Sustainable residential building issues in urban heat islands

– the potential of albedo and vegetation

Alamah Misni and Penny Allan

School of Architecture

Victoria University of Wellington, New Zealand

New Zealand Sustainable Building conferenceSB10 Innovation and transformation

26 – 27 May 2010 Te Papa Wellington New Zealand

1

Issue

oThe ability of the city to generate an urban heat island

oThe urban heat island effect contributes to biophysical hazard, air pollution and increased air conditioning load

oThe implementation of urban heat island mitigation strategies

oTwo basic strategies to mitigate urban heat island: high-albedo surface material and increase vegetative cover

2

Background

3

90 ºF

86

Background

Sketch of an Urban Heat island Profile (adapted from http://www.epa.gov/hiri)/3

90 ºF

86

Building heat gain

o Convection – walls, doors and windows

- through cracks around doors and windows, and through small pores in walls

o Conduction - opaque surfaces

- through the walls, roof, and windows

o Radiation - windows

- the main effect of solar radiation is usually by entering directly through windows

4

Light colour surfaces

o For areas have large amount of sunshine

o Reflect solar radiation and reduce the heat

absorbed

o The suitable value of albedo for building

envelope - can reduce the ambient temperature

o Walls - the biggest surfaces of buildings

o Roof - a third of a unwanted heat comes in

through5

Albedo

o The measure of a surface’s reflectivity

- the fraction of sunshine reflected from any

surface on the earth back into space

o Albedo can assume any value between

0.0 and 1.0

o Light-coloured surfaces have high

albedo and dark-coloured surfaces have

low albedo

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Albedo

No. Materials Albedo Materials Albedo Saving

1. Elastomeric

coating (light)

0.72 Black coating 0.08 45ºC

cooler

2. Light-coloured

insulated building

(Washington DC)

Dark-coloured

insulated building

(Washington DC)

8ºC

cooler

3. White surfaces 0.61 Ambient air 5ºC

warmer

4. Conventional

gravel

0.09 Ambient air 30ºC

warmer

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(Adapted from Connor 1985, and Taha 1997)

Albedo

o Local –reflective building envelopes

Individual typical house increase albedo from 0.30 to 0.90 can reduce air-conditioning bill 20%

o Global – microclimate changes

Cooling energy saving 3-5% for each 0.01 increase in overall city albedo

o Local + global saving of albedo were combined – 50% average hour, and 31% peak cooling period

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Potential of urban vegetation

o Vegetation has the potential to reduce ambient temperature and energy consumption

o Influence solar radiation, air temperature, humidity and air flow

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Vegetation can influence urban microclimate

Vegetation is all plant life includes trees, shrubs, grasses, and lawn

Vegetation

Directly

ShadingWind

control

Indirectly

Evapotranspiration

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Shading o Tree shading is the cover or shelter provided by

the interception by trees and solar radiation

o Using shade effectively depends on:

- the species,

- the density of the tree and the size, shape,

- the location of the moving shadow that it

casts

o Tree shade reduces cooling energy use in three

ways:

- shading window

- shading walls, windows, and roofs

- shading the soil around the building

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Summer

12

Summer Winter

In 40°S Latitude, deciduous trees on the northwest side (Adapted from Ha, 2009)

121

Shading and energy use

o Solar heat passing through windows and being

absorbed through the roof is the major reason

for air-conditioner use

o Shading is the most cost-effective way to

reduce solar heat gain and air-conditioning

costs

o The shade of trees and shrubs planted

immediately adjacent to buildings can directly

reduce cooling loads

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Shading and energy use

No. Researcher Year Location Energy saving

(year)

1. Akbari et al. 1986 Los Angeles 34%

2. Santamouris 2001 Cities across the

USA

10%

3. McPherson et al. 1988 Buffington, Florida US$108 (14%)

4. Akbari et al. 1986 Palm Springs,

California

US$60

5. Haque et al. 2000 South Carolina US$100–250

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Wind channeling o Wind is an asset;

- temperature are above 29°C

and relative humidity above 50%

o Landscaping around building;

- can act as a wind channel and

- have a substantial impact on a

residential building’s energy performance

o Airflow can be increased by strategically locating

a combination of hedgerows, and tree canopies

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17

Use evergreen trees in two to three rows in staggered order and shrubs

to deflect cold winds; and deciduous trees for channel summer breeze (Ha, 2009)1716

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|---Shrub----|-------- Tree-----------|----------------House--------------|(Ha, 2009)

The arrangement of trees and shrubs influence the

movement of wind around and through a building (Adapted from Ha, 2009)

1917

Wind and energy use

No. Researcher Year Location Energy

saving (year)

1. Heisler 1986 Central Pennsylvania 12%

2. Reimann Buechner

Partnership

1986 Radisson, New York 20%

3. McGinn 1982 Davis, California 35%

4. Huang et. al. 1992 Minneapolis,

Pittsburgh, Chicago,

Washington,

Sacramento, Miami,

and Phoenix

US$75–175

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EvapotranspirationThe evapotranspiration process contributes

significantly to cooling cities and save energy

o It is the combined loss of water from

vegetation to the atmosphere by evaporation

and transpiration

o Can create oases that are 2-8ºC cooler than

their surrounding

o The change is manifested in lower

temperatures and higher relative humidity

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(Paul & Dieter, 1993)

Selected

tropical

trees

and their

“cooling

factor”20

2222The model for a tree on the south side of the house (10%)

(Adapted from Huang et al. 1987)

21

23

The model for three trees on the south and west side of the house

(Adapted from Huang et al. 1987)

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2525

0

10

20

30

40

50

60

70

80

90

100Tem

pera

ture

(ºF

)

Base

10% cover

25% add. Cover

2 4 6 8 10 12 14 16 18 20 22 24

Temperature reduction in Sacramento due to added

tree cover on a typical summer day in July

(Adapted from Huang et al., 1987)

Evapotranspiration

2423

25

40

60

80

100Te

mp

era

ture

(ºF

)

Base

10% cover

25% add. Cover

2 4 6 8 10 12 14 16 18 20 22 24

Hour

Temperature reduction in Phoenix due to added tree

cover on a typical summer day in July: Increasing tree

cover can significantly decrease city-wide temperature (Adapted from Huang et al.,1987)

Evapotranspiration

2524

25 10

20

30

40

50

60

70

80

90

Tem

pera

ture

(F

)

Base

10% cover

25% add. Cover

2 4 6 8 10 12 14 16 18 20 22 24 hour

Temperature modifications calculated by microclimate

vegetation model for s typical summer day (2 June) in

Lake Charles with increase of 10% and 25% in a urban

tree canopy density(Adapted from Huang et al. 1987)

Evapotranspiration

2625

Saving a house a year No. Location 10% increase trees 25% increase trees

1. Sacramento 261 kWh 24% 603 kWh 57%

2. Phoenix 725 kWh 12% 1766 kWh 17%

3. Lake Charles 412 kWh 12% 1071 kWh 23%

Total saving a

house per year US$40–90 US$100–250

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(Adapted from Huang et al. 1987)

ConclusionoSimple changes in albedo levels

- can reduce home energy use by 10-50%

- a decrease of up to 2ºC in air temperature

oVegetation can reduce temperature and energy use

by doubling the amount of albedo modification

oUsing high albedo surface material and by combining

planted of every type of vegetation

- save energy and produce a comfortable house

- reduce urban heat island effect

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