analisis termico peatonal

7/31/2019 analisis termico peatonal

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ImprovedPedestrian

ThermalComfortThroughUrban

DesignJennifer Love

Master of Urban and

Environmental Planning

Professional Project

May 2009

[email protected]

INTRODUCTION

Open space in urban environments, be it a formal park, an informal

gathering place or inviting streetscape, contributes to a sense of

community and can encourage pedestrian activity. The challenge to

creating inviting open spaces in hot, arid climates such as Phoenix is

mitigating extreme summer temperatures to allow year-round pedestrian

and outdoor activity. In this study, a microscale climate analysis was

conducted using the software ENVI-met to examine the effects of

various urban design elements on thermal comfort with the goal of

creating open spaces that are usable year round. Specically, this

project develops a set of site scenarios with various physical, design,

and materials differences, and applies the ENVI-met program to each

scenario to analyze ambient site temperature.

TECHNICAL CONTEXT

There has been a rise in concern regarding Urban Heat Island

(UHI), which results in the increase in nighttime temperatures due

to the absorption of heat in materials such as asphalt. Urban design,

architecture techniques, and heat reducing materials can be used toreduce both the UHI effect and increase daytime thermal comfort,

allowing for pedestrian spaces to be comfortable year round, day and

night.

Recent research has examined the impact of UHI in Phoenix, which has

become a particular concern as the urbanized area grows. The number

of hours per day with temperatures exceeding 100F during July and

August (the peak summer months) have increased signicantly, from

3.6 hours to 6.4 hours in the last 50 years. Additionally, nighttime

temperatures have increased by 12 F over the last 20 years in the

Phoenix area (Gober, 2006). This exacerbates the need for urban designstandards which can improve the outdoor environment, especially since

the nighttime temperature is projected to increase approximately 15F by

2015 and 25F by 2100 (Brazel, 2003). Fortunately, both the UHI effect

and thermal comfort can be improved through urban design.

STUDY APPROACH

This study looked to build upon the open space concepts of the

Urban Form Project, but also to adapt these concepts at a microscale

level, integrating practical urban form to the existing structure of the

downtown and retrotting existing public spaces. Various urban designelements which impact thermal comfort were examined in this study,

applied to a specic test site in downtown Phoenix, and subsequently

analyzed using the software ENVI-met.

Urban Design Elements Inuencing Thermal Comfort

Urban design can be used to create a pleasant outdoor space while

reducing outdoor ambient air temperature. As building placement and

design have previously been studied extensively, this project focused

on landscaping type and placement, air circulation, and material type,


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Improved Pedestrian Thermal Comfort Through Urban Design

which can all contribute to creating acceptable ambient temperatures for

outdoor urban site activities during periods of extreme heat.

The following are key design elements which may be used to improve

thermal comfort:

Shaded Pedestrianways Clustered/Broad Canopy Trees

Grass

Water

Material Type

Shaded Pedestrianways

Building shade, covered walkways and awnings, depicted in Figure 1,

are important urban design elements that should be utilized to provide

shaded open space and pedestrianways. These design elements reduce

the amount of direct sun, thus improving thermal comfort and reducing

sun exposure, which can improve the ambient air temperature by up to

15% (Urban Form Project Working Paper #8, 2007).

At the Phoenix latitude, buildings can be used to provide shade on the

south side of the street in the summer months during the hottest part of

the day by casting a shadow to the north of the building. To balance this,

shade trees should be planted on the north side of the street, which is the

south side of a building.

Covered walkways and canopies extending from buildings can also

be used to shade and cool pedestrians. These can be built along east-west streets for extended lengths or for individualized locations. These

amenities can provide pedestrian refuge and shade along the north side

of the street, while the south side requires less intensive pedestrian

oriented shading due to the building shadow.

Clustered/Broad Canopy Trees

Increasing the cover of trees in an urban environment is a simple and

effective way to improve thermal comfort. Street trees will be most

effective in providing true shade, commanding visual contrast between

dense landscaping and open space when planted in groups. Trees should

be located to maximize shade for pedestrians, such as along walkwaysand sidewalks, depicted in Figure 2.

Planting trees to provide building shade is important to reduce

the radiant temperature of buildings and objects such as benches.

Additionally, evapotranspiration from a trees leaves in hot weather

cools the air around the tree with 65% of the heat from full sunlight on a

tree being dissipated by the trees canopy (Coder, 1996).

Figure 1

Shaded Pedestrianways in Phoenix

Figure 2

Shade Trees in Phoenix


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Clusters of trees are ideal for providing pedestrian and open space shade

to facilitate comfortable pedestrian circulation and outdoor seating

areas. For example, palm trees cast long shadows and are very useful

for providing pedestrian shade when grouped together, but they must be

used thoughtfully due to their long shadow.

GrassGrass can be used to cool an area and can be appropriately used in a

desert climate if designed in conjunction with summer shade. Creating

open spaces, with or without grass, improves the air circulation

throughout the core area of a downtown, which can help to mitigate hot

air trapped within dense developments. Open space areas with grass

will achieve more pleasant conditions given the cooling effect of grass

through evapotranspiration. Temperatures over grass areas are cooler

than over bordering areas, as illustrated in Figure 3.

Water Features

The use of water features, such as fountains or waterfalls, can have

a signicant impact on thermal comfort, potentially improving

temperatures up to 7.5% (UFP, WP#8, 2007). In an arid climate, water

promotes cooling through evaporation as well as absorbing heat from

surrounding materials. Flowing water promotes air movement which

can cool a space. Water also has a psychological cooling effect, making

a space seem cooler even if there is no measurable temperature change

(UFP WP#8, 2007).

Material Type

Materials used for pavement and amenities such as benches should beselected based on their color, permeability, conductivity and emissivity

(ability of objects to shed heat). Lighter color materials with low

emissivity retain less heat and contribute to lowering the temperature in

a space. Suggested material types and benets are:

Concrete lighter color reects energy from sun

Recycle crumb rubber added to material mix lower heat retention

Porous pavement less dense, able to transmit air and moisture to the

environment encouraging healthy growth of trees (UFP, 2008).

The heat retention of different materials is visible in Figure 4. Areas

paved with asphalt appear red indicating higher temperatures fromretained heat. By contrast, the areas paved with lighter colored concrete

appear green indicating lower temperatures.

Existing Conditions

Prior to conducting the scenario analysis as part of this study, detailed

temperature measurements were obtained for two sites and material

surfaces throughout downtown Phoenix. Key temperature ndings are

Figure 3

Cooling Effect of Grass

Figure 4

Temperature Difference in Paving

Materials


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presented in Figure 5 and Table 1. These temperatures were collected

using a Raytek infrared heat sensor on July 28, 2008, between 12:00

p.m. and 2:00 p.m. All of the listed materials in Table 1 are commonly

used throughout the Phoenix area.

Figure 5 Existing Site Conditions Field Measurements

Te

mpera

ture

(Fa

hren

he

it)

Material Temperature F

Textured Pavement (medium gray), Filtered Shade 108.9

Sandstone Bench (pink), Filtered Shade 104.0Concrete (light gray), Filtered Shade 103.1

Wooden Bench, Filtered Shade 97.6

Dirt and Gravel walkway, Filtered Shade 95.5

Concrete (light gray), Solid Shade 88.2

Grass, Solid Shade 85.4

Table 1 Existing Surface Conditions Field Measurements

The rst site measured was the Arizona Center, which has

successfully implemented many urban design techniques which cool

the public space. This site served as the study control, evaluating

the success of the proposed techniques on the test site. The second

site, in downtown Phoenix, was selected to test the microclimate

conditions of various urban design scenarios. The site, shown in

Figure 6, is a 5,260 square foot underutilized open space at the

northeast corner of 3rd Avenue and Jefferson Street adjacent to


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several municipal buildings.

Methodology

Figure 6

Northeast Corner of 3rd Avenue and Jefferson

The test site layout established for the ENVI-met model was extended

to include each of the structures at the corners of the intersection to

incorporate the air ow through the space, as well as the heat retention

of the street pavement. This site was selected to test various scenarios,

applying urban design principles and propose site changes in order to

improve pedestrian thermal comfort.

The ENVI-met modeling software can be used for macro- or

microscale analysis to evaluate daytime climate. Material type,

building conguration, air ow, climate conditions, and landscape areinput into the software with the output being a climate analysis of a

site. The software allows for the evaluation of various scenarios and

congurations by performing multiple analyses, as outlined in Figure 7.

Figure 7 ENVI-met Model Process


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Scenarios

Ten scenarios were modeled using ENVI-met in addition to the existing

conditions. The scenarios were iterative and cumulative, measuring the

impacts of incremental changes in material type, landscape, and shading.

The overall approach to the scenario modeling was to examine the

impact of minor changes to the existing site, adding additional shading

and changing the paving materials, and modifying site layout. Thesescenarios, described in Table 2, examined:

Impact of maximizing shade within the space,

Improving only the area between the street and open space by

creating a buffer from the heat radiating from the street,

Creating a shade buffer from the sun angles of the east and west,

Shade circumventing the open space while not actually providing

any improvements to the core of the space.

Scenario

1 Existing infrastucture Add significant shade trees

2

Existing infrastucture Add significant shade trees Change hardscape surfaces to light coloredpermeable pavers

3

Existing infrastucture Add significant shade trees and a Add permeable paver pathway through turf area

ScenarioDescription Description

7

Fill the entire space with turf and shade trees Leave only minimal walkways around theperimeter utilizing light colored permeable

pavers

8

Retain existing core space Add shade trees and use light coloredpermeable pavement along the southernand western edges

9

Retain existing core space Add shade trees along the eastern andwestern edges

4

Existing infrastucture Add significant shade trees Add light colored permeable paver pathwaythrough turf area

Change hardscape surfaces to light coloredpermeable pavers

5

Modify layout to increase turf and shade treearea adjacent to roadway

Add significant shade trees Add permeable paver pathway through turf area Change hardscape surface

6

Modify layout to increase turf and shade treearea adjacent to roadway

Add significant shade trees Add permeable paver pathway through turfarea

Change hardscape surface Add a water feature

10 Retain existing core space Add shade trees around the perimeter

Table 2 Scenario Descriptions

RESULTS

The characteristics and results for each scenario are detailed in Figure

8. This summarizes the improvements made, estimated investment

required, and temperature improvement.


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Landscaping/Building Inp ut

GrassBroad Canopy TreePalm TreeBuildingNo Vegetation

ConcretePermeable PaverBuildingSoil

104.3 F105.4 F106.4 F107.5 F108.6 F109.7 F110.8 F111.8 F112.9 F114.3 FT

emperature(F)

Mater ial Temperature Temperature Range Output

Key

Existin

g

Scenario1

Scenario2

Scenario

3

Scenario4

Scenario5

Scenario6

Scenario7

Scenario8

Scenario9

Scenario10

% Low (103-106F)% Medium (106-109F)

% High (109-112F)

% Very High (112-115F)

Total CostAverage Site Temper atur e (F)

Peak Site Temperature (F)

Temperatur e Change (F)

0

070

30

-

112.9114

-

0

08614

$4,400

113.1113.8-2.1

3

23704

$65,400

108.6113-6.6

0

48511

$23,400

109.4113.5-5.8

2466

100

$180,400105.8

109.9-9.4

086

140

$144,100106.5

109.9-8.7

685

90

$157,100106

110-9.2

07624

0

$154,200108.4110.2

-6.8

05347

0

$118,600109.1111.9

-6.1

00

93

7

$3,300110.7113

-4.5

0

7930

$24,400

110.3111.8-4.9

Figure 8 Temperature Difference in Paving Materials


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Figure 9 outlines the peak, average and low temperatures by scenario.

The largest temperature improvement is Scenario 4, with a nearly 10

F reduction in average site temperatures and a 4F reduction in the

peak site temperature. This scenario utilized a combination of elements

consisting of cool pavement types, shade, and buffering the heat

owing into the site from the street. An important nding is that the

peak temperature can be signicantly reduced through urban designimprovements.

Scenario

Ex

isting

Scenario

2

Scenario

1

Scenario

3

Scenario

4

Scenario

5

Scenario

6

Scenario

7

Scenario

8

Scenario

9

Scenario

10

Figure 9 Peak, Average and Lowest Temperature, by Scenario

Figure 10 outlines the intensity of heat by scenario. The results indicate

that Scenario 4 is the most effective conguration to reduce the overall

intensity of temperatures in the site. This scenario has the highest

percentage oflow temperatures and the lowest percentage of the space in

the high temperature range.

The maximum average site temperature reduction in the scenario

analysis is comparable with the measurements taken at the Arizona

Center in downtown Phoenix. The Arizona Center is an example of a


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site which utilizes design techniques that improve thermal comfort.

As shown previously, the difference between the existing baseline

temperatures at the Arizona Center and the 3rd Avenue and Jefferson

Street site is approximately 12F, which is similar to that reected in

the scenario analysis. This conrms that the ENVI-met model can

approximate temperature reductions due to urban design improvements

and validates the scenario modeling for this study.

Tempera

ture

Range

%

Ex

isting

Scenario

2

Scenario

1

Scenario

3

Scenario

4

Scenario

5

Scenario

6

Scenario

7

Scenario

8

Scenario

9

Scenario

10

Scenario

Figure 10 Intensity of Heat, by Scenario

Cost estimates for each scenario were developed based on past bid

tabulations from the City of Phoenix and are included in Figure 8. While

the costs of the scenarios vary, several cost effective recommendations

can be implemented to provide an improvement to thermal comfort.

The most expensive scenario, #4 at $185,000, was the scenario with the

greatest temperature improvement. Several other scenarios provided an

improvement in thermal comfort at a lower cost. Scenario #8, which

buffered the open space from the street provided a 6F reduction in

average temperature at a cost of $122,000. Providing additional shade

and lighter, porous material in the interior of the space, as modeled in

Scenario #3, cooled the space approximately 6F at the cost of $71,000.

The least expensive option, Scenario #9, examined the sun angle impact

to the space and showed a temperature reduction of 4.5F, although the


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intensity of the heat was not reduced as much as the other scenarios with

93% of the space in the high temperature range.

Key ndings of the study, grouped by individual and cumulative effects,

were:

Individual:1. The use of lighter colored and porous material can reduce the

temperatures radiating from the paving materials by nearly 7F

2. Increasing shade through vegetation in a space can reduce the

temperature by 2F.

3. Simply buffering the air ow from hotter street pavement can

decrease the temperature 6 F.

Cumulative:

1. Site conguration which maximizes shade, use of turf and lighter,

porous paving materials can decrease the site temperatures between

8-9F.

2. Shade used to provide protection from sun angles can cool the space

approximately 4F, although it does not signicantly reduce the

intensity of the heat in the space.

3. Both peak temperature and intensity or percentage of high

temperature areas in the space can be reduced.

CONCLUSIONS

Overall, the analysis results show that retrotting existing spaces can

provide substantial improvements and that costly, major infrastructure

investment is not required to improve thermal comfort in open spaces.Inexpensive improvements, such as protecting a site from sun angles,

can have a cooling impact to a space at a relatively low cost. Improving

or protecting a site from hotter air owing in can also reduce the overall

intensity of the heat in a space. The use of signicant tree coverage

results in improved thermal comfort in urban spaces to encourage

day time use. Pavement material type is signicant, and the use light

colored, porous materials has been demonstrated to reduce heat retention

and site temperature.

The implications of this study for hot, urban communities are that

material types are key factors in thermal comfort in urban environments

and that adjacent spaces, such as intersections or streets, can impact the

temperature of a space due to airow. The model results can be used to

project potential temperature reductions in other spaces such as plazas

and courtyards. The cooling concepts can also be applied to linear

spaces such as pedestrianways, as the radiative material qualities and

shade impacts are much the same, with a greater impact from airow

into a space.


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This project contributes to the creation of a more inviting pedestrian

network in the interest of establishing a sense of identity for downtown

Phoenix as well as other urban areas. The results from this project,

specically the resulting temperature analysis, can be applied to

understand the impacts urban design and site planning have on

microclimates in order to create more climate appropriate spaces.

The results are intended for use by the City of Phoenix to understand

the impact of the built environment on daytime pedestrian activity

in the urban core of the city. Although this study examined a specic

site, ndings can be transferred to other sites to allow communities

to implement measures to reduce the overall daytime temperatures of

spaces in the urban core and thereby improving thermal comfort. The

City will potentially expand upon this research, requiring developers

and municipal infrastructure projects to meet a minimum temperature

threshold.

This study shows that the ENVI-met software is a useful tool for

planners to analyze and predict the thermal comfort impacts of various

developments. This research also translates to other communities, both

in arid climates similar to Phoenix and seemingly dissimilar climates

which can adapt the modeling process for their needs. Additional study

would be required to determine urban design impacts in other types of

climates.

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