Impact of native evergreen trees on the visual comfort … of native evergreen trees on the visual...

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.

Impact of native evergreen trees on the

visual comfort in an office space in

Ahmedabad, India

Ankit Bhalla Sanyogita Manu

[GRIHA Council, TERI] [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

30th INTERNATIONAL PLEA CONFERENCE 16-18 December 2014, CEPT University, Ahmedabad

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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 (Te) at 6m, 9m and12m one by one and comparing it with a no-tree

scenario (Tn). 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

measured readings being higher than the simulated

values (Table 1).

Figure 1: Plan of the calibration model


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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


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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.

Figure 3: (a) Tn (= No Tree) interior view of the office layout with the analysis grid, (b) Te (= 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.

Table 2 Material properties for simulation inputs

Objects 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

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.

(a) (b)


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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 Te). 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 Te 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 Te 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 (Tn 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 Te 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 no-

tree scenario.

It is further observed that in case of Tn 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 Te 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.


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Figure 4: Useful Daylight Index, Te on Mar 21 Figure 5: Useful Daylight Index, Te on Jun 21

Figure 7: Useful Daylight Index, Te on Dec 27 Figure 6: Useful Daylight Index, Te on Sep 22


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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 Te 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. Te at a distance of 9m demonstrated visual comfort for

further more hours on Mar 21, Jun 21 and Sep 22 and when Te 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 Tn 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 Te helped achieve better visual comfort and the desired

contrast on the task screen as compared to the Tn 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.

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.

Figure 8: Visual comfort for Te scenario evaluated on the basis of the target luminance ratios

Figure 9: Visual comfort for Tn scenario evaluated on the basis of the target luminance ratios

Yes

No

Yes

No


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NOMENCLATURE

Te = Evergreen Tree

Tn = No Tree

UDI = Uniform daylight illuminance

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