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Simulating a Cooler LA

Urbanized Los Angeles covers 10,000 square kilometers and includes about 1,250 square kilometers of roof and another 1,250 square kilometers of pavement. Obviously, we cannot instantly replace these with cooler-colored materials. Nor can we quickly plant the 10 million shade trees that would make a difference. We can, however, simulate these actions using computer models. In our own simulation, we raise the city albedo-the reflected fraction of incident solar heat-by a modest 7.5 percent and cover 5 percent of its area with 10 million trees.

The models indicate that our “cool community” strategy has a lucrative benefit/cost ratio. The use of white roofs and shade trees in Los Angeles would lower the need for air conditioning by 18 percent, or 1.04 billion kilowatt-hours, for the buildings directly affected by the roofs and shaded by the trees. If we assume a price of peak electricity of 10 cents per kilowatt-hour-not uncommon-this translates into savings of $100 million per year.

Because white shingles show discoloration by fungus, the manufacturer must add fungicide, raising the cost. The difference, however, is not large. For a 1,000-square-foot roof, the cost premium of cooler shingles is less than $25. If lighter tiles raise the albedo 35 percentage points, the additional investment pays for itself in less than one summer’s worth of lowered air-conditioning bills.

There is also a large indirect benefit. If an entire community drops a degree or so in temperature, thanks to lighter roofs and pavement and to the evapotranspiration from trees, then everyone’s air-conditioning load goes down-even those buildings that are not directly shaded or that still have dark roofs. This indirect annual savings would total an additional 12 percent-0.7 billion kilowatt-hours, or $70 million. As shown in the table below, implementing these cool community measures would lower the need for peak electrical generating capacity by about 1,500 megawatts-equivalent to two or three large power plants.

The cooler temperature would lower smog, too. Smog “exceedance”-the amount by which ozone levels top the California standard of 90 parts per billion-would drop 12 percent. Ozone can irritate the eyes, inflame the lungs, trigger asthma attacks, and lower the respiratory system’s ability to fight off infection. While other components of air pollution also exact a toll on health-especially particulates and sulfur dioxide-ozone is figured to be responsible for about $3 billion in health-related costs every year in the Los Angeles basin. Thus a 12 percent reduction in ozone exceedance could save $360 million.

The benefits of light surfaces and shade trees extend beyond Los Angeles. The 18 percent direct savings of air conditioning attained by shading and lightening individual buildings do not depend on the size of the city, only on its climate; Atlanta, for example, would enjoy the same percentage reduction as Los Angeles. The indirect savings, on the other hand, will be significant only in large cities with significant heat islands. Since about half the U.S. population lives in heat islands, we estimate that the annual direct plus indirect U.S. air-conditioning energy savings, after 20 years, might be 10 percent. Peak air-conditioning demand would probably drop by 5 percent.

Benefits to Los Angeles of “Cool Communities” Measures

Direct Energy SavingsIndirect Energy SavingsSmog BenefitTotalsAvoided peak power (MW)A/C cost savings ($M/yr)Avoided peak power (MW)A/C cost savings ($M/yr)Avoided medical costs, 12% ozone reduction ($M/yr)Total avoided peak power (MW)Total cost savings ($M/yr)Cooler roofs4004620021104600171Trees6005830035180900273Cooler pavement00100157610091Total1000104600713601600535

Trees and light-colored roofing materials could save energy and clean the air, computer models show. “Direct” savings refer to the cooling effect on individual buildings. “Indirect” savings refer to cuts in air conditioning load for all buildings as the temperature of the surrounding community drops. The figures assume the planting of 10 million new trees and the lightening of 2,500 square kilometers of roofs and pavement.

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Credit: S. Stetson, Global Environmental Management

Tagged: Energy

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