Saving energy over the long haul: the role of polymers in modern cool roof technology

Transcription

1 Saving energy over the long haul: the role of polymers in modern cool roof technology Kurt Wood (Arkema, Inc.) Polymer Science of Everyday Things ACS 2012 Summer meeting

2 Outline The case for cool roofs The building owner s perspective A global perspective The physics of cool roofs- energy and color Key energy and radiation considerations Pigment selection and reflectivity Advanced polymer technology for cool roofs Boosting reflectivity through stay-clean technologies Boosting reflectivity through polymer choice Multiplying the ethic of conservation through extended service life Expanding the range of cool color options

3 Types of cool roofs Cool roof principles can be used with most every kind of roofing material Cool roof technology can be applied in the factory or over an existing roof

4 Benefits of reflective cool white roofing Reduces building heat-gain EPA estimates equivalent of up to R19 insulation value 15-30% HVAC savings 500 sq ft reflective roof 15yr cost savings: North ~$10,000 South ~$60,000 Increases HVAC and roof membrane longevity Results in lower greenhouse gas emissions 178 F 93 F

5 Grain elevator in China- Passive cooling from a reflective roof coating

6 MGM Grand Hotel and Casino- Las Vegas, NV Enhancing MGM Grand the efficiency Hotel of air conditioning The casino received significantly fewer complaints about high room temperature in the upper floors after the repair, a casino manager reported. Project: MGM Grand Hotel Las Vegas, NV Coating Manufacturer: United Coatings, Inc. Kymax Date Completed: 2006

7 Building Energy Consumption The building sector accounts for almost half of the energy consumption in the U.S. Buildings consume 75% of power plantgenerated electricity

8 ADDING NEW PEAK ELECTRICITY CAPACITY IS VERY EXPENSIVE Cost of Peak Electricity Cost of Regular Electricity Cost of Energy Conservation 10,2 cents / kwh 3,9 cents / kwh California peak demand is growing at about 2.4 percent per year, roughly the equivalent of three new 500-megawatt power plants cents / kwh Slide courtesy of Prof. M. Santamouris, U. Athens, Greece, from Proceedings of PALENC 2010 Congress

9 The urban heat island effect makes things worse A satellite image of Atlanta, Georgia during a summer day. Notice how the red splotches (indicating higher temperature) in the infrared picture (bottom) are clustered around downtown Atlanta. (Image courtesy of NASA)

10 Reflective roof coatings embody many core values of the green movement In most cases, coatings are water-based and minimally polluting (very low VOCs) They enable roof systems to last longer, reducing environmental footprint from material use Reduce energy consumption from air conditioning Reduce global warming directly by radiating away more heat- reduced urban heat island effect Reduce global warming indirectly by lowering greenhouse gas production (for electricity from fossil fuels)

11 Outline The case for cool roofs The building owner s perspective A global perspective The physics of cool roofs- energy and color Key energy and radiation considerations Pigment selection and reflectivity Advanced polymer technology for cool roofs Boosting reflectivity through stay-clean technologies Boosting reflectivity through polymer choice Multiplying the ethic of conservation through extended service life Expanding the range of cool color options

12 The Energy Balance on Roofs Re-emitted Energy E (far infrared) Reflected Radiation R Convection Absorbed Energy Roof Surface Layer Net Heat Flux into Building

13 The coatings-based cool roof strategy: Increase the fraction of incoming energy reflected and emitted Re-emitted Energy E Solar Reflectance (ρ or TSR) is a ratio indicating the fraction of solar energy reflected by a surface Reflected Radiation R Convection Absorbed Energy Roof Surface Layer Emissivity (percentage of heat emitted vs. a black-body) is high for most materials, except bare metals Reflectance of materials varies widely and we can use this to make cooler roofs Net Heat Flux into Building

14 Half the solar energy intensity is in the near-infrared To have maximum solar reflectivity, we need to reflect BOTH visible and near-infrared light

15 Reflectivity vs. wavelength for different roofing materials UV Visible Infrared Color is determined by absorbance/reflectance in the visible part only

16 For coatings, the pigment properties dominate the solar reflectance UV Visible Infrared The color determines the visible reflectance There can be large differences in the nearinfrared reflectance, even for the same color Unlike absorbance, reflectance is NOT an inherent material property. It occurs because of light transmission plus multiple light scattering events

17 Lab experiments relating TSR and measured temperature rise, vs. pigment type Total Solar Refelctance Rutile TiO2 Black pigment A Black pigment B Black pigment C Black pigment D Black pigment E KYNAR 500 PVDFbased paints (25 micron thickness) on aluminum substrate Temp. Rise in Degrees Celsius Rutile TiO2 Black pigment A Delta T vs. pigment grade Black pigment B Black pigment C Black pigment D Black pigment E Temperature rise measured in light box with IR lamp illumination

18 The highest reflectivity is always from a white material (all visible wavelengths plus near-ir)

19 Outline The case for cool roofs The building owner s perspective A global perspective The physics of cool roofs- energy and color Key energy and radiation considerations Pigment selection and reflectivity Advanced polymer technology for cool roofs Boosting reflectivity through stay-clean technologies Boosting reflectivity through polymer choice Multiplying the ethic of conservation through extended service life Expanding the range of cool color options

20 9 Years, Florida Solar Energy Center PVDF based Cool White Metal Roof PVC Membrane

21 What is PVDF? H H H H H H H H H H C C C C C C C C C C F F F F F F F F F F PVDF (Polyvinylidene fluoride) [CH 2 -CF 2 ] n Semi-crystalline polymer; Melting point T m ~ C Unique among fluoropolymers for its miscibility with other resins, e.g. certain acrylics Commercial coating grade introduced in 1965; given KYNAR 500 trademark Traditional PVDF coatings are non-aqueous dispersions, require baking KYNAR, KYNAR 500 FSF and KYNAR Aquatec are registered trademarks of Arkema, Inc.

22 KYNAR 500 PVDF licensed finishes (typically 70% PVDF/30% acrylic), are the industry standard for metal cool roof coatings, with 20+ year deco property retention Vinylidene fluoride monomer

23 New waterborne PVDF-acrylic hybrid technology Thick acrylicbased topcoat Thinner PVDF hybrid- based topcoat

24 Improved Acrylic Dirt Pick-up Resistance through Surface Crosslinking Technology Elastomeric Acrylic Coating, Initial Standard acrylic coating, after accelerated DPR testing Coating with surface crosslinking, after accelerated DPR testing

25 PVDF coatings for stay-clean surfaces At least three mechanisms contribute to the stay-clean advantages of PVDFbased coatings: PVDF crystallinity and barrier properties Fluoropolymer surface properties Inhospitality to fungal growth H H H H H H C C C C C C F F F F F F Poly(vinylidene fluoride)

26 Aluminum Panels in Florida, 5º South Facing 1 ½ Year, no mildewcide in formulations unexposed Exposed Standard Elastomeric Acrylic PVDF-acrylic hybrid

27 Real life stay clean comparison- in China Elastomeric Acrylic Initial SR 0.85 After 3 Years 0.55 Kynar Aquatec Initial SR 0.87 After 3 Years 0.77

28 Enhanced Reflectance of PVDF Coatings 100 Reflectance (%) Film thickness is 1.4 mils neat PVDF tint blue paint neat acrylic tint blue paint Wavelength (nm) measured with a UV-VIS-NIR Spectrometer with an integrating sphere (Perkin-Elmer Lambda 19).

29 Resin contribution to Solar Reflectance 0.66 Total Solar Reflectance Dry film thickness 1.4 mils Dry film thickness 1.0 mils Dry film thickness 0.7 mils Regulatory TSR threshold KYNAR 500 PVDF content on total resin (wt%) TSR as function of PVDF:acrylic ratio Tint blue coatings at constant pigment volume concentration

30 Annual Cooling Energy Savings of a Cool White Roof Coating versus a Non-Cool Roof at 0.20 TSR 1.4 kwh/ft >60% Elastomeric Acrylic TSR: 0.55 Fluoropolymer Latex TSR: Arizona California Florida Texas

31 System approach to cool roofing- example of after-market roof coating for built-up roofing High performance fluoropolymer topcoat ~ 3 mils dft Elastomeric primer mils dft Substrate

32 Substrates and primers can greatly affect the near-ir component of the solar reflectance! Scattering events occur because of refractive index variations. They are much less likely when the pigment size is much smaller than the wavelength of the light Colored topcoat (pigment 1) Primer or basecoat (pigment 2) Substrate (pigment 3)

33 Extending the cool roof color space using a basecoat/topcoat approach Durable topcoat with cool pigment (Minimal pigment for visual hiding) High solar reflectance, opaque basecoat Substrate But to make this work: The topcoat must retain its decorative properties, over the multi-year service life of the object!

34 KYNAR PVDF-based paints show minimal chalk and color fade (with inorganic pigments) because the PVDF resin is not attacked by UV light Original color under flap Unwashed portion Showing chalking Bottom portion Washed yearly Comparison of color matched KYNAR 500 and polyester powder coatings, 12 year South Florida exposure Polyester 70% PVDF Polyester 70% PVDF

35 SR vs. Lightness, over gray cement tile An even larger boost is achieved over gray concrete- 20% in many cases Study done with Dr. Ronnen Levinson, LBL Heat Island Group

36 Colors achievable as a function of SR Concept samples with white basecoats + PVDFacrylic hybrid topcoats- Many darker colors can meet LEED cool roof standards for steep slope roofs! Study done with Dr. Ronnen Levinson, LBL Heat Island Group

37 Increasing the solar reflectance of roof tiles- White primer plus colored KYNAR Aquatec Topcoat

38 New polymer technologies: Greatly extend the service life of the roof Polymer surface is extremely durable; lasts longer with better retention of decorative properties Lower maintenance costs Less environmental impact from raw material manufacturing and disposal Underlayers last longer when roof is cooler; Save energy and money Less A/C is needed the more reflective the roof is Rooftop A/C is more efficient when roof is cooler Utilities and cities benefit

39 Acknowledgements The cool coatings community including: Lawrence Berkeley Lab Heat Island Group Dr. Ronnen Levinson Oak Ridge National Laboratory Prof. M. Santamouris, U. of Athens Industrial partners Quest, Inc. (United Coatings), APV Engineered Coatings, ER Systems, etc. Shepherd Color Company Eagle Rooftiles, Inc. Arkema, Inc. colleagues Drs. Scott Gaboury, Min Zheng, Ravi Gupta Wayne Skilton, Cindy Hess, Jerry Petersheim John Dockery (Arkema Coating Resins) KYNAR, KYNAR 500 FSF and KYNAR Aquatec are registered trademarks of Arkema, Inc.