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Ronnen Levinson, Ph.D.

Acting Leader, Heat Island GroupEnvironmental Energy Technologies Division

Lawrence Berkeley National Lab

RMLevinson@LBL.gov; tel. 510-486-7494; http://CoolColors.LBL.gov

Presented to the China NDRC Delegation to Berkeley Lab

22 April 2010

Cool Roofs Cool Cities

Cool Planet

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Summer in the city

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Bird’s 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

The surface of Sacramento, CAis about

• 20% roofs• 30% vegetation• 40% pavement

~ 1 km2

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Heat island mitigation strategies

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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

incidentsunlight

I

reflectedsunlightRsol I

net emittedthermalradiation

E σ (T4 - Tsky4)

opaque surface at temperature T

convection

conduction

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Sunlight — more than meets the eye

AM1GH =(clear sky)air mass 1

global horizontal

Solar reflectance Rsol = 6.6% ultraviolet reflectance Ruv + 44.7% visible reflectance Rvis + 48.7% near-infrared reflectance Rnir

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White, cool color, warm color

white roof

cool red roof

gray roof

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Types of cool roofs

flat, white

pitched, white

pitched, cool & colored

Old New

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Cool colored roofing

cool clay tileR ≥0.40

CourtesyMCA Clay Tile

cool metalR ≥0.30

CourtesyBASF Industrial

Coatings

CourtesyAmericanRooftile

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 shingleR ≥0.25Courtesy

Elk Corporation

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Prototype shingles

Solar reflectance >= 0.25

(for conventional shingles,SR ~ 0.05 – 0.25)

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Prototype concrete tiles

Solar reflectance >= 0.40

(for conventional concrete tiles, SR ~ 0.1 – 0.4)

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Vegetation• Plants cool air by evaporating

water– sensible heat → latent heat– most effective in arid climates

• Plant matter remarkably solar reflective– R ≈ 0.3 for leaves– R ≈ 0.4 – 0.5 for wood– reflectance results from

cellular structure

• Trees can shade buildings • Green roofs

– high thermal mass– moderate solar reflectance– evaporative cooling– rainwater control– extensive (shallow soil) or

intensive (deep soil)

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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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Cool pavement technology: asphalt concrete (?)

Fresh asphaltR = 0.05T = 51°C (123°F)

Aged asphaltR = 0.15T = 46°C (115°F)

Asphalt +white coatingR = 0.51T = 31°C (88°F)

Fresh asphaltR = 0.05T = 51°C (123°F)

Aged asphaltR = 0.15T = 46°C (115°F)

Asphalt +white coatingR = 0.51T = 31°C (88°F) • Cool asphalt concrete still in its infancy

• We seek to identify cool solutions for resurfacing asphalt concrete pavement

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Potential benefits of white roofson commercial buildings

• DOE-2.1E building energy simulations

• Two roof types– aged gray roof

(solar reflectance=0.20)– aged white roof

(solar reflectance=0.55)

• Four building prototypes– new office, old office– new retail, old retail

• 235 U.S. cities • Local energy prices• Local emission factors• Local building stock• Local population density• Results: local, state, national

– cooling energy saving– heating energy penalty– energy cost saving– reductions in emission

of CO2, NOx, SO2, Hg

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Annual energy cost saving ($/m2)

R-19 roofR-13 wallsEER10 A/C

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Annual CO2 emission reduction (kg/m2)

R-19 roofR-13 wallsEER10 A/C

(from energyconservation)

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Potential white-roof benefits to U.S.

• 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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Global cooling• “Global cooling” offers

further CO2 reductions– negative radiative forcing:

high Rsol lowers T, reducing both convection and thermal radiation into the atmosphere

– 80% of reflected sunlight escapes into space

– 100 m2 (1000 ft2) of white roof retrofit offsets 10 t of CO2 emission (once)

– retrofitting 80% of U.S. commercial buildings yields one-time offset of 200 Mt CO2 (= 4M cars x 10 years)

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Buildings, cities and planet (oh my!)

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On the web

• Cool Colors Project– CoolColors.LBL.gov

• Heat Island Group– HeatIsland.LBL.gov

• Cool Communities Project– CoolCommunities.LBL.gov

• Roof Savings Calculator– RoofCalc.com

• Cool Roof Rating Council– CoolRoofs.org

• Cool California– CoolCalifornia.org

• EPA Heat Islands– epa.gov/hiri

• Energy Star Cool Roofs– EnergyStar.gov

Thank You!

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Supplementalslides

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Electricity price ($/kWh)

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Natural gas price ($/therm)

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CO2 emission factor (kg/kWh)