Content. Introduction. Literature review. Background. Aim-Objectives Hypotheses

Transcription

1 Vegetation as a climatic component for the design of urban spaces in hot-dry climate Understanding a case of Gandhinagar Central Business District area

2 Introduction Content Literature review Background Aim-Objectives Hypotheses Methodology Site and climate Defining boundary and constraints Simulation tool Result and Discussion Urban configuration Overall site Road Urban Canyon Surface material Conclusion References

3 Commercial buildings consume 32% of energy in air conditioning. Introduction The increased use of HVAC rises the outdoor anthropogenic heat. Peak urban electric demand rises by 2 4% for each 1K rise in daily maximum temperature above a threshold of k (15-20 c) in the climate of America, leading to the rise in 5-10% of urban peak electric demand. The building envelope is a mediator between various external conditions and static indoor conditions. Changes in the outdoor conditions impacts the indoor temperature. Hence there is need to implement effective measure to minimize effect of outdoor temperature on indoor. A Central Business district; core of the city generally consist of high-rise urban canyons and devoid of vegetation. These canyons provide multiple surfaces for the reflection and absorption of heat, resulting in rise of outdoor temperature due to geometric effects. Urban air temperatures can be lowered by the implementation of vegetation. Vegetation reduces air temperatures both by direct shading of dark surfaces and through evapotranspiration, the conversion of sensible heat to latent heat. These process can create an oasis effect which reduces air temperatures throughout a region Iyer Maithili, LBNL report, India commercial building sector, Lawrence Berkeley National laboratory, California Akbari Hashem, Energy saving potentials and air quality benefits of urban heat island mitigation, Heat island group, Lawrence Berkeley national laboratory, pg-2. Valsson Sheeba, Bharat Alka, Urban Heat Island: Cause for micro-climatic variations, Architecture Time space people, April-2009, pg Kurn Dan et al., The potential of reducing urban air temperature & Energy consumption through vegetative cooling, 1994, Lawrence Berkeley laboratory, California

4 Literature Review The influence of topography or in some cases by vegetation or human action is strongest in lower 2mt. of atmosphere and upper 0.5 to 1mt. of soil. The climate in this zone is known as microclimate. The air temperature inside the site depends on the shading intensity partial shaded area, on the thermal properties of soil, and on the air temperature of its immediate background. During daytime hours the effect of the width of streets on temperature is quite different than at night. In fact, higher urban density reduces the amount of sun reaching the street and increases the amount of mass that absorbs the radiation and its surface area; thus it can effectively lower the urban daytime maximum temperature. Palladio recommended that in hot places the street should be narrow with high houses to achieve comfortable environment at street level. Hein Wong from the simulations by Envi-met found that, when the vegetation is replaced with hard pavement and buildings, the temperature of core area was raised at about 301.5K, & the surroundings had higher temperature of 301.8k 302.2k Yu Chen, The intervention of plants in the conflicts between building & climate- A case study in Singapore, National University of Singapore, Shashua-Bar & Hoffman, Vegetation as a climatic component in the design of an urban street- An empirical model for predicting the cooling effect of urban green areas with trees, Energy and Buildings 31(2000), , pg-222. Kakon Anisha et. al, Simulation of the urban thermal comfort in a high density tropical city: Analysis of the proposed urban construction rules for Dhaka, Bangladesh, BUILD SIMUL (2009) 2: , pg-1 Hien Wong & Yu Chen, The thermal effects of city greens on surroundings under the tropical climate., conference paper, Plea The 21th Conference on Passive and Low Energy Architecture. Eindhoven, The Netherlands, 19 22, September 2004, Page 1 of 6.

5 John Spangenberg compared the surface temperature of the asphalted street for the three cases (no trees, lowdensity canopy and high-density canopy). During the hours when the street receives solar radiation (from 12:00 to 15:00), the trees have a huge impact on the surface temperature of the ground. The less dense tree lowers the temperature by up to 5 C and the high-density tree by up to 12 C. The result indicated that surfaces with different material have various surface temperature but similar air temperature. (Chen 2009) Trees with large leaf area density (LAD), can block the cross wind around buildings if not properly planted. (Chen 2009) Vegetation contributes to the modification of urban climate in primarily providing shading, evapotranspiration and directing wind, either as a windbreak or as a wind funnel (Rosheidat Akram et.al. 2008). The tree s shade also reduces the materials heat absorption and stored energy and thereby reducing the amount of radiation it emits back onto the street pedestrians. Saito conducted a series of field experiments and found that the maximum difference between inside and outside of the small green area was 3 c. It indicated that the green area, regardless of the size, can effectively improve the thermal environment in the city Chen Zhuolun,Krarti Moncef, Zhai Zhiqiang, Meng Qinglin, Zhao Lihua, Sensitive analysis of Landscaping effects on outdoor thermal enviornment in a residential community of hot-humid area in China, The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan Rosheidat Akram, Hoffman Dan, Bryan Harvey, Visualizing Pedestrian comfort using Envi-met, 3 rd national conference, IBPSA USA, July 30- August 1, 2008 Kakon Anisha, Mishima Nobuo, Kojima Shoichi, Simulation of the urban thermal comfort in a high density tropical city: Analysis of the proposed urban construction rules for Dhaka, Bangladesh, Build Sim(2009) 2: Yang Feng & Lau Stephen, Factor urban canopy shading into outdoor thermal environmental assessment, Journal of Habitat engineering 2010, Vol.2, No. 2, 87-94

6 Urbanization and climate Rapid urbanization, often neglecting design issues related to urban climate are likely to increase the levels of discomfort in cities. Background There are 2 main reasons for rise in temperature. First, most urban building materials are impermeable & water tight. Second, dark materials with low reflectivity of buildings & pavements collect & trap more of sun s energy. The city becomes a complex character consisting of different surface materials of low albedo. Introducing vegetation helps to mitigate these effects. The direct benefit is that the temperature around the green area is lower than that within the dense built environment. Buildings, if not shaded properly can become source of increasing heat in indoors through conduction. It is observed that maximum heat is conducted through roofs, walls and ground surfaces. The effects of modifying the urban environment through vegetation is quantified in terms of "direct" and "indirect" contributions. The direct effect of planting trees around a building is to alter the energy balance and cooling requirements of that particular building. The indirect effects are observed, when trees are planted throughout an entire city, producing city-wide changes in climate as they indirectly affect energy use of the single building.

7 Hypotheses Through various studies, it is identified that large urban parks extend positive effect to the surrounding environment. The aim of this study is to understand the influence of different vegetation proportion and urban geometry on urban microclimate. Moreover, an approach is taken to understand the changes on micro-climatic due to the variation in surface material. Such a study will lead to the understanding of role of vegetation on reducing interior cooling load. Aim To understand the role of vegetation to achieve better micro-climate in urban areas. Objective To observe the range of cooling of trees on the surrounding area. To analyze the effect of different ground surface materials on ambient temperature. To compare the variation on micro-climate with changing tree type. To understand the cooling effects on urban area due to variation in density of vegetation. To understand effects on microclimate with respect to aspect ratio.

8 Scope and limitation The study focuses on the understanding of microclimate of Sector 11, Gandhinagar. Here only trees & lawns are taken into consideration for the case of vegetation Simulation is done for summer month In urban areas, due to less availability of space between 2 bldgs., the diameter of crown for particular tree found is between 6-7 mts. In this study two types of tree are studied which are typically found in the area of Gandhinagar. Emissivity & reflectivity of roof for all the urban canyons is kept constant. The study focuses on the result obtained through simulation model. The study does not focus on calculation of anthropogenic heat.

9 Research Methodology Literature Review Area Identification Tree Selection Gandhinagar Central Business District Azardica Indica Peltophorum Hypothetical model Simulation Built up as defined in Byelaws Tree modeling as per native tree ENVI-met beta V 3.1 June 18, 2010 Analysis

10 Study area (Gandhinagar) Location: N latitude & E longitude. Hottest month: May Coldest month: January Mean max. temperature: 41 C Relative humidity: 65%. Gandhinagar is a designed city divided into 30 sectors, each sector measuring 1km x 0.75km. Sector 11 is proposed as Central Business District consisting of all major administrative offices Unpublished thesis - Chadha Hardip, The garden city concept its origins, developments and the Indian experience. (CEPT University). Draft development plan, Gandhinagar 2011 AD, existing situation. Draft development plan, Gandhinagar 2011 AD, general development control regulation. Draft development plan, Gandhinagar 2011 AD, development plan proposal.

11 9% 6% 4% Existing Plan Area Analysis Road 9% Built (foot print) 6% green area 4% open 81% 81%

12 Ganhinagar (Sector 11) Neem Tree Plot Side Peltophorum Tree Neem Tree Base Case Road Side Peltophorum Tree Neem on Road Combination (Plot + Road) Neem Plot Peltophorum Plot Peltophorum on Road Neem on Road Peltophorum on Road Trees as per the byelaw of 1 tree /100 sq.mt. Buildings as per FAR and byelaws of Gandhinagar

13 ENVI-met is a three-dimensional computer model that analyzes micro-scale thermal interactions within urban environments. The software uses both the calculation of fluid dynamics characteristics, such as air flow and turbulence, as well as the thermodynamic processes taking place at the ground surface, at walls, at roofs and at plants. The input file can be edited into each cell of the model. The resolution of cells can be as large as 10mts. Or as fine as 0.5mts. The program has modules of working areas upto 250 x 250 cells. To minimize boundary effects which may distort the output data, the model uses an area of nesting grids around the core of the model to move the model boundary away from the area of interest. ENVI-met has two basic steps before the simulation is run. First, is editing the input of building area to be tested. Second, is editing configuration file. ENVI-met outputs binary files (.EDI/.EDT) that are visualized in program Leonardo Figure: Basic layout of the fluid dynamics of the ENVI-met model. (Source: Bruse 2007)

14 Base Case

15 Base Case

16 Road Side Plantation

17 Plot Side Plantation

18 Road & Plot Side Plantation

19 Gandhinagar: Latitude Longitude Reference time zone: Name: CET/GMT +05:30 Reference longitude: Configuration data (From weather data) North : (-60) Soil : Loamy Spatial grid: 168 x 141 x 30 Size of grid cell: 6.5 x 6.5 x 2 No. nesting grid: 3 Typical summer day (June) Simulation day 18 June 10 Simulation time 6am to 6:00 pm Simulation hours 12 Save model state each (min.) 60 min. Wind speed in 10m a.g.l. (m/s) 3.55 Wind direction 179 Roughness length (m) 0.1 Initial temperature ( K) K Specific humidity (g/kg) Relative humidity (%) 84%

20 Tree height is measured using Haga altimeter. LAI is a dimensionless value ranging from 0 to 6. LAD calculated for both trees from the equation as shown below. 10 values of LAD are calculated for different Z values. Haga altimeter for measuring tree height Trees selected Height LAI as per their shedding period, & solar radiation attentuation June Sw radiation attentuation Azardica Indica (Neem-Decidious) 12mt LAI-3 80% Peltophorum Pterocarpum (copper pod evergreen) 15mt LAI-5 93% Figure: Fahmy M., Sharples S., Yahiya M., LAI based trees selection for mid latitude urban developments: A microclimatic study in Cario, Egypt, Building & Environment, 42(2), pp , 2010.

21 Result and Discussion Analysis Road Simulation Output Urban Configuration Urban Canyon Surface material Ambient air temperature, Surface temperature Open/ Built spaces Built Geometry Sky View Factor Vegetation proportion Orientation, Time Compute difference Correlate with site parameters

22 Site observation It is identified that on whole site, average cooling of minimum 1.1 C & maximum 1.72 C is achieved with the addition of trees at harsh afternoon 15:00hour. Also from the analysis of different cases, it is identified that neem pelto performs as a best case in the entire situation.

23 Thermal Graph Thermal Graphs at 15:00 Hours Plot Neem Road Neem Neem neem Base Case Pelto Pelto Plot Pelto Road Pelto Neem Pelto

24 Thermal Graph Through thermal graphs from ENVI-met it is identified that road side plantation is more preferable compared to plot side plantation for achieving cooling in plot areas for sector-11. Plot side plantation Peltophorum tree at 15:00hours Road side plantation Peltophorum tree at 15:00hours

25 Temperature Temperature Road Analysis NE -SW Roads NW-SE Roads No veg 2 Plot Neem 3 Plot Pelto 4 Road neem 5 Road pelto 6 Neem Neem 7 Neem Pelto 8 Pelto neem 9 Pelto pelto Temperature remains higher at 15:00 hours Case 7, Neem pelto proves as best case for NE-SW roads, whereas road pelto, is effective in minimizing temperature on NW-SE road. minimum of 0.44 C & maximum 2.17 C average ambient air temperature difference is achieved with the addition of vegetation.

26 Spread Graph along road End to street no veg plot neem plot pelto road neem road pelto neem neem neem pelto pelto neem pelto pelto street Parking at 15:00 hrs Graph 3 Case1: Road on NE-SW direction Entrance to street Urban canyon entrance to street Urban canyon node Graph 4 Case 2: Road on NW-SE direction Urban canyon Node Node Entrance to street Node urban canyon Graph 5 Case3: Road on NE-SW direction

27 Road Analysis The roads were analysed for four different cardinal direction at 15:00 hours. Here the difference in temperature is identified with the case without vegetation. Effect on roads is also governed by the street orientation and its geometry. Road with built canyon Road with built canyon configuration have lower temperature compared to open area. With the addition of trees on plot, difference of approximately 0.5 C is achieved on road as seen in graph 4 & graph 5. With the road side plantation, road pelto proves to be best case for NW-SE direction road with a reduction in temperature upto 2 C. In the NE-SW direction with the building aspect ratio of 0.6 (H:W), Combination case for Neem pelto reduces temperature upto 2 C. Node Point Node points are found to be warmer in all the cases due to absence of shade of trees. In NW-SE direction the temperature rises up to 0.5 C more compared to the case without vegetation as seen in Graph 4. Contrast effect In contrast to the cooling effect, trees on roads exchange heat with the surrounding air and constitute a significant source of heat on site as seen in Graph 4. This rise in temperature ranges upto 0.5 C at 15:00 hrs & upto 1 C at 12 hours compared to the case without vegetation. Rise in temperature below tree canopy is found more in case 9. The rise in temperature is normally found when built canyon is next to tree & road in the study area.

28 Road Sectional Graphs Graph 7 Road - parking - building Graph 9 Building - road - open area Graph 8 Building - Road Graph 10 Building - road - building (0.7 H:W ratio) Graph 11 Building - road building (0.23 H:W ratio) Graph 12 Building - road - garden

29 Road Mean Graph SW 9:00 NW NE 15:00 SW NW NE SE SE 12:00 SW NW NE 18:00 SW NW NE no veg plot neem plot pelto road neem road pelto neem neem neem pelto pelto neem pelto pelto SE SE

30 Urban Canyons Graph 14 Road - building - building (1.8 H:W ratio) Graph 15 Building open - building (0.15 H:W ratio) Graph 16 Urban canyons & open space Graph 17 Shop area

31 Temperature C Difference C Temperature C Difference C Temperature C Difference C Surface Material Graph 19 Above road-1, ambient & surface temperature Graph 21 Above loamy soil, ambient & surface temperature Graph 23 Between shady canyons, ambient & surface temperature Graph 20 Difference map above road Graph 22 Difference map above loamy soil Graph 24 Difference map between shady canyons

32 Surface Material loamy soil Neem pelto & pelto pelto reduces average ambient air temperature upto 1.56 C through-out the day at 15:00 hour. Trees on plot reduces ambient air temperature not more than 0.5 C at 12:00 hour. Road plantation on an average reduces ambient air temperature upto 1.6 C at 15:00 hours. At 15:00 surface temperature decreases to 7.25 C & 3.15 C at 18:00 on an average for all cases other than plot trees. Road 1 All combination cases reduce ambient air temperature on an average to 1.89 C, where Neem pelto performs best by reducing 2 C at 15:00 At 12:00, road1 have higher surface temperature ranging from1.65 C (plot neem) to 0.12 C (neem pelto) Between two canyons During afternoon 15:00 hour surface temperature decreases up to 6.65 C for all cases with tree on plot, whereas ambient temperature reduces up to 1.30 C for combination cases.

33 Conclusion Major observation reveal that shading, orientation, wind movement, aspect ratio are the main parameters that affects the sites micro climate. On the terms of design it is also identified that Strategic plantation of trees is important Shading of road is un-avoidable. Shading on road is more effective than shading on plot. The important factor in tree which makes difference is: Tree density(amount of leaf cover) Tree height Height at which canopy starts. Effect of trees for reducing temperature is also found in surrounding area. Effect of trees is found more profound after afternoon 12:00 hour.

34 Conclusion Road Sketch 1 section showing building - road- building (NE-SW) when buildings are at H:W of 0.53, it is ideal to plant peltophorum tree on both side of the road. Temperature reduction of 2.03 C at 15:00 hour can be achieved through it. In the same scenario, reverse effect with increasing the no. of trees. Sketch 2 showing open - road - parking -building Average dense tree on plot & dense tree on road. Ambient air temperature reduction of 0.98 C & 1.86 C at 12:00 & 15:00 hours respectively. Dense tree on both sites is also preferable as it can achieve cooling upto 1.30 C & 2.23 C at 9:00 & 18:00 hours. To achieve more cooling, parking space can be shaded with trees.

35 Conclusion Road Sketch 3 Building - road - road In the case with building on one side of the road, it is preferable to keep average dense tree next to building & shade of dense tree on road. Ambient air temperature reduction upto 1.29 C at 15:00 hour is achieved. In the same scenario if the plantation is done other way, then reverse effect can be found, with increasing air temperature upto 0.72 C at 12:00hour. Sketch 4 Road - building - road In this case when the building are placed at a distance, then it is preferable to plant dense tree on road & on plot, to shade the open areas. Ambient air temperature reduction of 2.78 C can be achieved at 15:00 hours.

36 Conclusion Urban canyon Sketch 5 Building & trees With the change in distance between buildings, plantation type is changing. When buildings are placed at a distance ratio, then dense tree is effective, when buildings are closely spaced, neem tree on plot is effective. Temperature reduction upto 2.5 C is achieved between all the cases between canyons. Sketch 6 Section showing open area & shady space between canyon It is preferable to shade open area with the dense tree, while in the shady space with closely spaced buildings, average dense tree is effective. Temperature reduction upto 2.2 C can be achieved in both the spaces.

37 References Chen Zhuolun,Krarti Moncef, Zhai Zhiqiang, Meng Qinglin, Zhao Lihua, Sensitive analysis of Landscaping effects on outdoor thermal enviornment in a residential community of hot-humid area in China, The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan Devi Suryadevara, Urban heat islands and Environmental impact, Andhra University, Visakhapatnam, India Hien Wong, Yu Chen, The thermal effcts of city greens on surroundings under the tropical climate, PLEA, 21 st conference, Eindhoven, The Netherlands, 19-22, September Jorg Spangenberg, Paula Shinzato, Erik Johansson, Denise Duarte, simulation of the influence of vegetation on microclimate thermal comfort in the city of são paulo, Rev. SBAU, Piracicaba, v.3, n.2, jun. 2008, p J. R. Simpson and E. G. McPherson, Simulation of tree shade impacts on residential energy use for space conditioning in Sacraamento, Atmospheric Environment Vol. 32, No. 1, pp , 1998 Jusuf Steve, Hien Wong, La Win Aung, Thu Htun, Negara To, Xuchao Wu, Study on effect of Greenery in campus area, PLEA, conference paper, Geneva, Switzerland, 6-8 September Jauregui, E., Influence of a large urban park on temperature and convective precipitation in tropical city. Energy and Buildings, 15-16, , Johansson E. Influence of urban geometry on outdoor thermal comfort in a hot dry climate: A study in Fez, Morocco. Building and Environment, 41: , Kakon Anisha, Mishima Nobuo, Kojima Shoichi, Simulation of the urban thermal comfort in a high density tropical city: Analysis of the proposed urban construction rules for Dhaks, Bangladesh, Build Simul (2009) 2: Markogiannakis G., Giannakidis G., Lampropoulou L., Implementation of EPBD in Greece Energy Performance of Buildings, CRES,2010.

38 References Rosheidat Akram, Hoffman Dan, Bryan Harvey, Visualizing Pedestrian comfort using Envi-met, 3 rd national conference, IBPSA USA, July 30- August 1, 2008 Shashua-Bar, L.Hoffman, M.E. Vegetation as a climatic component in the design of an urban street, An empirical method for predicting the cooling effect of urban green areas with trees. Energy and Buildings, 31, , Saito,I. Study of the effect of green areas on thermal enviornment in an urban area. Energy & Buildings, 15-16, , 1990 Valsson Sheeba, Bharat Alka, Urban heat island: cause for microclimate variation, Architecture Time, space and people, April 2009, pg Yang Feng & Lau Stephen, Factor urban canopy shading into outdoor thermal environmental assessment, Journal of Habitat engineering 2010, Vol.2, No. 2, Gartland Lisa, Heat Islands understanding and mitigating heat in urban areas, earthscan, London, Sterling, VA, 2008 Draft development plan, Gandhinagar 2011 AD, existing situation. Draft development plan, Gandhinagar 2011 AD, general development control regulation. Draft development plan, Gandhinagar 2011 AD, development plan proposal. Bruse, M. ENVI-met Beta IV version, Unpublished thesis - Chadha Hardip, The garden city concept its origins, developments and the Indian experience. (CEPT University).