Cool roofs and trees -- cheap energy savings!

Energy Saving Potentials and Air Quality Benefits of
Urban Heat Island Mitigation

Simple ways to cool the cities are the use of reflective surfaces (rooftops and pavements) and planting of urban vegetation. On a large scale, the evapotranspiration
from vegetation and increased reflection of incoming solar radiation by reflective surfaces will cool a community a few degrees in the summer.

(1) Resurface pavement and rooftops with reflective surfaces
(2) Plant 3 trees per house

As an example, computer simulations for Los Angeles, CA show that resurfacing about two-third of the pavements and rooftops with reflective surfaces and planting three trees per house can cool down LA by an average of 2–3K. This reduction in air temperature will reduce urban smog exposure in the LA basin by roughly the same amount as removing the basin entire onroad vehicle exhaust.

Heat island mitigation is an effective air pollution control strategy, more than paying for itself in cooling energy cost savings. We estimate that the cooling energy savings in U.S. from cool surfaces and shade trees, when fully implemented, is
about $5 billion per year (about $100 per air-conditioned house). Another benefit of a light-colored roof is a potential increase in its useful life. Expansion and contraction of a lightcolored roof is smaller than that of a dark one. Also, the degradation of materials resulting from the absorption of ultra-violet light is a temperature-dependent process.

Urban areas have typically darker surfaces and less vegetation than their surroundings (HIG 2005). These differences affect climate, energy use, and habitability of cities. At the building scale, dark roofs heat up more and thus raise the summertime cooling demands of buildings. Collectively, dark surfaces and reduced vegetation warm the air over urban areas, leading to the creation of urban "heat islands." On a clear summer afternoon, the air temperature in a typical city is as much as 2.5K higher than in the surrounding rural areas. Research shows that peak urban electric demand rises by 2–4% for each 1K 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.

Not only do summer heat islands increase system-wide cooling loads, they also increase smog production because of higher urban air temperatures (Taha et al. 1994). Smog is created by photochemical reactions of pollutants in the air; and these reactions are more likely to intensify at higher temperatures. For example, in Los Angeles, for every 1°C the temperature rise above 22°C, incident of smog increases by 5%.

The simulations of the effects of higher albedo on smog formation indicate that an albedo change of 0.3 throughout the developed 25% of the city would yield a 12%.

It has been estimated (Hall et al. 1992) that residents of L.A. would be willing to pay about $10 billion per year to avoid the medical costs and lost work time due to air pollution. The greater part of pollution is particulates, but the ozone contribution averages about $3 billion/yr. Assuming a proportional relationship of the cost with the amount of smog exceedance, the cooler-surfaced city would save 12% of $3 billion/yr, or $360M/yr.


ROOFS

Use of high-albedo urban surfaces and planting of urban trees are inexpensive measures that can reduce summertime temperatures.

Most high-albedo roofing materials are light colored, although selective surfaces that reflect a large portion of the infrared solar radiation but absorb some visible light can be dark colored and yet have relatively high albedos.

Examples: coating roofs with a white elastomer with a reflectivity of 0.70.

Application:

retail stores in a strip mall in Florida before and after applying a high-albedo coating to the roof measured a 25% drop in seasonal cooling energy use.

energy savings of 28% for a school building in Georgia which had an unpainted galvanized roof that was coated with white acrylic.

Caution:
Glare and visual discomfort can result if the number of light, reflective roofs are not kept to a reasonable level. Fortunately, the glare for flat roofs is not a major problem for those who are at street level. For sloped roofs, the problem of glare should be studied in detail before proceeding with a full-scale implementation of this measure.

Materials:
Roofing shingles are available in a variety of colors, including white, at the same price.

Reflective mineral granules or gravelcan be chosen material at the time of installation without adding to the cost of the
roof.
NOTE: other alternatives are "green roofs" that are covered with living vegetation such as grass, sedge, and wildflowers.

STREET and PARKING LOT PAVEMENT

Paved urban surfaces that are lighter in color reflect more light back into space and the surfaces and the air are cooler. Good maintenance practice calls for resurfacing a new road after about 10 years and the lifetime of resurfacing is only about 5 years. Hence, within 10 years, all the asphalt concrete surfaces in a city can be made light colored.

Materials: Application of products with an albedo of about 0.35, similar to that of cement concrete.

TREES

Shade trees intercept sunlight before it warms a building. The urban forest cools the air by evapotranspiration. Trees also decrease the wind speed under their canopy and shield buildings from cold winter breezes. Urban shade trees offer significant benefits by both reducing building air conditioning and lowering air temperature, and thus improving urban air quality by reducing smog. Over the life of a tree, the savings associated with these benefits vary by climate region and can be up to $200 per tree. The cost of planting trees and maintaining them can vary from $10 to $500 per tree.

Data on measured energy savings from urban trees are scarce.
Shading and microclimate effects of the trees at two monitored houses yielded seasonal cooling energy savings of 30%.

Rosenfeld (1998) studied the potential benefits of planting 11M trees in the Los Angeles Basin. They estimate an annual total savings of $270 million from direct and indirect energy savings and smog benefit; about 2/3 of the savings resulted from the reduction in smog concentration resulting from meteorological changes due to the evapotranspiration of trees. It also has been suggested that trees improve air quality by dry-depositing NOX, O3, and PM10 particulates. Rosenfeld et al. (1998) estimate that 11M trees in LA will reduce PM10 by less than 0.1%, worth only $7M, which is disappointingly smaller than the benefits of $180M from smog reduction.

Savings:
a homeowner who plants three shade trees would have a
present value of about $200 per home ($68/tree). The present value of indirect savings was about $72/home ($24/tree). The PV of smog savings was about $120/tree. Total PV of all benefits from trees was thus $210/tree.

Other benefits associated with urban trees include improvement in the quality of life, increased value of properties, decreased rain run-off water and hence a protection against floods. Trees also directly sequester atmospheric carbon dioxide.

Cautions:
Some trees emit volatile
organic compounds (VOCs) that exacerbate the smog problem. Obviously, selection of low-emitting trees should be considered in a large-scale tree-planting program.

Benjamin (1996) has prepared a list of several hundred tree species with their average emission rate.

Drought-resistant trees are recommended. Some trees need significant maintenance that may entail high costs over
the life of the trees. Tree roots can damage underground pipes, pavements and foundations. Proper design is needed to minimize these effects. Also, trees are a fuel source for fire; selection of appropriate tree species and planting them strategically to minimize the fire hazard is necessary.

Cost:
A promotional planting of trees 5–10 feet high costs about $10 per tree, whereas a professional tree-planting program using fairly large trees could amount to $150–470 a tree.

The cost elements include planting, pruning, removal of dead trees, stump removal, waste disposal, infrastructure repair, litigation and liability, inspection, and program administration. The life-cycle cost for trees located in parks, in yards, and along streets, highways, and houses
costs (including planting) is $300–500 per tree. Over 90% of the cost is associated with professional planting, pruning, tree and stump removal.

The best programs are probably the information programs that provide data on energy and smog savings of trees to the communities and homeowners that are considering planting trees for other reasons.

Program sources:

American Society for Testing of Materials (ASTM)
Cool Roof Rating Council (CRRC)
Building Energy Performance Standards of ASHRAE (American Society of Heating Refrigeration, and Airconditioning
Engineers)
California Title 24
California South Coast's Air Quality Management Plans.
The South Coast Air Quality Management District and the United States Environmental Protection Agency (EPA) now recognize that air temperature is as much a cause of smog as NOX or volatile organic compounds.










Source: Hashem Akbari
Heat Island Group
Lawrence Berkeley National Laboratory