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Deriving roof albedo for California cities using remote sensing
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George Ban-Weiss Assistant Professor [email protected]
Astani Dept of Civil and Environmental Engineering University of Southern California
June 11, 2015
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The team
Ronnen Levinson
Jordan Woods
Sponsor: California Air Resources Board
George Ban-Weiss
References: Ban-Weiss GA, Woods J, Levinson R, 2015. Using remote sensing to quantify albedo of roofs in California’s seven largest cities, Part 1: Methods. Solar Energy, 115, 777-790. Ban-Weiss GA, Woods J, Millstein D, Levinson R, 2015. Using remote sensing to quantify albedo of roofs in California’s seven largest cities, Part 2: Results and application to climate modeling. Solar Energy, 115, 791-805.
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• Buildings are responsible for ~40% of total energy use in U.S. (EIA, 2013)
• Many cities are “urban heat islands” with higher temperatures than surrounding areas
• Global climate change is projected to warm cities
• 81% of people in the U.S. live in cities as of 2010 (U.S. Census Bureau)
– Worldwide rate is ~50%
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Motivation
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The summer afternoon urban heat island effect
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California cities are projected to warm over the next century
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Sun et al. (2015)
# o
f d
ays
per
yea
r w
ith
T
> 3
5 °
C (
95
°F)
Extreme heat days by mid-century
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On the need for local solutions for dealing with climate change
• Mitigating global climate change requires vast decreases in CO2 emissions worldwide (top priority!)
• Given the lack of progress thus far, cities need local solutions to deal with local impacts of climate change
Davis et al. (2013) ERL 6
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Methods to reduce urban heat
1) Dark surfaces 2) Lack of vegetation 3) Anthropogenic heating 4) Thermally massive
materials Pavements
Roofs
Vegetation
Other
39%
19%
29%
14%
Reverse the local climate consequences of:
Fabric of a typical city
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Some “cool community” strategies
• Cool roofs
• Cool pavements • Vegetative roofs
• Street level
vegetation
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Solar reflectance (a.k.a. albedo)
Definition: The ratio of reflected to incident sunlight
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Solar reflective roofs can reduce building energy use AND lower urban temperatures
44 °C 80 °C
~20-25% of urban areas
are roof tops
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Cool roofs reduce energy use in most climates
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Cool concrete tile roof, albedo 0.51 Dark asphalt shingle roof, albedo 0.07
• Cool roofs can reduce cooling energy use by 10-20% or more
Rosado et al. (2014)
Roof footprint: 188 m2 (2020 ft2) Annual HVAC energy cost savings: US$167 (20%)
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Cool roofs can reduce urban temperatures
Georgescu et al. (2014)
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Primary research question
Q: What is the maximum increase in roof albedo attainable in cities? To answer, need to characterize current stock of roofs at city scale.
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Possible data sources
Satellite
1) NASA products (free) – Spatial resolution too low
2) Commercial satellites – Too expensive for our large areas of
analysis
Orthoimagery (acquired via airplane) – Not radiometrically calibrated
– Usually no near-infrared information
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National Agricultural Imagery Program (NAIP)
• Aerial imagery for all of California
• 1 meter spatial resolution
• 4 spectral bands
• red, green, blue, near-IR
• Available for free via USGS
• BUT, not radiometrically calibrated
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Possible data sources
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2009 was a special year …
• Northwest Group used a radiometrically calibrated sensor similar to those used on satellites
• Obtained raw sensor data and reprocessed
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Spatial coverage of acquired data
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Overview of our approach
Source: Ban-Weiss et al. (2015a) 18
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Example sensor data for each band
Band 1- Blue
Band 2- Green
Band 3- Red
Band 4- Near IR
19 Source: Ban-Weiss et al. (2015a)
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Extracting roof pixels using building outlines
20 Source: Ban-Weiss et al. (2015a)
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Developing an empirical model to predict albedo from remotely sensed reflectances
21 Source: Ban-Weiss et al. (2015a)
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Lab testing roofing products
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Predicting albedo from remotely sensed reflectances
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RMS = 0.016
Source: Ban-Weiss et al. (2015a)
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Ground-truthing remotely sensed roof albedo
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Results
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Mean albedo for each rooftop
26 Source: Ban-Weiss et al. (2015b)
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In Los Angeles most roofs have low albedo
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Source: Ban-Weiss et al. (2015b)
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Source: Ban-Weiss et al. (2015b)
0.18
0.18
0.20
0.17
0.18
0.00 0.05 0.10 0.15 0.20 0.25
San Francisco
San Jose
Bakersfield
Los Angeles
Long Beach
Average roof albedo
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Assessing precision of our approach using multiple fly-overs: high precision!
Source: Ban-Weiss et al. (2015b)
Scaled arithmetic difference = (Refl1 – Refl2)/√(2)
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Most city-wide roof area is “small” roofs
Source: Ban-Weiss et al. (2015b)
Fraction of total roof area made up of roofs with
area < roof area threshold
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Most city-wide roof area is “small” roofs
Source: Ban-Weiss et al. (2015b)
Fraction of total roof area made up of roofs with
area < roof area threshold
Most city-wide roof area is “small” roofs
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Small roofs have low albedo while larger roofs have higher albedo
Mean albedo of roofs with:
area < roof area threshold area > roof area threshold
Source: Ban-Weiss et al. (2015b)
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Mean albedo of roofs with:
area < roof area threshold area > roof area threshold
Source: Ban-Weiss et al. (2015b)
Small roofs have low albedo while larger roofs have higher albedo
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Mean albedo of roofs with:
area < roof area threshold
Source: Ban-Weiss et al. (2015b)
area > roof area threshold
Small roofs have low albedo while larger roofs have higher albedo
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albedomap.lbl.gov shows roof albedos for five California cities
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• Many commercial and residential buildings in California have roofs that could be made cooler (more reflective) to save energy
• Buildings with small roofs (primarily
homes) constitute a large fraction of city roof area and have low albedos
Policy relevant conclusions
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Therefore, efforts to increase urban-scale albedo for heat island mitigation and climate change adaptation should include cool roof requirements for residential buildings where appropriate (while maintaining desired aesthetics)
Policy relevant conclusions
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Latest policy developments
for cool roofs
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Los Angeles becomes the first major city to require cool roofs on residential buildings
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Min aged
SR
Min aged
TE
Min aged SRI
Nonres or high-rise res, low slope, all CZ
0.63 0.75 75
Nonres or high-rise res, high slope, all CZ
0.20 0.75 16
Res, low slope, CZs 13 & 15 0.63 0.75 75
Res, high slope, CZ 10 - 15 0.20 0.75 16
climate zone (CZ)
SR = solar reflectance (fraction of incident sunlight reflected, 0 - 1) TE = thermal emittance (efficiency emitting thermal radiation, 0 - 1) SRI = solar reflectance index (0 = reference black, 100 = reference white)
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2013 Title 24 prescribes cool roofs for all nonres buildings, and some res buildings
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Current and future work
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Life Cycle Assessment and Environmental Co-benefits of Cool Pavements
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Project just wrapping up
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Determining optimal urban heat mitigation strategies for vulnerable populations in a changing climate
Surface air temperature
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1
2
1: Vertical sensor location for detection of wide-area UHIE2: Vertical sensor location for detection of local UHIE
Conceptual framework for weather station siting. Shaded boxes rep-resent possible heights of sensors for detection of (1) wide-area urban heat island effect (UHIE) (about 10 meters above ground level) and (2) local UHIE (about 2 meters above ground level). Diagram not to scale.
Upcoming LBNL-USC-Altostratus study will measure UHI in Los Angeles Basin
Los Angeles City
Los Angeles County
First-guess monitoring region
Los Angeles JJA
First guess of monitoring region (red oval). At bottom-right, the top 5 percent areas (census tracts) in CalEnviroScreen are highlighted. Top-right figure shows prevailing wind direction for June, July, and August in Los Angeles based on 30 years of data.
44 44
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Cat’s eye retroreflector
Evaluate co-benefits Evaluate savings Assess technology
Develop technology Develop infrastructure 45
Solar-reflective “cool” walls: benefits, technologies, and implementation
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Thank you!
46
George Ban-Weiss [email protected] Assistant Professor
Astani Dept of Civil and Environmental Engineering University of Southern California
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Supplementary slides
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a Area weighted mean ± area weighted standard deviation
City-wide mean albedos (and other stats)
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Statistics for roofs with area > 5,000 m2
Source: Ban-Weiss et al. (2015b)
Provides an estimate for the fraction of commercial
buildings that obtained high albedo roofs due to Title-24
(as of 2009)
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Estimated error (accuracy) at 90% confidence interval
Source: Ban-Weiss et al. (2015b)