Reflective surfaces (geoengineering)

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The albedo of several types of roofs

Reflective surfaces are artificially-altered surfaces that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the surface) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy).[1] Reflective surfaces are a form of geoengineering.

The most well-known type of reflective surface is the cool roof. While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today's "cool roof" pigments allow metal roofing products to be EnergyStar rated in dark colors, even black. They aren't as reflective as whites or light colors, but can still save energy over other paints.

Solar reflective cars or cool cars reflect more sunlight than dark cars, reducing the amount of heat that is transmitted into the car’s interior. Therefore, it helps decreasing the need for air conditioning, fuel consumption, and emissions of greenhouse gases and urban air pollutants.[2]

In California, over 95% of the cars and small trucks are equipped with air conditioners, burning fossil fuels and producing greenhouse gas emissions. The Heat Island Group at Lawrence Berkeley National Laboratory (LBNL) conducted the cool cars project since 2010, sponsored by the California Energy Commission (CEC), with the goal of reducing air conditioning usage of cars by lowering cabin air temperatures.

Cool color parking lots are parking lots made with a reflective layer of paint. The project is being undertaken by Jordan Woods of the Berkeley Lab.[3]

Benefits of cool roofs

Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (90 °F), reaching temperatures of 70 to 90 °C (158 to 194 °F). This heat increase can cause negative effects on cooling energy use and environments.

Cool roofs, in those hotter climates, can offer both immediate and long-term benefits including:

  • Reduced building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day
  • Savings of up to 15% the annual air-conditioning energy use of a single-story building[4]
  • Extended service life of roofs
  • Improved energy efficiency of roofs, especially when there isn't adequate insulation provided in the roof envelope
  • Improved thermal comfort in buildings that do not have air conditioning
  • Help in mitigating the urban heat island effect.[5]
  • Reduced air pollution and greenhouse gas emissions, as well as a significant offsetting of the warming impact of greenhouse gas emissions.[6]

Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters.[7] Therefore, the net energy saving of cool roofs varies depending on climate. However, a 2010 energy efficiency study [8] looking at this issue for air conditioned commercial buildings across the USA found that the summer cooling savings typically outweigh the winter heating penalty even in cold climates near the Canadian border giving savings in both electricity and emissions. Without a proper maintenance program to keep the material clean, the energy savings of cool roofs can diminish over time due to albedo degradation and soiling.[9]

Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations, seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation.[10]

If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20-year lifetime.[11] In a real-world 2008 case study [12] of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6°C over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.

When sunlight falls on a white roof much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths that we call "heat" which cannot pass back through the atmosphere because they are absorbed by the greenhouse gases. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet.[13]

A 2012 study by researchers at Concordia University included variables similar to those used in the Stanford study (e.g., cloud responses) and estimated that worldwide deployment of cool roofs and pavements in cities would generate a global cooling effect equivalent to offsetting up to 150 Gigatonnes of carbon dioxide emissions - enough to take every car in the world off the road for 50 years.[14][15]

Disadvantages

A 2011 study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the "urban heat island effect," they may actually increase global temperature.[16][17] The study noted that it did not account for the reduction in greenhouse gas emissions that results from building energy conservation (annual cooling energy savings less annual heating energy penalty) associated with cool roofs (meaning that you will need to use more energy to heat the living space due to reduction in heat from sunlight in winter.) However that apply only to those area with low winter temperature but not the tropical area. Also homes in area receiving snow in winter are unlikely to get more heat due to darker roof since snow will cover it more of the winter. A response paper titled "Cool Roofs and Global Cooling," by researchers in the Heat Island Group at Lawrence Berkeley National Laboratory, raised additional concerns about the validity of these findings, citing the uncertainty acknowledged by the authors, statistically insignificant numerical results, and insufficient granularity in analysis of local contributions to global feedbacks.[18]

Also, 2012 research at University of California, San Diego's Jacobs School of Engineering into the interaction between reflective pavements and buildings found that, unless the nearby buildings are fitted with reflective glass or other mitigation factors, solar radiation reflected off light-colored pavements can increase the temperature in nearby buildings, increasing air conditioning demands and energy usage.[19]

Properties

When the sunlight strikes a dark rooftop, about 15% of it gets reflected back into the sky but most of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect significantly more sunlight and absorb less heat than traditional dark-colored roofs[20]

There are two properties that are used to measure the effects of cool roofs:

  • Solar reflectance, also known as albedo, is the ability to reflect sunlight. It is expressed either as a decimal fraction or a percentage. A value of 0 indicates that the surface absorbs all solar radiation, and a value of 1 represents total reflectivity.
  • Thermal emittance is the ability to emit absorbed heat. It is also expressed either as a decimal fraction between 0 and 1, or a percentage.

Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI measures the roof's ability to reject solar heat, defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100.[21]

Savings calculators

Calculating cost savings resulting from the use of cool roofs can be done using several tools developed by federal agencies.[22]

This tool developed by U.S. Department of Energy's Oak Ridge National Laboratory estimates cooling and heating savings for low slope roof applications with non-black surfaces.[23]

This tool was developed by Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory in order to provide industry-consensus roof savings for both residential and commercial buildings. It reports the net annual energy savings (cooling energy savings minus heating penalties) and thus is only applicable to the buildings with a heating and/or cooling system.

Types of cool roofs

Cool roofs for commercial and industrial buildings fall into one of three categories: roofs made from inherently cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green planted roofs.

Inherently cool roofs

White vinyl roofs, which are inherently reflective, achieve some of the highest reflectance and emittance measurements of which roofing materials are capable.[citation needed] A roof made of thermoplastic white vinyl, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning.

Coated roofs

An existing (or new) roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 1000 reflective roof products can be found in the Roofs Rating Council.[24]

Green roofs

Green roofs provide a thermal mass layer which helps reduce the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5).[25] Because of the lower solar reflectance, green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roofs is trapped by the greenhouse effect and then cooled by evapotranspiration.

Cool climates

In some climates where there are more heating days than cooling days, white reflective roofs may not be effective in terms of energy efficiency or savings because the savings on cooling energy use can be outweighed by heating penalties during winter. According to the U.S. Energy Information Administration, 2003 Commercial Buildings Energy Consumption Survey, heating accounts for 36% of commercial buildings' annual energy consumption, while air conditioning only accounts for 8% in United States.[26] However, according to the Cool Roofs Rating Council and other sources, "The roof is an insignificant source for heat gain in winter. While cool roof owners may pay slightly more to heat their homes, this amount is usually insignificant compared to the cooling energy savings during the summer". Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates. The energy trade-off is therefore not clear cut. Additionally, higher R values for insulating materials in the roof assembly and snow covering on roofs can lessen the impact of roof surface color.

Case study

In a 2001 federal study, the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand associated with a cool roof’s surface reflectance.[27] LBNL found that, compared to the original black rubber roofing membrane on the Texas retail building studied, a retrofitted vinyl membrane delivered an average decrease of 24 °C (75.2 °F) in surface temperature, an 11% decrease in aggregate air conditioning energy consumption, and a corresponding 14% drop in peak hour demand. The average daily summertime temperature of the black roof surface was 75 °C (167 °F), but once retrofitted with a white reflective surface, it measured 52 °C (126 °F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $0.07/square foot.

Instruments measured weather conditions on the roof, temperatures inside the building and throughout the roof layers, and air conditioning and total building power consumption. Measurements were taken with the original black rubber roofing membrane and then after replacement with a white vinyl roof with the same insulation and HVAC systems in place.

Promotional programs

Across the U.S. Federal Government

In July 2010, the United States Department of Energy announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the country.[28] As part of the new efforts, DOE will install a cool roof, whenever cost effective over the lifetime of the roof, during construction of a new roof or the replacement of an old one at a DOE facility.

Energy Star

Energy Star is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to reduce greenhouse gas emissions and help businesses and consumers save money by making energy-efficient product choices.

For low slope roof applications, a roof product qualifying for the Energy Star label under its Roof Products Program must have an initial solar reflectivity of at least 0.65, and weathered reflectance of at least 0.50, in accordance with EPA testing procedures.[29] Warranties for reflective roof products must be equal in all material respects to warranties offered for comparable non-reflective roof products, either by a given company or relative to industry standards.

Certification requirements for different cool roof programs
Slope Min. Solar Reflectance Min. Emittance Min. Solar Reflectance Index
ENERGY STAR
Low, initial 0.65
Low, aged 0.50
Steep, initial 0.25
Steep, aged 0.15
Green Globes
Low Slope 78
Steep Slope 29
USGBC LEED
Low Slope 78
Steep Slope 29

Cool Roof Rating Council

Cool Roof Rating Council [30] (CRRC) has created a rating system for measuring and reporting the solar reflectance and thermal emittance of roofing products. This system has been put into an online directory of more than 850 roofing products and is available for energy service providers, building code bodies, architects and specifiers, property owners and community planners. CRRC conducts random testing each year to ensure the credibility of its rating directory.

CRRC’s rating program allows manufacturers and sellers to appropriately label their roofing products according to specific CRRC measured properties. The program does not, however, specify minimum requirements for solar reflectance or thermal emittance.

Green Globes

The Green Globe system is used in Canada and the United States. In the U.S., Green Globes is owned and operated by the Green Building Initiative (GBI). In Canada, the version for existing buildings is owned and operated by BOMA Canada under the brand name 'Go Green' (Visez vert).

Green Globe uses performance benchmark criteria to evaluate a building’s likely energy consumption, comparing the building design against data generated by the EPA’s Target Finder, which reflects real building performance. Buildings may earn a rating of between one and four globes. This is an online system; a building’s information is verified by a Green Globes-approved and trained licensed engineer or architect. To qualify for a rating, roofing materials must have a solar reflectance of at least 0.65 and thermal emittance of at least 0.90. As many as 10 points may be awarded for 1-100 percent roof coverage with either vegetation or highly reflective materials or both.

LEED

The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system is a voluntary, continuously evolving national standard for developing high performance sustainable buildings. LEED provides standards for choosing products in designing buildings, but does not certify products.

Under the LEED 2009 version, to receive Sustainable Sites Credit 7.2 Heat Island Effect-Roof, at least 75% of the surface of a roof must use materials having a Solar Reflective Index (SRI) of at least 78. This criterion can also be met by installing a vegetated roof for at least 50% of the roof area, or installing a high albedo and vegetated roof in combination that meets this formula: (Area of Roof meeting Minimum SRI Roof/0.75)+(Area of vegetated roof/0.5) ≥ Total Roof Area.[31]

Examples of LEED-certified buildings with white reflective roofs are below.[32]

Building Name Owner Location LEED Level
Donald Bren School of Environmental Science & Management University of California, Santa Barbara Santa Barbara, California Platinum
Frito-Lay Jim Rich Service Center Frito-Lay, Inc. Rochester, New York Gold
Edifice Multifunction Travaux Public et Services Gouvernementaux Canada Montreal, Quebec Gold
Seattle Central Library City of Seattle Seattle, Wash. Silver
National Geography Society Headquarters Complex National Geographic Society Washington, D.C. Silver
Utah Olympic Oval Salt Lake City Olympic Winter Games 2002 Organizing Committee Salt Lake City, Utah Certified
Premier Automotive Group North American Headquarters Ford Motor Company Irvine, California Certified

Cool Roofs Europe and other countries

This project is co-financed by the European Union in the framework of the Intelligent Energy Europe Programme.

The aim of the proposed action is to create and implement an Action Plan for the cool roofs in EU. The specific objectives are: to support policy development by transferring experience and improving understanding of the actual and potential contributions by cool roofs to heating and cooling consumption in the EU; to remove and simplify the procedures for cool roofs integration in construction and building’s stock; to change the behaviour of decision-makers and stakeholders so to improve acceptability of the cool roofs; to disseminate and promote the development of innovative legislation, codes, permits and standards, including application procedures, construction and planning permits concerning cool roofs.[33] The work will be developed in four axes, technical, market, policy and end-users.

In tropical Australia, zinc-galvanized (silvery) sheeting (usually corrugated) do not reflect heat as well as the truly "cool" color of white, especially as metallic surfaces fail to emit infrared back to the sky.[34] European fashion trends are now using darker-colored aluminium roofing, to pursue consumer fashions.

NYC °CoolRoofs

NYC °CoolRoofs is a New York City initiative to coat rooftops white with volunteers.[35] The program began in 2009 as part of PlaNYC,[36] and has coated over 5 million square feet of NYC rooftops white.[37] On Wednesday, September 25, 2013 Mayor Michael R Bloomberg declared it "NYC °CoolRoofs Day" in New York City with the coating of its 500th building and reducing the carbon footprint by over 2000 tons. Volunteers use paintbrushes and rollers to apply an acrylic, elastomeric coating to the roof membrane.[38] A 2011 Columbia University study of roofs coated through the program found that white roofs showed an average temperature reduction of 43 degrees Fahrenheit when compared to black roofs.[39]

White Roof Project

White Roof Project is nationwide initiative that educates and empowers individuals to coat rooftops white. The program's outreach has helped complete white roof projects in more than 20 states and 5 countries, engaged 1,000's in volunteer projects, and sponsored the coating of 100's of nonprofit and low-income rooftops.

Urban heat island effect

An urban heat island occurs where the combination of heat-absorbing infrastructure such as dark asphalt parking lots and road pavement and expanses of black rooftops, coupled with sparse vegetation, raises air temperature by 1 to 3°C higher than the temperature in the surrounding countryside.[40]

Green building programs advocate the use of cool roofing to mitigate the urban heat island effect and the resulting poorer air quality (in the form of smog) the effect causes. By reflecting sunlight, light-colored roofs minimize the temperature rise and reduce cooling energy use and smog formation. A study by LBNL showed that, if strategies to mitigate this effect, including cool roofs, were widely adopted, the Greater Toronto metropolitan area could save more than $11 million annually on energy costs.[41]

See also

References

  1. ↑ California Energy Commission (2008). Title 24, Part 6, of the California Code of Regulations: California's Energy Efficiency Standards for Residential and Nonresidential Buildings. Sacramento, CA: California Energy Commission. 
  2. ↑ "Cool Cars". Heat Island Group, Lawrence Berkeley National Laboratory. Retrieved 1 December 2011. 
  3. ↑ Cool color parkings
  4. ↑ U.S. Department of Energy (2010). Cool roof fact sheet.
  5. ↑ Urban, Bryan; Kurt Roth (2011). Guidelines for Selecting Cool Roofs. U.S. Department of Energy. 
  6. ↑ Akbari, Hashem; Surabi Menon and Arthur Rosenfeld (June 2009). "Global cooling: increasing world-wide urban albedos to offset CO2". Climatic Change 94 (3): 275–286. doi:10.1007/s10584-008-9515-9. 
  7. ↑ United States Environmental Protection Agency (2011). Reducing Urban Heat Islands: Compendium of Strategies. 
  8. ↑ Levinson, R; Hashem Akbari (2010). "Potential benefits of cool roofs on commercial buildings: conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants.". Energy Efficiency (3): 53–109. 
  9. ↑ Bretz, Sarah; Hashem Akbari (1997). "Long-term performance of high albedo roof coatings". Energy and Buildings 25 (2): 159–167. doi:10.1016/S0378-7788(96)01005-5. 
  10. ↑ Maxwell C Baker (1980). Roofs: Design, Application and Maintenance. Polyscience Publications. ISBN 0-921317-03-4. 
  11. ↑ California Energy Commission (2005). Residential Compliance Manual For California's 2005 Energy Efficiency Standards. Sacramento, CA: California Energy Commission. 
  12. ↑ Campra, Pablo; Monica Garcia, Yolanda Canton, Alicia Palacios-Orueta (2008). "Surface temperature cooling trends and negative radiative forcing due to land use change toward greenhouse farming in southeastern Spain". Journal of Geophysical Research 113. doi:10.1029/2008JD009912. 
  13. ↑ http://www.energy.ca.gov/commissioners/rosenfeld_docs/2010-10-11_Cool_Roofs_Science_at_Theater_Berkeley.ppt
  14. ↑ Akbari, Hashem; H Damon Matthews and Donny Seto (2012). "The long-term effect of increasing the albedo of urban areas". Environ. Res. Lett. (7): 159–167. doi:10.1088/17489326/7/2/02400. 
  15. ↑ http://www.independent.co.uk/environment/climate-change/painting-roofs-white-is-as-green-as-taking-cars-off-the-roads-for-50-years-says-study-7640770.html
  16. ↑ http://www.stanford.edu/group/efmh/jacobson/Articles/Others/HeatIsland+WhiteRfs0911.pdf
  17. ↑ http://www.jubbling.com/featured_jubbling/the-roof-your-wife-painted-white-last-summer-should-be-painted-back-to-black
  18. ↑ Menon, Surabi; Ronnen Levinson, Marc Fischer, Dev Millstein, Nancy Brown, Francisco Salamanca, Igor Sednev, and Art Rosenfeld (2011). Cool Roofs and Global Cooling. 
  19. ↑ Yaghoobian, N.; Kleissl, J. (2012). "Effect of reflective pavements on building energy use". Urban Climate 2: 25. doi:10.1016/j.uclim.2012.09.002. 
  20. ↑ Urban, Bryan; Roth, Kurt. "Guidelines For Selecting Cool Roofs". U.S. Department of Energy. Retrieved 30 December 2013. 
  21. ↑ Levinson, Ronnen (2009). "Cool Roof Q & A (draft)". Retrieved 10 December 2011. 
  22. ↑ DOE Cool Roof Calculator
  23. ↑ Roofing Comparison Calculator (RSC)
  24. ↑ CRRC (Cool Roofs Rating Council) website
  25. ↑ Levinson, Ronnen (2010). "Cool Roofs, Cool Cities, Cool Planet" (PowerPoint Slides). Retrieved 10 December 2011. 
  26. ↑ Energy Information Administration. "Table E1A. Major Fuel Consumption by End Use for All Buildings, 2003". Commercial Buildings Energy Consumption Survey. U.S. Energy Information Administration. Retrieved 10 December 2011. 
  27. ↑ Konopacki, Steven J.; Hashem Akbari (2001). Measured energy savings and demand reduction from a reflective roof membrane on a large retail store in Austin. Lawrence Berkeley National Laboratory. LBNL-47149. 
  28. ↑ "DOE Takes Steps to Implement Cool Roofs across the Federal Government". United States Department of Energy. 2010. Retrieved 10 December 2011. 
  29. ↑ "Roof Products Key Product Criteria". United States Environmental Protection Agency. Retrieved 10 December 2011. 
  30. ↑ Cool Roof Rating Council
  31. ↑ U.S. Green Building Council (2009). LEED 2009 for New Construction and Major Renovations Rating System. Washington, DC: United States Green Building Council, Inc. p. 20. 
  32. ↑ "Voluntary Green Building Programs". VinylRoofs.org. Retrieved 10 December 2011. 
  33. ↑ "Market challenges on cool roofs". EU Cool Roofs Council. Retrieved 10 December 2011. 
  34. ↑ H. Suehrcke, E. L. Peterson and N. Selby (2008). "Effect of roof solar reflectance on the building heat gain in a hot climate". Energy and Buildings 40: 2224–35. doi:10.1016/j.enbuild.2008.06.015. 
  35. ↑ "NYC °CoolRoofs". 
  36. ↑ "White Trumps Black in Urban Cool Contest". 
  37. ↑ "Cool Roofs Planned Across CUNY’s Rooftops". 
  38. ↑ http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120009506_2012009395.pdf
  39. ↑ "Bright Is The New Black: New York Roofs Go Cool". 
  40. ↑ Oke, TR. Thompson, R.D. & Perry, A., ed. Urban Climates and Global Environmental Change. New York, NY: Applied Climatology: Principles & Practices. pp. 273–287. 
  41. ↑ Konopacki, Steven; Hashem Akbari (2001). Energy impacts of heat island reduction strategies in the Greater Toronto Area, Canada. Lawrence Berkeley National Laboratory. 

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