3-17-14 Shepardson Verbal1
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Summer in the city
Summer Urban Heat Island
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Cool the city, save electricity
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30,000 deaths in European heat wave (2003)
739 deaths in Chicago heat wave (1995)
In Chicago, virtually all of the deaths occurred
on the top floors of black-roofed buildings without air conditioning
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Sunlight does not directly heat the air.
Opaque surfaces (e.g., pavements & roofs) absorb some of the sunlight.
How is air heated?
See GG bridge 9 mi away = 2x thickness of atmosphere. Satellites see people from 100 mi up, and 2 passes thru atmosphere.
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Solar reflectance (SR)
SR = fraction of sunlight reflected =
reflected sunlight ÷ incident sunlight
Scale is 0 - 1 (or 0 - 100%)
Higher SR is cooler (usually)
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Reflective roofs stay cooler in the sun
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Atmospheric greenhouse effect (i/ii)
Sun
Earth
Sun is about 10,000 F or Rankine, Earth is about 50 F or 500 Rankine. Factor about 20 in abs. temp. explains wlength ratio of 20 btwn peaks.
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Atmospheric greenhouse effect (ii/ii)
Sunlight heats the Earth.
Earth re-radiates thermal infra-red radiation (TIR).
Nearly all of the TIR is trapped by gases
(H2O, CO2, O2, …).
This is the “atmospheric greenhouse” effect.
Increasing greenhouse gas concentrations warms the earth.
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What can we make cooler?
Sacramento ≈ 1 km2
There are many man-made dark surfaces.
Pavements
Other
Urban fabric above tree canopy
This varies slightly for different cities
The %s are for the Sacramento square and measure the “above tree canopy” area
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Strategies for cool communities…
(Levinson)
(Pomerantz)
HIG is after 6 degrees of UHI – pavement is about 2 degrees
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…and a cooler planet!
If we reflect sunlight, it mostly passes back out of the atmosphere without heating the air.
Lowering the air temperature by reflecting sunlight is thermally equivalent to removing greenhouse gases from the atmosphere.
Reflective surfaces delay global warming.
(Rosenfeld)
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White roofs, cool-colored roofs
flat, white
pitched, white
pitched, cool & colored
OLD
NEW
AC savings ≈ 15%
AC savings ≈ 10%
AC savings ≈ 5%
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Potential U.S. white-roof benefits
Retrofitting 80% of U.S. air-conditioned commercial buildings (2.1B m2) would annually save
$735M
6.2 Mt CO2 (=1.2M cars)
9.9 kt NOx (=0.6M cars)
26 kt SO2
126 kg Hg
through energy conservation
Product lifetime energy savings has present value of $11B
New York Times, 30 July 2009
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Sunlight — more than meets the eye
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White, cool color, warm color
white roof
cool red roof
gray roof
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Cool colored roofs available today
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Advanced cool-colored asphalt shingles
Prototype cool-colored asphalt shingles
reflect up to 35% of sunlight
can save up to 60% more energy
Solar reflectance ≥ 25%
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Advanced white roof coatings, membranes
White roof coatings, membranes soil rapidly, lose solar reflectance (SR)
initial SR ≈ 0.80
aged SR ≈ 0.55
How to keep white roofs clean and reflective?
reduce leaching of plasticizers
decrease surface roughness & stickiness
photocatalytic
self-cleaning
photoinduced hydrophilicity
White metal roof stays clean,
saving 70% more energy than
soiled white coating.
Both roofs exposed
for 9 years in Florida
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Types of pavement
asphalt concrete
=
“asphalt”
cement concrete
=
“concrete”
Asphalt vs. cement concrete
composite
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Pavements cover about one-third of cities
Of that third, about
50% are streets (usually asphalt concrete)
40% are exposed parking (usually asphalt concrete)
10% are sidewalks (usually cement concrete)
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Asphalt concrete – solar reflectance
Fresh asphalt concrete has an SR about 5%.
As it ages, its SR increases to about 15%.
Need photos
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Cooler asphalt pavements
New pavement: Use light-colored aggregate
Aggregate shows as the binder rubs off
Light-colored rock
Sea shells
Porcelain, etc.
Older pavement: Use light-colored aggregate in surface coatings (“chip seals”)
Depends on availability of suitable aggregate
Don’t want to ship heavy rocks over long distances
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Light-colored chip seal in San Jose, CA
The side streets are resurfaced with light-colored chip seals.
The main road is “black-topped”.
blacktop
chip seal
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Cement concrete – solar reflectance
(traditional gray-cement concrete)
NEW
AGED
Fresh cement concrete has an SR about 35%.
As it ages, its SR decreases to about 20%.
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Cooler cement concretes
Cool. Gray-cement concrete with light colored fine aggregate: initial SR ≈ 40%
Cooler. Slag concrete, in which slag replaces about 50% of gray cement: initial SR ≈ 60%
Coolest. White-cement concrete: initial SR ≈ 70%
White-cement concrete
Traditional gray-cement concrete
Courtesy Concrete Technology Laboratory
The solar reflectance of the cement has more effect on the solar reflectance of the
concrete than any other constituent material. The solar reflectance of the supplementary
cementitious material (in this study, fly ash or slag) has the second greatest effect.
Source: Medgar L. Marceau and Martha G. VanGeem (2007)
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White is “cool” in Bermuda
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…in Santorini, Greece
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…in Hyderabad, India
…and widely
in the state of Gujarat, India.
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Walmart store in Northern California
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Congratulations to UC Davis
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White roofs are popular in Tucson, AZ
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…and in Punta Gorda, FL
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Washington, DC (Federal) has problems
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Pentagon
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Solar-reflective surfaces cool the globe via “negative radiative forcing”
Source: Intergovernmental Panel on Climate Change (IPCC)
This shows the average reflection and absorption of the Earth in the left oval and the lower frequency infrared emission and GHG trapping of radiation in the right oval. The Earth reflects
30% of incoming radiation (i.e. it has an albedo = 0.3). Its surface absorbs 45% of incoming sunlight on average and eventually reemits it in the far infrared part of the spectrum.
Such radiation is partially trapped by greenhouse gases.
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GLOBAL COOLING: Making 100 m2 (1000 ft2) of gray roofing white offsets emission of 10 t of CO2
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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.
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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.
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Progress in energy efficiency standards
In 2005, California’s “Title 24” energy efficiency standards prescribed white surfaces for low-sloped roofs on commercial buildings.
In 2008, California prescribed “cool colored” surfaces for steep residential roofs in its 5 hottest climate zones.
Arizona, Florida and Georgia followed.
Other U.S. states & all countries with hot summers should follow.
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Recent cool roof progress (2005 – 2011)
2005
California Title 24 – “Flat roofs shall be white” (15 climate zones)
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 scratches head
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 will offer, at G20 Energy Ministers meeting, technical assistance to developing countries who join a cool roof initiative.
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100 Cool Cities would unite many initiatives and trade associations
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What makes a surface cool?
High solar reflectance (Rsol) lowers solar heat gain (0.3 - 2.5 µm)
High thermal emittance (E) enhances thermal radiative cooling (4 - 80 µm)
high solar reflectance + high thermal emittance = low surface temperature
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Surface temperature in Atlanta, GA
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Cool pavement technology: cement concrete
Study by Portland Cement Association shows that cement concretes have solar reflectances of 0.30 – 0.65
LEED compliant (SRI ≥ 29)
Solar reflectances of 45 concrete mixes
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Resin binders
SR: varies depending on aggregate
Uses:
new construction, preventive maintenance
streets, sidewalks, parking lots, plazas, playgrounds
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Clear
resin
binder
Resin binders
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Derives cooling from entrapped water
Uses:
Rain water control, streets, shoulders, sidewalks, paths, alleys, parking lots, plazas, playgrounds
Can be made with any binder. (Use one size of aggregate so there are channels through the pavement.)
SR: varies depending on pavement type
Pervious / porous / permeable pavements
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Water flows very freely through a
pervious pavement
Permeability = leads to increase evaporation of water in pavements which cools
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White roofs cool your buildings and cities and (this is NEW)… our planet!
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3 papers and 1 memo estimate tonnes of CO2 offset by 100 m2 (1000 ft2) of white roofing
It is assumed that of 1 tonne of CO2 emitted only 0.55 tonnes remain in the atmosphere after one year,
so the atmospheric and emitted columns are just in the ratio of 0.55/1.
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Building standards
For planning purposes, follow the lead of the California Public Utilities Commission and internalize externalities. Started at $10/tonne CO2, and will increase to $30/tonne CO2.
Thus, California already plans to incorporate externalities when optimizing building standards.
Externalities justify cool roofs on non-air conditioned buildings.
Apply externalities also to cool pavements, vehicle roofs, even train roofs.
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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
White Roofs Alliance
WhiteRoofsAlliance.org
Cool Roof Rating Council
CoolRoofs.org
Cool California
CoolCalifornia.org
EPA Heat Islands
epa.gov/heatisland
Energy Star Cool Roofs
EnergyStar.gov
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