Cool Materials and Shade Trees
On a clear summer afternoon, the air in a typical city is about 3K (5°F) warmer than that in the surrounding countryside. This phenomenon, known as the summer urban heat island, results from a lack of vegetation and a prevalence of dark surfaces in cities. Urban heat islands can be uncomfortable, aggravate heat-related illnesses, and make heat waves more deadly. Higher air temperatures also accelerate smog formation and increase demand for air conditioning on a hot summer afternoon. This spike in air conditioning demand is expensive and polluting, and can induce brownouts or even blackouts by straining the electrical grid.
In the mid-1980s, Hashem Akbari, a Berkeley Lab scientist, began investigating the possibility that shade trees and reflective roofing and paving materials could mitigate urban heat islands and reduce energy consumed for air conditioning. Trees cool buildings and people by providing shade, and remove heat from the air by evaporating water. Solar-reflective roofs and pavements use materials that reflect much of the sun's radiation back to the sky, keeping their surfaces cool and reducing the flow of heat into the air. White roofs are coolest, but dark roofs can also be made to reflect the invisible "near-infrared" component of sunlight. These white or colored "cool" roofs reduce the need for cooling energy in air-conditioned buildings and make unconditioned buildings more comfortable.
Akbari and his research team, the Urban Heat Island Group, demonstrated the remarkable abilities of shade trees and cool roofs to lower building temperatures and reduce the need for air conditioning. His group also used computer simulations of urban airsheds to show that citywide application of cool roofs, cool pavements, and trees could cool the urban heat island and improve air quality.
Berkeley Lab researchers estimate that about 10% of current U.S. air conditioning demand results from the urban heat island. Eliminating the urban heat effect could save the U.S. more than $1 billion each year and reduce annual carbon emissions by about 8 million metric tons. In Los Angeles alone, mitigation measures that reduce the average air temperature by 3K (5°F) could reduce peak power demand by 1.5 MW, offer cooling energy savings worth $170 M/year, and yield a 12% reduction in smog (ozone) worth $360 M/year.
Changing Codes and Standards
Berkeley Lab has a long track record of working with the private sector and government agencies to ensure that energy-efficient innovations don't remain stalled in the lab. In 1998 Berkeley Lab scientists were instrumental in forming the Cool Roof Rating Council, an independent organization dedicated to evaluating and reporting the radiative properties (solar reflectance and thermal emittance) of roofing materials for the benefit of consumers, contractors, and building codes.
The Berkeley Lab team has helped add cool roofing credits to building energy efficiency standards in several U.S. cities and states, including Atlanta, GA (1995); Chicago, IL (2001); Florida (2001); and California (2001). The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) also updated their standards to incorporate cool roof credits for non-residential buildings in 1997 and for residential buildings in 1999.
In 2005, California added to its building energy code a prescriptive cool-roof requirement for new non-residential buildings with low-sloped roofs. Pacific Gas & Electric estimates that this measure will reduce demand for air-conditioning power within its service territory by about 350 megawatts, equivalent to the elimination of one average-sized power plant. The cool roof team is now developing similar code proposals for non-residential sloped roofs and for all residential roofs. The California Energy Commission estimates that the installation of cool roofing materials statewide could save over $150M worth of electricity each year.
Berkeley Lab scientists have advised a number of local authorities, including the Sacramento Municipal Utilities District and the South Coast Air Quality Management District, on how best to create incentives for homeowners and businesses to adopt cool roofs to reduce their energy use and slow the formation of smog.
Extending their work internationally, Berkeley Lab scientists have demonstrated their "cool" materials in such countries as Japan and India. Japan has been particularly active in heat island mitigation efforts. In 2004, the city of Osaka began a $1 billion program to implement heat-island mitigation technologies.
Working with Industry to Develop Cool Colored Roofing
White roofs reflect the most sunlight and are coolest. However, U.S. homeowners tend to select dark roofs. Most conventional dark roofs are hot because they absorb both the visible and invisible (near infrared) components of sunlight. Berkeley Lab scientists, with funding from the California Energy Commission and the Department of Energy, partnered with the roofing industry and the Oak Ridge National Laboratory to develop cool nonwhite roofing materials for homes.
Cool color roofing technology makes solar-reflective roofing available in any color (dark or light) by selectively reflecting the invisible component of sunlight. Cool colors are not intended to replace light-colored roofing, but to improve the solar reflectance of the dark-colored roofing products that dominate the residential market. Cool color roofing can reduce a home's solar heat gain and air-conditioning energy use in a warm climate by about 10 to 20%. These roofs also lower a home's peak-hour cooling power demand by about 10 to 20%, helping prevent blackouts and brownouts on hot summer afternoons. Cool dark roofs yield a net annual energy savings (decrease in cooling energy minus increase in heating energy) of about 6 to 11%.
Berkeley Lab scientists used pigment spectroscopy to identify solar-reflective pigments of different colors and developed software for the design of cool color coatings. They formed and collaborated with a consortium of U.S. pigment, coating and roofing manufacturers to develop novel methods to manufacture asphalt shingle, clay tile, concrete tile, and metal roofing in a wide palette of cool colors. Berkeley Lab research estimates that applying cool-colored roofs to residences in 11 U.S. cities could achieve a net energy savings in the U.S. worth over $400 million per year. In California, the net energy savings would be worth more than $40 million per year.
The cool roof team now works with sixteen industrial partners who represent more than 95% of the residential roofing products produced in the U.S. Berkeley Lab's roofing partners have had such remarkable success that other industries are interested in developing partnerships with Berkeley Lab. The research team is interested in bringing cool materials to the paving industry, and working with automobile manufacturers on cool colors for automotive paints.
- Berkeley Lab researchers collaborated with the following pigment manufacturers and roofing materials manufacturers:
- 3M Industrial Minerals (St. Paul, MN)
- Akzo Nobel (Macungie, PA)
- American Rooftile Coatings (Fullerton, CA)
- BASF Industrial Coatings (Colton, CA)
- CertainTeed Corporation (Valley Forge, PA)
- Custom-Bilt Metals (Chino, CA)
- Elk Corporation (Ennis, TX)
- Ferro Corporation (Cleveland, OH)
- GAF Materials (Wayne, NJ)
- Hanson Roof Tile (Fontana, CA)
- ISP Minerals (Hagerstown, MD)
- MCA Clay Tile (Corona, CA)
- Monier Lifetile (Thousand Oaks, CA)
- Owens Corning (Granville, OH)
- Shepherd Color Company (Cincinnati, OH)
- Steelscape Inc. (Kalama, WA)