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1 © 2010 Ronnen Levinson Ronnen Levinson, Ph.D. Acting Leader, Heat Island Group Environmental Energy Technologies Division Lawrence Berkeley National Lab tel ; Presented to the China NDRC Delegation to Berkeley Lab 22 April 2010 Cool Roofs Cool Cities Cool Planet
© 2010 Ronnen Levinson 2 Summer in the city
© 2010 Ronnen Levinson 3 Bird’s eye view of urban land use The surface of Sacramento, CA is about 20% roofs 30% vegetation 40% pavement ~ 1 km 2
© 2010 Ronnen Levinson 4 Heat island mitigation strategies
© 2010 Ronnen Levinson 5 What makes a surface cool? High solar reflectance (R sol ) lowers solar heat gain ( µm) High thermal emittance (E) enhances thermal radiative cooling ( µm) high solar reflectance + high thermal emittance = low surface temperature incident sunlight I reflected sunlight R sol I net emitted thermal radiation E σ (T 4 - T sky 4 ) opaque surface at temperature T convection conduction
© 2010 Ronnen Levinson 6 Sunlight — more than meets the eye AM1GH = (clear sky) air mass 1 global horizontal Solar reflectance R sol = 6.6% ultraviolet reflectance R uv % visible reflectance R vis % near-infrared reflectance R nir
© 2010 Ronnen Levinson 7 White, cool color, warm color white roof cool red roof gray roof
© 2010 Ronnen Levinson 8 Types of cool roofs flat, white pitched, white pitched, cool & colored Old New
© 2010 Ronnen Levinson 9 Cool colored roofing cool clay tile R ≥0.40 Courtesy MCA Clay Tile cool metal R ≥0.30 Courtesy BASF Industrial Coatings Courtesy American Rooftile Coatings cool concrete tile R ≥0.40 standard concrete tile (same color) solar reflectance gain = cool fiberglass asphalt shingle R ≥0.25 Courtesy Elk Corporation
© 2010 Ronnen Levinson 10 Prototype shingles Solar reflectance >= 0.25 (for conventional shingles, SR ~ 0.05 – 0.25)
© 2010 Ronnen Levinson 11 Prototype concrete tiles Solar reflectance >= 0.40 (for conventional concrete tiles, SR ~ 0.1 – 0.4)
© 2010 Ronnen Levinson 12 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)
© 2010 Ronnen Levinson 13 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
© 2010 Ronnen Levinson 14 Cool pavement technology: asphalt concrete (?) Cool asphalt concrete still in its infancy We seek to identify cool solutions for resurfacing asphalt concrete pavement
© 2010 Ronnen Levinson 15 Potential benefits of white roofs on 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 CO 2, NO x, SO 2, Hg
© 2010 Ronnen Levinson 16 Annual energy cost saving ($/m 2 ) R-19 roof R-13 walls EER10 A/C
© 2010 Ronnen Levinson 17 Annual CO 2 emission reduction (kg/m 2 ) R-19 roof R-13 walls EER10 A/C (from energy conservation)
© 2010 Ronnen Levinson 18 Potential white-roof benefits to U.S. Retrofitting 80% of U.S. air- conditioned commercial buildings (2.1B m 2 ) would annually save – $735M – 6.2 Mt CO 2 (=1.2M cars) – 9.9 kt NO x (=0.6M cars) – 26 kt SO 2 – 126 kg Hg through energy conservation Product lifetime energy savings has present value of $11B New York Times, 30 July 2009
© 2010 Ronnen Levinson 19 Global cooling “Global cooling” offers further CO 2 reductions – negative radiative forcing: high R sol lowers T, reducing both convection and thermal radiation into the atmosphere – 80% of reflected sunlight escapes into space – 100 m 2 (1000 ft 2 ) of white roof retrofit offsets 10 t of CO 2 emission (once) – retrofitting 80% of U.S. commercial buildings yields one-time offset of 200 Mt CO 2 (= 4M cars x 10 years)
© 2010 Ronnen Levinson 20 Buildings, cities and planet (oh my!)
© 2010 Ronnen Levinson 21 On the web Cool Colors Project – CoolColors.LBL.gov Heat Island Group – HeatIsland.LBL.gov Cool Communities Project – CoolCommunities.LBL.gov Roof Savings Calculator – RoofCalc.com Cool Roof Rating Council – CoolRoofs.org Cool California – CoolCalifornia.org EPA Heat Islands – epa.gov/hiri Energy Star Cool Roofs – EnergyStar.gov Thank You!
© 2010 Ronnen Levinson 22 Supplemental slides
© 2010 Ronnen Levinson 23 Electricity price ($/kWh)
© 2010 Ronnen Levinson 24 Natural gas price ($/therm)
© 2010 Ronnen Levinson 25 CO 2 emission factor (kg/kWh)
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