HomeMy Public PortalAbout20130523CRCtoCC.pdfTybee Police Vehicles
Part 2
May 23, 2013
Community Resource Committee
This Report
•Summary of Last Report
•Concerns regarding alternative vehicles
•Costs – Total Cost of Ownership
•Maintenance & Warranty Information
•Safety
•Size & Comfort
•Perceptions
•Fuel Savings
•Recommendations
Q & A – Introduce Chief Mike Holman, White Bluff, TN
Police Department
Summary of Last Report
•TIPD patrol fleet is aging &
inefficient
•TIPD accounts for half of
city’s gasoline budget
•With expanded activities, gas
use is rising
•Newer patrol cars are no
more efficient
•Current patrol fleet:
– Avg. 15 mpg/vehicle
– 223,724 miles driven ‘12
– At $3.60/gal = $53,933
Opportunities for Savings
TIPD Current Fleet
(Crown Vics,
Chargers)
Diesel Sedan
(VW Passat TDI)
Gas/Electric Hybrid
(Toyota Prius)
*EPA city
Case Study avg.
15 mpg
(calculated)
31 mpg
(38 mpg, Belle
Meade)
51 mpg
(52 mpg, White
Bluff)
*Annual Fuel Cost
per vehicle
(11,000 mi,
$3.60/gallon)
$2,640
$1,454
($4.10/gallon)
$776
Cost of Ownership
Base Price 5 Yr. Totals* Grand Total
Dodge Charger
SRT-8 Sedan
$25,995
$45,172
(significant
depreciation)
$71,167
VW Passat TDI
SE Sedan
$26,225 $37,211
$63,436
Toyota Prius
Hybrid
$24,200 $27,899
$52,099
*5-year totals from Motortrend.com for 2013 models. Include maintenance,
repairs, depreciation, fuel, financing, insurance, state fees. Figures are not for
police use, but allow for standardized comparison between vehicles.
Maintenance & Warranty
Basic Powertrain/Hybrid
Drive
Maintenance
Dodge Charger
SRT-8 Sedan
36/36,000
60/100,000
N/A
VW Passat TDI
SE Sedan
36/36,000 96/100,000
36/36,000
Toyota Prius
Hybrid
60/60,000 96/100,000
24/25,000
Safety
Both proposed alternatives meet or exceed
the highest safety standards set by the IIHS
(Insurance Institute for Highway Safety)
Size & Comfort
Overall
Length
Overall
Width
Luggage
Volume
Leg Room
Front/Rear
Head Room
Front/ Rear
Dodge
Charger
199.9” 75.0" 15.4 cu. ft. 41.8”/40.1” 38.6”/36.6”
VW
Passat
TDI
191.6” 72.2” 15.9 cu. ft. 42.4”/39.1” 38.3”/37.8”
Prius II 176.4 “
68.7 “
21.6 cu. ft.
42.5”/36.0” 38.6”/37.6”
Perception
•The alternatives don’t look like police cars,
which can also be an advantage
•Both alternatives have been seen as
acceptable options by other police
departments
Fuel Savings
If entire fleet was converted, annual savings in fuel alone would be:
Dodge Charger (actual) 15 mpg x $3.60 = $53,933
VW Passat TDI (city EPA) 31 mpg x $4.10 = $29,589 ($24,344)
Toyota Prius II (city EPA) 51 mpg x $3.60 = $15,792 ($38,141)
Based on 223,724 annual miles driven (2012 actual)
Recommendations
•Replace at least oldest existing vehicles -- not just
add to the fleet
•The more vehicles that are replaced, the more fuel
and maintenance savings will occur
•Consider zero to low interest financing rather than
outright purchase to replace at least 6 vehicles
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De
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Da
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Do
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44
2
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St
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Pa
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Av
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N/
A
Re
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(a
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)
ke
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)
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(a
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Ke
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Sa
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To
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y
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Ba
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(m
t
h
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l
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s
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36
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3
6
,
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36
/
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60
/
6
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0
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n
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Po
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Tr
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(m
t
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m
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l
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s
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60
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0
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96
/
1
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Wa
r
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‐
Ma
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(m
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/
m
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)
N/
A
36
/
3
6
,
0
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24
/
2
5
,
0
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An
t
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n
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wi
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d
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d
wi
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d
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d
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Bo
d
y
ma
t
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Pa
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Bo
d
y
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N/
A
bo
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y
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N/
A
Gr
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Fo
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N/
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Do
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Re
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Ti
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Re
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fi
x
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fi
x
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Mi
r
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a
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Av
a
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St
a
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St
a
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Su
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N/
A
N/
A
Ti
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R
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1
5
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R
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P
1
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5
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Wh
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s
17
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si
l
v
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m
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m
1
7
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m
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m
15
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m
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Ex
t
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Po
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20
1
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Do
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Ch
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20
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3
Vo
l
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Pa
s
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2.
0
L
TD
I
SE
(D
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e
s
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l
)
20
1
3
To
y
o
t
a
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Sp
a
r
e
ti
r
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an
d
wh
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l
co
m
p
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c
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st
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l
co
m
p
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c
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st
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l
co
m
p
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c
t
steel
Wi
n
d
s
h
i
e
l
d
wi
p
e
r
s
‐
fr
o
n
t
va
r
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a
b
l
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in
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m
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t
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v
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a
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l
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Re
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p
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s
N/
A
fi
x
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s
N/
A
Av
a
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Av
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En
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3.
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2.
0
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1.
8
L
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En
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DO
H
C
DO
H
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(i
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c
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l
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tu
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)
DO
H
C
Br
a
k
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4 ‐wh
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disc
An
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l
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c
k
br
a
k
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sy
s
t
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m
(A
B
S
)
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l
4 ‐wh
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l
4 ‐wh
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l
Br
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k
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as
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St
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St
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d
St
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n
d
a
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d
Re
g
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n
e
r
a
t
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v
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br
a
k
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N/
A
St
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Dr
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p
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fr
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En
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St
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A
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steel
St
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& pi
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A
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Fr
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Ownership Summary - 5 Year Ownership Cost Breakdown
2013 Dodge Charger SE Sedan Price: $25,995 | MPG: 14 city/31 hwy
Over 5 years this car costs 4.49% more to own than similar vehicles.
Year 1 Year 2 Year 3 Year 4 Year 5 5 YEAR
TOTAL
Maintenance $97 $321 $466 $1,044 $362 $2,290
Repairs $0 $0 $114 $247 $273 $634
Fuel $2,246 $2,311 $2,378 $2,447 $2,518 $11,900
Depreciation $9,042 $2,436 $2,312 $2,129 $1,701 $17,620
Financing $1,019 $807 $587 $367 $131 $2,911
Insurance $1,872 $1,872 $1,872 $1,872 $1,872 $9,360
State Fees $128 $86 $85 $79 $79 $457
Yearly Totals $14,404 $7,833 $7,814 $8,185 $6,936 $45,172
$43,146
$2,026
Poor
Read more: http://www.motortrend.com/cars/2013/dodge/charger/cost_of_ownership/#ixzz2Txv0kBMg
2013 Volkswagen Passat TDI SE Sedan Price Range: $26,225 | MPG: 31 city/43 hwy
Over 5 years this car costs 6.68 less to own than similar vehicles.
Year 1 Year 2 Year 3 Year 4 Year 5 5 YEAR
TOTAL
Maintenance $0 $58 $386 $1,168 $520 $2,132
Repairs $0 $0 $117 $254 $515 $886
Fuel $1,567 $1,612 $1,659 $1,707 $1,757 $8,302
Depreciation $7,174 $2,338 $2,214 $2,026 $1,566 $15,318
Financing $1,007 $798 $580 $363 $130 $2,878
Insurance $1,448 $1,448 $1,448 $1,448 $1,448 $7,240
State Fees $126 $86 $85 $79 $79 $455
Yearly Totals $11,322 $6,340 $6,489 $7,045 $6,015 $37,211
$39,698
($2,487)
Average
http://www.motortrend.com/cars/2013/volkswagen/passat/tdi_se_sedan/3405/cost_of_ownership/
2013 Toyota Prius Two Price Range: $24,200 | MPG: 51 city/48 hwy
Over 5 years this car costs 25.25% less to own than similar vehicles.
Year 1 Year 2 Year 3 Year 4 Year 5 5 YEAR
TOTAL
Maintenance $0 $18 $299 $635 $299 $1,251
Repairs $0 $0 $75 $162 $328 $565
Fuel $1,028 $1,058 $1,089 $1,120 $1,153 $5,448
Depreciation $3,574 $2,139 $2,060 $1,929 $1,543 $11,245
Financing $822 $651 $473 $296 $106 $2,348
Insurance $1,323 $1,323 $1,323 $1,323 $1,323 $6,615
State Fees $117 $81 $80 $75 $74 $427
Yearly Totals $6,864 $5,270 $5,399 $5,540 $4,826 $27,899
$34,944
5-Year Cost of Similar Vehicles
5-Year Cost of Similar Vehicles
Value Rating
5-Year Cost of Similar Vehicles
Difference
Difference
Value Rating
($7,045)
Excellent
Read more: http://www.motortrend.com/cars/2013/toyota/prius/cost_of_ownership/#ixzz2TrgoxWur
Difference
Value Rating
Situation: The Tybee Police Department is faced with a growing demand for services, but
has an aging and very inefficient vehicle fleet. The cost of fleet maintenance will
continue to increase until older vehicles are rotated out of service. Rising fuel costs and
ever-increasing overhead could cause serious financial strain on Government departments
and Taxpayer's wallets.
Actions: The CRC (Committee Resource Committee) has researched several potential
solutions for the aging fleet of police vehicles and has two case studies to present:
VW Passat Diesel
Toyota Prius Hybrid
Both vehicles are currently being used in other police departments around the country
with good success (so neither would mean breaking new ground) and we have been able
to speak in depth to the Chiefs of Police in two of those municipalities—which both
happen to be in the state of Tennessee—to learn more about feasibility, costs, retrofitting
and their overall satisfaction with their different choices.
Case Study – Police Department of Belle Meade, TN
Vehicle: VW Passat Type: Turbo Charged Diesel Fuel: Diesel
Background
Belle Meade has approximately 4000 permanent residents and is approximately the same
geography as Tybee Island in terms of square mileage and layout. Tim Eads, the Police Chief of
Belle Meade, has described through emails and conversations with David Turner the complete 2-
year changeover to the VW Passat TDI Clean Diesel Sedan. The changeover has been problem-
free with a huge reduction in fuel expenditure even though the diesel fuel is typically $0.60 more
than 87 grade gasoline. Police officers appreciate the size, power and comfort of the vehicle,
while taxpayers enjoy the financial benefits of fuel efficiency reliability of the vehicle.
Specific comparison data provided by Belle Meade PD, Tennessee
Top speed in their district 45MPH (same as Tybee)
Vehicles in Use: 16 VW Passat TDI Clean Diesel (2 more being added this year)
Average Fuel Economy: 34mpg (compared to 31/43 City/Hwy on Cars.com)
Maintenance: Included for 3 years, except for oil changes (done by Police Dept.)
Rationale/Advantages
Low maintenance engine; oil and air filter are directly accessible; no spark plugs or leads
as diesel combustion is based on compression.
“Low-end torque” allows quick acceleration from 0 to 45-50 MPH (necessary in pursuit
or intent to stop situations) and electronic stability control (required for high-speed or
cornering high-speed control situations)
Largest trunk space of any vehicle being considered; lockable and capable of handling
equipment slides for modern communication equipment and most rear and front legroom
combined and the most rear legroom in its class, allowing for more space for prisoner
transport
VW has had superb reputation for high quality high performance vehicles since 1937.
The Jetta, Jetta Sportwagon and Passat Sedan are built by American workers in
Tennessee (a target audience for tourists coming to Tybee); the majority of parts are
sourced in the USA, engine and transmission manufactured in Germany.
Outfitting can be performed Belle Meade’s preferred vendor for $10,500 excluding
Police Radio (which would be removed from one of Tybee's old Cruisers) and includes
two way radar, light bar etc., and all other requirements to meet Georgia Police Cruiser
code. The Department of Public Works can maintain the outfitting of these vehicles and
will not require special maintenance training as it would with a hybrid vehicle.
Exceeds CAFE requirements and has proven very low emissions.
While it does not provide, on paper, the best fuel economy, the Passat is an ATTRACTIVE
COMPROMISE based on all the criteria that our Police Chief has stated is required and delivers
fuel economy that is 2 + times better than the Dodge Charger or Ford Crown Victoria.
Challenges
Diesel costs $0.60-0.80/gallon more than 87 grade gasoline, however the efficiency of
diesel engines gives double or more fuel economy to the same size and class of vehicle.
Diesel is still perceived as “dirty”, however modern ultra-low sulphur diesel fuels
actually pollute less than their petroleum-based counterparts.
Case Study – Police Department of White Bluff, TN - Contact: Chief Mike Holman 615-797-
3131 or 615-425-8238 cell
Vehicle: Toyota Prius Type: Hybrid Drive Fuel: Gasoline, 87
Background
White Bluff, TN, a suburb of Nashville which has 3,219 residents and is very similar to Tybee in terms of
area, converted its entire fleet of 4 police vehicles to the Toyota Prius Hybrid over 2 years ago, and they
are now looking to add more. In conversations with Police Chief Mike Holman, the Chief emphasized
that they have been very pleased with the selection of the Prius: the changeover has gone smoothly on all
fronts and has slashed the department’s fuel budget by almost 75%. Although there was some initial
reticence on the part of officers to move to a non-traditional vehicle, most are now very happy with the
Prius and agree that nothing else even comes close to its combination of versatility, room and low
operating costs, not even taking into account its environmental benefits.
Specific comparison data provided by White Bluff PD, Tennessee
Top speed in their district 45MPH (same as Tybee)
Vehicles in Use: 4 Toyota Prius Hybrids (2 more being requested this year)
Average Fuel Economy: 50-52 mpg (compared to 51/48 City/Hwy on Cars.com)
Maintenance: Included for 5 years, except for oil changes. Hybrid drive fully warrantied for 8
years/100k miles)
Rationale/Advantages
Toyota has consistently ranked as one of the top car manufacturers for initial quality,
performance, reliability and vehicle safety. The Prius has been manufactured since 2004, so its
hybrid technology has been thoroughly tested and road-proven over a long period. Has won
numerous awards, including the NHTSA 5-Star Overall Safety Rating.
Interior space is only ¾” shorter and 3” narrower than the Crown Victoria. Chief Holman also
reports interior and cargo space are more than adequate for their needs and more flexible than
other traditional police vehicles.
Fuel performance is optimum at speeds averaging less than 35 mph—which fits perfectly within
the profile of use on Tybee Island. White Bluff PD reduced their fuel costs from over
$30,000/year to approximately $8,000.
Superior off-the-line acceleration and maneuverability. Before making their purchase decision,
Chief Holman and his officers test-drove the Prius to a top speed of 116 mph and took it to the
EVOC (Emergency Vehicle Obstacle Course) training where it “blew away” the competition.
Hybrid Synergy Drive engine automatically cycles on and off when the vehicle is stopped while
allowing all systems to function fully (draws from large battery supply), saving on fuel and
carbon output.
Challenges
Perception/Acceptance—“Doesn’t look like a police car!” But Chief Holman and other police
departments report that this actually works to their advantage, giving officers an element of
surprise.
Outfitting – Local vendor may not be able/willing to do, however there are plenty of other
vendors who can.
Additional Resources
White Bluff PD, TN, converts to Prius:
Boca Raton PD uses Prius for unmarked police cars
NYC uses Prius as police cars
Potential benefits of solar reflective car shells: Cooler cabins, fuel savings
and emission reductions
Ronnen Levinson ⇑, Heng Pan, George Ban-Weiss, Pablo Rosado, Riccardo Paolini 1, Hashem Akbari 2
Heat Island Group, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
article info
Article history:
Received 15 February 2011
Received in revised form 29 April 2011
Accepted 2 May 2011
Available online 29 July 2011
Keywords:
Cool colored car
Solar reflective shell
Vehicle air conditioning
Vehicle fuel economy
Vehicle emission reduction
ADVISOR
abstract
Vehicle thermal loads and air conditioning ancillary loads are strongly influenced by the absorption of
solar energy. The adoption of solar reflective coatings for opaque surfaces of the vehicle shell can decrease
the ‘‘soak’’ temperature of the air in the cabin of a vehicle parked in the sun, potentially reducing the vehi-
cle’s ancillary load and improving its fuel economy by permitting the use of a smaller air conditioner. An
experimental comparison of otherwise identical black and silver compact sedans indicated that increas-
ing the solar reflectance (q) of the car’s shell by about 0.5 lowered the soak temperature of breath-level
air by about 5–6 C. Thermal analysis predicts that the air conditioning capacity required to cool the cabin
air in the silver car to 25 C within 30 min is 13% less than that required in the black car. Assuming that
potential reductions in AC capacity and engine ancillary load scale linearly with increase in shell solar
reflectance, ADVISOR simulations of the SC03 driving cycle indicate that substituting a typical cool-col-
ored shell (q = 0.35) for a black shell (q = 0.05) would reduce fuel consumption by 0.12 L per 100 km
(1.1%), increasing fuel economy by 0.10 km L 1 [0.24 mpg] (1.1%). It would also decrease carbon dioxide
(CO2) emissions by 2.7 g km 1 (1.1%), nitrogen oxide (NOx) emissions by 5.4 mg km 1 (0.44%), carbon
monoxide (CO) emissions by 17 mg km 1 (0.43%), and hydrocarbon (HC) emissions by 4.1 mg km 1
(0.37%). Selecting a typical white or silver shell (q = 0.60) instead of a black shell would lower fuel con-
sumption by 0.21 L per 100 km (1.9%), raising fuel economy by 0.19 km L 1 [0.44 mpg] (2.0%). It would
also decrease CO2 emissions by 4.9 g km 1 (1.9%), NOx emissions by 9.9 mg km 1 (0.80%), CO emissions
by 31 mg km 1 (0.79%), and HC emissions by 7.4 mg km 1 (0.67%). Our simulations may underestimate
emission reductions because emissions in standardized driving cycles are typically lower than those in
real-world driving.
2011 Elsevier Ltd. All rights reserved.
1. Introduction
Over 95% of the cars and small trucks sold in California have
air conditioning [1,2]. Use of air conditioning (AC) in cars has
been estimated to increase carbon monoxide (CO) emissions
by 0.99 g km 1 (71%), increase nitrogen oxide (NOx) emissions
by 0.12 g km 1 (81%), and reduce fuel economy by 2.0 km L 1
[4.6 mpg] (22%)[3]. Air conditioning is the major ancillary load
for a light-duty vehicle. The AC is sized to cool the cabin air
from its ‘‘hot soak’’ condition (i.e., vehicle parked in the sun,
facing the equator, on a summer afternoon) to a comfortable
quasi-steady temperature, such as 25 C. Reducing the peak
cooling load lowers the required cooling capacity, reducing
ancillary load, improving fuel economy, and decreasing tailpipe
emissions.
The current study focuses on the decrease in soak temperature,
reduction in AC capacity, and improvement in fuel economy attain-
able through the use of solar reflective shells. Here ‘‘shell’’ refers to
the opaque elements of the car’s envelope, such as its roof and
doors. First, we experimentally characterize component tempera-
tures and cooling demands in a pair of otherwise identical dark
and light colored vehicles, the former with low solar reflectance
and the latter with high solar reflectance. Second, we employ a
thermal model to predict the AC capacity required to cool each
vehicle to a comfortable final cabin air temperature. Third, we
use the AVL ADVISOR vehicle simulation tool to estimate the
dependence on ancillary load of the fuel consumption and pollu-
tant emissions of a comparable prototype vehicle in various stan-
dard drive cycles. Finally, we calculate the fuel savings and
emission reductions attainable by using a cool shell to reduce
ancillary load.
0306-2619/$ - see front matter 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apenergy.2011.05.006
⇑Corresponding author. Tel.: +1 510 486 7494.
E-mail address:RML27@cornell.edu (R. Levinson).
1 Present address: Department of Building Environment Science & Technology,
Politecnico di Milano, Milano, Italy.
2 Present address: Department of Building, Civil, and Environmental Engineering,
Concordia University, Montreal, Canada.
Applied Energy 88 (2011) 4343–4357
Contents lists available at ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
2. Literature review
Extensive research over the past two decades has focused on
reducing air conditioner ancillary loads. Most studies consider ca-
bin air temperature, AC cooling load, and/or occupant comfort.
2.1. Technology performance
Technologies to reduce AC load include solar reflective glazing,
solar reflective shells, ventilation, insulation and window shading
[4–8]. Past research has identified the use of solar reflective glazing
as an especially effective strategy for reducing cooling loads, since
sunlight transmitted through glazing accounts for 70% of cabin
heat gain in hot soak conditions [9]. For example, Rugh et al.[4]
measured that solar reflective glazing in a Ford Explorer decreased
cabin air (‘‘breath’’) temperature by 2.7 C, lowered instrument pa-
nel temperatures by 7.6 C, and reduced windshield temperatures
by 10.5 C. They also reported that this decrease in cabin air soak
temperature would permit an 11% reduction in AC compressor
power. More generally, they estimated that AC compressor power
could be decreased by about 4.1% per 1 C reduction in cabin soak
temperature.
Akabane et al.[10]experimentally determined that in a vehicle
traveling 40 km h 1 (25 mph) on a hot day (outdoor air tempera-
ture 38 C, horizontal solar irradiance 0.81 kW m 2), about 42% of
the vehicle heat load resulted from transmission through the glaz-
ing, with about 48% from conduction through the shell and about
10% from engine heat and air leaks. We note that load fractions
may vary with window opacity and with the ratio of window area
to shell area.
Rugh and Farrington [7]found that ventilation and window
shading during soak can be effective in reducing AC loads. They
concluded that natural ventilation (achieved using appropriately
placed inlets to allow for natural convection) can be almost as
effective as forced ventilation.
Solar reflective shells have also been reported to reduce soak
temperatures. Hoke and Greiner [6]used the RadTherm and
UH3D modeling tools to simulate soak conditions for a sport utility
vehicle (SUV) parked on a hot summer day in Phoenix, AZ. They
concluded that each 0.1 increase in the solar reflectance q of the
shell reduces the cabin air soak temperature by about 1 C. For
example, cabin air soak temperature in a vehicle with a white shell
(q = 0.50) was predicted to be 4.6 C lower than that in a compara-
ble vehicle with a black shell (q = 0.05). Rugh and Farrington [7]
measured for several vehicles the reduction in cabin air (breath)
soak temperature versus increase in shell solar reflectance.
Increasing the shell solar reflectance of a Ford Explorer mid-size
SUV by 0.44 lowered cabin air soak temperature by 2.1 C
(0.47 C reduction per 0.1 gain in shell solar reflectance), while
increasing that of a Lincoln Navigator full-size SUV by 0.45 lowered
cabin air soak temperature by 5.6 C (1.2 C reduction per 0.1 gain
in shell solar reflectance). Increasing the solar reflectance of only
the roof of a Cadillac STS full-size sedan by 0.76 lowered cabin
air temperature by 1.2 C (0.15 C reduction per 0.1 gain in roof so-
lar reflectance).
A 3-D computational fluid dynamics (CFD) simulation by Han
and Chen [8]estimated that increasing body insulation reduces
steady state thermal load, but raises air cabin temperature during
soaking and cooling.
2.2. Modeling tools
Simulations of thermal load and thermal comfort in vehicles
typically use either lumped-parameter models [5,11–13]or tran-
sient CFD models [14–18]. A study by the National Renewable
Energy Laboratory (NREL) concluded that transient CFD tools are
best suited for this task [19].
Bharathan et al.[20]provide a definitive overview of the models
that have been adopted or developed by NREL to simulate environ-
mental loads, thermal comfort, and AC fuel use. We summarize
some findings below.
2.2.1. Environmental forcing
The GUI-driven MATLAB application Vehicle Solar Load Estima-
tor (VSOLE), developed by NREL [21], calculates the solar radiation
transmitted, absorbed, and reflected by glazing as a function of
glazing properties and location, vehicle geometry and orientation,
time, and radiation source.
2.2.2. Thermal modeling
The commercial CFD tool RadTherm can be used to simulate so-
lar heat load, interior and exterior convection, and conduction
through the envelope, while the commercial CFD tool Fluent can
be used to simulate convective heat transfer and fluid flow in the
cabin.
2.2.3. AC performance
An NREL model uses transient analysis to optimize vehicle AC
performance [22].
2.2.4. Fuel economy
The AVL ADVISOR vehicle simulator originally developed by
NREL [23,24]can simulate the effect of vehicle ancillary load on
fuel consumption and pollutant emissions.
2.2.5. Thermal comfort
NREL has applied two models from the University of California
at Berkeley—the Human Thermal Physiological Model and the Hu-
man Thermal Comfort Empirical Model—to evaluate thermal com-
fort in vehicles.
3. Theory
3.1. Cabin air temperature model
The cabin air heating rate, or rate at which the internal energy
of the cabin air U(t) increases with time t,i s
dU
dt ¼ma cv
dT a
dt ð1Þ
where ma is the cabin air mass,cv is the specific heat of air at con-
stant volume, and Ta(t) is the cabin air temperature. The cabin air is
assumed to be transparent to both sunlight and thermal radiation,
but exchanges heat with the air conditioner and the cabin surface.
If the air is well mixed, a simple model for the variation of cabin
air temperature with time is
dT a
dt ¼a½T v ðtÞ T a ðtÞ þ b½T s ðtÞ T a ðtÞ ;ð2Þ
where Tv(t) is the temperature of the air flowing into the cabin from
the AC vent,Ts(t) is the mean temperature of the cabin’s surface,
and a and b are fitted constants.
As the cabin air is mechanically cooled, it may reach a quasi-
steady state in which Ta(t) asymptotically approaches a final value.
In this condition, denoted by the superscript ,(dTa/dt)⁄0 and
thus
T a T v þ b
a ðT s T aÞ :ð3Þ
We may need to lower the vent air temperature if the final cabin air
temperature T a exceeds some design target T 0
a , such as 25 C. If the
4344 R. Levinson et al./Applied Energy 88 (2011) 4343–4357
difference between the cabin surface temperature and the cabin air
temperature (Ts Ta) is insensitive to the vent air temperature Tv,
then
dT a
dT v
1:ð4Þ
That is, reducing T v by DT will lower T a by approximately DT. Resiz-
ing the AC to yield a new vent air temperature T 0
v ðtÞ T v ðtÞ DT re-
sults in a new cabin air temperature T 0
a ðtÞ that can be computed by
numerically integrating
dT 0
a
dt ¼a½T 0
v ðtÞ T 0
a ðtÞ þ b½T s ðtÞ T a ðtÞ ð5Þ
subject to the initial condition T 0
a ð0Þ¼T a ð0Þ. Note that the second
term on the right hand side of Eq.(5)is the same as that in Eq.
(2)because we have assumed that T 0
s ðt Þ T 0
a ðtÞ¼T s ðtÞ T a ðtÞ.
3.2. AC capacity model
In recirculation mode, the rates at which the original and re-
sized air conditioners remove heat from the cabin air are
qAC ðtÞ¼_mcp ½T a ðt Þ T v ðtÞ ð6Þ
and
q0
AC ðtÞ¼_mcp ½T 0
a ðt Þ T 0
v ðtÞ ;ð7Þ
respectively, where _m is the AC air mass flow rate and cp is the spe-
cific heat of air at constant pressure. To meet peak cooling load, the
capacity of the resized AC must be at least
Q max q0
AC ðtÞ ¼_mcp max T 0
a ðtÞ T 0
v ðtÞ :ð8Þ
3.3. Fuel saving and emission reduction model
Consider two vehicles that differ only in shell solar reflectance q
and required AC capacity Q. The reduction in AC capacity attainable
by substituting the high-reflectance shell (subscript ‘‘H’’) for the
low-reflectance shell (subscript ‘‘L’’) is
DQ H Q L Q H ð9Þ
and the reduction in vehicle ancillary power load P is
DPH ¼DQ H =COP ð10Þ
where COP is the coefficient of performance of the AC system.
Let F denote fuel consumption rate (volume of fuel per unit dis-
tance traveled) and E represent pollutant emission rate (mass of
pollutant per unit distance traveled). If reductions in F and E are
each linearly proportional to reduction in P, then
DF H ¼cF DQ H =COP ð11Þ
Table 1
General properties of test vehicles.
Make and model 2009 Honda Civic 4DR GX
Cabin volume (m3)2.57
Engine idle speed (RPM)700
AC air flow rate (m3 s 1)0.1
Black Silver
Odometer distance (mi) [km]4300 [6900] 6200 [10,000]
AC line high pressure (psi) [MPa] 165 [1.13] 175 [1.20]
AC line low pressure (psi) [MPa] 35 [0.24]40 [0.28]
Table 2
Vehicle surface properties.
Surface Area (m2) Solar reflectance Thermal emittance
Roof 2.0 0.05 (black)0.83 (black)
0.58 (silver)0.79 (silver)
Ceiling 2.0 0.41 n/a
Dashboard 0.6 0.06 n/a
Windshield 0.9 0.06 0.88
Seat 2.4 0.38 n/a
Door 3.0 0.11 n/a
1. cabin air
2. roof
3. windshield
4. dashboard
5. ceiling
6. door
7. seat
8. vent air
(a)
8
2
1 3
4,8
4 3
2
6 157
5
6 7
(b)
Fig. 2.Locations of the eight cabin temperature sensors (thermistors), shown in (a)
top view and (b) side view.
Fig. 1.Experimental vehicles parked facing south in Sacramento, CA on July 17, 2010. Tower between vehicles (black car, solar reflectance 0.05, left; silver car, solar
reflectance 0.58, right) supports a Davis Instruments Vantage Pro weather station (upper mount) and an Eppley Laboratory Precision Spectral Pyranometer (lower mount).
R. Levinson et al./Applied Energy 88 (2011) 4343–4357 4345
and
DEH ¼cE DQ H =COP ð12Þ
where cF dF/dP and cE dE/dP are constant coefficients.
(We will show that for the drive cycles simulated in this study,
cF and cE are indeed nearly constant within the ancillary power
load ranges considered. However, past studies have raised doubt
about the degree to which standardized driving cycles represent
vehicle emissions from real-world driving [25]. Specifically, a dis-
proportionate fraction of emissions occur from ‘‘off-cycle’’ driving
characterized by high speed and/or acceleration. Bevilacqua [3]
has shown that NOx and CO emissions are almost doubled with
the operation of AC. Here the linearity assumption offers a very
conservative estimate of pollutant reductions.)
Finally, consider a cool colored vehicle (subscript ‘‘C’’) that dif-
fers from the first two vehicles only in shell solar reflectance and
required AC capacity. Since DOE-2 simulations indicate that reduc-
tion in a building’s annual peak demand for cooling power is line-
arly proportional to gain in roof solar reflectance [26], we assume
that reduction in required AC capacity scales with increase in shell
solar reflectance, such that
DQ C Q L Q C ¼qC qL
qH qL
DQ H :ð13Þ
It then follows that the rates of fuel savings and emission reduction
attainable by substituting the cool colored shell for the low-reflec-
tance shell are
DF C ¼cF DQ C =COP ð14Þ
and
DEC ¼cE DQ C =COP ð15Þ
respectively.
Table 3
Input parameters for the two vehicle prototypes simulated with ADVISOR. Simulation
results from these two cars were interpolated to match the 84 kW power rating of the
Honda Civics used in our experiments.
Prototype 1 Prototype 2
Vehicle type Compact Compact
Vehicle power rating (kW)63 102
Vehicle mass (kg)1466 1601
Drivetrain configuration Conventional Conventional
Fuel converter FC_SI63_emis FC_SI102_emis
Table 4
US EPA [31]and ADVISOR fuel economy (mpg) estimates for the 2009 Honda Civic GX.
EPA sticker ADVISOR (0–4 kW ancillary load)
City 24 19–27
Highway 36 31–39
Combined city/highway 28 24–32
(a)
0
10
20
30
40
50
60
70
80
90
100
18:30 18:40 18:50 19:00 19:10 19:20
Local standard time (16 Jul 2010)
Ca
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
-5
0
5
10
15
20
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(b)
0
10
20
30
40
50
60
70
80
90
100
18:30 18:40 18:50 19:00 19:10 19:20
Local standard time (16 Jul 2010)
Ve
n
t
a
i
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
-5
0
5
10
15
20
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
Fig. 4.Comparisons of (a) cabin air temperature and (b) vent air temperature in
each car during indoor HVAC calibration.
(a)
0
5
10
15
20
25
30
35
40
45
50
Local standard time (17 Jul 2010)
Te
m
p
e
r
a
t
u
r
e
(
°
C
)
0
10
20
30
40
50
60
70
80
90
100
Re
l
a
t
i
v
e
h
u
m
i
d
i
t
y
(
%
)
outside air temperature
outside air relative humidity
(b)
0
200
400
600
800
1000
1200
08:00 10:00 12:00 14:00 16:00
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
So
l
a
r
i
r
r
a
d
i
a
n
c
e
(
W
/
m
2 )
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Wi
n
d
s
p
e
e
d
(
m
/
s
)
solar irradiance
wind speed
Fig. 3.Weather during soaking and cooling trials, including (a) outdoor air
temperatureandhumidityand(b)globalhorizontalsolarirradianceandwind speed.
4346 R. Levinson et al./Applied Energy 88 (2011) 4343–4357
4. Experiment (thermal study)
4.1. Overview
A pair of otherwise identical light duty vehicles, one with a
black shell and the other with a silver shell, were instru-
mented with surface and air temperature sensors. AC perfor-
mance was calibrated with an indoor heating and cooling
trial. The vehicles were then parked outdoors on a sunny sum-
mer day and subjected to a series of five soaking and cooling
trials.
4.2. Vehicles
Two 2009 Honda Civic 4DR GX compact sedans, one black and
one silver, were loaned by California’s Department of General Ser-
vices (Fig. 1). Apart from shell color, the vehicles were essentially
identical, with only minor differences in odometer distance and
AC line pressures (Table 1).
The air mass one global horizontal solar reflectance [27,28]of
each exterior surface (roof) and interior surface (ceiling, dashboard,
windshield, seat and door) was measured with a solar spectrum
reflectometer (Devices & Services SSR-ER, version 6; Dallas, TX).
The hemispherical thermal emittance of each roof and windshield
was measured with an emissometer (Devices & Services AE1; Dal-
las, TX). The solar reflectances of the black and silver roofs were
0.05 and 0.58, respectively, while their thermal emittances were
0.83 and 0.79 (Table 2).
4.3. Instrumentation
The roof, ceiling, dashboard, windshield, seat, door, vent air and
cabin air temperatures in each car were measured with thermis-
tors (Omega SA1-TH-44006-40-T [surfaces], Omega SA1-TH-
44006-120-T [air]; Stamford, CT) and recorded at 1 Hz with a por-
table data logger (Omega OM-DAQPRO-5300; Stamford, CT). The
vent air thermistor was suspended in front of a central HVAC out-
let, while the cabin air thermistor was suspended at breath level
midway between the front seat headrests. Top and side views of
the eight temperature measurement points in each vehicle are
shown in Fig. 2.
Each vent air and cabin air thermistor was wrapped in alumi-
num foil (low solar absorptance; low thermal emittance) to mini-
mize both solar absorptance and radiative coupling to the cabin.3
Interior surface thermistors (ceiling, dashboard, windshield, seat,
and door) were wrapped in foil and secured with clear adhesive tape.
Clear tape over foil yields high solar reflectance and high thermal
emittance, minimizing solar absorptance while retaining radiative
0
10
20
30
40
50
60
70
80
90
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Te
m
p
e
r
a
t
u
r
e(
°
C
)
roof
dashboard
ceiling
windshield
seat
door
cabin air
black
(a)
(b)
0
10
20
30
40
50
60
70
80
90
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Te
m
p
e
r
a
t
u
r
e(
°
C
)
roof
dashboard
ceiling
windshield
seat
door
cabin air
silver
Fig. 5.Roof, dashboard, ceiling, windshield, seat, door and cabin air temperatures
measured during soaking and cooling trials in (a) the black car and (b) the silver car.
(a)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Te
m
p
e
r
a
t
u
r
e
(
°
C
)
-10
0
10
20
30
40
50
60
70
80
90
Di
f
f
e
r
e
n
c
e
(
°
C
)
cabin surface
cabin air
vent air
cabin air - vent air
cabin surface - cabin air
black
(b)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Te
m
p
e
r
a
t
u
r
e
(
°
C
)
-10
0
10
20
30
40
50
60
70
80
90
Di
f
f
e
r
e
n
c
e
(
°
C
)
cabin surface
cabin air
vent air
cabin surface - cabin air
cabin air - vent air
silver
Fig. 6.Cabin surface, cabin air and vent air temperatures measured during soaking
and cooling trials in (a) the black car and (b) the silver car. Also shown are
differences between cabin surface and cabin air temperature and between cabin air
and vent air temperature.
3 Increasing the thermal emittance of the sensor by replacing the foil with white
paint (low solar absorptance, high thermal emittance) would tend to increase, rather
than decrease, the apparent cabin air temperature by radiatively coupling the sensor
to warm cabin surfaces. To illustrate, we note that the windshield in the black car has
low solar absorptance and high thermal emittance (as would a white-coated sensor),
but runs about 4 C warmer than the cabin air during the soak and about 13 C
warmer than the cabin air during cooldown. These elevated windshield temperatures
result from radiative coupling to the hot dashboard and ceiling.
R. Levinson et al./Applied Energy 88 (2011) 4343–4357 4347
coupling to the cabin. Roof thermistors were affixed with reflec-
tance-matched opaque adhesive tape. Black tape of solar reflectance
0.05 was used on the black roof (q = 0.05), and a light-colored tape of
solar reflectance 0.62 was used on the silver roof (q = 0.58).
(a)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Ro
o
f
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
5
10
15
20
25
30
35
40
45
50
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(b)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Ce
i
l
i
n
g
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
5
10
15
20
25
30
35
40
45
50
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(c)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Da
s
h
b
o
a
r
d
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
2
4
6
8
10
12
14
16
18
20
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(d)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Wi
n
d
s
h
i
e
l
d
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
2
4
6
8
10
12
14
16
18
20
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(e)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Se
a
t
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
5
10
15
20
25
30
35
40
45
50
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(f)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Do
o
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
5
10
15
20
25
30
35
40
45
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(g)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Ve
n
t
a
i
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
5
10
15
20
25
30
35
40
45
50
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
(h)
0
10
20
30
40
50
60
70
80
90
100
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Ca
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
0
2
4
6
8
10
12
14
16
18
20
Di
f
f
e
r
e
n
c
e
(
°
C
)
black
silver
black - silver
Fig. 7.Comparisons of (a) roof, (b) ceiling, (c) dashboard, (d) windshield, (e) seat, (f) door, (g) vent air and (h) cabin air temperatures measured during soaking and cooling
trials.
4348 R. Levinson et al./Applied Energy 88 (2011) 4343–4357
A weather station (Davis Instruments VantagePro2; Hayward,
CA) mounted between the vehicles at a height of 2 m recorded
1 min averages of outside air temperature, relative humidity,
global horizontal solar irradiance, and wind speed (Fig. 1). Solar
irradiance was also measured with a first class pyranometer
(Eppley Laboratory Precision Spectral Pyranometer; Newport, RI)
to check the solar irradiance reported by the weather station’s
silicon radiometer. The first class pyranometer shared a datalogger
channel with the black car’s vent air thermistor. During daytime
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
Ca
b
i
n
a
i
r
h
e
a
t
i
n
g
r
a
t
e
(
k
W
)
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
Di
f
f
e
r
e
n
c
e
(
k
W
)
black
silver
black - silver
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
08:00 10:00 12:00 14:00 16:00
Local standard time (17 Jul 2010)
AC
c
o
o
l
i
n
g
r
a
t
e
(
k
W
)
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Di
f
f
e
r
e
n
c
e
(
k
W
)
black
silver
black - silver
(a)
(b)
Fig. 8.Comparisons of (a) cabin air heating rates measured during soaking and
cooling trials and (b) AC cooling rates measured during cooling trials.
Table 5
Cooling trial measurements. Temperature differences are black car–silver car.
Cycle Cool1 Cool2 Cool3 Cool4 Cool5
Start (LST)09:32 11:02 12:33 14:02 15:32
End (LST)09:59 11:32 13:01 14:32 16:00
Duration (min)28 30 28 30 28
Mean outdoor air temperature (C)25.0 28.8 32.9 35.8 37.5
Mean solar irradiance (kW m 2)0.83 0.98 1.00 0.86 0.64
Black cabin air temperature after soaking (C)47.7 55.9 61.8 64.4 63.6
Silver cabin air temperature after soaking (C)43.3 50.7 56.1 58.0 57.3
Cabin air temperature difference after soaking (C)4.4 5.2 5.7 6.4 6.4
Black cabin air temperature after cooling (C)22.1 27.7 32.0 34.3 33.7
Silver cabin air temperature after cooling (C)19.6 24.9 28.2 29.9 29.5
Cabin air temperature difference after cooling (C)2.5 2.8 3.8 4.4 4.2
Black cabin surface temperature after soaking (C)48.5 53.3 58.0 62.2 66.0
Silver cabin surface temperature after soaking (C)45.3 48.4 52.5 56.4 59.9
Cabin surface temperature difference after soaking (C)3.2 4.9 5.5 5.8 6.1
Black cabin surface temperature after cooling (C)28.7 33.7 38.3 40.9 42.7
Silver cabin surface temperature after cooling (C)27.4 30.5 34.2 36.5 38.3
Cabin surface temperature difference after cooling (C)1.3 3.2 4.1 4.4 4.4
Black vent air temperature after cooling (C)9.6 14.2 17.4 20.5 20.2
Silver vent air temperature after cooling (C)9.1 13.0 15.4 16.8 17.0
Vent air temperature difference after cooling (C)0.5 1.2 2.0 3.7 3.2
(a)
cabin air:
y = 0.70x - 10.8
R2 > 0.99
vent air:
y = 0.56x - 15.8
R2 = 0.95
0
5
10
15
20
25
30
35
40
45
50
40 45 50 55 60 65 70
Cabin air temperature after soaking (°C)
Te
m
p
e
r
a
t
u
r
e
a
f
t
e
r
c
o
o
l
i
n
g
(
°
C
)
cabin air (black)
cabin air (silver)
vent air (black)
vent air (silver)
cabin air (fit)
vent air (fit)
(b)after soaking:
y = 0.90x + 4.8
R2 = 0.87
after cooling:
y = 0.72x - 5.1
R2 = 0.93
0
10
20
30
40
50
60
70
80
90
100
40 45 50 55 60 65 70
Cabin air temperature after soaking (°C)
Ca
b
i
n
s
u
r
f
a
c
e
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
after soaking (black)
after soaking (silver)
after cooling (black)
after cooling (silver)
after soaking (fit)
after cooling (fit)
Fig. 9.Variations with cabin air final soak temperature of (a) cabin air and vent air
final cooldown temperatures and (b) cabin surface final soak and final cooldown
temperatures. Soak and cooldown intervals were approximately 60 and 30 min,
respectively.
R. Levinson et al./Applied Energy 88 (2011) 4343–4357 4349
trials, the shared channel recorded vent air temperature while the
vehicle was being cooled, and solar irradiance at other times.
4.4. AC calibration (16 July 2010)
On the evening of 16 July 2010, each vehicle was parked under a
carport to shield it from sunlight. All windows were closed. At
18:38 LST, maximum heating (highest HVAC temperature setting,
top fan speed, recirculation mode) was used to raise the cabin air
temperature in each vehicle to about 60 C in 16 min. Each cabin’s
air temperature was then reduced to about 20 C after 20 min of
maximum cooling (lowest HVAC temperature setting, top fan
speed, recirculation mode). The cabin air and vent air temperatures
during the cooling cycle in the black car were compared to those in
the silver car to verify that the AC systems performed similarly.
4.5. Soaking and cooling (17 July 2010)
At 08:00 LST on the following day (17 July 2010), the vehicles
were removed from the carport and parked outdoors, side by side,
facing due south (Fig. 1). All windows were closed.
The weather was warm and sunny, with the outside air temper-
ature rising steadily from 21 C at 08:00 LST to 38 C at 16:00 LST.
Global horizontal solar irradiance reached about 1.0 kW m 2
shortly after noon, and wind speed ranged from about 0.5 to
1.3 m s 1 (Fig. 3). Solar irradiances measured with the silicon radi-
ometer closely matched those measured with the first class
pyranometer.
From 08:30 to 16:00 LST, each parked car was run through five
rounds of soaking and cooling in which an approximately 60 min
soak (HVAC off) was followed by about 30 min of maximum cool-
ing. The soaking and cooling intervals were closely synchronized
car-to-car.
Table 6
Characteristics of each cooling trial, including fit parameters a and b; coefficient of
determination R2; measured final cabin air temperature T a ; and AC cooling capacity Q
needed to attain a final cabin air temperature of 25 C.
Trial a (s 1)b (s 1)R2 T a (C)Q (kW)
black_cool1 0.017 0.030 0.86 22.1 2.60
black_cool2 0.012 0.024 0.90 27.7 3.05
black_cool3 0.010 0.023 0.88 32.0 3.66
black_cool4 0.012 0.024 0.93 34.3 3.83
black_cool5 0.015 0.022 0.93 33.7 3.64
silver_cool1 0.018 0.025 0.95 19.6 2.35
silver_cool2 0.012 0.024 0.86 24.9 2.61
silver_cool3 0.011 0.023 0.90 28.2 3.03
silver_cool4 0.011 0.023 0.89 29.9 3.34
silver_cool5 0.015 0.022 0.90 29.5 3.25
(a)
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0 5 10 15 20 25 30
Cooling time (min)
d[
c
a
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
]
/
d
t
(
°
C
/
s
)
measured
fit
black_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
black
(b)
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0 5 10 15 20 25 30
Cooling time (min)
d[
c
a
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
]
/
d
t
(
°
C
/
s
)
measured
fit
silver_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
silver
Fig. 10.Measured and fitted rates of change of cabin air temperature
versus cooling time in Trial 4, shown for (a) the black car and (b) the
silver car.
(a)
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Cooling time (min)
Ca
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
r
e
d
u
c
t
i
o
n
(
°
C
)
black_cool1 (09:32-09:59 LST, 28 min, 25°C, 0.83 kW/m²)
black_cool2 (11:02-11:32 LST, 30 min, 29°C, 0.98 kW/m²)
black_cool3 (12:33-13:01 LST, 28 min, 33°C, 1.00 kW/m²)
black_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
black_cool5 (15:32-16:00 LST, 28 min, 37°C, 0.64 kW/m²)
black
(b)
0
5
10
15
20
25
30
0 5 10 15 20 25 30
Cooling time (min)
Ca
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
r
e
d
u
c
t
i
o
n
(
°
C
)
silver_cool1 (09:32-09:58 LST, 27 min, 25°C, 0.83 kW/m²)
silver_cool2 (11:02-11:32 LST, 30 min, 29°C, 0.98 kW/m²)
silver_cool3 (12:33-13:00 LST, 28 min, 33°C, 1.00 kW/m²)
silver_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
silver_cool5 (15:32-15:59 LST, 28 min, 37°C, 0.64 kW/m²)
silver
Fig. 11.Reduction in cabin air temperature versus cooling time in each of five trials,
shown for (a) the black car and (b) the silver car. Cooling trial interval, duration,
mean outside air temperature and mean solar irradiance are listed in parentheses.
4350 R. Levinson et al./Applied Energy 88 (2011) 4343–4357
5. Simulations (fuel savings and emission reductions)
We used the vehicle simulation tool AVL ADVISOR (version
2004.04.09 SP1) to relate rates of fuel consumption, nitrogen oxide
(NOx) emission, carbon monoxide (CO) emission, and hydrocarbon
(HC) emission to ancillary power load. ADVISOR was first devel-
oped in November 1994 by the National Renewable Energy Labora-
tory. It was designed as an analysis tool to help the US Department
of Energy (DOE) quantify the potential of hybrid vehicles to save
fuel and reduce emissions. ADVISOR simulates vehicle powertrains
and power flows among its components [23,24]. Fuel use and
tailpipe emissions can be simulated in a variety of standard driving
cycles. In ADVISOR, AC power load is added as an accessory
mechanical load.
In this study we focus on ADVISOR simulations of the EPA Speed
Correction (SC03) driving cycle, a transient test cycle with an
average speed of 34.8 km h 1 (21.6 mph) and a maximum speed
of 88.2 km h 1 (54.8 mph). We also show results for the EPA
Urban Dynamometer Driving Schedule (UDDS) (average speed =
31.5 km h 1 = 19.6 mph), and the EPA Highway Fuel Economy Test
(HWFET) driving cycle (average speed = 77.7 km h 1 = 48.3 mph)
[29]. Simulations were performed with ancillary power load rang-
ing from 0 to 4 kW at a resolution of 0.2 kW.Since we did not have
access to an ADVISOR vehicle prototype for the Honda Civic, each
ADVISORsimulationwasrunfortwoavailableprototypes:onewith
a 63 kW engine, and the other with a 102 kW engine. Results were
then interpolated to match the engine power rating of the Honda
Civic GX (84 kW). Note that while the Honda Civic GX is fueled by
natural gas, our ADVISOR simulations represent an equal-power
vehicle fueled by gasoline.Table 3 presents additional details of
the ADVISOR simulations.
We estimate CO2 emission reduction from fuel savings at the
rate of 2321 g CO2 per L of gasoline [30].
We compared ADVISOR fuel economy predictions to EPA gaso-
line gallon equivalent estimates for the 2009 Honda Civic GX [31].
EPA estimates are derived from a formula that weights results from
five different drive cycles [32]. ADVISOR simulations of the EPA Ur-
ban Dynamometer Driving Schedule (UDDS) and the EPA Highway
Fuel Economy Test (HWFET) were used to estimate city and high-
way fuel economies, respectively. A ‘‘combined’’ fuel economy was
computed as a weighted average (55% UDDS, 45% HWFET).
Table 4 presents results for ancillary loads ranging from 0 to
4 kW. The EPA city, highway, and combined fuel economies lie
within the range of our simulation results. We also note that ADVI-
SOR predicts an SC03 drive cycle fuel economy of 19–26 mpg, sim-
ilar to that of the UDDS urban drive cycle.
6. Results
6.1. AC calibration (16 July 2010)
The heater in the silver car was slightly more powerful than that
in the black car, yielding 2–3 C higher peak values of vent air
(a)
0
1
2
3
4
5
6
7
8
9
10
11
12
0 5 10 15 20 25 30
Cooling time (min)
Ca
b
i
n
s
u
r
f
a
c
e
t
e
m
p
-
c
a
b
i
n
a
i
r
t
e
m
p
(
°
C
)
black_cool1 (09:32-09:59 LST, 28 min, 25°C, 0.83 kW/m²)
black_cool2 (11:02-11:32 LST, 30 min, 29°C, 0.98 kW/m²)
black_cool3 (12:33-13:01 LST, 28 min, 33°C, 1.00 kW/m²)
black_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
black_cool5 (15:32-16:00 LST, 28 min, 37°C, 0.64 kW/m²)
black
(b)
0
1
2
3
4
5
6
7
8
9
10
11
12
0 5 10 15 20 25 30
Cooling time (min)
Ca
b
i
n
s
u
r
f
a
c
e
t
e
m
p
-
c
a
b
i
n
a
i
r
t
e
m
p
(
°
C
)
silver_cool1 (09:32-09:58 LST, 27 min, 25°C, 0.83 kW/m²)
silver_cool2 (11:02-11:32 LST, 30 min, 29°C, 0.98 kW/m²)
silver_cool3 (12:33-13:00 LST, 28 min, 33°C, 1.00 kW/m²)
silver_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
silver_cool5 (15:32-15:59 LST, 28 min, 37°C, 0.64 kW/m²)
silver
Fig. 12.Difference between cabin surface and cabin air temperatures versus
cooling time in each of five trials, shown for (a) the black car and (b) the silver
car.
(a)
20
25
30
35
40
45
50
55
60
65
70
0 5 10 15 20 25 30
Cooling time (min)
Ca
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
measured
fit
after resizing AC
cooldown target (25°C)
black_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
black
(b)
20
25
30
35
40
45
50
55
60
65
70
0 5 10 15 20 25 30
Cooling time (min)
Ca
b
i
n
a
i
r
t
e
m
p
e
r
a
t
u
r
e
(
°
C
)
measured
fit
after resizing AC
cooldown target (25°C)
silver_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
silver
Fig. 13.Measured and fitted cabin air temperatures versus cooling time in Trial 4,
shown for (a) the black car and (b) the silver car. Each graph also shows the cabin air
temperature time series predicted after the AC is resized to attain a target final
cabin air temperature of 25 C.
R. Levinson et al./Applied Energy 88 (2011) 4343–4357 4351
temperature and cabin air temperature. However, the AC systems
performed comparably: after just 2 min of cooling, the vent air
temperatures matched to within 1 C, and the cabin air tempera-
tures agreed to within 0.5 C(Fig. 4).
6.2. Soaking and cooling (17 July 2010)
6.2.1. Temperatures profiles within each car
Fig. 5 shows the evolution of the exterior surface, interior sur-
face, and cabin air temperatures in each car over the course of its
five soaking and cooling cycles. The following remarks will focus
on the middle three soaking and cooling cycles, which span
10:00–14:30 LST and are centered about solar noon (12:15 LST).
While soaking, the warmest surfaces of the black car are usually
its black roof (solar absorptance A =1 q = 0.95) and black dash-
board (also A = 0.95), both of which are directly heated by the
sun. Its next warmest surfaces are the ceiling (conductively heated
by the roof and radiatively heated by the dashboard and wind-
shield), followed very closely by the windshield (radiatively and
convectively heated by the dashboard). The seat, which is heated
primarily by radiative exchange with the ceiling, is markedly cool-
er. The coolest interior surface in the black car is the door, which
from geometric considerations can be expected to receive less than
half its thermal radiation from the ceiling.
The warmest surfaces of the silver car while soaking are its
black dashboard (A = 0.95), which is directly heated by the sun,
and its windshield, which is radiatively and convectively heated
by the dashboard. The next warmest surfaces are its silver roof (di-
rectly heated by the sun, but absorbing only 42% of sunlight) and
its ceiling (conductively heated by the roof and radiatively heated
by the dashboard and windshield). As in the black car, the seat is
markedly cooler, and the door is coolest.
We note that in each car, abnormally high door temperatures
are observed during the first and last soaking cycles, and abnor-
mally high seat temperatures are seen in the last soaking cycle.
This is simply due to direct solar illumination of the door exterior
and seat surface, which does not occur at other times.
Air conditioning rapidly cools the cabin air and all interior sur-
faces in each vehicle. Dashboard and windshield temperatures re-
main well above the cabin air temperature because the dashboard
is still heated by the sun and the windshield is radiatively coupled
to the dashboard. Air conditioning has little effect on roof surface
temperature, indicating that the conductive heat flow through
the lined ceiling is small compared to the roof’s solar heat gain.
We approximate each car’s cabin surface temperature Ts as the
area-weighted average of its ceiling, dashboard, windshield, seat
and door surface temperatures. This estimate of mean interior sur-
face temperature neglects unmonitored surfaces, including the
floor, rear window and side windows.Fig. 6 shows the evolution
of Ts,Ta and Tv in each car, as well as that of Ta Tv and Ts Ta.
(Since the vent air temperature is relevant only when the AC is
on, zero values drawn for Tv and Ta Tv during the soak cycles
should be ignored.)
6.2.2. Black car versus silver car
Fig. 7 compares the roof, ceiling, dashboard, windshield, seat,
door, vent air and cabin air temperatures in the black car to those
in the silver car. As expected, the greatest temperature difference is
observed at the roof, where the black car was up to 25 C warmer
than the silver car. While soaking, the ceiling temperature differ-
ence (black–silver) peaked at 11 C, while the dashboard tempera-
ture difference was less than 5 C and the windshield temperature
difference was less than 2 C. The seat and door temperature differ-
ences reached 7 C and 5 C, respectively. The vent air and cabin air
temperature differences each peaked around 5–6 C.
Fig. 8 compares the cabin air heating rate dU/dt and AC cooling
rate qAC in the black car to those in the silver car. While cooling, the
difference in dU/dt is roughly centered about zero and less than
0.03 kW in magnitude. The difference in qAC is much larger, peak-
ing around 0.3 kW. During the cooling cycle,qAC is one to two or-
ders of magnitude larger than dU/dt, suggesting that most of the
heat removed by the AC comes from the cabin surface, rather than
the cabin air.
6.2.3. Cooldown temperature versus soak temperature
The five cooling cycles are denoted ‘‘cool1’’ through ‘‘cool5’’.Ta-
ble 5 summarizes the properties of each cooling cycle, including its
start and end time, duration, primary weather conditions, cabin air
and surfaces temperatures after soaking and after cooling, and vent
(a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 5 10 15 20 25 30
Cooling time (min)
AC
c
o
o
l
i
n
g
r
a
t
e
(
k
W
)
measured
after resizing AC
black_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
black
(b)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 5 10 15 20 25 30
Cooling time (min)
AC
c
o
o
l
i
n
g
r
a
t
e
(
k
W
)
measured
after resizing AC
silver_cool4 (14:02-14:32 LST, 30 min, 36°C, 0.86 kW/m²)
silver
Fig. 14.Measured AC cooling rate in Trial 4 shown for (a) the black car and (b) the
silver car. Each graph also shows the cooling rate predicted after the AC is resized to
attain a target final cabin air temperature of 25 C.
Table 7
Coefficients of proportionality c relating changes in rates of fuel consumption F,N Ox
emission ENOx , CO emission ECO and HC emission EHC in each of three drive cycles to
change in ancillary power load.
Coefficient UDDS SC03 HWFET
cF (L per 100 km per kW)0.884 0.830 0.403
cE;NOx (mg km 1 kW 1)33 3924
cE;CO (mg km 1 kW 1)60 123 29
cE;HC (mg km 1 kW 1)2229 10
4352 R. Levinson et al./Applied Energy 88 (2011) 4343–4357
air temperatures after cooling. Each temperature is reported first
for the black car, then for the silver car, followed by the black - sil-
ver temperature difference.
The cabin air and vent air temperatures attained after 30 min
of cooling each strongly and linearly correlate to the cabin air tem-
perature reached after 60 min of soaking, with coefficient of
determination R2 > 0.99 for the former and R2 = 0.95 for the latter
(Fig. 9a). The (area weighted mean) cabin surface temperatures at-
tained after soaking and after cooling also linearly correlate to the
cabin air soak temperature, with R2 = 0.87 and R2 = 0.93, respec-
tively (Fig. 9b). The same linear relationships work equally well
for both cars. This indicates that under the strictly controlled con-
ditions of these experiments, cabin air soak temperature captures
the thermal history of the soaking interval sufficiently well to pre-
dict cabin air and cabin surface temperatures after cooling.
6.2.4. Applicability of cabin air temperature model
The validity of the cabin air temperature model in Eq.(2)was
tested by regressing the rate of change of the cabin air temperature,
dTa/dt, to the temperature differences Tv Ta and Ts Ta. Regres-
sioncoefficients a and b for eachcar and coolingcycleare presented
in Table 6, along with each fit’s coefficient of determination R2. Val-
ues of R2 were fairly high, ranging from 0.86 to 0.93 for the black car
and 0.86 to 0.95 for the silver car.Fig. 10 shows the measured and
fitted values of dTa/dt for the fourth cooling cycle in each car.
Fig. 11 shows the variation with cooling time of the cabin air
temperature reduction Ta(0)Ta(t) in each vehicle. Cooling is rapid
at the start of each 30 min cycle and slow near its end. For example,
during the fourth cooling cycle, the cabin air temperature in the
black car falls 16 C in the first two minutes (8 C min 1), another
11 C in the next 18 min (0.6 C min 1), and just 2 C in the final
10 min (0.2 C min 1). This indicates that Ta asymptotically ap-
proaches a quasi-steady value T a toward the end of the cooling
cycle.
Fig. 12 shows the variation with cooling time of Ts Ta in each
car. In the three middle cooling cycles (cool2, cool3 and cool4), this
temperature difference varies little after the first two minutes of
cooling. For example, during the final 28 min of the fourth cooling
cycle,Ts Ta decreases by 1.2 C in the black car and 0.5 C in the
silver car, while the vent air temperatures each fall by about 7 C.
This indicates that Ts Ta depends only weakly on Tv.
6.2.5. Resizing AC to attain 25 C final cabin air temperature
The black car attained a final cabin air temperature below 25 C
in the first cooling cycle, and the silver car did so in the first and
second cooling cycles. Otherwise, neither vehicle’s cabin air tem-
perature was reduced to 25 C or lower after approximately
30 min of maximum cooling. For example, at the end of the fourth
cooling cycle, the cabin air temperatures in the black and silver
cars were 34.3 C and 29.9 C, respectively (Table 6).
To attain a lower final cabin air temperature T 0
a , Eq.(4)indicates
that the vent air temperature Tv(t) must be decreased by the differ-
ence DT between the cabin air final temperature T a (approximated
by the cabin air temperature measured at the end of the cooling
(a)
(c) (d)
0
5
10
15
20
25
30
01234
Fr
a
c
t
i
o
n
a
l
f
u
e
l
s
a
v
i
n
g
s
(
%
)
Ancillary load of cool car (kW)
(b)
0
2
4
6
8
10
12
14
01234
Fr
a
c
t
i
o
n
a
l
N
O
x
re
d
u
c
t
i
o
n
(
%
)
Ancillary load of cool car (kW)
0
2
4
6
8
10
12
14
01234
Fr
a
c
t
i
o
n
a
l
C
O
r
e
d
u
c
t
i
o
n
(
%
)
Ancillary load of cool car (kW)
0
2
4
6
8
10
12
14
01234
Fr
a
c
t
i
o
n
a
l
H
C
r
e
d
u
c
t
i
o
n
(
%
)
Ancillary load of cool car (kW)
Fig. 15.Fractional reductions in rates of (a) fuel consumption, (b) NOx emission, (c) CO emission and (d) HC emission as a function of ancillary loads of the standard (black)
and cool (nonblack) cars for the SC03 driving cycle. Each curve represents a different value for ancillary load of the standard car. Results represent a vehicle with a power
rating of 84 kW.
R. Levinson et al./Applied Energy 88 (2011) 4343–4357 4353
cycle) and T 0
a . For example, in the fourth cooling cycle DT would be
9.3 C for the black car and 4.9 C for the silver car if T 0
a =2 5C.
New vent air temperature profiles T 0
v ðtÞ T v ðtÞ DT were com-
puted for each car and cooling cycle based on the value of DT re-
quired to cool the cabin air to 25 C. Eq.(5)was then numerically
integrated to compute the new cabin air temperature profile
T 0
a ðtÞ.Fig. 13 compares the measured and fitted values of Ta(t)i n
the fourth cooling cycle to the values of T 0
a ðtÞ computed after
decreasing the black and silver cars’ vent air temperatures by
DT = 9.3 C and DT = 4.9 C, respectively. Also drawn for reference
is the cooldown target temperature (25 C).
AC cooling rates before and after vent temperature reduction
were computed from Eqs.(6)and (7).Fig. 14 shows for the fourth
cooling cycle in each car the measured AC cooling rate and the AC
cooling rate after lowering the vent air temperature to attain a final
cabin air temperature of 25 C.
The AC cooling capacity Q (peak AC cooling rate) required to at-
tain T 0
a =2 5C was computed from Eq.(8)for each car and cooling
cycle (Table 6). For example, in the fourth cooling cycle Q was
3.83 kW in the black car, and 3.34 kW in the silver car. The ratio
of Qsilver to Qblack ranged from 0.83 to 0.87 over the three middle
cooling cycles.
6.3. Fuel savings and emission reductions
6.3.1. Fuel consumption and pollutant emission versus ancillary power
load
Table 7 shows values of c obtained by linearly regressing ADVI-
SOR simulations of fuel consumption, NOx emission, CO emission
and HC emission rates to ancillary power load. The variations of
fuel consumption and emissions with ancillary load were highly
linear within the simulated range (0–4 kW) and the minimum
coefficient of determination (R2) was 0.96.Fig. 15 relates reduc-
tions in fuel consumption and emission to the ancillary loads of
the standard (black) and cool (nonblack) cars. Each curve repre-
sents a different value for ancillary load of the standard car. For
brevity, we present charts only for the SC03 driving cycle.
6.3.2. Fuel savings and emission reductions versus cool car solar
reflectance
Since roof and cabin air soak temperatures peaked in the fourth
cycle (Table 5), AC capacity requirements QL (black car) and QH (sil-
ver car) were based on values computed for the fourth cooling cy-
cle. The following analysis assumes AC capacities QL = 3.83 kW and
QH = 3.34 kW and shell solar reflectances qL = 0.05 and qH = 0.58,
for a capacity reduction of 92.5 W per 0.1 increase in shell solar
reflectance.
Hendricks [33]obtained a maximum COP of 1.6 when optimiz-
ing the COP of a mechanically driven compressor for the SC03 cy-
cle. Here we select a COP of 2 to conservatively estimate reduction
in ancillary power load, which is inversely proportional to COP.
Table 8,Fig. 16 and Fig. 17 present fuel savings and emissions
reductions attained when a cool (solar reflective) car shell is
substituted for a standard (black) car shell (q = 0.05). Dashed ver-
tical lines in Figs. 16 and 17 mark the shell solar reflectances of a
typical cool colored car (q = 0.35), a typical white or silver car
(q = 0.60), and a hypothetical super-white car (q = 0.80)4.Fig. 16
shows fractional fuel savings and emission reductions and Fig. 17
shows absolute fuel savings and emission reductions.
Results from our model with c values from the SC03 drive cycle
indicate selecting a typical cool colored shell (q = 0.35) would re-
duce fuel consumption by 0.12 L per 100 km (1.1%), increasing fuel
economy by 0.10 km L 1 [0.24 mpg] (1.1%). It would also decrease
CO2 emissions by 2.7 g km 1 (1.1%), NOx emissions by 5.4 mg km 1
(0.44%), CO emissions by 17 mg km 1 (0.43%), and HC emissions by
4.1 mg km 1 (0.37%). Selecting a typical white or silver shell
(q = 0.60) instead of a black shell would lower fuel consumption
by 0.21 L per 100 km (1.9%), raising fuel economy by 0.19 km L 1
[0.44 mpg] (2.0%). It would also decrease CO2 emissions by
4.9 g km 1 (1.9%), NOx emissions by 9.9 mg km 1 (0.80%), CO emis-
sions by 31 mg km 1 (0.79%), and HC emissions by 7.4 mg km 1
(0.67%). A hypothetical super-white car shell (q = 0.80) could save
0.29 L per 100 km (2.6%), increasing fuel economy by 0.25 km L 1
[0.59 mpg] (2.7%) and decreasing CO2,N Ox, CO and HC emissions
by 6.7 g km 1 (2.6%), 13 mg km 1 (1.1%), 43 mg km 1 (1.1%), and
10 mg km 1 (0.91%), respectively.
As discussed previously, emissions in standardized driving cy-
cles are typically lower than those in real-world (off-cycle) driving.
Hence, our simulation results may underestimate emission
reductions.
We can compare fuel and emission reductions of urban versus
highway driving by observing results for the UDDS and HWFET
Table 8
Variations with shell solar reflectance of rates of fuel consumption, fuel savings,
pollutant emission and emission reduction for a compact sedan (engine power
84 kW). Results are presented for three different drive cycles simulated using
ADVISOR. Parenthetical results indicate percent reductions in fuel consumption and
emission rates relative to the black car.
Driving
cycle
Black car
(q = 0.05)
Cool colored
car (q = 0.35)
Silver or
white car
(q = 0.60)
Hypothetical super-
white car (q = 0.80)
Fuel consumption (L per 100 km)
SC03a 10.95 10.84 10.74 10.67
UDDSb 10.59 10.46 10.36 10.28
HWFET c 6.86 6.80 6.76 6.72
Fuel savings (L per 100 km)
SC03 NA 0.12 (1.1%) 0.21 (1.9%) 0.29 (2.6%)
UDDS NA 0.12 (1.2%) 0.23 (2.1%) 0.31 (2.9%)
HWFET NA 0.056 (0.82%) 0.10 (1.5%) 0.14 (2.0%)
CO2 emission reduction (g km 1)d
SC03 NA 2.7 4.9 6.7
UDDS NA 2.9 5.2 7.1
HWFET NA 1.3 2.4 3.3
NOx emission (g km 1)
SC03 1.22 1.22 1.22 1.21
UDDS 0.70 0.69 0.69 0.69
HWFET 0.53 0.53 0.53 0.52
NOx emission reduction (mg km 1)
SC03 NA 5.4 (0.44%) 9.9 (0.80%) 13 (1.1%)
UDDS NA 4.7 (0.67%) 8.5 (1.2%) 12 (1.7%)
HWFET NA 3.3 (0.62%) 6.1 (1.1%) 8.3 (1.6%)
CO emission (g km 1)
SC03 3.99 3.98 3.96 3.95
UDDS 2.07 2.06 2.06 2.05
HWFET 1.69 1.69 1.69 1.68
CO emission reduction (mg km 1)
SC03 NA 17 (0.43%) 31 (0.79%) 43 (1.07%)
UDDS NA 8.4 (0.41%) 15 (0.74%) 21 (1.01%)
HWFET NA 4.0 (0.24%) 7.3 (0.43%) 10 (0.59%)
HC emission (g km 1)
SC03 1.12 1.11 1.11 1.11
UDDS 0.61 0.61 0.61 0.60
HWFET 0.46 0.46 0.46 0.46
HC emission reduction (mg km 1)
SC03 NA 4.1 (0.37%) 7.4 (0.67%) 10 (0.91%)
UDDS NA 3.1 (0.50%) 5.6 (0.92%) 7.6 (1.3%)
HWFET NA 1.4 (0.30%) 2.5 (0.55%) 3.4 (0.75%)
a SC03 simulates transient driving (average speed = 34.8 km h 1, 21.6 mph).
b UDDS simulates urban driving (average speed = 31.5 km h 1 = 19.6 mph).
c HWFET simulates highway driving (average speed = 77.7 km h 1 = 48.3 mph).
d CO2 emission reduction calculated at the rate of 2321 g CO2 per L gasoline [30].
4 Many white metal roofing products have initial solar reflectances in the range of
0.7–0.8 [34]. We present the super-white shell (q = 0.80) as a limiting case.
4354 R. Levinson et al./Applied Energy 88 (2011) 4343–4357
driving cycles (Fig. 16). Relative to the SC03 drive cycle, fuel sav-
ings are larger for the UDDS cycle (urban driving) and smaller for
the HWFET cycle (highway driving). Further, relative to the SC03
cycle, emissions reductions for NOx, CO, and HC are smaller for
both the UDDS cycle and HWFET cycle. Emissions reductions are
larger for UDDS than HWFET for NOx, CO, and HC. This may be
due to the fact that emissions are more sensitive to transients
(e.g., simulated vehicle accelerations) in driving cycles [31].
7. Summary
In this study we estimated the decrease in soak temperature,
potential reduction in AC capacity, and potential fuel savings and
emission reductions attainable through the use of solar reflective
car shells. First, we experimentally characterized component tem-
peratures and cooling demands in a pair of otherwise identical
dark and light colored vehicles, the former with low solar reflec-
tance (q = 0.05) and the latter with high solar reflectance
(q = 0.58). Second, we developed a thermal model that predicted
the AC capacity required to cool each vehicle to a comfortable final
temperature of 25 C within 30 min. Third, we used the ADVISOR
vehicle simulation tool to estimate the fuel consumption and pol-
lutant emissions of each vehicle in various standard drive cycles
(SC03, UDDS, and HWFET). Finally, we calculated the fuel savings
and emission reductions attainable by using a cool shell to reduce
ancillary load.
The air conditioners in the experimental vehicles were in most
trials too small to lower cabin air temperature to 25 C within
30 min. We estimate that if the vehicle ACs were resized to meet
this target, the AC cooling capacity would be 3.83 kW for the car
with low solar reflectance and 3.34 kW for the car with high solar
reflectance (Table 6).
Assuming that potential reductions in AC capacity and engine
ancillary load scale linearly with increase in shell solar reflectance,
ADVISOR simulations of the SC03 driving cycle indicate that substi-
tuting a typical cool-colored shell (q = 0.35) for a black shell
(q = 0.05) would reduce fuel consumption by 0.12 L per 100 km
(1.1%), increasing fuel economy by 0.10 km L 1 [0.24 mpg] (1.1%).
It would also decrease CO2 emissions by 2.7 g km 1 (1.1%), NOx
emissions by 5.4 mg km 1 (0.44%), CO emissions by 17 mg km 1
(0.43%), and HC emissions by 4.1 mg km 1 (0.37%). Selecting a typ-
ical white or silver shell (q = 0.60) instead of a black shell would
lower fuel consumption by 0.21 L per 100 km (1.9%), raising fuel
economy by 0.19 km L 1 [0.44 mpg] (2.0%). It would also decrease
CO2 emissions by 4.9 g km 1 (1.9%), NOx emissions by 9.9 mg km 1
(0.80%), CO emissions by 31 mg km 1 (0.79%), and HC emissions by
7.4 mg km 1 (0.67%).Ahypotheticalsuper-whitecarshell(q = 0.80)
could save 0.29 L per 100 km (2.6%), increasing fuel economy by
0.25 km L 1 [0.59 mpg] (2.7%) and decreasing CO2,N Ox, CO and
(a)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
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Fr
a
c
t
i
o
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a
l
f
u
e
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s
a
v
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s
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%
)
Solar reflectance of cool car
UDDS
SC03
HWFET
co
o
l
c
o
l
o
r
e
d
c
a
r
wh
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/
s
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i
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a
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(b)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 0 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 9 1
Fr
a
c
t
i
o
n
a
l
N
O
x
re
d
u
c
t
i
o
n
(
%
)
Solar reflectance of cool car
UDDS
HWFET
SC03
co
o
l
c
o
l
o
r
e
d
c
a
r
wh
i
t
e
/
s
i
l
v
e
r
c
a
r
su
p
e
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-
wh
i
t
e
c
a
r
(c)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fr
a
c
t
i
o
n
a
l
C
O
r
e
d
u
c
t
i
o
n
(
%
)
SC03
UDDS
HWFET
wh
i
t
e
/
s
i
l
v
e
r
c
a
r
co
o
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c
o
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o
r
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d
c
a
r
su
p
e
r
-
wh
i
t
e
c
a
r
(d)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Fr
a
c
t
i
o
n
a
l
H
C
r
e
d
u
c
t
i
o
n
(
%
)
Solar reflectance of cool carSolar reflectance of cool car
UDDS
SC03
HWFET
wh
i
t
e
/
s
i
l
v
e
r
c
a
r
co
o
l
c
o
l
o
r
e
d
c
a
r
su
p
e
r
-
wh
i
t
e
c
a
r
Fig. 16.Fractional reductions in rates of (a) fuel consumption, (b) NOx emission, (c) CO emission and (d) HC emission versus solar reflectance of the cool car shell, assuming a
vehicle power rating of 84 kW. Reference values of solar reflectance for a typical cool colored car, a typical white or silver car, and a hypothetical super-white car are shown as
dashed vertical lines.
R. Levinson et al./Applied Energy 88 (2011) 4343–4357 4355
HC emissions by 6.7 g km 1 (2.6%), 13 mg km 1 (1.1%), 43 mg km 1
(1.1%), and 10 mg km 1 (0.91%), respectively. These results may
underestimate emission reductions in real-world driving.
Acknowledgments
This work was supported by the California Energy Commission
through its Public Interest Energy Research Program. It was also
supported by the Assistant Secretary for Energy Efficiency and
Renewable Energy, Office of Building Technology, State, and Com-
munity Programs, of the US Department of Energy under Contract
No. DE-AC02-05CH11231. We wish to thank the California Depart-
ment of General Services for use of their vehicles and facility, with
special appreciation to Kimberly Harbison for her assistance; John
Rugh of the National Renewable Energy Laboratory, for technical
advice; former California Energy Commissioner Arthur Rosenfeld,
for his support; and Philip Misemer of the California Energy Com-
mission, for guiding our project.
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c
a
r
(b)
0
5
10
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20
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Ab
s
o
l
u
t
e
N
O
x
re
d
u
c
t
i
o
n
(
m
g
k
m
-1
)
Solar reflectance of cool car
SC03
UDDS
HWFET
co
o
l
c
o
l
o
r
e
d
c
a
r
wh
i
t
e
/
s
i
l
v
e
r
c
a
r
su
p
e
r
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wh
i
t
e
c
a
r
(c)
0
10
20
30
40
50
60
70
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Ab
s
o
l
u
t
e
C
O
r
e
d
u
c
t
i
o
n
(
m
g
k
m
-1
)
Solar reflectance of cool car
SC03
UDDS
HWFET
wh
i
t
e
/
s
i
l
v
e
r
c
a
r
co
o
l
c
o
l
o
r
e
d
c
a
r
su
p
e
r
-
wh
i
t
e
c
a
r
(d)
0
2
4
6
8
10
12
14
16
18
20
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Ab
s
o
l
u
t
e
H
C
r
e
d
u
c
t
i
o
n
(
m
g
k
m
-1
)
Solar reflectance of cool car
SC03
UDDS
HWFET
wh
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e
/
s
i
l
v
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c
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r
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o
l
o
r
e
d
c
a
r
su
p
e
r
-
wh
i
t
e
c
a
r
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