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Cool Surfaces and Shade Trees to Reduce Energy Use and Improve Air Quality in Urban Areas

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Elevated summertime temperatures in urban ‘heat islands’ increase cooling-energy use and accelerate the formation of urban smog. Except in the city’s core areas, summer heat islands are created mainly by the lack of vegetation and by the high solar radiation absorptance by urban surfaces. Analysis of temperature trends for the last 100 years in several large U.S. cities indicate that, since ∼1940, temperatures in urban areas have increased by about 0.5–3.0°C. Typically, electricity demand in cities increases by 2–4% for each 1°C increase in temperature. Hence, we estimate that 5–10% of the current urban electricity demand is spent to cool buildings just to compensate for the increased 0.5–3.0°C in urban temperatures. Downtown Los Angeles (L.A.), for example, is now 2.5°C warmer than in 1920, leading to an increase in electricity demand of 1500 MW. In L.A., smoggy episodes are absent below about 21°C, but smog becomes unacceptable by 32°C. Because of the heat-island effects, a rise in temperature can have significant impacts. Urban trees and high-albedo surfaces can offset or reverse the heat-island effect. Mitigation of urban heat islands can potentially reduce national energy use in air conditioning by 20% and save over $10B per year in energy use and improvement in urban air quality. The albedo of a city may be increased at minimal cost if high-albedo surfaces are chosen to replace darker materials during routine maintenance of roofs and roads. Incentive programs, product labeling, and standards could promote the use of high-albedo materials for buildings and roads. Similar incentive-based programs need to be developed for urban trees.
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Solar Energy Vol. 70, No. 3, pp. 295–310, 2001
Published by Elsevier Science Ltd
Pergamon PII: S0038–092X(00)00089–X Printed in Great Britain
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COOL SURFACES AND SHADE TREES TO REDUCE ENERGY USE AND
IMPROVE AIR QUALITY IN URBAN AREAS
H. AKBARI , M. POMERANTZ and H. TAHA
Lawrence Berkeley National Laboratory, Heat Island Group, Berkeley, CA, USA
Abstract—Elevated summertime temperatures in urban ‘heat islands’ increase cooling-energy use and
accelerate the formation of urban smog. Except in the city’s core areas, summer heat islands are created mainly
by the lack of vegetation and by the high solar radiation absorptance by urban surfaces. Analysis of
temperature trends for the last 100 years in several large U.S. cities indicate that, since |1940, temperatures in
urban areas have increased by about 0.5–3.08C. Typically, electricity demand in cities increases by 2–4% for
each 18C increase in temperature. Hence, we estimate that 5–10% of the current urban electricity demand is
spent to cool buildings just to compensate for the increased 0.5–3.08C in urban temperatures. Downtown Los
Angeles (L.A.), for example, is now 2.58C warmer than in 1920, leading to an increase in electricity demand of
1500 MW. In L.A., smoggy episodes are absent below about 218C, but smog becomes unacceptable by 328C.
Because of the heat-island effects, a rise in temperature can have significant impacts. Urban trees and
high-albedo surfaces can offset or reverse the heat-island effect. Mitigation of urban heat islands can
potentially reduce national energy use in air conditioning by 20% and save over $10B per year in energy use
and improvement in urban air quality. The albedo of a city may be increased at minimal cost if high-albedo
surfaces are chosen to replace darker materials during routine maintenance of roofs and roads. Incentive
programs, product labeling, and standards could promote the use of high-albedo materials for buildings and
roads. Similar incentive-based programs need to be developed for urban trees. Published by Elsevier Science
Ltd.
1. INTRODUCTION cities were cooler than their surroundings. After
1940, when built-up areas began to replace vege-
Modern urban areas have typically darker surfaces tation, the urban centers became as warm or
and less vegetation than their surroundings. These warmer than the suburbs, and the warming trend
differences affect climate, energy use, and became quite obvious, so that, from 1965 to 1989,
habitability of cities. At the building scale, dark urban temperatures have increased by about 18C.
roofs heat up more and, thus, raise the summer- Regardless of whether or not there is a tem-
time cooling demands of buildings. Collectively, perature difference from rural conditions, data
dark surfaces and reduced vegetation warm the air suggest that temperatures in cities are increasing.
over urban areas, leading to the creation of urban Fig. 1 depicts the summertime monthly maximum
‘heat islands’. On a clear summer afternoon, the and minimum temperatures between 1877 and
air temperature in a typical city is as much as 1997 in downtown Los Angeles. It clearly indi-
2.58C higher than in the surrounding rural areas. cates that the maximum temperatures at dow-
We have found that peak urban electric demand ntown Los Angeles are now about 2.58C higher
rises by 2–4% for each 18C rise in daily maxi- than they were in 1920. The minimum tempera-
mum temperature above a threshold of 15 to tures are about 48C higher than they were in 1880.
208C. Thus, the additional air-conditioning use In Washington, DC, temperatures increased by
caused by this urban air temperature increase is about 28C between 1871 and 1987. The data
responsible for 5–10% of urban peak electric indicate that this recent warming trend is typical
demand, at a direct cost of several billion dollars of most U.S. metropolitan areas, and exacerbates
annually. demand for energy.
In California, Goodridge (1987, 1989) showed Akbari et al. (1992) have found that peak urban
that, before 1940, the average urban–rural tem- electric demand in six American cities (Los
perature differences for 31 urban and 31 rural Angeles, CA; Washington, DC; Phoenix, AZ;
stations in California were always negative, i.e., Tucson, AZ; and Colorado Springs, CO) rises by
2–4% for each 18C rise in daily maximum
temperature above a threshold of 15 to 208C (the
Author to whom correspondence should be addressed. Tel.: case of Los Angeles is shown in Fig. 2). For the
11-510-486-4287; fax: 11-510-486-4673; e-mail:
h akbari@lbl.gov Los Angeles Basin, it is estimated that the heat
]
295
296 H. Akbari et al.
Fig. 2 also shows the probability of smoggy days
in Los Angeles, as measured by ozone concen-
tration vs. temperature. At maximum daily tem-
peratures below 228C, the maximum concentra-
tion of ozone is below the California standard of
90 parts per billion (ppb); at temperatures above
358C, practically all days are smoggy.
2. HEAT ISLAND MITIGATION
1
Use of high-albedo urban surfaces and the
planting of urban trees are inexpensive measures
that can reduce summertime temperatures. The
effects of modifying the urban environment by
planting trees and increasing albedo are best
quantified in terms of ‘direct’ and ‘indirect’
contributions. The direct effect of planting trees
Fig. 1. Ten-year running average summertime monthly maxi- around a building or using reflective materials on
mum and minimum temperatures in Los Angeles, California roofs or walls is to alter the energy balance and
(1877–1997). The ten-year running average is calculated as cooling requirements of that particular building.
the average temperature of the previous four years, the current However, when trees are planted and albedo is
year, and the next five years. Note that the maximum modified throughout an entire city, the energy
temperatures have increased by about 2.58C since 1920. balance of the whole city is modified, producing
city-wide changes in climate. Phenomena associ-
island increases power consumption by about 1– ated with city-wide changes in climate are re-
1.5GW, costing the rate-payers over $100 million ferred to as indirect effects, because they indirect-
per year. Nationwide, the additional air-condition- ly affect the energy use in an individual building.
ing use caused by urban air temperature increase Direct effects give immediate benefits to the
is responsible for 5–10% of urban peak electric building that applies them. Indirect effects achieve
demand, at a direct cost of several billion dollars benefits only with widespread deployment.
annually. There is an important distinction between direct
Not only do summer heat islands increase and indirect effects: while direct effects are
system-wide cooling loads, but they also increase recognized and accounted for in present models of
smog production because of higher urban air building-energy use, indirect effects are ap-
temperatures (Taha et al
.
, 1994). For example, preciated far less. Accounting for indirect effects
is more difficult and the results are comparatively
less certain. Understanding these effects and
incorporating them into accounts of energy use
and air quality is the focus of our current research.
It is worth noting that the phenomenon of summer
urban heat islands is itself an indirect effect of
urbanization.
The issue of direct and indirect effects also
enters into our discussion of atmospheric pollu-
tants. Planting trees has the direct effect of
reducing atmospheric CO because each individ-
2
ual tree directly sequesters carbon from the at-
mosphere through photosynthesis. However,
planting trees in cities also has an indirect effect
1
When sunlight hits an opaque surface, some of the energy is
reflected (this fraction is called the albedo 5a), and the
Fig. 2. Ozone levels and peak power for Southern California rest is absorbed (the absorbed fraction is 1 2a). Low-a
Edison versus 4 p.m. temperature in Los Angeles, California. surfaces of course become much hotter than high-a sur-
(Source: Akbari et al
.
, 1990). faces.
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 297
on CO . By reducing the demand for cooling meteorology and its sensitivity to changes in
2
energy, urban trees indirectly reduce emission of surface properties. The Urban Airshed Model
CO from power plants. Akbari et al
.
(1990) (UAM) was used to simulate the impacts of the
2
showed that the amount of CO avoided via the changes in meteorology and emissions on ozone
2
indirect effect is considerably greater than the air quality. The CSUMM and the UAM essential-
amount sequestered directly. Similarly, trees di- ly solve a set of coupled governing equations
rectly trap ozone precursors (by dry-deposition), a representing the conservation of mass (continui-
direct effect, and indirectly reduce the emission of ty), potential temperature (heat), momentum,
these precursors from power plants (Taha, 1996). water vapor, and chemical species continuity to
obtain for prognostic meteorological fields and
pollutant species concentrations.
3. TOOLS FOR ANALYSIS The CSUMM is a hydrostatic, primitive-equa-
Fig. 3 depicts the overall methodology used in tion, three-dimensional Eulerian model that was
analyzing the impact of heat-island mitigation originally developed by Pielke (1974). The model
measures on energy use and urban air pollution. is incompressible (uses incompressibility assump-
The DOE-2 building-energy simulation program tion to simplify the equation for conservation of
is used to calculate the energy use and energy mass), and employs a terrain-following coordinate
savings in buildings. To calculate the direct system. It uses a first order closure scheme in
effects, prototypical buildings are simulated with treating sub-grid scale terms of the governing
dark- and light-colored roofs, and with and with- differential equations. The model’s domain is
out shade trees. Typical weather data for each about 10 km high with an underlying soil layer
climate region of interest are used in these that is about 50 cm deep. The CSUMM generates
calculations. To calculate the indirect effects, the three-dimensional fields of prognostic variables as
typical weather data input to DOE-2 are first well as a boundary layer height profile that can be
modified to account for changes in the urban input to the UAM.
climate. The prototypical buildings are then simu- The UAM is a three-dimensional, Eulerian,
lated with the modified weather data to estimate photochemical model that is capable of simulating
savings in heating and cooling energy consump- inert and chemically reactive atmospheric pollu-
tion. tants. It has been recommended by the U. S.
To understand the impacts of large-scale in- Environmental Protection Agency (EPA) for
creases in albedo and vegetation on urban climate ozone air quality modeling studies of urban areas
and ozone air quality, mesoscale meteorological (EPA, 1986). The UAM simulates the advection,
and photochemical models are used. For example, diffusion, transformation, emission, and deposi-
Taha et al
.
(1995) and Taha (1996, 1997) used tion of pollutants. It treats about 30 chemical
the Colorado State University Mesoscale Model species and uses the carbon bond CB-IV mecha-
(CSUMM) to simulate the Los Angels Basin’s nism (Gery et al
.
, 1988). The UAM accounts for
Fig. 3. Methodology to analyze the impact of shade trees, cool roofs, and cool pavements on energy use and air quality (smog).
298 H. Akbari et al.
emissions from area and point sources, elevated 4. COOL ROOFS
stacks, mobile and stationary sources, and vegeta- At the building scale, a dark roof is heated by
tion (biogenic emissions). For a detailed discus- the sun and, thus, directly raises the summertime
sion of the use and adaptation of these models and cooling demand of the building beneath it. For
the study of the impact of the heat island mitiga- highly absorptive (low-albedo) roofs, the differ-
tion strategies in L.A. Basin, see Taha (1996, ence between the surface and ambient air tem-
1997). peratures may be as high as 508C, while for less
Examples of outputs from these simulations are absorptive (high-albedo) surfaces with similar
shown in Figs. 4 and 5. Fig. 4 shows the predicted insulative properties, such as roofs covered with a
reduction in air temperature in Los Angeles at 2 white coating, the difference is only about 108C
p.m. on August 27 as a result of increasing the (Berdahl and Bretz, 1997). For this reason, ‘cool’
urban albedo and vegetation cover by moderate surfaces (which absorb little ‘insolation’) can be
amounts (average increases of 7%). Fig. 5 shows effective in reducing cooling-energy use. Highly
corresponding changes in ozone concentrations. absorptive surfaces contribute to the heating of
Because of the combined effects of local emis- the air, and thus indirectly increase the cooling
sions, meteorology, surface properties, and topog- demand of (in principle) all buildings. Cool
raphy, ozone concentrations increase in some surfaces incur no additional cost if color changes
areas and decrease in others. The net effect, are incorporated into routine re-roofing and re-
however, is a decrease in ozone concentrations. surfacing schedules (Bretz et al
.
, 1997 and Rosen-
The simulations also predict a reduction in feld et al
.
, 1992).
population-weighted exceedance exposure to Most high-albedo surfaces are light colored,
ozone (above the California and National Ambient although selective surfaces that reflect a large
Air Quality Standards) of 10–20% (Taha, 1996). portion of the infrared solar radiation but absorb
This reduction, for some smog scenarios, is some visible light may be dark colored and yet
comparable to ozone reductions obtained by have relatively high albedos (Berdahl and Bretz,
replacing all gasoline on-road motor vehicles with 1997).
electric cars.
Fig. 4. Temperature difference (from the base case) for a case with increased surface albedo and urban forest. The temperature
difference is at 2 p.m. on a late-August day in Los Angeles.
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 299
Fig. 5. Ozone concentrations difference (from the base case) for a case with increased surface albedo and urban forest. The
difference is shown for 2 p.m. on a late-August day in Los Angeles.
4.1.
Energy and smog benefits of cool roofs modeled with the DOE-2.1E simulation program.
Akbari et al
.
(1993) and Gartland et al
.
(1996)
4.1.1.
Direct energy savings. There is a sizable report that the simulations underestimated the
body of measured data documenting the direct cooling-energy savings and peak-power reduc-
energy-saving effects of light-colored roofs. In the tions by as much as twofold.
summers of 1991 and 1992, Akbari et al
.
(1993, Parker et al
.
(1995) monitored nine homes in
1997) monitored peak power and cooling-energy Florida before and after applying high-albedo
savings from high-albedo coatings on one house coatings to their roofs. Air-conditioning energy
and two school bungalows in Sacramento, Cali- use was reduced by 10–43%, with average sav-
fornia. They collected data on air-conditioning ings of 7.4 kWh/day (savings of 19%). Peak
electricity use, indoor and outdoor temperatures demand between 5 and 6 p.m. was reduced by
and humidities, roof and ceiling surface tempera- 0.2–1.0 kW, with an average reduction of 0.4 kW
tures, inside and outside wall temperatures, inso- (savings of 22%). The amount of energy savings
lation, and wind speed and direction. roughly inversely correlated with the amount of
Applying a high-albedo coating to one house ceiling insulation and the location of the duct
resulted in seasonal savings of 2.2 kWh/day (80% system: large savings in poorly insulated homes
of base-case use), and peak demand reductions of and those with the duct systems in the attic space,
0.6 kW (about 25% of base-case demand). In the and smaller savings in well insulated homes.
school bungalows, cooling-energy was reduced by Akbari et al
.
(1998) and Konopacki et al
.
3.1 kWh/day (35% of base-case use), and peak (1998) monitored the impacts of light-colored
demand by 0.6 kW (about 20% of base-case roofs on cooling-energy use of three commercial
demand). (It is important to note that altering the buildings in northern California. Increasing the
albedo starts to pay for itself immediately through reflectance of the roofs from an initial albedo of
the direct effect.) The buildings were also about 0.20 to 0.60 dropped the roof temperature
300 H. Akbari et al.
on hot summer afternoons by about 258C. Sum- Angeles, Dallas/Fort Worth, Houston, Miami/
mertime, standard-weekday, average daily air- Fort Lauderdale, New Orleans, New York City,
conditioning savings were 18% in a medical Philadelphia, Phoenix, and Washington, DC/
office building, 13% in a second medical office Baltimore. Cooling-energy savings and heating-
building, and 2% in a drug store. In another energy penalties were then obtained from the
demonstration project in Florida, Parker et al
.
difference in the simulated energy use of the
(1998) measured cooling electricity savings re- prototype buildings with light- and dark-colored
sulting from the application of light-colored roofi- roofs.
ng in a small strip mall; they reported savings of The study also estimated how much energy and
about 20 to 40%. The Sacramento Municipal money could be saved if all the roofs of existing
Utility District (SMUD) reports similar savings, building stocks in large metropolitan areas were
measured in about ten commercial buildings in changed from dark to light. This was done by
Sacramento (Hildebrandt et al
.
, 1998). scaling the simulated energy savings of the in-
Computer simulations are used to obtain esti- dividual prototype buildings by the amount of
mates of year-round effects for a variety of air-conditioned space immediately beneath all
building types and climates. A recent study made roofs in an entire MSA. For this purpose, we used
quantitative estimates of peak demand and annual data on the stock of commercial and residential
cooling-electricity use and savings that would buildings in each MSA, the saturation of heating
result from increasing the reflectivity of the roofs and cooling systems, the current roof reflec-
(Konopacki et al
.
, 1997). The estimates of annual tivities, and the local costs of electricity and gas.
net savings in cooling electricity are adjusted for Results for the 11 metropolitan areas are sum-
the penalty of increased wintertime heating- marized in Tables 1 and 2. Sum totals for all 11
energy use. The analysis is based on simulation of MSAs were: electricity savings, 2.6 tera-watt
2
building-energy use, using the DOE-2 building- hours (TWh) (200 kilowatt hours per 100 m roof
energy simulation program. The study specified area of air-conditioned buildings); heating energy
22
11 prototypical buildings: single-family residen- penalty, 6.9 TBtu (5 therms per 100 m ); net
2
tial (old and new), office (old and new), retail savings in energy bills, $194 M ($15 per 100 m );
store (old and new), school (primary and sec- and savings in peak demand 1.7 gigawatt (GW)
2
ondary), health care (hospital and nursing home), (135 W per 100 m ). Six building types account
and grocery store. Most prototypes were simu- for over 90% of the annual electricity and net
lated with two heating systems: gas furnace and energy savings: old residences accounted for more
heat pumps. DOE-2 simulations were performed than 55%, new residences for about 15%, and
for the prototypical buildings, with light and dark four other building types (old/new offices and
roofs, in a variety of climates, to obtain estimates
of the energy use for air conditioning and heating.
Weather data for 11 U.S. Metropolitan Statistical
2
Areas (MSAs) were used: Atlanta, Chicago, Los One therm is 100,000 Btu.
Table 1. Estimates of metropolitan-scale annual cooling electricity savings (GWh), net energy savings ($M), peak demand
electricity savings (MW), and annual natural gas penalty (GBtu) resulting from application of light-colored roofing on residential
and commercial buildings in 11 Metropolitan Statistical Areas
Metropolitan area Residential Commercial Commercial and residential
Elec Gas Net Peak Elec Gas Net Peak Elec Gas Net Peak
(GWh) (GBtu) (M$) (MW) (GWh) (GBtu) (M$) (MW) (GWh) (GBtu) (M$) (MW)
Atlanta 125 349 8 83 22 55 1 14 147 404 9 97
Chicago 100 988 6 89 84 535 4 56 183 1523 10 145
Los Angeles 210 471 18 218 209 154 18 102 419 625 35 320
Dallas/Ft Worth 241 479 16 175 71 113 4 36 312 592 20 211
Houston 243 284 21 127 79 62 6 30 322 347 27 156
Miami/Ft Lauderdale 221 4 18 115 35 3 2 11 256 7 20 125
New Orleans 84 107 6 27 33 28 3 16 117 135 9 42
New York 35 331 3 56 131 540 13 95 166 871 16 151
Philadelphia 44 954 21 108 47 292 4 49 91 1246 3 157
Phoenix 299 74 32 106 58 31 5 18 357 105 37 123
DC/Baltimore 182 845 6 183 45 184 2 31 227 1029 8 214
Total 1784 4886 133 1287 814 1997 62 458 2597 6884 194 1741
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 301
2
Table 2. Estimates of savings or penalties per 100 m of roof area of air-conditioned buildings resulting from application of
light-colored roofing on residential and commercial buildings in 11 Metropolitan Statistical Areas: annual cooling electricity
savings (kWh), net energy savings ($), peak demand electricity savings (W), and annual natural gas penalty (therms)
Metropolitan area Residential Commercial and residential Commercial
Elec Gas Net Peak Elec Gas Net Peak Elec Gas Net Peak
(kWh) (therms) ($) (W) (kWh) (therms) ($) (W) (kWh) (therms) ($) (W)
Atlanta 153 4 10 102 239 6 11 152 162 4 10 107
Chicago 131 13 8 116 228 15 11 152 162 13 9 128
Los Angeles 182 4 16 189 350 3 30 171 239 4 20 183
Dallas/Ft Worth 166 3 11 121 224 4 13 114 176 3 11 119
Houston 198 2 17 103 261 2 20 99 211 2 18 102
Miami/Ft Lauderdale 259 0 21 135 340 0 19 107 267 0 21 131
New Orleans 199 3 14 64 287 2 26 139 218 3 17 78
New York 104 10 9 166 211 9 21 153 173 9 17 158
Philadelphia 81 18 22 199 232 14 20 241 122 17 4 211
Phoenix 314 1 34 111 409 2 35 127 327 1 34 113
DC/Baltimore 137 6 5 138 221 9 10 152 148 7 5 140
old/new retail stores) together accounted for tential savings were valued at $104M /year
(Rosenfeld et al
.
, 1998).
about 25%.
The results for the 11 MSAs were extrapolated
4.2.
Other benefits of cool roofs
to estimate the savings in the entire United States. Another benefit of a light-colored roof is a
The study estimates that, nationally, light-colored potential increase in its useful life. The diurnal
roofing could produce savings of about 10 TWh/ temperature fluctuation and concomitant expan-
year (about 3.0% of the national cooling-electrici- sion and contraction of a light-colored roof is
ty use in residential and commercial buildings), an smaller than that of a dark one. Also, the degra-
increase in natural gas use by 26 GBtu/year dation of materials due to absorption of ultra-
(1.6%), a decrease in peak electrical demand of violet light is a temperature-dependent process.
7 GW (2.5%) (equivalent to 14 power plants each For these reasons, cooler roofs may last longer
with a capacity of 0.5 GW), and a decrease in net than hot roofs of the same material.
annual energy bills for the rate-payers of $750M.
4.3.
Potential problems with cool roofs
4.1.2.
Indirect energy and smog benefits. Using
the Los Angeles Basin as a case study, Taha Several possible problems may arise from the
(1996, 1997) examined the impacts of using cool use of reflective roofing materials (Bretz and
surfaces (cool roofs and pavements) on urban air Akbari, 1994, 1997). A drastic increase in the
temperature and, thus, on cooling-energy use and overall albedo of the many roofs in a city has the
smog. If higher albedo surfaces are thoroughly potential to create glare and visual discomfort if
applied, an urban heat island can be limited or not kept to a reasonable level. Besides being
reversed at negligible expense. In these simula- unpleasant, extreme glare could possibly increase
tions, Taha estimates that about 50% of the the incidence of traffic accidents. Fortunately, the
urbanized area in the L.A. Basin is covered by glare for flat roofs is not a major problem for
roofs and roads, the albedos of which can realisti- those who are at street level. For sloped roofs, the
cally be raised by 0.30 when they undergo normal problem of glare should be studied in detail
repairs. This results in a 28C cooling at 3 p.m. before proceeding with a full-scale implementa-
during an August episode. This summertime tion of this measure.
temperature reduction has a significant effect on In addition, many types of building materials,
further reducing building cooling-energy use. The such as tar roofing, are not well adapted to
annual savings in L.A. are estimated at $21M painting. Although such materials could be spe-
(Rosenfeld et al
.
, 1998). cially designed to have a higher albedo, this
Taha has also simulated the impact of urban- would be at a greater expense than painting.
wide cooling in Los Angeles on smog; the results Additionally, to maintain a high albedo, roofs
show a significant reduction in ozone concen- may need to be recoated or rewashed on a regular
tration. The simulations predict a reduction of basis. The cost of a regular maintenance program
10–20% in population-weighted smog (ozone). In could be significant.
L.A., where smog is especially serious, the po- A possible conflict of great concern is the fact
302 H. Akbari et al.
that building owners and architects like to have shade air-conditioning equipment would also like-
the choice as to what color to select for their ly be beneficial.
rooftops. This is particularly a concern for sloped
roofs.
5.1.
Energy and smog benefits of shade trees
5.1.1.
Direct energy savings. Data on mea-
4.4.
Cost of cool roofs sured energy savings from urban trees are scarce.
Increasing the overall albedo of roofs is an In one experiment, Parker (1981) measured the
attractive way of reducing the net radiative heat cooling-energy consumption of a temporary build-
gains through the roof, and, hence, reducing ing in Florida before and after adding trees and
building cooling loads. To change the albedo, the shrubs and found cooling-electricity savings of up
rooftops of buildings may be painted or covered to 50%. In the summer of 1992, Akbari et al
.
with a new material. Since most roofs have (1997) monitored peak-power and cooling-energy
regular maintenance schedules or need to be re- savings from shade trees in two houses in Sac-
roofed or recoated periodically, the change in ramento, California. The collected data included
albedo should be done then to minimize the costs. air-conditioning electricity use, indoor and out-
High-albedo alternatives to conventional roofi- door dry-bulb temperatures and humidities, roof
ng materials are usually available, often at little or and ceiling surface temperatures, inside and out-
no additional cost. For example, a built-up roof side wall temperatures, insolation, and wind speed
typically has a coating or a protective layer of and direction. The shading and microclimate
mineral granules or gravel. Under such condi- effects of the trees at the two monitored houses
tions, it is expected that choosing a reflective yielded seasonal cooling-energy savings of 30%,
material at the time of installation should not add corresponding to average savings of 3.6 and 4.8
to the cost of the roof. Also, roofing shingles are kWh/day. Peak-demand savings for the same
available in a variety of colors, including white, at houses were 0.6 and 0.8 kW (about 27% savings
the same price. The incremental price premium in one house and 42% in the other).
for choosing a white rather than a black single-ply A few other studies have focused on the wind-
membrane roofing material is less than 10%. Cool shielding effect of trees. DeWalle et al
.
(1983)
roofing materials that require an initial investment used mobile homes to measure the windbreaking
may turn out to be more attractive in terms of effects of trees on energy use. In a follow-up
life-cycle cost than conventional dark alternatives. experiment, Heisler (1989) measured the effect of
Usually, the lower life-cycle cost results from trees on wind and solar radiation in a residential
longer roof life and/or energy savings. neighborhood. Huang et al
.
(1990) used the data
provided by Heisler (1989) and simulated the
impact of shading and wind reduction on residen-
5. URBAN TREES tial buildings’ heating- and cooling-energy use.
The benefits of trees can also be divided into Their simulations indicated that a reduction in
direct and indirect effects: shading of buildings infiltration because of trees would save heating-
and ambient cooling (urban forest). Shade trees energy use. However, in climates with cooling-
intercept sunlight before it warms a building. The energy demand, the impact of windbreak on
urban forest cools the air by evapotranspiration. cooling is fairly small compared to the shading
Trees also decrease the wind speed under their effects of trees and, depending on climate, it
canopy and shield buildings from cold winter could decrease or increase cooling-energy use. In
breezes. Urban shade trees offer significant bene- cold climates, the wind-shielding effect of trees
fits by both reducing building air-conditioning, can substantially reduce heat-energy use in build-
lowering air temperature, and thus improving ings. Akbari and Taha (1992) simulated the wind-
urban air quality by reducing smog. Over the life shielding impact of trees on heating-energy use in
of a tree, the savings associated with these four Canadian cities. For several prototypical
benefits vary by climate region and can be up to residential buildings, they estimated heating-
$200 per tree. The cost of planting trees and energy savings in the range of 10 to 15%.
maintaining them can vary from $10 to $500 per In a recent study, Taha et al
.
(1996) simulated
tree. Tree-planting programs can be designed to the meteorological impact of large-scale tree-
be low cost, so they can offer savings to com- planting programs in ten U.S. metropolitan areas:
munities that plant trees. We are considering here Atlanta, GA; Chicago, IL; Dallas, TX; Houston,
trees that shade buildings. Placing trees in order to TX; Los Angeles, CA; Miami, FL; New York,
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 303
Table 3. Number of additional trees planted in each metropolitan area and their simulated effects in reducing the ambient
temperature. (Source: Taha et al
.
, 1996)
Location Millions of Millions of Max air temperature
addional trees additional trees reduction in the
in the simulation in the metropolitan hottest simulation
domain area cell (8C)
Atlanta 3.0 1.5 1.7
Chicago 12 5.0 1.4
Los Angeles 11 5.0 3.0
Fort Worth 5.6 2.8 1.6
Houston 5.7 2.7 1.4
Miami 3.3 1.3 1.0
New York City 20 4.0 2.0
Philadelphia 18 3.8 1.8
Phoenix 2.8 1.4 1.4
Washington, DC 11 3.0 1.9
NY; Philadelphia, PA; Phoenix, AZ; and simulation cells, the temperature was decreased
Washington, DC). Table 3 shows the number of by up to 38C (see Table 3). The corresponding
added trees simulated in each city and impact on air-conditioning savings resulting from ambient
air temperature. The number of trees in each grid cooling by trees in hot climates ranges from $5 to
2
cell varied from the low hundreds to the high tens $10 per year per 100 m of roof area of residential
of thousands. The DOE-2 building simulation and commercial buildings. Indirect effects are
program was then used to estimate the direct and smaller than direct shading, and, moreover, re-
indirect impacts of trees on saving cooling-energy quire that the entire city be planted.
use for two building prototypes: a single-family Based on the results of Taha et al
.
(1996),
residence and an office. The calculations ac- Rosenfeld et al
.
(1998) estimated the potential
counted for a potential increase in winter heating- benefits of trees, specifically in the Los Angeles
energy use. Table 4 shows that, in most hot cities, Basin. The study assumed planting 11M trees
shading a building can save annually $5 to $25 according to the following plan: three shade trees
2 2
per 100 m of roof area of residential and (each with a canopy cross section of 50 m ) per
commercial buildings. Savings in residential air-conditioned house, for a total of 5.4M trees;
2
building are higher than in commercial buildings. about one shade tree for each 250 m of non-
residential roof area for a total of 1M trees; 4.6M
5.1.2.
Indirect energy and smog benefits. Taha trees to shade non-air-conditioned homes or to be
et al
.
(1996) estimated the impact on ambient planted along streets, in parks, and in other public
temperature resulting from a large-scale tree- spaces. The results of that analysis are shown in
planting program in the selected ten cities. They Table 5. Note that about two-thirds of the savings
used a three-dimensional meteorological model to in L.A. result from the reduction in smog con-
simulate the potential impact of trees on ambient centration resulting from meteorological changes
temperature for each region. The mesoscale simu- due to the evapotranspiration of trees. It has also
lations showed that, on average, trees can cool been suggested that trees improve air quality by
down cities by about 0.3 to 18C at 2 p.m.; in some dry-depositing NO , O , and PM10 particulates.
x3
Table 4. DOE-2 simulated HVAC annual energy savings from trees. Three trees per house and per office are assumed. All
2
savings are $/100 m . (Source: Taha et al
.
, 1996)
Location Old residence New residence Old office New office
Direct Indirect Indirect Direct Indirect Direct Indirect Direct
Atlanta 5 2 3 1 3 2 2 2
Chicago 3 2 1 0.5 1 1 2 1
Los Angeles 12 8 7 5 6 12 4 10
Fort Worth 6 6 5 4 4 5 2 4
Houston 10 6 6 4 3 5 3 3
Miami 9 3 6 3 3 2 2 2
New York City 3 2 2 1 3 3 2 2
Philadelphia 250 2702111
Phoenix 27 8 16 5 9 5 6 4
Washington, DC 3 2 1 1 3 1 2 1
304 H. Akbari et al.
Table 5. Energy savings, ozone reduction, and avoided peak power resulting from use of urban trees in the Los Angeles Basin
(Source: Rosenfeld et al
.
, 1998)
Benefits Direct Indirect Smog Total
1 Cost savings from trees (M$/ year) 58 35 180 273
2DPeak power (GW) 0.6 0.3 0.9
3 Present value per tree ($) 68 24 123 211
Rosenfeld et al
.
(1998) estimate that 11M trees in other benefits of trees are not considered in the
L.A. will reduce PM10 by less than 0.1%, worth cost benefit analysis shown in this paper.
only $7M, which is disappointingly smaller than
5.3.
Potential problems with shade trees
the benefits of $180M from smog reduction. There are some potential problems associated
The present value (PV) of savings is calculated with trees. Some trees emit volatile organic
to find out how much a homeowner can afford to compounds (VOCs) that exacerbate the smog
pay for shade trees. Rosenfeld et al
.
(1998) problem. Obviously, selection of low-emitting
estimate that, on this basis, the direct savings to a trees should be considered in a large-scale tree-
home owner who plants three shade trees would planting program. Benjamin et al
.
(1996) have
have a present value of about $200 per home prepared a list of several hundred tree species
($68/tree). The present value of indirect savings with their average emission rates.
was smaller, about $72/home ($24/tree). The PV In dry climates and areas with a serious water
of smog savings was about $120/tree. Total PV of shortage, drought-resistant trees are recom-
all benefits from trees was then $210/tree. mended. Some trees need significant maintenance
Reducing smog by citywide cooling can be that may entail high cost over the life of the trees.
considered equivalent to reducing the formation Tree roots can damage underground pipes, pave-
of smog precursors at constant temperature. We ments and foundations. Proper design is needed to
estimate that shade trees will reduce the maxi- minimize these effects. Also, trees are a fuel
mum smog concentration by 5%. Using the ozone source for fire; selection of appropriate tree
3
‘isopleths’ (such as Milford’s), a 5% reduction in species and planting them strategically to mini-
smog is equivalent to reducing precursors by mize the fire hazard should be an integral com-
approximately 12%, i.e., reducing NO in L.A. by
x
ponent of a tree-planting program.
175 tons/day, a very significant drop and 25 times
5.4.
Cost of trees
more than the 4 tons/day through reduced power-
plant emissions. The cost of a citywide tree-planting program
depends on the type of program offered and the
5.2.
Other benefits of shade trees types of trees recommended. At the low end, a
There are other benefits associated with urban promotional planting of trees 5–10 feet high costs
trees. Some of these include improvement in the about $10 per tree, whereas a professional tree-
quality of life, increased value of properties, planting program using fairly large trees could
decreased rain run-off water and, hence, a protec- amount to $150 to $470 a tree (McPherson et al
.
,
tion against floods (McPherson et al
.
, 1994). 1994). McPherson has collected data on the cost
Trees also directly sequester atmospheric carbon of tree planting and maintenance from several
dioxide, but Rosenfeld et al
.
(1998) estimate that cities. The cost elements include planting, prun-
the direct sequestration of carbon dioxide is less ing, removal of dead trees, stump removal, waste
than one-fourth of the emission reduction re- disposal, infrastructure repair, litigation and
sulting from savings in cooling-energy use. These liability, inspection, and program administration.
The data provide details of the cost for trees
located in parks, yards, along streets, highways,
3
and houses. The present value of all of these
Milford et al
.
(1989) have carried out detailed calculations
analyzing the changes in the maximum ozone concen- life-cycle costs (including planting) is $300 to
tration reached in Los Angeles vs. initial concentration of $500 per tree. Over 90% of the cost is associated
NO and VOCs (volatile organic compounds). They pre-
x
with professional planting, pruning, and tree and
sented their calculations in the form of ‘isopleths’ of equal stump removal. On the other hand, a program
maximum smog concentration for various levels of NO
x
administered by the Sacramento Municipal Utility
and VOCs concentration (typically shown as a percent
reduction of emissions) for a typical summer episode. District (SMUD) and Sacramento Tree Founda-
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 305
tion in 1992–1996 planted 20-foot tall trees at an 6. COOL PAVEMENTS
average cost of $45 per tree. This only includes The practice of widespread paving of city
the cost of a tree and its planting; it does not streets with asphalt began only within the past
include pruning, removal of dead trees, and hundred years. The advantages of this smooth and
stump. With this wide range of costs associated all-weather surface for the movement of bicycles
with trees, in our opinion, tree costs should be and automobiles is obvious, but some of the
justified by other amenities they provide beyond associated problems are perhaps not so well
air-conditioning and smog benefits. The best appreciated. One consequence of covering streets
programs are then probably the information pro- with dark asphalt surfaces is the increased heating
grams that provide data on energy and smog of the city by sunlight. A dark surface absorbs
savings of trees to the communities and home light, and, therefore, it gets warmer. The pave-
owners that have decided to plant trees for other ments in turn heat the air and help create the
reasons. ‘urban heat island’. If urban surfaces were lighter
Even trees planted along streets and in parks in color, more of the incoming light would be
where they do not offer direct shade to air-con- reflected back into space and the surfaces and the
ditioned buildings exert an ambient cooling effect air would be cooler. This tends to reduce the need
sufficient to have a substantial impact on smog for air conditioning.
reduction. Simulations for Los Angeles indicate Urban pavements are made predominantly of
that trees account for net savings (energy and asphalt concrete. In this discussion, we will not
smog savings) of about $270M annual benefit, of deal with the common alternative, cement con-
which, $58M comes from their contribution to crete, and the ongoing debate as to whether it is
shading (Table 5). preferable because of its longer life-time. The
At another level, our calculations suggest that questions we address are whether there are ways
urban trees play a major role in sequestering CO
2
to reduce the heating of cities caused by asphalt
and thereby delaying global warming. Rosenfeld
et al
.
(1998) showed that a tree planted in Los concrete and whether this can be economical and
Angeles avoids the combustion of 18 kg of carbon practical.
annually, even though it sequesters only 4.5 kg (as In Fig. 6, we show some measurements of the
it would if growing in a forest). In that sense, one effect of albedo on pavement temperature. The
shade tree in Los Angeles is equivalent to four data clearly indicate that significant modification
forest trees. of the pavement temperature can be achieved: a
Fig. 6. Dependence of pavement surface temperature on albedo. Data were taken at about 3 p.m. in Berkeley, California, on new,
old, and light-color coated asphalt pavements. The data from San Ramon, California, were taken at about 3 p.m. on four asphalt
concrete and one cement concrete (albedo50.35) pavements.
306 H. Akbari et al.
108C decrease in temperature for a 0.25 increase ozone exceedance of the California air-quality
in albedo. standard (Taha, 1997). It has been estimated (Hall
et al
.
, 1992) that L.A. people would be willing to
pay about $10 billion per year to avoid the
6.1.
Energy and smog benefits of cool medical costs and lost work time due to air
pavements pollution. The greater part of pollution is par-
An estimate of the benefits can be deduced by ticulates, but the ozone contribution averages
first finding the temperature decrease that would about $3 billion/year. Assuming a proportional
result if a city were resurfaced with more reflec- relationship of the cost with the amount of smog
tive paving materials. Cool pavements provide exceedance, the cooler-surfaced city would save
only indirect effects through lowered ambient 12% of $3 billion/year, or $360M/year. As
temperatures. Lower temperature has two effects: above, we attribute about 21% of the saving to
(1) reduced demand for electricity for air con- pavements. Thus, smog improvement from alter-
2
ditioning and (2) decreased production of smog ing the albedo of all 1250 km of pavements by
(ozone). Rosenfeld et al
.
(1998) estimated the cost 0.25 saves about $76M/year. Per unit area, this is
2
savings of reduced demand for electricity and of worth about $0.06/m per year.
the externalities of lower ozone concentrations in
the Los Angeles Basin.
6.2.
Other benefits of cool pavements
6.1.1.
Electric power savings in Los Angeles.
6.2.1.
Increased life expectancy of pavements.
Simulations for Los Angeles indicate that a It has long been known that the temperature of a
reasonable change in the albedo of the city could pavement affects its performance (Yoder and
cause a noticeable decrease in temperature. Taha Witzak, 1975). This has been emphasized by the
(1997) predicted a 1.58C decrease in temperature
4
new system of binder specification advocated by
of the downtown area. The lower temperatures in the Strategic Highway Research Program
the city are calculated for the condition that all (SHRP). Beginning in 1987, this program led
roads and roofs are improved. From the pavement experts to carry out the task of re-
meteorological simulations of 3 days in each searching and then recommending the best meth-
season, the temperature changes for every day in a ods of making asphalt concrete pavements. A
typical year were estimated for Burbank, typical result of this study was the issuance of spe-
of the hottest one-third of L.A. The energy cifications for the asphalt binder. The temperature
consumptions of typical buildings were then range that the pavement will endure is a primary
simulated for the original weather and also for the consideration (Cominsky et al
.
, 1994). The per-
modified weather. The differences are the annual formance grade (PG) is specified by two tempera-
energy changes due to the decrease in ambient tures: (1) the average 7-day maximum tempera-
temperature. The result is a city-wide annual ture that the pavement will likely encounter, and
saving of about $71M (million), due to combined (2) the minimum temperature the pavement will
albedo and vegetation changes. The kWh savings likely attain. Note, importantly, that it is the
attributable to the pavement are $15M/year, or
2
pavement temperature and not the air temperature
$0.012/m year. Analysis of the hourly demand that is considered. There is a rule of thumb in the
indicates that cooler pavements could save an industry, the ‘rule of 90’, that, when the sum of
estimated 100 MW of peak power in L.A. Fewer the absolute values of these temperatures is
power plants, required to handle the growth of greater than 908C, some kind of modification of
peak load, need be built if cooler pavements are the asphalt will be needed; this adds to the cost.
installed, saving money, resources, and pollution. For example, if a binder is specified as PG 58-22,
6.1.2.
Smog savings in Los Angeles. The simu- it is intended to function between 58 and 2228C.
lations of the effects of higher albedo on smog The sum of the absolute values, 581u222u580.
formation indicate that an albedo change of 0.3 An ordinary grade of asphalt will suffice; its cost
throughout the developed 25% of the city would is about $125 per ton. If, however, the pavement
yield a 12% decrease in the population-weighted must function between 76 and 2168C, or PG
76-16, the sum 761u216u592, a modified
asphalt is recommended. This higher grade costs
42
The model assumes that all roofs (1250 km ) have albedo about $165 per ton (Bally, 1998), a 30% increase
increased by 0.35, from about 0.15 to 0.5, and all
2
in price.
pavements (1250 km ) have albedo increased from about
0.1 to 0.35. As an example of a temperature-dependent
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 307
source of pavement failure, we consider ‘tertiary hard and loosen the aggregate. Aggregate is
creep’. Pavements gradually accumulate perma- sometimes thrown by tires, but, when installed
nent distortions as tires roll over them repeatedly. properly, this seems to be rare.
‘Tertiary creep’ refers to the phenomenon of the
accelerated rate of distortion after many such
6.4.
Cost of cool pavements
repetitions. This signals gross failure of the It is clear that cooler pavements will have
pavement. Cominsky et al
.
(1994) gave evidence energy, environmental and engineering benefits.
that cooler pavements can have a significantly The issue is then whether there are ways to
higher resistance to tertiary creep and, thus, can construct pavements that are feasible, economical,
last much longer. Using Cominsky’s data, we and cooler. The economic question is whether or
estimate that a 108C decrease in pavement tem- not the savings generated by a cool pavement
perature can result in a 25-fold increase in its life over its lifetime are greater than its extra cost.
expectancy against this type of failure, a factor so Properly, one should distinguish between initial
large that we suspect it is an overestimate. cost and lifetime costs (including maintenance,
repair time, and length of service of the road).
6.2.2.
Improved visibility. Reflectivity of pave- Often the initial cost is decisive, so we will
ments is also a safety factor in visibility at night consider only that cost here.
and in wet weather, affecting the demand for
streets’ electric lighting. Street lighting is more
6.4.1.
Thick pavements. A typical asphalt con-
effective if pavements are more reflective, which crete contains about 7% of asphalt by weight, or
can lead to greater safety; or, alternatively, less about 17% by volume; the remainder is rock
lighting could be used to obtain the same visibili- aggregate, except for a few percent of voids. The
ty. These benefits have not yet been monetized. cost of ordinary asphalt (1998 prices) is about
$125 per ton, and the price of aggregate is about
6.3.
Potential problems with cool pavements $20 per ton, exclusive of transportation costs.
A practical drawback of high reflectivity is Thus, in one ton of mixed asphalt concrete, the
glare, but this does not appear to be a problem. cost of materials only is about $28/ton, of which
We suggest a change in resurfacing using not about $9 is in the binder and $19 is in the
black asphalt, with an albedo of about 0.05–0.12, aggregate. For a pavement about 10 cm thick (4
3
but the application of a product with an albedo of inches), with a density of 2.1 ton/m , the cost of
2
about 0.35, similar to that of cement concrete. The the binder alone is about $2 per m and aggregate
2
experiment to test whether this will be a problem costs about $4.2 per m .
has already been performed: every day, millions Experimentally, the albedo of a fresh asphalt
of people drive on cement concrete roads, and we concrete pavement is about 0.05 (Pomerantz et
rarely hear of accidents caused by glare, or of al
.
, 1997) because the relatively small amount of
people even complaining about the glare on such black asphalt coats the lighter colored aggregate.
roads. Thus, every reader of this paper likely As an asphalt concrete pavement is worn down
knows the answer from experience. and the aggregate is revealed, we observed an
There is also a concern that, after some time, albedo increase to about 0.15 for ordinary aggre-
light-colored pavement will darken because of gate. If it were made with a reflective aggregate,
dirt. This tends to be true, but again, experience we could expect the long-term albedo to approach
with cement concrete roads suggests that the light that of the aggregate.
color of the pavement persists after long usage. How much money might such a reflective
Most drivers can see the difference in reflection pavement save? Using the assumptions for Los
between an asphalt and a cement concrete road Angeles, a cooler pavement would generate a
2
when they drive over them, even when the roads stream of savings of $0.07/m per year for the
are old. More studies are needed to quantify the lifetime of the road, about 20 years. At a real
effect of aging. interest rate of 3% per year, this has a present
The use of chip seals is a promising method of value about 15 times the current saving (Rosen-
resurfacing that achieves lighter color. (Chip feld et al
.
, 1998). Thus, the potential savings are
2
sealing is the pressing on of aggregate into soft worth $1.1/m at present. This saving would
2
binder, as a resurfacing technique (Asphalt Insti- allow for purchase of a binder, instead of $2/m ,
2
tute, 1989).) Although this is popular in many costing $3/m , or 50% more expensive. Or, one
2
districts, some have reservations about the use of could buy aggregate; instead of spending $4.2/m ,
2
chip seals in cul de sacs, where tires are turned one can now afford $5.2/m (a 20% more expen-
308 H. Akbari et al.
sive, whiter, aggregate). It is doubtful that such surface, achieved by using a chip seal. The costs
2
of both of these are about the same, $0.60/m
modest increases in costs can buy much whiter (Means, 1996). For a chip seal, about half the
pavements. materials cost is aggregate and half is the binder.
In the special case of a climate in which the If special light-colored aggregate is used in the
pavement is subjected to such wide temperature chip seal, there will be an extra cost. For example, if
swings that the ‘rule of 90’ is violated, the cost of
2
the aggregate costs 50% more, instead of $0.30/m
binder is increased by about 30%, if SHRP
2
it will cost $0.45/m , and the price of the chip
recommendations are followed. For a 10 cm thick
2
2
seal will rise by $0.15/m . If the energy, en-
new road, the cost of ordinary asphalt is $2/m
2
vironmental and durability benefits over the life-
and higher grade asphalt costs $2.60/m . Instead
2
time of the resurfacing exceed $0.15/m , the
of buying the higher grade binder, one could
2
cooler pavement pays for itself. Again, this de-
apply a chip seal, which costs about $0.60/m . pends on local circumstances: the climate and
Chip seals comprise a binder onto which aggre- smog conditions vs. the cost of light-colored
gate is pressed. The aggregate is visible from the aggregate. For Los Angeles, we have estimated
outset, and, if it is reflective, the pavement stays that energy and environmental savings alone are
cooler. It might be sufficiently cool that it is
2
about $0.36/m (present value over the lifetime
unnecessary to use the higher grade binder. For of 5 years for a resurfacing), and, thus, one could
example, the data of Fig. 6 show that a 0.25 afford to pay twice the usual price for aggregate
increase in albedo can reduce the pavement and still have no net increase in cost. Lifetime
temperature by 108C. This suggests that the benefits would also accrue in addition to energy
maximum temperature specification for the pave- and smog benefits.
ments might be reduced by 108C. A lower grade
of binder might then be acceptable. The reduced
cost of the binder cancels the cost of the chip seal, 7. CONCLUSIONS
and one enjoys the cooling benefit at no extra Cool surfaces (cool roofs and cool pavements)
cost. and urban trees can have a substantial effect on
Thus, for thick pavements, the energy and urban air temperature and, hence, can reduce
smog savings may not pay for whiter roads. cooling-energy use and smog. We estimate that
However, if the lighter-colored road leads to about 20% of the national cooling demand can be
substantially longer lifetime, the initial higher cost avoided through a large-scale implementation of
may be offset by lifetime savings. An example of heat-island mitigation measures. This amounts to
this is to be seen when a higher grade binder is 40 TWh/year savings, worth over $4B per year
replaceable by a white surface. This must be by 2015, in cooling-electricity savings alone.
evaluated according to the demands on the road Once the benefits of smog reduction are ac-
and the climate. counted for, the total savings could add up to over
6.4.2.
Thin pavements. At some times in its $10B per year.
life, a pavement needs to be maintained, i.e., Achieving these potential savings is conditional
resurfaced. This offers an opportunity to get on receiving the necessary federal, state, and local
cooler pavements economically. Good mainte- community support. Scattered programs for plant-
nance practice calls for resurfacing a new road ing trees and increasing surface albedo already
after about 10 years (Dunn, 1996) and the lifetime exist, but to start an effective and comprehensive
of resurfacing is only about 5 years. Hence, campaign would require an aggressive agenda.We
within 10 years, all the asphalt concrete surfaces have started to collaborate with the American
in a city can be made light-colored. As part of this Society for Testing of Materials (ASTM) and the
regular maintenance, any additional cost of the industry, to create test procedures, ratings, and
whiter material will be minimized. Note also that labels for cool materials. We have also initiated
because the lifetime of the resurfacing is only plans to incorporate cool roofs and trees into the
about 5 years, the present value of the savings is Building Energy Performance Standards of AS-
five-times greater than the annual savings. Thus, HRAE (American Society of Heating Refrigera-
2
for LA, the present value is about $0.36/m tion, and Airconditioning Engineers), California
2
($0.03/ft ). Can a pavement be resurfaced with a Title 24, and the California South Coast’s Air
light color at an added cost less than this saving? Quality Management Plans. We also plan to
For resurfacing, there are the options of a black demonstrate savings in selected ‘Cool Com-
topping, such as a slurry seal, or a lighter-colored munities’, including federal facilities, particularly
Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas 309
Bretz S., Akbari H. and Rosenfeld A. (1997) Practical issues
military bases. A related effort involves expand- for using high-albedo materials to mitigate urban heat
ing the Los Angeles Basin’s Regional Clean Air islands. Atmospheric Environment 32(1), 95–101.
Bretz S. and Akbari H. (1997) Long-term performance of
Incentive Market (RECLAIM) NO -credit trading
x
high-albedo roof coatings. Energy and Buildings Special
market to include air temperature reduction by Issue on Urban Heat Islands and Cool Communities 25(2),
cool surfaces. The South Coast Air Quality Man- 159–167.
Bretz S. and Akbari H. (1994) Durability of high-albedo roof
agement District and the EPA now recognize that coatings. In Proceedings of the ACEEE 1994 Summer Study
air temperature is as much a cause of smog as on Energy Efficiency in Buildings, Vol. 9, p. 65.
NO or volatile organic compounds, so that cool Cominsky R. J., Huber G. A., Kennedy T. W. and Anderson
x
M. (1994). The Superpave Mix Design Manual for New
surfaces and shade trees should be monetized on Construction and Overlays
.
SHRP-A-
407
, National Re-
RECLAIM along with NO . Finally, EPA is
x
search Council, Washington, DC.
DeWalle D. R., Heisler G. M. and Jacobs R. E. (1983) Forest
considering mechanisms that would allow inclu- home sites influence heating and cooling energy. Journal of
sion of cool surface and trees in State Im- Forestry 81(2), 84–87.
plementation Plans (SIPs) for ozone compliance. Dunn B. H. (1996) What you need to know about slurry seal.
Better Roads March, 21–25.
EPA (1986). Guideline on Air Quality Models
(
Revised
)
, U.S.
Environmental Protection Agency EPA-450/2-78-027R.
Acknowledgements—This work was supported by the Assis- Gartland L. M., Konopacki S. J., Akbari H. (1996) Modeling
tant Secretary for Energy Efficiency and Renewable Energy, the effects of reflective roofing. In Proceedings of the
1996
Office of Building Technologies of the U.S. Department of ACEEE Summer Study on Energy Efficiency in Buildings,
Energy, and the U.S. Environmental Protection Agency under Vol. 4, p. 117.
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and Testing of the CBM-IV for Urban Abd Regional
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Implementation of light-colored surfaces: profits for utilities
... The replacement of green vegetation cover by artificially hardened surfaces in the urban fabric is considered to be the leading cause of the Urban Heat Island (UHI) [2], and the "canyons" formed by high-density buildings primarily affect the ventilation of the inner city, leading to heat accumulation and increasing the regional heat risk [3]. The temperature difference between cities and surrounding suburban areas can be as high as 8 • C [4], and every 1 • C increases in daily maximum temperature increases electricity demand by 2-4%, with the use of cooling facilities such as air conditioners accounting for 5-10% of peak urban electricity demand [5]. According to the World Health Organization, cities are responsible for 60-80% of global energy consumption [6], with the building sector accounting for approximately 40% of the world's total energy consumption [7]. ...
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... Still, the causes of the UHI remain difficult to identify and isolate (Manoli et al., 2019). However, it is generally attributed to a reduction in evaporative cooling with other factors including a heat release associated with human activity (Akbari et al., 2001) and an increased storage of energy associated with a reduction in albedo (Taha, 1997;Sailor and Lu, 2004;Collier, 2006;Zhao et al., 2014). ...
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Cities such as Detroit, MI in the post-industrial Rust Belt region of the United States, have been experiencing a decline in both population and economy since the 1970's. These “shrinking cities” are characterized by aging infrastructure and increasing vacant areas, potentially resulting in more green space. While in growing cities research has demonstrated an “urban heat island” effect resulting from increased temperatures with increased urbanization, little is known about how this may be different if a city shrinks due to urban decline. We hypothesize that the changes associated with shrinking cities will have a measurable impact on their local climatology that is different than in areas experiencing increased urbanization. Here we present our analysis of historical temperature and precipitation records (1900–2020) from weather stations positioned in multiple shrinking cities from within the Rust Belt region of the United States and in growing cities within and outside of this region. Our results suggest that while temperatures are increasing overall, these increases are lower in shrinking cities than those cities that are continuing to experience urban growth. Our analysis also suggests there are differences in precipitation trends between shrinking and growing cities. We also highlight recent climate data in Detroit, MI in the context of these longer-term changes in climatology to support urban planning and management decisions that may influence or be influenced by these trends.
... As for another type of passive roof, the cool roof, Rosenfeld's early test (Rosenfeld et al., 1998) in Los Angeles showed that the increase in roof reflectivity per 0.1 could lead to a decrease in the average ambient temperature of 0.51°C. Levinson and Akbari of the heat island group of the Lawrence Berkeley National Laboratory in the United States have conducted a lot of research on the urban heat island effect caused by the selection of different pavement/roof materials and the corresponding environmental and ecological impacts from the heat island effect model calculation and experimental research for many years (Akbari et al., 2016(Akbari et al., , 2001Levinson et al., 2007;2005a;2005b). Santamouris conducted extensive research on the heat island effect, from the cause of the heat island to the impact on energy and the environment, and from analysis and calculation tools to the implementation of response measures, providing a comprehensive theoretical foundation for the study of the urban heat island effect (Santamouris et al., 2015(Santamouris et al., , 2011Santamouris, 2013b;Santamouris, 2013a;Santamouris, 2014). ...
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Building energy, particularly air conditioning energy, makes for a significant fraction of overall societal energy usage. The heat island effect is a common urban environmental concern that threatens human sustainable development. Roofs, which cover more than 20% of the total metropolitan area, play an essential role in mitigating the urban heat island effect and lowering building energy use. To examine the triple beneficial benefits of cool roofs and green roofs on building energy conservation in different temperature zones in China, a simulation model based on a typical residential community is established. The results reveal that minimizing direct heat acquisition from the roof, reducing heat transfer from the enclosure, and enhancing the coefficient of performance have a surprising triple effect on building energy saving. In cold areas, hot summer and cold winter areas, and hot summer and warm winter areas, cool roofs may lower the regional ambient temperatures by 2°C, 2.3°C, and 2.6°C, respectively, whereas green roofs can reduce the regional ambient temperatures by 1°C, 1.1°C, and 1.2°C. The triple saving impact of cool roofs and green roofs may accomplish 11.0%, 11.5%, 12.6%, and 9.4%, 8.1%, and 9.3%, respectively, for building energy conservation. Because of the increased solar radiation, cool roofs perform better in low-latitude zones, whereas green roofs function consistently.
... 4 urban heat island effect. According to previous research, UHI has a close association with meteorological indices and earth surface features such as climate change [5], a shift in the direction of the local wind [6], rising energy use in cities [7], ground-level ozone generation pollutes the air [29] and as a result, affects individuals' comfort and physical health temperature and cooling rates are two key markers of the urban heat island [9]. Based on this temperature and cooling rates data, it was established that urban physical variables, particularly the floor area ratios and buildings coverage ratio, had a significant impact on the quality of life in cities. Building attributes that influence the presence of the urban heat island phenomena had a substantial influence on the size of the UHI. ...
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In the twenty-first century, urban heat islands (UHIs) have become a major problem for humanity as a consequence of urbanization and industrialization. The main causes of UHI are the vast amounts of heat generated by urban structures as they consume and re-radiate solar energy and anthropogenic heat sources. The two heat sources cause an urban area's temperature to rise above its surroundings, a phenomenon known as Urban Heat Island (UHI). Many approaches, methods, models, and investigative tools have been implemented to study and analysis this phenomenon. In general, green areas in cities are thought to be an effective approach to mitigate urban heat island effects and bring comfort to residents. The improvement of microclimatic conditions in urban environments is mostly influenced by evapotranspiration. Most of the studies show a rising trend in the UHI, which is linked to decreased plant cover and land-use changes. The main objectives of this paper were to explain the concept, formation factors, and influential factors of UHI. In addition, the most common strategies and tools that are applied in mitigating rising temperatures in urban areas were reviewed and summarized. The finding of several studies showed that increasing urban vegetation areas in addition to optimizing their spatial distribution and configuration is an effective strategy to reduce the impact of UHI.
... The process of heat island development reflects the unbalanced energy exchange between human activities and natural systems (Voogt and Oke 2003). This unbalanced energy exchange in turn affects the urban thermal environment (Voogt 2002;Huang et al. 2005), which will have an impact on human health and thermal comfort (Akbari et al. 2001). ...
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... White is the most general colour for "cool roofs". A white roof can reflect 55-80% of incident sunlight making its roof surface cool on a clear summer day [2]. This decreases heat transfer through the roof and makes the space below the roof more comfortable in unconditioned buildings. ...
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... Since 1750, when Industrial Revolution took place, climate change is strongly contributed by the emissions of Aerosols and Greenhouse gases (GHGs), resulting in a rise in global temperature. Deforestation, mining, artificial cloud seeding, excessive land use etc. led to other problems such as an increase in frequency and intensity of storms, droughts, floods, landslides, decline of ice sheets, melting of glaciers etc. (Akbari et al, 2001;Bala et al, 2007;Crutzen et al, 2000). Therefore, apart from natural and man-induced climate change, there could be one more category where the cause of CC appears to be natural but actually it is triggered by the cumulated effects of anthropogenic activities. ...
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... Elevated land surface heat (LST) is one of the direct consequences of converting natural lands to artificial surfaces in the process of urbanization that intensifies anthropogenic heat. This phenomenon has been posing critical problems to communities, as abnormal temperatures significantly alter local climate, influence water and energy consumption, aggravate the negative impacts on residents' physical and mental health, and increase the level of ground-level ozone that leads to smog formation (Akbari et al., 2001;Bornstein & Lin, 2000;Cao et al., 2010;Hondula et al., 2014;Imhoff et al., 2010;Kolokotroni et al., 2007;Singh et al., 2017;Stone, 2005). These adverse impacts of high LST are especially significant in highly developed areas where urban heat islands (UHIs) and extreme heat events frequently occur (Habeeb et al., 2015;Oke, 1982). ...
... They allow limiting the temperature by absorbing solar radiation, evaporating water through evapotranspiration, and creating shady areas. Additionally, they help purify the air (Eliasson, 2000;Akbari et al., 2001;Bowler et al., 2010). In city parks, the temperature can drop by 4 degrees compared to other areas of the city (Eliasson, 1996). ...
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"Climate change effects are becoming increasingly noticeable especially in the dynamic and overcrowded city areas. The frequency of occurrence and ways of appearance of natural risks along with the negative effects of intensive economic activities, energy conventional production, unsustainable transportation and energy consumption determine increases in dysfunctions that must be managed by the local authorities in the long term. This study addresses climate change in relation to policy and regulatory framework for urban planning. The article portrays several climate change-related threats that usually occur in urban areas, which are emphasized in the scientific literature, but also exemplifies practical solutions formulated by planners in their strategy for sustainable urban development as counteracting the current specific threats. The selected case studies are the cities of Kraków in Poland and of Cluj-Napoca in Romania. Results of the literature review show that the main risks related to sustainability of the urban areas, as effects of the climate change and man-made actions, are correspondingly visible and addressed in the main strategic plans at the local level, adopted by the city authorities as practical measures and actions to be implemented by 2030 aiming to limit the effects of the climate changes that arise, as well as to limit the causes that generate these changes. By implementing the proposed measures and by achieving the objectives described in the presented action plans, both of the selected cities prove their engagement in the European mission of climate change adaptation and mitigation. "
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This paper reports on the first results from tests on a reflective roofing system on a commercial building in Florida. The building is a elementary school with a sloped, modified bitumen roof. Air-conditioning power was measured in a base configuration prior to the roofing system being changed to a white color. Roof, decking, and plenum air temperatures were strongly affected by the change to a white roof system. The school, which was monitored for a full year in both the pre- and post-condition, saw the measured annual chiller electric power reduced by 10%, or 13,000 kWh/yr. Cooling-load reductions during the utility summer peak were substantially greater, more than 30% during the afternoon hours.
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Slurry seal is defined as a mixture of continuously graded fine aggregate, mineral filler, emulsified asphalt, and water properly proportioned, mixed, and spread as a surface treatment. When applied, the cured slurry seal should have a homogeneous appearance, fill cracks, adhere firmly to the sureface, and provide a weather proof, high-friction seal. This paper provides a vast information about slurry seal - including its gradation, use, effectiveness, properties, etc.
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Incluye bibliografía e índice
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ooling energy savings of 10 to 70% have been achieved by applying high-albedo coatings to residential buildings in California and Florida. Since dirt accumulation can alter the performance of high-albedo roofs as an energy efficiency measure, we examined some high-albedo coatings at various stages of exposure to determine the magnitude of this effect. We conclude that most of the albedo degradation of coatings occurred within the first year of application, and even within the first two months of exposure. On one roof, 70% of the drop in albedo for the entire first year occurred within the first two months. After the first year, the degradation slowed, with data indicating small losses in albedo after the second year. We use measured data to estimate the effects of weathering of white roofs on seasonal cooling energy savings and estimate a 20% reduction from first year energy savings for all subsequent years (2–10). Although washing the roofs with soap is effective at restoring original albedo, calculations show that it is not cost-effective to hire someone to clean a high-albedo roof only to achieve energy savings. Instead, it would be useful to develop and identify dirt-resistant high-albedo coatings.
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Twenty-six spot albedo measurements of roofs were made using a calibrated pyranometer. The roofs were surfaced with either an acrylic elastomeric coating, a polymer coating with an acrylic base, or a cementitious coating. Some of the roofs` albedos were measured before and after washing to determine whether the albedo decrease was permanent. Data indicated that most of the albedo degradation occurred within the first year, and even within the first two months. On one roof, 70% of one year`s albedo degradation occurred in the first two months. After the first year, the degradation slowed, with data indicating small losses in albedo after the second year. Measurements of seasonal cooling energy savings by Akbari et al. (1993) included the effects of over two months of albedo degradation. We estimated â¼20% loss in cooling-energy savings after the first year because of dirt accumulation. For most of the roofs we cleaned, the albedo was restored to within 90% of its initial value. Although washing is effective at restoring albedo, the increase in energy savings is temporary and labor costs are significant in comparison to savings. By our calculations, it is not cost-effective to hire someone to clean a high-albedo roof only to achieve energy savings. Thus, it would be useful to develop and identify dirt-resistant high-albedo coatings.
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A series of field experiments in Florida have examined the impact of reflective roof coatings on air conditioning energy use in occupied homes. The tests were conducted on nine residential buildings from 1991 to 1994 using a before and after protocol where the roofs were whitened at mid-summer. Measured AC electrical savings in the buildings during similar pre- and post-retrofit periods averaged 19%, ranging from a low of 2% to a high of 43%. Utility peak coincident peak savings averaged 22%. Cooling energy reductions appear to depend on ceiling insulation level and roof solar reflectance, air duct system location and air conditioner sizing relative to load. A complementary thermal study of the effect of reflective roofing systems has been conducted in a side-by-side roof test facility. Ceiling heat flux reductions up to (60%) were measured from reflective roofing in these experiments. However, the test results have also shown degradation in solar reflectance and associated thermal performance after a year of exposure.
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Dark roofs raise the summertime air-conditioning demand of buildings. For highly-absorptive roofs, the difference between the surface and ambient air temperatures can be as high as 90 F, while for highly-reflective roofs with similar insulative properties, the difference is only about 20 F. For this reason, cool roofs are effective in reducing cooling energy use. Several experiments on individual residential buildings in California and Florida show that coating roofs white reduces summertime average daily air-conditioning electricity use from 2--63%. This demonstration project was carried out to address some of the practical issues regarding the implementation of reflective roofs in a few commercial buildings. The authors monitored air-conditioning electricity use, roof surface temperature, plenum, indoor, and outdoor air temperatures, and other environmental variables in three buildings in California: two medical office buildings in Gilroy and Davis and a retail store in San Jose. Coating the roofs of these buildings with a reflective coating increased the roof albedo from an average of 0.20--0.60. The roof surface temperature on hot sunny summer afternoons fell from 175 F--120 F after the coating was applied. Summertime average daily air-conditioning electricity use was reduced by 18% (6.3 kWh/1000ft{sup 2}) in the Davis building, 13% (3.6 kWh/1000ft{sup 2}) in the Gilroy building, and 2% (0.4 kWh/1000ft{sup 2}) in the San Jose store. In each building, a kiosk was installed to display information from the project in order to educate and inform the general public about the environmental and energy-saving benefits of cool roofs. They were designed to explain cool-roof coating theory and to display real-time measurements of weather conditions, roof surface temperature, and air-conditioning electricity use. 55 figs., 15 tabs.
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This book is a practical guide that presents the current state of knowledge on potential environmental and economic benefits of strategic landscaping and altering surface colors in our communities. The guidebook, reviews the causes, magnitude, and impacts of increased urban warming, then focuses on actions by citizens and communities that can be undertaken to improve the quality of our homes and towns in cost-effective ways.
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This report summarizes paving materials suitable for urban streets, driveways, parking lots and walkways. The authors evaluate materials for their abilities to reflect sunlight, which will reduce their temperatures. This in turn reduces the excess air temperature of cities (the heat island effect). The report presents the compositions of the materials, their suitability for particular applications, and their approximate costs (in 1996). Both new and resurfacing are described. They conclude that, although light-colored materials may be more expensive than conventional black materials, a thin layer of light-colored pavement may produce energy savings and smog reductions whose long-term worth is greater than the extra cost.