CBS Newsletter
Spring 1994
pg. 7

Heat Islands - And How to Cool Them

Albedo modification results: Regions within the modeling domain that have been identified for simulated albedo augmentation. Dark green areas are unmodified, light green areas represent modification of less than 0.10, and white is a modification in excess of 0.10. The average albedo increase over the 394 modified cells is 0.16. The second graphic in this document (depicting the central part of the first map) shows the difference between the high-albedo case and the base-case simulation at noon.

The desert oasis is often represented in movies as an island of cool green palms and a running spring or pool amid a sea of sand. The urban oasis is in some ways its opposite, a dark "heat island" whose temperature profile stands out from the cool greenery of the surrounding countryside.

Since 1985, a group of LBL researchers, including Hashem Akbari, Art Rosenfeld, Sarah Bretz, Beth Fishman, Dan Kurn, and Haider Taha, has been studying urban heat islands and ways to mitigate their high temperature. They have found that on a summer day, the average temperature in a typical American city is about 3 to 5 degrees F hotter than the surrounding area; they also estimate that air conditioning to cool cities from this effect accounts for 5 to 10% of urban peak electric demand. In Los Angeles alone, the additional electricity costs more than $100 million per year, not counting the costs of the added smog concentration caused by this heat.

Islands in the Sun

The elevated temperatures of urban heat islands are increasing with population and new building growth. Since 1940, temperatures of many cities have climbed steadily by 0.25 to 1 degree F per decade. A hot summer afternoon can raise peak cooling demands throughout the U.S. by about 10 GW, which costs several billion dollars each year. Los Angeles has experienced one of the largest observed rises: each 1 degree F rise there increases peak cooling demand by 1.5%.

Heat islands compromise air quality through two mechanisms. First, power plants that generate the additional electricity to meet the load produce pollution. Second, higher air temperature enhances the formation of smog-in Los Angeles, the probability increases 2 to 4% per degree F. When the city is below 70 degrees F, smog episodes are rare. Smog appears more than 50% of the time when the temperature reaches 90 degrees F. Reducing the daily high temperature by 5 degrees F in Los Angeles could eliminate one-third of its smog episodes.

The Comfort of Shady Trees and Lightened Surfaces
Inexpensive ways of mitigating heat-island effects are as old as human civilization: planting shade trees and changing the color of surfaces so that they reflect more incoming solar radiation, for example, by painting them or covering them with lighter materials. The high "albedo" of a light-colored surface is good at reflecting the sun's energy. Shade trees reduce heat gain by directly shading buildings as well as through evapotranspiration. Results indicate that shade trees can reduce cooling energy use in buildings by about 10% of the capital cost of avoided power plants and air conditioning equipment. Light-colored surfaces can cool even more effectively with more immediate results than shade trees, which take time to grow. The cost of saved energy is less than 1¢/kWh and 2¢/kg of carbon respectively. Assuming an average of 5¢/kWh for electricity, the net cost is 4¢/kWh. The approximate net cost of avoided CO2 is about -$200/ton of carbon.

To simulate the effects of lightening and greening a city, the LBL scientists used a three-dimensional meteorological model of the Los Angeles Basin consisting of 2600 cells, each 25 km2. They identified 394 of the 2,600 cells as "developed areas" where lightening agents could increase the albedo of the cells' impermeable surfaces. When the albedo is increased by about 0.16, the average difference between the current and lightened Los Angeles at 3 p.m. is 4 degrees F.

Looking for Energy Savings
Seeking to quantify the energy saved from mitigation techniques, Center researchers gathered data during the summers of 1991 and 1992 at residences and school bungalows in Sacramento, California. In 1992, the team placed shade trees at one house for four weeks and measured the home's energy use. After moving the shade trees to the other site, they made the same measurements. A comparison of the homes suggests that the trees saved 30% of cooling energy use in the unshaded building. By changing the albedo of one house's roof from a dark 0.16 to a very light 0.78, the team measured a seasonal air-conditioning savings of about 40% (330 kWh/yr). This work is giving the Sacramento Municipal Utility District the background data for establishing a demand-side management program to save air-conditioning energy. Already, there is sufficient evidence to claim that utility-sponsored DSM programs could save perhaps $100 million per year in energy costs through these simple, inexpensive mitigation methods.

—Allan Chen

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Hashem Akbari
Heat Island Project
Energy Analysis Program
(510) 486-4287; (510) 486-6996 fax


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