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Cool Roofs Cool Cities Cool Planet

Cool Roofs Cool Cities Cool Planet

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Cool Roofs Cool Cities Cool Planet

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  1. Cool Roofs Cool Cities Cool Planet Ronnen Levinson, Ph.D. Acting Leader, Heat Island Group Environmental Energy Technologies DivisionLawrence Berkeley National Lab RMLevinson@LBL.gov; tel. 510-486-7494; http://CoolColors.LBL.gov Presented to the China NDRC Delegation to Berkeley Lab22 April 2010

  2. Summer in the city

  3. Bird’s eye view of urban land use • The surface of Sacramento, CAis about • 20% roofs • 30% vegetation • 40% pavement ~ 1 km2

  4. Heat island mitigation strategies

  5. What makes a surface cool? net emittedthermalradiationE σ (T4 - Tsky4) reflectedsunlightRsolI incidentsunlightI convection opaque surface at temperature T conduction • 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

  6. 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

  7. White, cool color, warm color white roof cool red roof gray roof

  8. Types of cool roofs New Old flat, white pitched, cool & colored pitched, white

  9. Cool colored roofing cool concrete tile R ≥0.40 CourtesyAmericanRooftileCoatings standard concrete tile (same color) +0.37 +0.26 +0.23 +0.15 +0.29 +0.29 solar reflectance gain = cool clay tile R ≥0.40 CourtesyMCA Clay Tile cool metal R ≥0.30 CourtesyBASF IndustrialCoatings cool fiberglass asphalt shingleR ≥0.25 CourtesyElk Corporation

  10. Prototype shingles Solar reflectance >= 0.25 (for conventional shingles, SR ~ 0.05 – 0.25)

  11. Prototype concrete tiles Solar reflectance >= 0.40 (for conventional concrete tiles, SR ~ 0.1 – 0.4)

  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)

  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

  14. Cool pavement technology: asphalt concrete (?) • Cool asphalt concrete still in its infancy • We seek to identify cool solutions for resurfacing asphalt concrete pavement

  15. 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 emissionof CO2, NOx, SO2, Hg

  16. Annual energy cost saving ($/m2) R-19 roof R-13 wallsEER10 A/C

  17. Annual CO2 emission reduction (kg/m2) R-19 roof R-13 wallsEER10 A/C (from energyconservation)

  18. 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

  19. 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)

  20. Buildings, cities and planet (oh my!)

  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!

  22. Supplementalslides

  23. Electricity price ($/kWh)

  24. Natural gas price ($/therm)

  25. CO2 emission factor (kg/kWh)