Post on 16-May-2020
Cool Roofs to Cool the World(Global Use, Potential and Policies)
.
Jayant SathayeSenior Scientist
Lawrence Berkeley National Laboratory
University of California
Berkeley
jasathaye@LBL.gov
With inputs from
Drs. Art Rosenfeld and Surabi Menon, LBNL, and Prof. Vishal Garg, IIIT
IGBC – Green Building Congress
21 October, 2011 22
BirdBird’’s eye view of urban land uses eye view of urban land use
Tree Cover 13%
Sidewalk 5%
Barren Land 8%
Misc. 6%
Road 22%
Parking Area 12%
Grass 15%
Roof 19%
Area by Land-Cover Category Above the Canopy
~ 1 km2
The surface of
Sacramento, CA
is about
• 20% roofs• 30% vegetation
• 40% pavement
2
3
Reflective roofs stay cooler in the sun
0.0 0.2 0.4 0.6 0.8 1.0
Te
mp
era
ture
Ris
e (
°C
)
Galvanized Steel
IR-Refl. Black
Bla
ck P
ain
t
Gre
en
Asp
ha
lt S
hin
gle
Re
d C
lay
Til
e
Lt.
Re
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ain
t
Lt.
Gre
en
Pa
int
Wh
ite A
sp
ha
lt S
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Wh
ite A
sp
ha
lt S
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Al
Ro
of
Co
at.
Op
tic
al
Wh
ite
Op
tic
al
Wh
ite
Wh
ite P
ain
tW
hit
e P
ain
t
Wh
ite C
em
en
t C
oat.
Wh
ite C
em
en
t C
oat.
Solar Absorptance = 1 – Solar Reflectance
Solar Reflectance1.0 0.8 0.6 0.4 0.2 0.0
50
40
30
20
10
0
4
White roofs, cool-colored roofs save money
flat, white
pitched, white
pitched, cool & colored
OLD NEW
AC savings ≈ 15%
AC savings ≈ 10%
AC savings ≈ 5%
Chicago Heat Wave 1995, 739 Deaths
The most lethal floor was the top floor of a
building with a black roof
5
European Heat Wave 2003, 30,000 Deaths
Moscow-Centered Heat Wave 2010,~10,000 Deaths
6
White roofs around the world
88
…in Santorini, Greece
8
99
Walmart store in northern California
910
10
Congratulations to Univ. of California, Davis
10
1111
White roofs are popular in Tucson, AZ
1112
12
Washington, DC (Federal) has problems
12
• Use of broken china
mosaic and lime wash in
traditional Indian roofs
• Traditional Knowledge
• Increasing awareness
• Development of codes and
other voluntary programs
Ahmedabad Mumbai
New Delhi Bangalore
1414
…in Hyderabad, India
14
Cooling our planet
1616
Solar-reflective surfaces cool the globe
via “negative radiative forcing”
Source: Intergovernmental
Panel on Climate Change (IPCC)16
1717
GLOBAL COOLING: making 100 m2 (1000 ft2) of gray
roofing white offsets the emission of 10 t of CO2
18
How much CO2 equivalent is offset if we whiten all
eligible urban flat roofs world-wide? (i/ii)
• Answer: 24 Gigatonnes (Gt)
– 2/3 of a year’s worldwide emission
– Gigatonne = billion metric tons
• If implemented over 20 years (the life of a roof or a program) this is ≈ 1.2 Gt/year.
19
How much CO2 equivalent is offset if we whiten
all eligible urban flat roofs world-wide? (ii/ii)
• Offset is equivalent to taking 300 million cars off the road for 20 years.
– There are about 600 million passenger cars world wide, and they each emit ≈ 4 t CO2/year.
In terms of avoided power plants
Full white roof potential can offset the
emissions from 500 medium-sized coal
fired power plants or 1,000 medium-
sized gas fired power plants.
That is just the albedo effect – if the building
is air conditioned, it will avoid comparable
real CO2 back at the power plant.
20
In terms of avoided power plants
Full white roof potential can offset the
emissions from 500 medium-sized coal
fired power plants or 1,000 medium-
sized gas fired power plants.
That is just the albedo effect – if the building
is air conditioned, it will avoid comparable
real CO2 back at the power plant.
21
22
Two almost identical buildings – West Bldg and East Bldg
West Bldg East Bldg
Roof shifted from concrete to white (Reflectivity: 0.70-0.30 = 0.40)
Annual savings kWh/m2 13 14
Percentage Savings 10% 19%
24
Cool colored roofs available today
cool clay tile
R ≥0.40
Courtesy
MCA Clay Tile
cool metal
R ≥0.30
Courtesy
BASF Industrial
Coatings
Courtesy
American
Rooftile
Coatings
+0.37 +0.29+0.15+0.23+0.26 +0.29
cool concrete tile
R ≥0.40
standard concrete tile
(same color)
solar reflectance gain =
cool fiberglass asphalt shingle
R ≥0.25Courtesy
Elk Corporation
25 26
Progress in energy efficiency standards
• In 2005, California’s “Title 24” energy efficiency
standards prescribed white surfaces for low-sloped
roofs on commercial buildings. Several hot states are
following.
• In 2008, California prescribed “cool colored” surfaces
for steep residential roofs in its 5 hottest climate
zones.
• Other U.S. states & all countries with hot summers
should follow.
27
US -- Recent cool roof progress (2005 – 2011)
• 2005
– California Title 24 – “Flat roofs shall be white” (15 out of 16 climate
zones). Walmart adopts white roofs for ALL stores.
– EPA ENERGY STAR lists Cool Roof Materials
• 2010
– June 1st, 2010 – Memo from U.S. Energy Secretary Steven Chu
calls for all DOE Buildings to have white roofs, if cost-effective
– June 16th, 2010 – Marine Corp follows suit, Pentagon following slowly
– June 19th, 2010 – RetroFIT Philly announces winner of “coolest block”
contest to white-coat black roofs of row houses.
• 2011
– 100 Cool Cities launched – see www.WhiteRoofsAlliance.org
– Spring 2011 – US launched, at G20 Energy Ministers meeting, a
voluntary Cool Roofs & Pavements initiative and offered technical
assistance to developing countries who join early. Joined so far: India,
Japan, Mexico, US.
To come 2012…
• Model codes will be modified to prescribe “flat roofs shall
be white”
– ASHRAE for commercial buildings
– EECC for residential buildings
• But states and cities have to adopt model codes
28
India Technology Scope
� Cool Roof technology would be
effective in almost 90% of the Indian
region.
� It has wide scope of being an
effective energy savings solution for
both new and retrofit buildings.
India Policy and Code Bodies
�ECBC-2007 - Bureau of Energy Efficiency
(BEE)
�LEED India- Indian Green Building Council
(IGBC)
�GRIHA - The Energy and Research
Institute (TERI/ MNRE)
Indian Green Building Council - IGBC
• Use roofing materials with a solar reflectance index (SRI) equal to or
greater than the values in the table below for a minimum of 75% of the
roof surface. Roofing materials having a lower SRI value than those
listed below may be used if the weighted rooftop SRI average meets
the following criteria: Credit - Heat Island effect: Roof
Area of Roof Meeting Minimum SRI SRI of Installed Roof
—————————— x —————— ≥
75%
Total Roof Area Required SRI
Roof Type Slope SRI
Low-sloped roof ≤ 2:12 (15%) 78
Steep-sloped roof > 2:12 (15%) 29
Refers to Cool Roof Rating Council for product information. Default value in IGBC
32
Global Cool Cities Alliance could unite
many initiatives and trade associations
33
Resources on the web
• Art Rosenfeld’s website
– ArtRosenfeld.org
• Cool Colors Project
– CoolColors.LBL.gov
• Heat Island Group
– HeatIsland.LBL.gov
• Cool Communities Project
– CoolCommunities.LBL.gov
• Roof Savings Calculator
– RoofCalc.com
• Global Cool Cities Alliance
− GlobalCoolCitiesAlliance.org
• Cool Roof Rating Council
− CoolRoofs.org
• Cool California
– CoolCalifornia.org
• EPA Heat Islands
– epa.gov/heatisland
• Cool Roof Calculator
– Coolroof.cbs.iiit.ac.in
February 2011
34
Other Slides
February 2011
353535
PentagonPentagonRostov on Don, white roofs
Baku, dark roofs
39
Sunlight — more than meets the eye
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 500 750 1000 1250 1500 1750 2000 2250 2500
Wavelength (nm)
No
rma
lize
d S
ola
r Ir
rad
ian
ce
near-infrared visibleUV
Solar Irradiance DistributionSolar Irradiance DistributionSolar Irradiance DistributionSolar Irradiance Distribution
■ ■ ■ ■ 6.6% ultraviolet (300 - 400 nm)
■ 44.7% visible (400 - 700 nm)
■ 48.7% near-infrared (700 - 2500 nm)
Air Mass 1 Global Horizontal (AM1GH) Solar Irradiance
40
White, cool color, warm color
white roof
cool red roof
gray roof0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
250 500 750 1000 1250 1500 1750 2000 2250 2500
Wavelength (nm)
Re
fle
cta
nce
0.0
0.5
1.0
1.5
2.0
Irrad
ian
ce
(W m
-2 nm
-1)
nir visuv
cool red (Rsol=0.39)
standard white (Rsol=0.82)
solar irradiance
dark gray (Rsol=0.23)
4141
Summer in the citySummer in the city
41
summer urban heat island