Cooling Our Communities: An Overview of Heat Island Project Activities
H. Akbari
Modern urban areas usually have dark surfaces and less vegetation than their surroundings. Such differences affect the climate, energy use, and habitability of cities. Dark roofs on buildings are heated by the summer sun and raise the summertime cooling demands of buildings. The dark surfaces and reduced vegetation collectively warm the summer air over urban areas, leading to the creation of the summer urban heat island. On a clear summer afternoon, the air temperature in a typical city is about 2.5°C (5°F) hotter than the surrounding rural area. We have found that peak urban electric demand in five American cities (Los Angeles, CA; Washington, D.C.; Phoenix, AZ; Tucson, AZ; and Colorado Springs, CO) rises by 2-4% for each 1°C rise in daily maximum temperature above a threshold of 15- 20°C. Thus, the additional air-conditioning use caused by this urban air temperature increase is responsible for 5-10% of urban peak electric demand.
What can be done to counteract the heat island effect? The Heat Island Project has examined both building- and city-scale effects of the urban surface on energy use and climate. At the building scale, cool roofs reduce air conditioning loads. Numerous experiments on individual buildings in California and Florida show that painting the roof white reduces the air conditioning load between 10 and 50%, depending on the thickness of insulation under the roof. At the community scale, increasing the albedo (solar reflectance) of urban surfaces and planting trees in urban areas can limit or reverse the urban heat island effectively and inexpensively. An estimate of the national impact of cool surfaces and shade trees (combining the cooling effect at the building level and community-wide cooling) is summarized in the Table.
For highly absorptive (low-albedo) surfaces, the difference between the surface and ambient air temperature, may be as high as 50°C (100°F), while for less absorptive (high-albedo) surfaces, such as white paint, the difference is about 10°C. For this reason, shade trees (i.e., trees that directly shade buildings) and cool surfaces (which absorb little of the incident insolation) are effective means of cooling buildings and reducing energy use. Through direct shading and evapotranspiration, trees reduce summer cooling energy use in buildings at about 1% of the capital cost of avoided power plants plus air-conditioning equipment. Cool surfaces are more effective than trees and cost little if color changes are incorporated into routine re-roofing and resurfacing schedules. In addition, the results from light-colored surfaces are immediate, while it may be ten or more years before a tree is large enough to produce significant energy savings.
Reflective urban surfaces and shade trees also reduce smog. We simulated the cooling achieved by increasing the solar reflectance of roofs and roadways in the Los Angeles Basin. The results show a 2°C (4°F) cooling by noon, when smog is forming rapidly. Putting these results into the Los Angeles smog model then predicts a reduction in population-weighted smog of 10-20%.
Table. Basecase U. S. air-conditioning use and savings potential of cool surfaces and shade trees. We estimate that 20% of air conditioning can be avoided by the year 2015.
| 1995 Basecase | 2015 Basecase | Savings * | |
| Electricity (TWh) | 440 | 540 | 108 |
| CostÝ (billon $) | 44 | 54 | 11 |
| CO2 (MtC) | 110 | 135 | 27 |
*Assuming 1 kWh costs 10¢ in 1994 dollars.
ÝPotential savings in 20 years when roof resurfacing is completed and shade trees have matured.
The Table describes the potential savings. However, achieving this potential is conditional on receiving the necessary federal support. Programs for planting shade trees already exist, but starting an effective and comprehensive program requires research and material development, wholesale technology transfer and implementation guidelines, and outreach activities. In the following articles we briefly discuss some of the progress in these areas in FY 1995. Taha examines the impact of large-scale albedo and vegetation modifications on ozone air quality in the Los Angeles Basin. Pomerantz et al. present the results of a project to estimate direct energy cost savings from cool roofs in Sacramento, CA. His work is complemented by Taha et al., who present the result of their meteorological modeling and estimating energy impacts of cool surfaces in several U.S. regions. In our field measurement activities, Levinson and Akbari discuss preliminary data on the rate of evapotranspiration from a tree while Smith and Akbari present a study of microclimate variation around a house in a suburban setting. Berdahl et al. discuss a database for cool materials, focusing on roofing materials. Discussion of cool paving materials is presented by Pomerantz et al. Our database activities are complemented by the development of American Society for Testing of Materials (ASTM) standards for measuring solar reflectance and thermal emittance of construction materials and comparing their steady-state surface temperatures, as presented by Akbari.
References
Akbari H, Davis S, Dorsano S, Huang J, Winnett S (eds.). Cooling Our Communities: A Guidebook on Tree Planting and Light-Colored Surfacing. U.S. Environmental Protection Agency, Office of Policy Analysis, Climate Change Division. Lawrence Berkeley National Laboratory Report No. LBL-31587, 1992.
Akbari H, Rosenfeld A, Taha H. Summer heat islands, urban trees, and white surfaces. In: Proceedings of American Society of Heating, Refrigeration, and Air Conditioning Engineers, February 10-14, 1990. Atlanta, GA: American Society of Heating, Refrigeration, and Air Conditioning Engineers. Also published as Lawrence Berkeley National Laboratory Report LBL-28308, 1990.
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