field of research pertaining to outdoor thermal comfort conditions especially urban thermal comfort is relatively new. Matzarakis and Mayer (1998) investigated the thermal component of different urban microclimates in Freiburg, Germany and found that the heat stress levels of the human beings depend mainly on the shading effects and clothing factors. Spagnolo and de Dear (2003) found that the value of the outdoor comfort index (OUT_SET*) was higher than the indoor comfort index (SET*). Also, the larger outdoor comfort zone has been attributed to the existence of large spatial and temporal variations. Ali-Toudert et al (2005) found that the heat stress in unobstructed locations is high, when compared to sheltered urban sites in Beni-Isguen, Algeria. Also, the significant role of building materials in determining the heat stress conditions was identified. Ali-Toudert and Mayer (2006) found the dependence of thermal comfort on the design of the street, including geometry, orientation and other design strategies, such as the galleries and horizontal overhangs. Gulyas et al (2006) examined the outdoor thermal comfort conditions in the complex urban environment of Szeged, Hungary using the RayMan model and found a difference of 15oC to 20oC in the PET index due to the radiation differences in sites that were shaded differently by buildings. Emmanuel and Fernando (2007) insisted that the mitigation strategies adopted by urban designers should be based on human comfort (determined by both MRT and air temperature), rather than on simply attempting to control air temperature alone. Mayer et al (2009) also identified that the shading effect of trees can reduce the MRT and PET values significantly, when compared to air temperatures.
Fewer urban climate studies have been conducted in tropical climates (Arnfield 2003). Also, the studies deal mostly with urban-rural temperature differences and microclimate variations within urban areas (Johansson 2006). Studies relating to the impact of urban climate on human comfort conditions are very few and deal mostly with indoor comfort conditions. Studies conducted by the India Meteorological Department pertain primarily to the variations in climatic parameters, and does not include the impact of urban morphology on comfort trends. The consideration of these parameters will aid in the reduction of the urban heat island effect. This study attempts to contribute to the understanding of the relationship between urban built form, air temperatures and comfort conditions in the hot humid city of Chennai, India. 3 AREA OF STUDY The Chennai Metropolis lies between 12°50’49”N and 13°17’24” N latitude and 79°59’53”E and 80°20’12” E longitude, along the south eastern coast of India, representing the hot humid type of tropical climate. Six different residential neighbourhoods, ranging from dispersed low-rise suburban areas to the densely populated city core, were selected for the study. The urban parameters that were considered in the site selection include the amount of vegetation, percentage of urban built-up in terms of buildings, roads and pavements, and canyon geometry (H/W ratio). The thermal properties of the urban surfaces were similar in all locations. Table 1 shows the characteristics of the selected residential neighbourhoods in Chennai. Table 1
Site
Characteristics of the residential neighbourhoods
Description
Land use
Nature of buildings
Old city core located in the continuous building Mixed Medium rise area (Wall to wall George Town construction) with high Residential (2-3 storeys) density urban built-up and narrow streets High density urban Mixed Medium rise Purasawakkam built-up with few trees Residential (3-4 storeys) and heavy traffic Medium density urban built-up in the south of the Medium rise Besant Nagar Residential city in close proximity to (3-4 storeys) the coast Medium density residential with Medium rise T-Nagar considerable amount of Residential (3-4 storeys) greenery in the center of the city Low-rise residential Low- rise Ambattur neighbourhood in the Residential (2 storeys) southern suburbs Low-rise Institutional Anna zone with dense Institutional Low-rise University vegetation 3
Ground cover (%) SVF H/W Built-up
Roads/ Green paving cover
0.28
1.1
90.4
9.1
0.5
0.19
1.8
84.9
12.7
2.4
0.35
0.7
29.8
11.1
59.1
0.31
0.6
28.4
17.8
53.8
0.28
0.5
26.7
10.3
63.0
0.35
0.3
11.8
22.4
65.8
METHODOLOGY
The air temperature and relative humidity data were measured continuously on an hourly basis using HOBO dataloggers ((HOBO U10 Temp/RH) in the selected neighbourhoods. The characteristics of the urban
morphology of the residential urban environments were estimated based on a grid of 200m × 200m surrounding the respective in-situ measurement location. The microclimate variation between the selected neighbourhoods was analyzed for a hottest day in summer (18th May 2008). The PET index has been chosen for the calculation of outdoor thermal comfort conditions in the micro level study. The RayMan Pro model (Matzarakis et al 2007, 2010) has been used to calculate the Physiologically Equivalent Temperatures. The parameters that define the urban built-up are vast and this micro-level study is confined to the basic determinants – Height to width (H/W) ratio of the street canyon and the percentage of built up area. The 200m X 200m grid of the residential neighbourhoods are shown in Figures 1 (a to f). Figure 2 shows the urban built form of the street canyons, showing the position of the instrument used in the field measurements.
(a)
(d) Figure 1
(b)
(e)
(c)
(f)
200m x 200m grid of the residential neighbourhoods (a) George Town (b) Purasawakkam (c) Besant Nagar (d) T-Nagar (e) Ambattur (f) Anna University
4 RESULTS AND DISCUSSIONS The urban parameters that define the urban built form were analyzed with respect to air temperatures and the calculated thermal comfort. The intra urban air temperature variations at the six residential neighbourhoods are shown in Fig. 3 and the PET variation is shown in Fig.4. “Urban building structures and urban processes modify the atmospheric background conditions with a temperature increase or decrease and can be regarded as a function depending on different factors like the weather, time of the day and year, urban land use, street design and type of building structure” (Mayer et al 2009). Also, the parameters that define the urban built form are vast and this study is confined to the basic determinants – the percentage of built-up area and the H/W ratio of the street canyon. Amongst the six urban sites considered for the study, George Town and Purasawakkam site had similar urban built form but their thermal recordings were different due to the increased vehicular traffic at the Purasawakkam site and are not considered in the analysis.
Figure 2
The urban built form of the street canyons showing the position of the instrument used in the field measurements
43 41
Temperature in oC
39 37 35 33 31 29 27 25 0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23
Time Purasawakkam
George Town
Besant Nagar
Ambattur
T-Nagar
Anna University
Figure 3 The intra urban air temperature variations 4.1
Intra-urban air temperature variations
Between 0:00hrs and 6:00hrs, the intra urban air temperature variation ranged from 1.7oC to 2.7oC, and the maximum variation occurred at 6:00hrs. Between 6:00hrs and 18:00hrs, the presence of intense solar radiation increased the intra urban variations, and it ranged between 3.1oC and 7.0oC. Between 18:00 hrs and 0:00 hrs, when the sky was mostly cloudy, the intra urban variation ranged from 3.1oC to 3.5oC. The re-emission of the absorbed radiation in the evenings was extremely reduced by the overcast sky, resulting in the higher intra urban differences compared to early mornings (0:00 hrs to 6:00hrs). During the clear nights (0:00hrs to 6:00hrs), the lowest temperatures were recorded in the Anna University and Besant Nagar sites, due to the presence of a high degree of vegetation and the increased sky view factor (refer figures 1, 2 and table 1), the exception being the warming at Besant Nagar that started just before 6:00hrs, due to its proximity to the coast in the east and the early sunrise (5:43am). The Purasawakkam and George Town sites were the warmest pockets during nights, attributed to the dense urban built-up and higher H/W ratio. The Purasawakkam and George Town sites experienced the maximum nocturnal heat islands during the minimum temperature epoch (6:00hrs). During the day, the Purasawakkam site experienced maximum temperatures irrespective of its higher H/W ratio, attributed to the increased vehicular traffic. Although, the Purasawakkam and George Town sites with higher H/W ratio are exposed to direct solar radiation only for a short duration (10:00hrs to 13:00hrs), the impact on the air temperatures is high due to the absence of vegetation shading (Refer table 1 and figure 1). The George Town site remained cooler in the afternoons due to the shading of buildings. During the maximum temperature epoch (14:00hrs), the higher H/W ratio in the George Town (35.1oC) site resulted in a cool island when compared to the shallow site (39.8oC) of Ambattur, with an intra urban air temperature difference of about 4.7oC. 4.2 PET variations Between 18:00hrs and 6:00hrs, the intra urban PET variations ranged from 2.1oC to 4.0oC, and between 6:00hrs and 18:00hrs they ranged from 3.3oC to 8.9oC. The PET values were within the upper limit of the comfort zone (33oC) during night time, with Purasawakkam as an exception, where it exceeded the upper limit of the comfort zone, till 23:00hrs in the night. The daytime comfort conditions between 9:00hrs and 17:00 hrs
exceeded the upper limit (33oC) of the comfort zone. The higher PET values during the daytime are attributed to the exposure to solar radiation. The PET values reached a maximum of 53.3oC at around 12 noon but the intra urban variations in comfort conditions during this period were not significant. The intra urban comfort conditions were found to be higher at 10:00hrs (8.9oC) and 14:00hrs (7.3oC). At 10:00hrs, the direct solar radiation and the absence of vegetation shading in the street canyons of Purasawakkam and George Town sites, resulted in higher PET values, attributed to the increase in intra urban variation, but the variation between the other measurement sites was not significant.
54 52
Purasawakkam
50
George Town
48
Besant Nagar
46
Ambattur T- Nagar
44
Anna University
o
PET C
42
upper comfort zone limit
40 38 36 34 32 30 28 26 24 0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Time
Figure 4 PET variations The PET values at 14:00hrs revealed a significant intra urban variation (7.3oC) between the measurement sites. The low-rise profile of the Anna University and Ambattur sites reduced the comfort conditions significantly during daytime, resulting in the higher PET values of 47.5oC and 46.6oC respectively. The dense urban fabric of George Town experienced the lowest PET value (40.2oC), attributed to the higher H/W ratio. The PET values of the Besant Nagar (43.4oC) and T-Nagar (42.6oC) sites were marginally higher, when compared to those of George Town. The PET variations at 6:00hrs are not as significant as those of the day (3.1oC). The Anna University (26.4oC) site experienced the minimum PET values, and the Purasawakkam (29.5oC) and George Town (29.3oC) sites experienced the maximum PET values. 4.3
Influence of the Urban Built Form on the Outdoor Comfort Conditions
Effect of the Percentage of Built-up Area: Figure 5 (a and b) shows the effect of the percentage of built-up area on thermal comfort. During daytime, the increase in the built-up area reduced the PET values resulting in the increase of the day time comfort conditions. During night time, the increase in the built-up area increased the PET values resulting in the reduction of the night time comfort conditions. Also, the analysis revealed that the discomfort caused due to the increased built-up area during nights is extremely significant (R2 = 0.89) when compared to the comfort caused during daytime (R2 = 0.22).
48 47 46
o
PET in C
45 y = -0.0392x + 46.127 2 R = 0.2248
44 43 42 41 40 39 0
10
20
30
40
50
60
70
80
90
100
Built-up area in % George Town
Besant Nagar
T-Nagar
Ambattur
Anna University
Linear (PET)
(a) 30
29 PET in oC
y = 0.0349x + 26.401 2 R = 0.8982
28
27
26 0
10 George Town
20
30
Besant Nagar
40 50 60 Built-up area in %
70
T-Nagar
Anna University
Ambattur
80
90
100
Linear (PET)
(b) Figure 5 The effect of percentage of built-up area on the thermal comfort (a) Daytime (b) Night time
Effect of the H/W ratio: Figure 6 (a and b) shows the effect of the H/W ratio on thermal comfort. During daytime, the increase in the H/W ratio reduced the PET values resulting in the increase of the day time comfort conditions. During night time, the increase in the H/W ratio increased the PET values resulting in the reduction of the night time comfort conditions. The analysis of the street geometry (H/W ratio) also revealed a similar trend as that of the built-up area. The daytime comfort conditions increased significantly (R2 = 0.76) with an increase in the aspect ratio of the street canyons. The increase in the aspect ratio also shows an adverse effect of reducing the night time comfort conditions (R2 = 0.98).
48 47 46
PET in oC
45 44 y = -8.8125x + 49.7
43
2
R = 0.7646
42 41 40 39 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
H/W ratio George Town
Besant Nagar
T-Nagar
Ambattur
Anna University
Linear (PET)
(a) 30
y = 3.6761x + 25.327 R2 = 0.9853
o
PET in C
29
28
27
26 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
H/W ratio George Town
Besant Nagar
T-Nagar
Ambattur
Anna University
Linear (PET)
(b) Figure 6 The effect of H/W ratio on the thermal comfort (a) Daytime (b) Night time 5
CONCLUSIONS
The air temperature and the thermal comfort trends in the residential neighbourhoods of the CMA revealed that the nights were comfortable. During the daytime, all the residential sites were uncomfortably hot with the PET values well above the upper limit of the comfort zone. As the daytime comfort was found to have a significant correlation with the street geometry and percentage of urban built-up, the study indicates the significance of improving the daytime comfort in residential areas, by stipulating appropriate urban built-form in the development regulations of the CMA. The thermal comfort analysis of the residential areas revealed, that the daytime comfort conditions can be improved significantly with an increase in the percentage of the built-up area and the H/W ratio. However, the increased aspect ratio and the urban built-up reduced the night time comfort conditions, and influences the energy demand for cooling during the nights. The H/W ratio influences the comfort conditions significantly when compared to the percentage of urban built-up area. Moreover, the study reveals that with increase in the H/W ratio, the daytime comfort increases and the night time comfort decreases. This indicates the need for arriving an optimum H/W ratio and percentage of built-up area in the CMA, in improving the outdoor thermal comfort.
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