Impact of native evergreen trees on the visual comfort in an office space in Ahmedabad, India

Impact of native evergreen trees on the visual comfort in an office space in Ahmedabad, India Ankit Bhalla [GRIHA Council, TERI] Sanyogita Manu [CARBSE, CEPT University] ABSTRACT This study investigated the impact of native evergreen trees on the daylight availability in office spaces in Ahmedabad, India. An evergreen tree, native to the hot and dry climate of Ahmedabad, was selected and its impact on daylighting in interior spaces is analyzed compared to a no-tree scenario. The distance of the tree from the window was varied to examine parameters such as contrast and brightness at the task plane for the equinox and solstice days. Desktop Radiance 2.0, which is a backward ray tracing daylight simulation software, was used, followed by a calibration study. Uniform and sunny sky conditions based on Ahmedabad climate data were considered. The results indicate that trees can be very effective in achieving visual comfort in conditions of harsh sunshine outdoors. The type of tree is of more importance for visual comfort than the distance between the tree and the window. The evergreen tree performed well to mitigate visual discomfort. Careful selection of the tree type and its positioning on the southern facade reduced illuminance levels but helped improve visual comfort by almost 50%. This study also explains in detail the method used for determination of Leaf Area Index and Leaf Area Density used for calculating the crown density of the tree, which may help future work attempting to study the impact of vegetation on the thermal or visual performance of building envelope. INTRODUCTION Daylight is considered the best source of light for good color rendering. It gives a sense of cheeriness and brightness which is known to have a significant positive impact on people. Therefore, people desire good natural lighting in their living environments (Li DHW et al, 2006). Ahmedabad is located at 23 N latitude and 72 E longitude, in close proximity to the Tropic of Cancer. It, therefore, falls in a region that receives the highest annual rate of solar radiation. In such a harsh climate characterized by high levels of solar radiation and intense sunlight, appropriate design of windows is critical to minimizing direct sunlight by means of shading and providing diffused daylight reflected from the ceiling. Previous studies recommend using systems that can help to redistribute and filter daylight coming from windows and skylights. Shrubs and trees can achieve this in addition to providing other benefits such as pleasing aesthetics, noise reduction, and passive cooling (Khaled & Ahmed, 2012). Another study states that the shade from the trees reduces not only the direct solar heat gain through the building envelope but also helps to diffuse the light reflected from the sky and surrounding surfaces (Lechner, 2002). A recent study indicates that plants and trees can provide solar shading in the same manner as the jalis, chajjas, awnings, louvres while improving the quality of daylight by scattering direct sunlight and moderating glare the bright sky (Khaled & Natheer, 2009). Trees provide summer shade yet Ankit Bhalla is a Project Officer at GRIHA Council, The Energy & Resources Institute, New Delhi. Sanyogita Manu is an Assistant Professor at CEPT University, and a Senior Research Associate at the Centre for Advanced Research in Building Science and Energy (CARBSE ), CEPT University, Ahmedabad, India. 1

allow winter access. The best locations for evergreen trees are on the south and south-west side of the building. When trees drop their leaves in winter the sunlight can reach inside to heat the interiors (Kamal, 2012). The penetration of daylight into a building depends on many factors, including the depth of the room from the window wall, ceiling height, internal reflectance value of the room surfaces, window orientation, shape and size, and the optical properties of the glazing. However, the most significant factor is the availability of daylight outside the building, which can be seriously affected by external obstructions like neighboring buildings or trees (Capeluto, 2003). Some studies such as of done by (Manglani, 2001; Gates, 1979; Reinhart & Jakubiec, 2012; Laband & Sophocleus, 2009; Yates & McKennan, 1988) have examined the effect of trees on the heating and cooling loads in buildings in various climatic types. These include quantitative and qualitative analysis of the effects of tree shading, evapotranspiration and wind control. The external radiative exchanges that took place between one tree and a west wall were studied. The study executed by (Manglani, 2001) proposed a methodology for collecting, analyzing and evaluating relevant data for the study of vegetation shade as a means of attenuating the incident solar radiation. The methodology consisted in collecting the values of solar radiation (incoming and outgoing), the air temperature, surface temperatures of the trees and the wall, both in direct sunlight and shade through field measurements and calculating the long-wave radiation flux. Quantification and measurement of the role of trees in scattering sunlight and providing quality daylight in buildings is an area of research that has not been examined closely in the earlier studies. This paper aims to emphasize on the importance of tree shading and provides a methodology to analyze the effect of tree shading on daylight performance and lighting quality. METHODOLOGY Simulation Model For the study, a room of 20m х15m, with fully glazed window of 3m х 20m on the southern facade and placing a mature evergreen tree (T e ) at 6m, 9m and12m one by one and comparing it with a no-tree scenario (T n ). Maximum distance of the tree from the window wall is calculated such that the highest point of the tree canopy makes a 45 degree angle with the center of the window sill. Diameter of the tree roots determines the minimum distance of the tree from the window wall. The minimum, maximum distances, and a mid-point between the two, are considered as three points for varying the location of the tree vis-à-vis the wall. For glare analysis, a computer screen is considered as the reference point positioned at the center of the room at task level (0.7m from the floor), facing south (the vertical task screen faces the window while the user faces the wall) in one scenario and facing north (the screen faces the wall while the user faces the window) in the other. A calibration study was also done using a 1:10 scale physical model of the office space, and the lux levels were logged on Mar 13, 2013 at hourly intervals from 0900-1800 hours at the 20 grid points shown in Figure 1. It was observed that from 0900 to 1800 hours the average deflection in the measurement ranges from 6-10%, with the Figure 1: Plan of the calibration model measured readings being higher than the simulated values (Table 1). 2

Table 1 Percentage difference between simulated and measured values Sensor Points Time (hours) 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 A1 10% 9% 13% 12% 13% 13% 11% 14% 13% 14% A2 7% 8% 3% 3% 12% 13% 14% 6% 9% 12% A3 12% 3% 3% 8% 0% 1% 1% 4% 1% 4% A4 4% 3% 4% 4% 7% 2% 0% 2% 4% 3% B1 13% 11% 14% 13% 11% 13% 10% 13% 10% 13% B2 9% 7% 4% 6% 13% 4% 3% 7% 12% 10% B3 11% 1% 3% 1% 8% 3% 6% 11% 3% 12% B4 6% 1% 0% 2% 6% 0% 5% 3% 1% 4% C1 12% 14% 15% 14% 12% 12% 10% 14% 11% 15% C2 11% 9% 6% 8% 11% 7% 0% 5% 11% 13% C3 11% 4% 1% 6% 6% 2% 5% 10% 5% 14% C4 3% 3% 6% 1% 7% 0% 9% 5% 3% 3% D1 13% 15% 14% 13% 12% 12% 11% 13% 9% 14% D2 8% 10% 9% 9% 12% 5% 3% 5% 12% 11% D3 2% 5% 4% 6% 5% 5% 4% 13% 6% 13% D4 0% 1% 2% 5% 5% 2% 3% 4% 4% 4% E1 11% 13% 14% 14% 11% 13% 10% 12% 11% 15% E2 11% 10% 13% 7% 10% 8% 5% 8% 10% 14% E3 4% 4% 3% 7% 3% 4% 6% 15% 3% 14% E4 4% 2% 6% 3% 1% 4% 4% 8% 4% 5% (a) (b) Figure 2: (a) Tn (= No Tree) view of the office layout with the analysis grid, (b) Te (= Evergreen Tree) view of the office layout with the tree placed at three distances (6m, 9m and 12m) from the building envelope Desktop Radiance 2.0 was used for daylighting simulation taking from similar studies (Khaled & Ahmed, 2012; Khaled & Natheer, 2009; Gandhi, 2011). Radiance uses accurate ray-tracing technique for generating an image by tracing the path of light through pixels in an image plane and simulating the 3

effects of its encounters with simulated objects. In the first phase of the study, from a list of all native evergreen trees, one is selected. A simulation model was then developed with tree placement and task location, along with the appropriate material properties for a typical office space. Tree canopy density input was based on the analysis of actual trees. The precision of the 3D models was maintained by matching the canopy density between actual and modeled trees, and using the Leaf Area Density formula (Fahmy et al, 2010; Cantin & Dubois, 2011). Finally, the daylight simulations were run and the impact of tree type and its distance from the window was studied on illuminance, illuminance range and visual quality. (a) Figure 3: (a) T n (= No Tree) interior view of the office layout with the analysis grid, (b) T e (= Evergreen Tree) interior view of the office layout with the tree placed at a distances of 6m from the building envelope. The simulations were run for sunny sky conditions for Ahmedabad to account for extreme conditions. Test times are selected as representative of conditions during the year. The use of all or any two days of the equinox or two days of solstice are adopted widely by previous studies in daylighting research (Khaled & Ahmed, 2007; Khaled & Natheer, 2009; Hongbing et al, 2010). For this study, two equinox days of Mar 21 and Sep 22, and two solstice days of Jun 21 and Dec 27 are studied in detail. The standard daily office working hours in Ahmedabad are 0900-1700 hours. Simulations were run for 0800, 1000, 1200, 1500 and 1700 hours. Simulation inputs for building material properties and selected trees are described in Table 2 below. Objects Table 2 Material properties for simulation inputs Properties Walls, Window, Ceiling White paint; Reflectance: 70% Floor White paint; Reflectance: 50% Outside Exposed Ground Green Grass; Reflectance: 34% Glazing 10 mm Single pane clear glass with aluminum frame; VLT: 73% Table and Wooden brown laminate; Reflectance: 30% LCD computer screen Single pane black glass; Reflectance: 95% Neem Tree (b) Canopy density: 60% (Mar 21), 70% (Jun 21), 80 % (Sep 22 ), 50% (Dec 27) Height: 12m; Crown diameter: 9m; Reflectance: 31% Selection of Trees References from literature (TCPO, 1980; Krishen, 2006) helped in developing specific criteria for selection of the appropriate tree type as follows: The tree should have the potential to scatter sunlight and improve lighting quality in indoor spaces; it should not block or significantly reduce illumination levels inside the space. 4

It should be able to grow and withstand harsh climatic conditions of Ahmedabad with the maximum day temperature reaching as high as 50 C. It should be able to grow well in areas with mean annual rainfall varying from 400-1200 mm. The tree roots should not cause damage to building foundations, or, in other words, not extend beyond 5m of the tree spine for the purpose of this study The tree selected for the study was Neem (Azadirachta Indica, henceforth referred to as T e ). Tₑ belongs to fabaceae family. It is a slow growing evergreen tree; the average height of the tree is 10-15m. The average canopy density ranges from 80-90% through the year. The tree is found in areas with mean annual rainfall as low as 300 mm (TCPO, 1980; Krishen, 2006). RESULTS Luminance contrast in the field of view should be comfortable and should improve the visual performance. The following luminance ratios within the field of view were used as the basis of evaluation: 3:1 between task and darker surrounding and 10:1 between task and remote darker surfaces (IESNA, 2000; CIBSE, 2008; Khaled, 2010; Khaled & Natheer, 2009; Khaled & Ahmed, 2007). From Figures 4 through 7, it can be observed that T e at a distance of 6m from the building envelope on Mar 21 (Figure 4) allowed UDI in a range of 62-94% under sunny sky conditions whereas on Jun 21 (Figure 5) the UDI range increased to 73-100%. UDI range on Sep 22 (Figure 6) was 75-95% and dropped to 34-86% on Dec 27 (Figure 7). After the T e was placed at a distance of 9m from the building envelop, a slight decrease in the UDI range (49-94%) on Mar 21 was noted of useful daylight illuminance from 49%-94% under sunny sky condition, but increased to 72-100% on Jun 21. It was stable at 84-90% on Sep 22 and dipped again to 44-86% on Dec 27. When the distance between the tree and the building is increased to 12m, a UDI range of 65-94% was observed on Mar 21, 79-93% on Jun 21, increased to 86-100% on Sep 22. On Mar 21 UDI was observed to be in the range of 0-39% under sunny sky conditions when there was no tree in front of the window (T n scenario) while on Jun 21 it to 0-42%. The UDI range on Sep 22 was 54-73% and Dec 27 was 30-61%. Planting an evergreen tree in front of an office window in Ahmedabad T e demonstrated about 50% higher UDI in the space for all the 4 days (Mar 21, Jun 21, Sep 22 and Dec 27) as compared to the notree scenario. It is further observed that in case of T n the daylighting levels are above 2000 lux for about 90% of the time in the month of March and June and almost 65% in the month of September and December, this leads to visual discomfort for the building users, on other hand plantation of tree T e at multiple distances 6m, 9m, & 12m showcased that the daylighting levels are achieved well within the range of 100-2000 lux confirming visual comfort in the space, the unwanted solar radiations were reflected by almost 67%. UDI graphs determines that trees can provide sun shading and improve the quality of daylight entering through windows by scattering direct sunlight and reducing its intensity while moderating glare coming from the bright sky and confirms that sunlight can be filtered and softened by plantation of trees in front of the fenestration of the building envelope. 5

Figure 4: Useful Daylight Index, T e on Mar 21 Figure 5: Useful Daylight Index, T e on Jun 21 Figure 6: Useful Daylight Index, T e on Sep 22 Figure 7: Useful Daylight Index, T e on Dec 27 6

From Figure 8 (the upper bar yes is the time when visual comfort was attained and the below bar no indicates the situation when visual comfort was not achieved), it is observed that T e when placed at a distance of 6m from the building envelope demonstrated visual comfort for most of the time on Mar 21 and Sep 22 as compared to Jun 21 and Dec 27. T e at a distance of 9m demonstrated visual comfort for further more hours on Mar 21, Jun 21 and Sep 22 and when T e was placed at 12m distance from the building envelop revealed that under sunny sky conditions the visually comfortable hours increased only for Mar 21 and Sep 22. The scenario where the user was facing the window (and the work screen was facing the wall) performed better on all four days as compared to that where the user was facing the wall. For the T n scenario in Figure 9, visual comfort was attained for very only a few hours on Sep 22 and Dec 27 under sunny sky conditions. For the rest of the time, both orientations of the work screen lead to discomfort. It is evident from Figures 8 and 9 that T e helped achieve better visual comfort and the desired contrast on the task screen as compared to the T n scenario due to reduction in luminance distribution area between the luminance iso-contours. The luminance ratio between the task, nearby surroundings and remote darker surface satisfied the recommendations when the user was facing the window. Thus it can be concluded from the figures 8 and 9 that trees have the potential to mitigate acute brightness of the sky perceived through the fenestration of the building envelope and reduce the contrasting luminance ratios, particularly at certain view angles to achieve visual comfort. Yes No Figure 8: Visual comfort for T e scenario evaluated on the basis of the target luminance ratios Yes No Figure 9: Visual comfort for T n scenario evaluated on the basis of the target luminance ratios CONCLUSION Daylight analysis (luminance ratio and illuminance levels) for a typical office space in Ahmedabad brought to the fore three sources of visual discomfort: acute contrast in luminance between the task surface and background surfaces, high brightness from the windows, and uneven distribution of daylight in the space. Accordingly, it is suggested to plant native evergreen trees at a distance of 9m in front of the southern window to maintain the UDI within the range of 70-75% and provide visual comfort. In future, a study of other types of trees, tree arrangements, and building orientation may be added for further study. It is also important to compare simulation results against actual measurements to validate the model, and develop more appropriate metrics to quantify visual comfort. ACKNOWLEDGMENTS The authors would like to acknowledge Prof. Rajan Rawal, Prasad Vaidya, Jalpa Gandhi, Wayne C. Zipperer, Prof. Khaled A. Al-Sallal, Dr. A. S. Sidhu, Dr. Santan Barthwal and Dr. Priyabrata Santra for their support and guidance. 7

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