Effect Of Urban Geometry On Solar Insolation Threat In Jsnac (pilot Study) 6b334g

Effect of Urban Geometry on Solar Insolation Threat in JSNAC (Pilot Study)

EFFECT OF URBAN GEOMETRY ON SOLAR INSOLATION THREAT IN JSNAC (PILOT STUDY) Jiih Kui Lam, Dilshan Remaz Ossen and Mohd. Hamdan Ahmad Faculty of Built Environment, Universiti Teknologi Malaysia, Skudai, MALAYSIA [email protected] ABSTRACT: Inappropriate orientation and proportioned outdoor plaza or open space generates thermal discomfort in less-winded areas, especially in equatorial tropics. Shadow umbrella theory has been proposed by Emanuel R. (2003) as a hypothetical planning solution to reduce or prevent solar radiation into public outdoors. The decisive factor is to optimize shaded public outdoor spaces using urban build form. The planning layout of Johor State New istrative Centre has similarities with Putrajaya istrative centre layout, which experiences an Urban Heat Island phenomenon. The most common feature is that the ‘office clusters’ planning in both cases are placed around an open plaza. The orientation of the plaza is based on the symmetric planning layout rather than any environmental principles. The ratio between the open plaza and the surrounding office blocks does not match with the previous research findings, i.e. the length to width ratio, and the height of buildings. This paper discusses and evaluates the effectiveness of existing plazas in the new istrative centre in Johor by using data from literature review and through software simulations. It is to investigate different width and length of the plazas, height of the surrounding buildings and different orientations of the plaza. Influence of solar radiation incident on the build form is determined using Ecotect 5.2 computer simulation program. The plaza in office cluster in Johor new istrative is later found to be oversized and inappropriately orientated. The results show that optimal ratio of building cluster geometry is 1: 1.5-2: 0.27-0.5 (W:L:H), which comply to ideal urban proportion theories suggested by researchers of this field. Keywords: shadow umbrella, solar insolation, urban geometry, JSNAC

1.

INTRODUCTION

The equatorial climate has almost unchanging weather patterns throughout a year. Thus, living in the tropical outdoors is relatively pleasant for most of the year. Unlike other climates, daily weather patterns dominate over seasonal weather. It is described that all seasons occur within a single day here. It is pointed out in Correa (1989) study that tropical living is a part indoors and part outdoor activities. Comfortable urban environment is important to attract occupants to public spaces. As such, shading to reduce the thermal stress and perceivable air movement to enhance the cooling effect are necessary in the outdoors. Often the urban planning regulations and design concepts used in tropics are imported from temperate climates and thus did not fully respond to the characteristics of tropical climate. As a consequence, urban areas often become unnecessarily uncomfortable, such as in Putrajaya – a newly and carefully planned federal government istrative centre, has been experiencing a 5oC higher as compared to surrounding rural area. It was projected to reach up to 40°C, a contributing factor to global warming that brings about heavy rain and flash floods. Ahmad Fuad Embi, Drainage and Irrigation Department deputy director-general, in The Star News (March 6, 2007) explained a few contributing factors of the UHI phenomenon. The main reason was the surface characteristics 293

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of our cities, which were heavily built up with concrete buildings, and grounds covered with tar and tiles that absorb and release heat. This study is a continuation of the undergraduate thesis research on projected solar radiation and heat in Johor New istrative Centre or JSNAC (Lam, 2008). JSNAC (Johor State New istrative Centre) has a similar planning concept with Putrajaya, which is the office cluster concept. Office blocks in both designs are located enclosing a plaza. The orientation of the plaza is based on the symmetric planning layout rather than any environmental principles. According to Emmanuel (2005), inappropriate orientation and proportioned outdoor plaza will generate thermal discomfort in less-winded areas, especially in equatorial tropics. The proportion of the plaza will be analyzed and investigated. Below are the objectives of this research:i. To investigate the current proportion of the plaza with literature review. ii. To find out the shading angle and building geometry for the particular location of JSNAC by using the shadow umbrella theory. iii. To find out the effectiveness of the existing plazas, based on solar insolation and shading percentage. 2.

URBAN OUTDOOR COMFORT IN THE TROPICS

2.1

Urbanization and Outdoor Comfort

Today many cities in the region experience rapid urban growth often without much reference to the evolving urban environment. The shape of a city tends to trap radiation near the surface. A lot of energy is stored in the city during the day-time and it is then gradually lost during the night. This slows down the night-time cooling of a city compared to non-urban areas (ESPERE Climate Encyclopaedia, 2006). In view of this common phenomenon of Urban Heat Island happening in most of the major cities around the world, it has put increased demand on the comfort requirements in the design of buildings and outdoor environment (Khandaker, 2003). Studies by Khandaker (2003) pointed out the problems of urban dwellers being inhibited to form any meaningful relationship with their present urban outdoor setting. Lifestyles are increasingly becoming introverted. This is due to the progressive degradation of the physical environment as an effect of urbanization. There are various effects of urbanization on climatic parameters, which can be summarized in table 1. It is a microclimate change which will later influence the global climate. Various studies and analyses were done on thermal effects of geometry and orientation of urban open space in different climate conditions, by different group of researchers. Geometry and thermal properties of urban surfaces had been found to be the two main parameters influencing urban climate (Oke T.R., 1987). Oke (1988) argued the role of the urban in of thermal comfort is to provide urban shelter, achieve warmth, maximize solar access and provide adequate natural daylighting. His study on the aspect ratio for a hypothetical mid-latitude city suggested that the range 0:4 < H/W < 0:6 represented an acceptable compromise in meeting thermal and pollution criteria. Ratio of 0.4 would provide shade and retain reasonable proportion of heat island warmth, while ratio of 0.60 would ensure both atmospheric dispersion and solar access to be maintained within the street canyons. This range was later found to be suitable for all latitudes in climates by Arnfield (1990). 294

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Table 1: Effect of urbanization on climatic parameters Climatic parameter

Effect of urbanization

Temperature (Khemani et al., 1973) Humidity (Chandler, 1976) Precipitation (Landsberg, 1981) Wind (Oke, 1987)

Rise in daily minimum temperature; changes in maximum temperature. Daytime humidity reduces, night-time value increases. The increase is attributed more to air pollution than heat emission. Increases in the number of calm periods. Up to 20 percent reduction in wind speeds are known. Though incoming radiation values are not changed, the apparent values are high due to the containment of reflected radiation by the heat dome.

Solar Radiation (Emmanuel, 1997)

Emmanuel (1993) pointed out that the urban design goals are radiation reduction during the day and ventilative cooling at night. He was based on the characteristics of Urban Heat Island in equatorial areas, where the daytime heat in urban is not transferred fast enough due to surface properties and urban geometry. Heat was stored up especially by high thermal capacity materials and was found to be hard to be released at night because of narrow urban canyon. Therefore, Emmanuel (1993) developed a hypothetical concept of shadow umbrella using urban geometry for radiation reduction in the tropical outdoors during the day. The issue that matters most in of urban thermal comfort and urban geometry is the “view factor” (the proportion of the total spherical field of view from a subject taken up by surfaces). The next important determinant is the temperature difference between the surfaces and the human being in the urban system. If surfaces are warm, the human body receives heat. If urban surfaces can be kept “cool”, people ing by will lose heat to them, and will therefore be comfortable. In keeping with these principles, the following steps are taken to establish an urban massing in a neighbourhood that will keep its surfaces ‘cool’ by creating shade. The shadow umbrella theory is also a preventive measure by offering an urban planning concept. L. Shashua-Bar (2000) carried out a study on geometry and orientation aspects in ive cooling of the canyon street climate with trees. He presented a calculation model of ive cooling and its characteristics and later indicated that the maximum rise in air temperature on a typical summer day in the Mediterranean coastal region had been contributed mainly by the direct solar radiation. The problem was governed mainly by the street orientation and geometry as measured by the aspect ratio of buildings height to street width courtyards. To reconcile in these aspects, he found out that the main strategy was through shading. He also highlighted the importance of openness of the cluster to the sky, given by the SVF factor which in turn depends on the street aspect ratio. The higher the latter, the smaller the SVF, the weaker the effect of the outgoing long wave radiation which would reduce the extent of air cooling. Khandaker (2003) defined the comfort for outdoors in wet tropics through his study. His field observation in various spatial types in the urban suggested the characteristics of shaded needed. During the early hours of the day, when air temperature is below the daily mean, open spaces or spaces with partial shading are desirable. From the mid hours of the day, when the temperature progressively rises above the mean to the late hours of the day when air temperature is maximum, spaces with complete shading are required. During late hours of the day (after 1700 hr), open fields

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or spaces with no enclosures are desirable. Shading in urban spaces by any means is of utmost priority, particularly between 1200 hr and 1500 hr, and preferably between 1030 hr and 1530 hr. Ahmed (2005) in his studies evaluated the different forms’ proportions and physical parameters on the capability to provide reasonable amounts of shadow in summer and sunlit area in winter in different climate regions. The optimum courtyard ratio was defined as that which allows the form to receive maximum release of radiation at night and minimum in day. For the tropics, the optimum courtyard height to obtain a reasonable performance was found to be three-storey, where it is 3 meter in every floor height. He suggested the width and length as well and the final aspect ratio he proposed for the tropics is W:L:H of 1 : 2 : 0.8. The maximum shaded area is obtained when the form is elongated along the north–south axis. Yezioro (2006) evaluated the ratio between the insolated area and the total examined area. In his findings, the design parameters that determined the insolation of the urban square were the height of the buildings around it, its orientation and proportions. He suggested a ratio of 1 : 1.5-2 : 0.6 to be ideal for a rectangular urban squares. These squares would be best to be elongated along the N–S direction. Second best were rectangular squares elongated along the NW–SE and NE–SW directions. O.D. Corbella’s study (2008) explained the consequences of both oversized and undersized plaza. For a narrow or undersized plaza, it would cause obstruction of natural winds to go through the urban area, trap heat either emitted by vehicles or heat island phenomenon and amplify noise and air pollution. An oversized plaza would cause heat and skin cancer due to overexposure of surfaces. His study also pointed out that the fundamental issue in the tropics was the reduction of solar radiation and surface temperatures, in order to decrease the infrared radiation liquid flow and the air temperature. Table 2 summarizes the key findings from various researches on ideal orientation and proportion of plaza. Table 2: Key findings on ideal plaza in tropical climate Research

Climate

Findings

Oke (1988)

Temperate (mid-latitude)

Emmanuel (1993)

Tropical

H:W ratio of 0.4 to 0.6, acceptable compromise range in meeting thermal and pollution criteria in urban. Hypothetical ‘shadow umbrella’ theory to create urban geometry based on sun position. Highlighted the importance of urban geometry as shading and openness of the cluster to the sky (the Sky View Factor) to strengthen the outgoing of long-wave radiation. During early morning, open spaces or spaces with partial shading are desirable. In noontime, complete shading are required. During late afternoon, open fields or spaces with no enclosures are desirable. (W:L:H of 1 : 2 : 0.8). Best orientation is N-S. (W:L:H of 1 : 1.5-2 : 0.6). Ideal orientation is N-S Effects of both oversized and undersized plaza: undersized plaza – obstruct natural winds, trap heat and pollution; oversized plaza – cause heat and skin cancer.

L. Shashua-Bar (2000) Temperate Khandaker (2003)

Tropical

Ahmed (2005) Yezioro (2006) Corbella (2008)

Tropical Temperate -

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2.2

Johor State New istrative Centre

Figure 1: Overall plan of JSNAC and location of 2 study areas – 1 (C2S), 2 (C3N)

JSNAC, the most recently planned and built government istrative centre is located at 1.4oN, 103.7oE. The office planning approach found in Putrajaya and JSNAC is popularly termed as “office cluster”. However, the term “office cluster” is not yet a formal and general label for such approach. But the two main components which can be identified in this planning approach are the central plaza and the office blocks. The overall concept plan will have a vast focal open space Dataran Mahkota. At both ends, stand the landmark buildings of Dewan Undangan Negeri and Menteri Besar’s complex, which is along the Qiblat Axis. There are 6 oval office clusters on the periphery in axis towards the Dataran Mahkota. Two office clusters were chosen as study areas, because of the different orientations. Bearings of each plaza were discussed in the following table. Table 3: Orientation of plazas of the study areas Study Area 1 2

Cluster

Orientation

C2S C3N

22o26’ 72o59’

Slightly NE-SW Almost E-W

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Figure 2: Site plan of the JSNAC office cluster

The above figure shows the C2S overall plan. The planning of office clusters are the same with only difference in the land shape and area. The office cluster is oval shaped and symmetrical. The office blocks are curvilinear in shape which consist of 3 office blocks at one side, with the centre flexible unit higher than the two front blocks. The central plaza is 125 m in length and 47 m in width.

Figure 3: Hourly solar insolation of Singapore on 21 June Source: Ecotect

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Effect of Urban Geometry on Solar Insolation Threat in JSNAC (Pilot Study)

The data which will be used as a reference and for simulation purpose is Singapore, due to the location of JSNAC which is near to this neighbouring country. Recorded solar insolation of 21 June from 0800 hr to 1700 hr is 4519.85 W/m2. The hourly solar insolation indicates that minimum temperature occur before 0700 hours and rapidly rise within the next three hours. By 1100 to 1200 hours, radiation gain reaches near maximum level and stays high until 1300 hr. It declines rapidly, starting at around 1400 hours. 3.

METHODOLOGY

3.1

The Plaza Investigation

Figure 4: Cross section of office cluster, JSNAC and the canyon geometry

Above figure shows the urban design guideline of the office cluster in JSNAC from the planner. The maximum allowable height is 37.8 m for the flexible unit and 31.8 m for the front blocks. In this study, height is considered fixed at the maximum allowable value. The changing parameters therefore are the width and length for this case. The width between the two office blocks is 98 m. The investigation work will be done by referring to the suggested aspect ratio from table 2, where simple calculation of the W: L: H will give a fast and early idea whether the plaza is in a good proportion. Shadow Umbrella The decisive factor of shadow umbrella theory (Emmanuel, 1993) is to optimize shaded public outdoor spaces using urban build form. The ratio between the building height (H) and width (W) and length (L) of the open space is important and will be determined through this theory. Shadow umbrella concept largely depends on the solar geometry – the solar altitude and solar azimuth. This implies that it depends on some basic elements of the specific site: date of year, time of day, location, building orientation and dimensions. The purpose is to shade a particular surface at all times, thus the lowest angles must first be established. The northernmost solar exposure occurs during the summer solstice (June 21). The southernmost exposure will be on the winter solstice (December 21). These two days will determine the northern and southern extremities of sun positions. From the data in Figure 3, the eastern and western extremities will be determined by cut-off times on each of these days. In tropics, the minimum temperature occurs before 0700 hours and rapidly rise within the next three hours. By 1100 hours, temperature reaches near maximum level 299

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Figure 5: Shadow umbrella urban block

and stays that way until it declines rapidly at around 1400 hours. Shading is needed during this period of time which we called ‘cut-off hours’ – 0800 hours to 1700 hours, determining eastern and western extremity respectively. The sun position of the summer solstice eastern extremity (0800 hr, 21 June), summer solstice western extremity (1700 hr, 21 June), winter solstice eastern extremity (0800 hr, 21 December) and winter solstice western extremity (1700 hr, 21 December) can be get from the sun path diagram. Figure 5 shows the ideal shadeable urban block developed from shadow umbrella theory for JSNAC (1.4 oN, 103.7 oE). It turns out that the block will be elongated along north/south, and have a proportion of roughly 1:2 (east/west : north/south). The height in the east will be 0.27 in ratio and 0.50 in the west. 3.2

Software Simulation Analyses

The software simulation analyses were carried out by using Ecotect software. Ecotect was a complete building design and environmental analysis tool that covers the full range of simulation and analysis functions required to truly understand how a building design will operate and perform. This included shading, solar, lighting, thermal, acoustics and so on. The definition of ‘insolation’ from Ecotect referred to Incident Solar Radiation and represented the amount of radiation incident on a point or surface over a specified period. First overshadowing masks were generated at each point due to surrounding buildings and objects, then hourly diffuse and direct radiation data was read directly from the climate data over a -set period. Assessment of solar insolation and percentage of shading were done on both plazas of the study clusters to understand the average hourly solar insolation received on the plaza between the cut-off hour of 0800 hr to 1700 hr.

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The studies were conducted on one day, representing the summer solstice which is 21st of June, as on 21st of June, the sun is at its northermost. It represented the lowest value of solar insolation (besides winter solstice on 21 December) of the year a tropical surface may experience. Singapore’s weather data was inputted as data source because of the location of the site which was near to Singapore. Simulations were conducted on the real dimension and condition on site – the width, length, and height (maximum allowable height of 31.8 m, 37.8 m), and orientation which according to planned footprint of the office blocks and centre plaza. Lastly, simulations were conducted on a rectified proportion of the plaza (which is complying to the aspect ratio theory) to get the performance of the plaza. 4.

ANALYSES & RESULTS

4.1

Plaza Proportion

Figure 6: Evaluation of the aspect ratio of JSNAC

Figure 6 shows the calculation of geometry in one of the office cluster, JSNAC. It uses the maximum allowed height given in the planning guideline and is divided by the width of the plaza. It is found to be slightly outside the suggested ideal figure by Oke (0.4 to 0.6). The plaza is slightly too wide exposed and in other words the geometry of the buildings are not able to generate desired shading to the space. 4.2

Solar Insolation

Located in the tropical area, the site is subjected to hot and humid climate, which solar radiation and heat as major climate threat. The use of green or nature as centre plaza in office cluster design will provide a comfortable public space and enhance working environment to the office. However, from the software simulations and analyses, a few problems or design errors are found. Figure 7 shows the average solar insolation value of the plaza (area of analysis). There is no much difference even though the value of E-W orientation is higher. This graph suggests that the plaza is oversized. Heat will build up quickly at the early of the cut-off hour and goes down rapidly in the afternoon. It reaches the peak at the hour between 1200 hr and 1300 hr. The afternoon solar radiation is higher than morning.

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Figure 7: Average solar insolation (W/m2) of plazas in C2S and C3N on 21 June

Figure 8: Solar insolation (W/m2) in the center of plazas in C2S and C3N on 21 June

Figure 8 gives the spot reading taken in the centre of the plaza. The values are almost the same for both orientations. This graph suggests that no much shade available in the centre of the plaza throughout the day. The plaza is too large to be shade by the geometry of the building. There is a significant decrease in solar insolation reading (Figure 9) for the rectified porportion of C2S. It is found that the rectified plaza achieved a performance of 10.22% and 13.15% lower than C2S and C3N respectively in the solar insolation analysis. This suggests that the proportion of the plaza, i.e. aspect ratio is effective in reducing the solar insolation. Comparing to Figure 8, it is predicted that the performance of shade by using parameter of orientation will only come into effect when the porportion is ideal or shade is adequate in other words.

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Figure 9: Rectified proportion of C2S according to Oke’s theory and its solar insolation (W/m2) performance

4.3

Shade Percentage

Figure 10 proved the plaza with NE-SW orientation (C2S) is better shaded, especially in the early of the day and in the late afternoon when the building blocks create shade for morning and afternoon sun. Up to 50% more shade during these hours of time in comparing to C3N. In average, C2S generates 27% more shade than C3N. Percentage of shade declines in the noon time. Although the percentage of shade in the afternoon is higher, the solar insolation is also higher in the afternoon (Figure 8), which means the average shading percentage and average solar insolation are not relative. This is because different areas are shaded or exposed to different level of solar radiation. West sun

Figure 10: Percentage of shading in the plazas in C2S and C3N on 21 June

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Figure 11: Rectified proportion of C2S and its shading (percentage) performance

is severe in the tropics and is important to be avoided. The graph of early morning and late afternoon is in same pattern with the 2 study plazas, and both meet in the noon time where the plaza is oversized and shade is inadequate. The rectified proportion of C2S achieves 67% and 112.26% more shade than C2S and C3N. The rectified plaza shows significant shade even when approaching and during noon time. 4.4

Model from shadow umbrella

Figure 12: Model showing ideal proportion of urban built form for JSNAC

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Effect of Urban Geometry on Solar Insolation Threat in JSNAC (Pilot Study)

The above ideal massing for C2S and its plaza is developed from the shadow umbrella diagram in Figure 5. The plaza is elongated along north/south, and have a proportion of roughly 1:2 (width : length). The height in the east will be 0.27 in ratio and 0.50 in the west. The optimum courtyard ratio is defined as that which allows the form to receive minimum radiation in the day and maximum radiation to be released at night. In short, the ideal proportion of urban geometry for office clusters in JSNAC is 1: 1.5-2 : 0.27-0.5. 5.

SUMMARY AND FINDINGS

From the analysis, it can be concluded that JSNAC experiences following problems;a. Oversized plaza – Low height to width ratio. This may lead to overexposure to heat and difficult in creating shade both to the plaza and building itself. On the other hand, the maximum allowable height of the buildings proposed by planners may have been too low. b. The centre of the plaza is inadequate in shading. The plaza is too large to be shade by the geometry of the building. c. There is no much difference in the value of C2S (NE-SW) and C3N (E-W) orientation. It is found that shade will only perform during noon time when the proportion of the plaza is ideal. d. Although the percentage of shade in the afternoon is higher, the solar insolation is also higher in the afternoon. West sun is severe in the tropics and is important to be avoided. The ideal proportion of urban geometry for office clusters in JSNAC is 1: 1.5-2 : 0.27-0.5 (W:L:H). 6.

CONCLUSION

This study is generally to make the equatorial urban outdoors thermally comfortable. Although life in the equatorial region is largely an outdoor phenomenon, modern urban development which is lack of environmental concern during planning, has failed to facilitate such climatically pleasant outdoor environment. Solar radiation and heat are found to be the major problem. Various studies unanimously suggested shading to be major concern in tropical urban to bring comfort to the spaces. The planning of the office clusters in JSNAC, which incorporated a centre plaza, has the potential to be a cool and comfortable outdoor urban environment. However, through the plaza investigation and software analyses, the orientation, building geometry (height to width ratio of the plaza), proportion and size of the plaza are found to be inappropriate, too wide and overexposed to the solar radiation. This rises a question of ‘will JSNAC become the next heated Putrajaya?’ It is always better to prevent than to cure. In view of these problems, measure should be taken during the planning stage before the construction to solve the problems. Solution is developed by a shadow umbrella model of the specific JSNAC site, with ideal building geometries and proportion, to improve the existing design and to prevent the plaza from becoming a ‘planned wasteland’ or ‘urban desert’ because of heat. The results show that optimal ratio of building cluster geometry is 1: 1.5-2: 0.27-0.5 (W:L:H).

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