2007-01-1194 

Reduction in Vehicle Temperatures and Fuel Use from Cabin 

Ventilation, Solar-Reflective Paint, and a New Solar-Reflective 

Glazing 

John P. Rugh, Lawrence Chaney, Jason Lustbader 

National Renewable Energy Laboratory 

John Meyer 

Visteon 

Mukesh Rustagi, Kurt Olson 

PPG Industries 

Rupert Kogler 

Webasto AG 

Copyright © 2006 SAE International

ABSTRACT 

A new type of solar-reflective glass that improves 

reflection of the near-infrared (NIR) portion of the solar 

spectrum has been developed. Also developed was a 

prototype solar-reflective paint that increases the NIR 

reflection of opaque vehicle surfaces while maintaining 

desired colors in the visible portion of the spectrum. 

Both of these technologies, as well as solar-powered 

parked car ventilation, were tested on a Cadillac STS as 

part of the Improved Mobile Air Conditioning Cooperative 

Research Program (I-MAC). Significant reductions in 

interior and vehicle skin temperatures were measured. 

The National Renewable Energy Laboratory (NREL) 

performed an analysis to determine the impact of 

reducing the thermal load on the vehicle. A simplified 

cabin thermal/fluid model was run to predict the potential 

reduction in A/C system capacity. The potential 

reduction in fuel use was calculated using a vehicle 

simulation tool developed by the U.S. Department of 

Energy (DOE). 

INTRODUCTION 

When operating, the A/C system is the largest ancillary 

load on an automobile’s engine. This increases the inuse fuel consumption experienced by a consumer. An 

NREL study1

 showed that the United States uses 7 billion 

gal (26.4 billion L) of fuel per year for light-duty vehicle 

A/C, which is equivalent to 5.5% of the total national 

light-duty vehicle fuel use. It takes 9.5% of the imported 

crude oil to produce this much gasoline. The fuel-use 

percentages are based on a total annual light-duty 

vehicle fuel use of 125.9 billion gal (477 billion L)2

 and 

imported oil of 73 billion gal (276 billion L)3

. 

The A/C system in a vehicle is sized to reduce the 

interior temperature to an acceptable level after a hot 

soak. Technologies, such as solar-reflective glass and 

paint, can reduce the thermal load on a vehicle, interior 

temperatures, and fuel use. 

The Society of Automotive Engineers (SAE) I-MAC 

Cooperative Research Program4

 was announced in April 

of 2004, with the first full meeting taking place July 1 

during the SAE-sponsored 2004 Alternate Refrigerant 

System Symposium. The goal of the program is to make 

significant reductions in all aspects of emissions resulting 

from the operation of an R134a vehicle A/C system. 

Specific targets for reductions were: 

1. Refrigerant leakage during operation and service 

2. Tailpipe emissions resulting from powering the 

system 

3. Thermal load on the vehicle after the vehicle is 

parked in the sun to minimize cooling requirements. 

 

The SAE I-MAC Cooperative Research Program is a 

joint effort funded by DOE, the U.S. Environmental 

Protection Agency (EPA), SAE, and 28 industry partners. 

The Thermal Load Reduction Team goal is to 

demonstrate advanced technologies that reduce cooling 

loads by 30% after a vehicle is parked in the sun. 

1

NREL/CP-540-40986. Posted with permission. 

Presented at the 2007 SAE World Congress 

April 16-19, 2007, Detroit, Michigan 
LOAD-REDUCTION TECHNOLOGIES 

During the summer of 2005, the I-MAC Thermal Load 

Reduction Team evaluated technologies to reduce the 

thermal load on the A/C system. The team picked the 

most promising technologies and continued testing in 

2006. The technologies incorporated into the Cadillac 

STS test vehicle are solar-reflective glazings, solarpowered parked car ventilation, and solar-reflective paint. 

SOLAR-REFLECTIVE GLAZINGS 

As vehicle design trends increase glazing surface area, a 

greater portion of the thermal load on vehicles enters 

through the glazings. Developing vehicle glazings that 

reflect the incident solar radiation is a critical step to 

making significant reductions in the thermal loads. The 

solar-reflective glazings used in the test vehicle were 

developed with the goal of reducing the solar heat load to 

the maximum extent possible given the Cadillac STS 

glazing area and transmissivity requirements. Since the 

infrared portion of the solar spectrum provides the 

highest amount of solar energy, the objective was to 

reflect as much of that energy away from the vehicle as 

possible, so that it is neither absorbed in the glass nor 

transmitted through the glass. 

PPG Industries’ glass technology group developed a 

technology, called Sungate® EP, that allows only 3% of 

the infrared (IR) energy to be transmitted through the 

glass. With this technology, only 33% of the total solar 

energy is transmitted through the glass, most of it in the 

visible spectrum. Since the federal requirements for 

visibility dictate a minimum of 70% visible light 

transmission, the Sungate EP glass is close to the 

highest performance possible within the constraints of 

the visibility requirements. 

SOLAR-POWERED PARKED CAR VENTILATION 

When the solar energy enters the vehicle, it goes into 

heating the interior mass of the passenger compartment 

including the air. Venting the warm air and pulling in 

cooler ambient air can reduce the temperature of the 

interior and reduce climate control thermal loads. 

Solar cells are integrated in the sunroof glass to provide 

power to the electric blower system that is integrated in 

the sunshade. Depending on the design requirements, 

the blowers can either vent hot air or pull in cool air 

through the roof while the car is parked. Opening the 

sunroof and pressing an ON/OFF switch activates the 

“solar ventilation.” As soon as the sun shines, the 

blowers system turns on. 

The solar panel consists of 24 cells over an area of 0.13 

m2

 which provides 12 V and 17 W in the system 

manufactured by Webasto. The solar panel can be 

linked to the electrical system of the car if desired. The 

blower system consists of six 1.7 W fans. By measuring 

the exit velocity at the fans, we estimated the blower 

assembly flow rate to be approximately 0.077 kg/s (137 

cfm). 

Since the Cadillac STS already had a sunroof, we were 

unable to perform a professional installation due to 

structural limitations. Instead, a bracket was built and 

secured to the roof around the existing opening. Shown 

in Figure 1, the sunroof with the solar cells was then 

installed above the existing sunroof. The fan assembly 

was installed in the existing Cadillac STS sunshade. A 

production installation would work just like a regular 

sunroof. 

Figure 1. Test (Non-Production) Installation of the 

Solar-Powered Parked Car Ventilation System 

SOLAR-REFLECTIVE PAINT 

The solar-reflective coating was made in the PPG 

Industries coatings research center in Allison Park, 

Pennsylvania. IR reflective pigments (available from 

Ferro Corporation) were incorporated into a commercial 

automotive refinish basecoat composition. The IR 

reflective black, yellow, and brown pigments were added 

to the basecoat at a total level of 22% by weight of the 

dry film. Other pigments, such as TiO2, were also added 

at a level of 22% by weight of the dry film. The 

thermoplastic-polymeric binder system used to make the 

basecoat consisted of acrylic copolymers and cellulose 

acetate butyrate. 

The basecoat was over-coated with a commercial 

refinish clearcoat available from PPG Industries (Low 

2
VOC Performance Clear D8-93). Basecoat film 

thickness was approximately 20-25μ. Clearcoat film 

thickness was about 45-55μ. The clearcoat consisted of 

acrylic polyols cross-linked with isocyanate functional 

resins. The basecoat pigmentation was chosen based on 

experience with commercial IR reflective roofing 

coatings, which are sold by PPG under the trade name of 

Duranar® SPF. The coatings were applied in a 

commercial body shop via standard spray techniques, 

which are well known in the industry. Further optimization 

of the IR reflectance properties of practical coatings 

systems continues to be a research interest within PPG. 

The control basecoat contained conventional pigments, 

such as iron oxide, carbon black, and TiO2. The TiO2 

level in the control basecoat was the same as in the 

experimental basecoat. The visual color was 

approximately the same as the experimental basecoat. 

The binder systems in the two basecoats were the same, 

as were the compositions of the clearcoats on both cars. 

Laboratory testing with a sun lamp yielded a 9-10°C 

difference in equilibrium panel temperature between 

experimental and control coatings. The measured solar 

absorptivity of the solar reflective and control coatings 

was 82% and 89% respectively. 

The control basecoat/clearcoat were applied to the 

control vehicle while the solar reflective 

basecoat/clearcoat were applied to the modified vehicle. 

VEHICLE SOAK TEST SETUP 

From July 23, 2006, to September 7, 2006, outdoor 

thermal soak tests were performed on a pair of Cadillac 

STSs. One vehicle was modified with the load-reduction 

technologies while the other vehicle was not modified 

except for the control paint that color-matched the solarreflective paint. The advantage of using two vehicles is 

that the impact of day-to-day environmental differences 

is minimized, although we still needed a high solar load 

and light winds. We measured the temperature 

difference at various locations between the vehicles. 

Table 1 outlines the four configurations we tested. 

Table 1. 2006 I-MAC Thermal Load Reduction Team 

Cadillac STS Test Matrix 

Config. Ventilation Solar-Ref. 

Windshield 

SolarRef. 

Backlite 

SolarRef. 

Sidelites 

SolarRef. 

Paint 

1 x x x x x 

2 x x x x 

3 x x x 

4 x x 

The tests were performed in Golden Colorado, at 

NREL’s South Table Mesa site (Figure 2). The ground 

surface was a mixture of crushed rock and dirt. The 

vehicles were oriented to 160o and were leveled to 

approximately the same tilt angle. The seats were 

adjusted to the same position. The Sungate EP 

windshield did not have a shade band, so the shade 

band region on both vehicles was covered with an 

opaque material. The windows were cleaned frequently 

to eliminate dust accumulation. 

Figure 2. Cadillac STSs at NREL Test Site 

General Motors (GM) provided the data acquisition 

system in both vehicles, and Nissan instrumented the 

vehicles. For the soak test, 41 type K thermocouples 

were used to measure surface and air temperatures. 

NREL radiation shields were added to the breath air 

temperature thermocouples. Adhesive thermocouples 

reinforced with metallic tape were used on most 

surfaces. The exterior roof surface thermocouple was 

secured using thermally conductive Omega epoxy. 

Environmental conditions were measured by a weather 

station at a nearby building. The parameters of interest 

were ambient temperature, global horizontal solar 

radiation, and wind speed/direction. The test day was 

determined to be good if the average wind speed was 

less than 2.5 m/s and if there were no significant clouds 

between 10:00 and 13:30. 

To determine the inherent temperature difference 

between the vehicles, data were gathered during the 

night. The ventilation system was not operating and 

there was no solar load. We assumed these offsets 

would apply during the day, and the daytime temperature 

differences were adjusted accordingly. The offsets were 

small, ranging from 0oC at the front passenger seat to 

0.7oC at the dashboard. 

The body leakage during the thermal soak was 

measured to determine if the vehicles had similar leak 

rates. A tracer gas decay method was implemented 

using two Bruel & Kjaer Multi-Gas Type 1302 Monitors. 

A small amount of SF6 was injected into both vehicles, 

and the concentrations were monitored for 2 hours. The 

air changes per hour were 0.59 and 0.69 for the control 

and modified vehicles respectively. This infiltration is 

equivalent to approximately 1.2 cfm, a fairly low leak 

rate. 

3
VEHICLE SOAK TEST RESULTS 

The data system was typically started around 8:00 a.m., 

and data were recorded every minute throughout the 

day. The reduction in temperature (Baseline Vehicle – 

Modified Vehicle) was then computed for each location. 

The time period 12:30 to 13:30 was determined to be the 

critical period because the temperature differences 

between the vehicles were fairly constant during this 

time. Figure 3 shows the temperature reductions versus. 

time for the solar-reflective glass in all locations and 

solar-reflective paint. The larger temperature reductions 

at 11:00 for the driver seat and 15:00 for the passenger 

seat are when the IR portion of sun load on the 

respective seat is being reflected by the solar-reflective 

sidelites in the modified vehicle. The larger variation in 

the roof skin temperature difference is due to changes in 

the wind speed and the associated changes in heat loss 

from the roof. Table 2 defines which thermocouples 

were used to calculate the average temperatures. 

0

5

10

15

20

25

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00

Time

Reduction in Temperature (oC)

0

9

18

27

36

45

(

oF)

Foot Air

Breath Air

Dashboard

Driver Seat

Pass Seat

Windshield

Roof Skin

Series8

Pass. Seat

Windshield

Foot Air

IP

Dr. Seat Breath Air

Roof Skin

Figure 3. Temperature Reduction vs. Time for 

Ventilation, Solar-Reflective Glass in All Locations, 

and Solar-Reflective Paint 

Table 2. Definition of Temperature Averages 

Location Description 

Air-Foot Average of driver, passenger, left rear and 

right rear air temperature in the footwell 

Air-Breath Average of driver, passenger, left rear and 

right rear air temperature at breath level 

Air Average of air-foot and air-breath 

Dashboard Average of 2 surface thermocouples on 

the instrument panel 

Roof 

Exterior 

Exterior surface of the roof 

Driver Seat 

& Pass 

Seat 

Average of internal and surface 

temperature of the seat cushion and seat 

back 

Windshield Interior surface temperature 

The average reduction in temperature for a given day is 

the time average from 12:30-13:30. For most of the 

configurations, we had multiple good test days, and 

these data were averaged. Only configuration 3 had a 

single day of good data, but it was an optimum day. 

Figure 4 shows the reduction in temperature for the four 

configurations. The numerical temperate reduction data 

are in Table 3. 

CONFIGURATION 1: SOLAR-REFLECTIVE GLASS 

(ALL LOCATIONS), SOLAR-POWERED VENTILATION, 

SOLAR-REFLECTIVE PAINT 

The combination of solar-reflective glass (all locations), 

solar-powered ventilation, and solar-reflective paint 

resulted in significant temperature reductions. Upon 

entering a vehicle, an occupant first makes contact with 

the air and seat. The breath air temperature was reduced 

12.0oC, and the seats were reduced 10-12oC. Thermal 

radiation from the dashboard and windshield also 

impacts thermal comfort. The dashboard temperature 

was reduced 16.8oC and the windshield was reduced 

20.4oC. The temperature of the vehicles with solarreflective glass, parked car ventilation, and solarreflective paint is located in Table 4. 

During the solar-soak tests, the difference in windshield 

temperature was easy to detect by touch. While the 

control windshield felt very hot, the Sungate EP 

windshield felt cool in comparison. Although the lower 

paint temperature was measurable, it was not as easy to 

detect by touching the surface. 

We found that the operation of the solar-powered 

ventilation affected the roof skin temperature. The roof 

temperature was reduced 9.8oC when the ventilation was 

on and 6.0oC on average when the ventilation was off. 

The flow exiting the vehicle entrained cool ambient air 

and caused increased convective heat loss from the roof 

skin. Since the roof skin thermocouple was located 

within a foot of the ventilation outlet (see Figure 1), we 

think the 6.0 average temperature reduction is a better 

estimate of the reduction in roof skin temperature due to 

solar-reflective paint. 

0

5

10

15

20

25

Air-Foot Air-Breath Air Dashboard Roof Exterior Front Driver

Seat

Front Pass

Seat 

Windshield

Reduction in Temperature (oC)

0

9

18

27

36

45

Reduction in Temperature (oF)

Solar Reflective Glass-all locations, Parked Car Ventilation

Solar Reflective Glass-all locations

Solar Reflective Windshield-Backlite

Solar Reflective Windshield

S i 5

Solar Reflective Paint-all tests

Figure 4. Reductions in Temperature (oC)

4
Table 3. Reductions in Temperature (oC) 

Air-Foot

AirBreath Air DashboardRoof 

Exterior

Front 

Driver Seat

Front Pass 

Seat Windshield

Solar Reflective 

Glass-all locations, 

Ventilation 5.6 12.0 8.8 16.8 9.8 / 6.0 10.3 11.9 20.4

Solar Reflective 

Glass-all locations 4.4 9.7 7.1 14.6 5.5 8.7 8.7 19.3

Solar Reflective 

Glass-W indshieldBacklite 3.3 7.7 5.5 13.0 6.1 6.4 6.6 19.6

Solar Reflective 

Glass-W indshield 3.0 6.7 4.9 12.5 6.4 6.0 6.1 20.2

Table 4. Temperatures for Configuration 1 (oC, 

except where noted)

Air-Foot Air-Breath Air Dashboard

Seat 

Driver Seat Pass Windshield Ambient

Wind 

Speed 

(m/s)

Control 36.4 57.0 46.7 77.0 51.1 54.2 68.7 27.7 2.0

Modified 30.8 44.9 37.9 60.1 40.6 42.5 48.3

CONFIGURATION 2: SOLAR-REFLECTIVE GLASS 

(ALL LOCATIONS), SOLAR-REFLECTIVE PAINT 

Without the solar-powered ventilation, the vehicle interior 

temperatures still were dramatically cooler. The average 

air temperature and seat temperature were reduced 

9.7oC and 8.7oC respectively. The windshield was 

19.7oC cooler, and the dashboard was 14.6oC cooler. 

CONFIGURATIONS 3 & 4: SOLAR-REFLECTIVE 

GLASS (WINDSHIELD & BACKLITE AND WINDSHIELD 

ONLY), SOLAR-REFLECTIVE PAINT 

Temperature reductions with the windshield/backlite and 

the windshield only were similar to each other. This is 

because the vehicles were oriented towards the sun and 

the solar load on the backlight was small. If the vehicles 

had been oriented away from the sun, the modified 

vehicle air temperatures would have been essentially the 

same as the control vehicle because the solar load on 

the north-facing windshields would have been lower. 

The windshield/IP/seats would have been cooler in both 

vehicles because there would have been shading from 

the roof and less direct sun load. 

SOLAR-POWERED VENTILATION ONLY 

During the summer of 2005, we tested the solar-powered 

ventilation by itself. With air being pulled out of the 

vehicle, the reduction in temperatures in Figure 5 was 

significant. The air temperature was reduced 5.6oC, and 

the seat temperatures were reduced 5-6oC. Because the 

glass was not solar reflective, the solar load was still high 

on the windshield and dashboard; therefore, the 

temperature reductions were due to convection heat loss 

driven by the interior airflow. Air was being pulled in 

through the HVAC heater/defroster ducts and any other 

natural body leakage areas. Figure 5 also shows the 

reductions in temperature with air being blown into the 

vehicle. For the Cadillac STS, this was not an effective 

way to cool the vehicle, and the temperature reductions 

were significantly lower. 

-2.9

7.1

2.1

6.2

1.5

0.7 0.3

2.3

3.0

8.3

5.6

8.3

1.0

6.0

5.3

2.3

-4

-2

0

2

4

6

8

10

Air-Foot Air-Breath Air Dashboard Roof Exterior Front Driver

Seat

Front Pass

Seat 

Windshield

Reduction in Temperature (oC) 

Air blown into vehicle

Air pulled out of vehicle

Figure 5. Reduction in Temperature for SolarPowered Ventilation Only (2005 Test Program)

SOLAR-REFLECTIVE ROOF ONLY 

During the summer of 2005, we also tested the Cadillac 

STS with a solar-reflective film on the roof. The film was 

an IR reflecting product (CI-100T) manufactured by 3M. 

The 6.7oC cooler roof temperature did not have a 

significant impact on the interior temperatures. Although 

we were not able to test the solar-reflective paint by itself 

in 2006, we think the reduction in interior temperatures 

would be similar to 2005 solar-reflective roof film results. 

0.5

1.2

0.8 0.9

6.7

0.4 0.4

-0.6 -1

0

1

2

3

4

5

6

7

8

Air-Foot Air-Breath Air Dashboard Roof Exterior Front Driver

Seat

Front Pass

Seat 

Windshield

Reduction in Temperature ( oC)

Figure 6. Reduction in Temperature for SolarReflective Film on Roof Only (2005 Test Program)

VEHICLE THERMAL ANALYSIS 

A numerical model of the STS test vehicle was 

developed using Fluent CFD software and RadTherm 

thermal analysis software. The CAD files of the vehicle 

interior geometry were obtained from GM. The CAD was 

imported into ANSA software where it was cleaned up, 

details were removed, and surface meshed. The model 

volume mesh was created using the Fluent TGrid 

hexcore mesh tool. The resulting volume mesh was 

approximately 1,300,000 cells; the surface mesh in 

RadTherm was approximately 209,000 elements. 

RadTherm models the solar load on the vehicle, 

convection losses on the interior and exterior surfaces, 

and conduction through the surfaces. Fluent CFD 

software was used to model the convective heat transfer 

5
and fluid flow in the cabin. During the analysis RadTherm 

and Fluent interact in the following way. RadTherm 

provides surface temperature boundary conditions to 

Fluent, and Fluent provides heat transfer coefficients and 

fluid temperatures to RadTherm. Several exchanges 

between RadTherm and Fluent were needed to achieve 

a consistent solution. 

The model was first validated against quasi steady-state 

soak data from the Cadillac STS test. Figure 7 

compares the model prediction to test data for the 

baseline vehicle. Several factors—such as the 

uncertainty in temperature measurement locations, 

material properties, and vehicle orientation—could 

contribute to the differences between measured 

temperatures and the model-predicted temperatures. 

Overall, the results show excellent agreement. Figure 8 

compares the model prediction to test data for the 

reduced load vehicle. In the reduced load model, 

uncertainty in the ventilation airflow rate, as well as the 

same factors cited in the baseline model, contribute to 

the differences between predicted and measured 

temperatures. The results for the reduced load vehicle 

also show good agreement. The validated model was 

then used to simulate the vehicle cooldown. 

30

40

50

60

70

80

90

IP Left

IP right

Dr seat

Pa Seat

windshield

Backlight

Dr side light

Pa side light

Pkg Shelf

Headliner

air foot

air breath

DATA

Model

-0.8

+1.4

+2.2

+7.2

-1.5

-2.6

+2.2

+1.7

-9.8

+7.0

+0.8

-2.9

Figure 7. Baseline Soak Results Comparison to Test 

Data 

30

40

50

60

70

80

90

IP Left

IP right

Dr seat

Pa Seat

windshield

Backlight

Dr side light

Pa side light

Pkg Shelf

Headliner

air foot

air breath

DATA

Model

-4.2

-1.9

+1.4

+5.2 -5.2 +1.3

-1.1

-3.7

+6.0

+8.5

+3.0

-7.5

Figure 8. Reduced Load Soak Results Comparison 

to Test Data

It was found that excessive run times were needed to 

achieve a cool-down simulation using the full 

Fluent/RadTherm model. Therefore, a simplified model 

was created for the cool-down simulations, which used 

only RadTherm. In the full Fluent/RadTherm simulation, 

the heat transfer coefficients and fluid temperatures are 

mapped to the RadTherm mesh. In the simplified model, 

the heat transfer coefficients are mapped to the 

RadTherm mesh and held constant during the cooldown. 

The fluid temperatures are now simulated by eight fluid 

nodes, making use of the advection capabilities in 

RadTherm. In essence the model was bisected in each 

plane into eight fluid volumes, in contrast to the 

1,300,000 fluid volumes in the Fluent model. This 

simplified model was validated to vehicle cool-down data. 

A typical baseline cool-down comparison is shown in 

Figure 9. As expected the results of the simplified model 

are not an exact match to the data, but the model has 

captured the overall behavior of the thermal 

environment. Some of the factors contributing to the 

data/model differences are the same as those already 

discussed under the steady-state model validation. In 

addition, the initial conditions were not an exact match to 

the data. This is not of great concern as the main focus 

of the cool-down model is to predict differences in 

cooling load required between the baseline and reduced 

load configurations. 

25

35

45

55

65

75

85

0 5 10 15 20 25 30 35 40 45

Time (Minutes)

Temperature (oC)

Driver Seat (Simulation)

Driver Seat (Data)

Pass. Seat (Simulation)

Pass. Seat (Data)

Driver Seat

Pass. Seat

Temperature (oC)

Figure 9. Baseline Cool-Down Comparison

A cool-down time of 30 minutes was chosen. That is the 

vehicle cabin was assumed to be at a comfortable 

temperature in 30 minutes. The approach that was used 

was to fix the cooling load in the baseline vehicle at 5.7 

kW. This results in a realistic cooldown with the interior 

mass average temperature reaching 30oC in 30 minutes. 

The cooling load in the reduced thermal load vehicle was 

then decreased until the vehicles had an equal 

cooldown, which was determined in two ways. The first 

method was to use a mass average temperature, and 

then determine at what cooling load the two vehicles 

achieved an equal mass average temperature of 30oC in 

30 minutes. The second method calculated a heat 

balance for each vehicle. 

Temperature (oC)

6
The mass average temperature calculation shows an 

equal cool-down time with a 4.0 kW A/C system, as is 

shown in Figure 10. This is a 30% reduction in cooling 

load. The heat balance calculation shows equal time to 

30oC with a 31% lower A/C load. To be conservative, the 

30% lower load (4.0 kW) was used in the fuel-use 

calculations. 

30

35

40

45

50

55

60

0 5 10 15 20 25 30

Minutes

Mass Average Temperature (oC)

Baseline (5.7kW)

Reduced Thermal Load (4.0 kW)

Figure 10. Mass Average Temperature

The A/C fuel-use calculation followed the procedure 

used in References 1 and 5. The ADVISOR vehicle 

simulator was used to determine the effect of the 

reduced A/C load on fuel use. The Cadillac STS was 

simulated on standard city and highway drive cycles, and 

the results were compared to published window sticker 

values. The A/C load on the vehicle was modeled by 

using a curve of compressor power as a function of 

compressor rpm obtained from the OEM. The vehicle 

fuel economy was then calculated in the simulator over 

the FTP cycle for the vehicle without A/C, baseline A/C, 

and 30% reduced A/C. The Vehicle Miles Traveled with 

A/C on (VMTAC) are the average miles traveled by a 

passenger car in the United States (11,998 miles6

) 

multiplied by the average percent of A/C use in the 

United States (32.6%1

). The VMTAC are 3,911 miles. 

Fuel used per vehicle is calculated by dividing the miles 

traveled by the fuel economy (miles per gallon). Fuel 

used for A/C is then the difference between fuel used 

with A/C on and fuel used with A/C off. For the Cadillac 

STS, the baseline A/C fuel use is 42.6 gal per year while 

the reduced thermal load vehicle A/C fuel use is 31.4 gal. 

These data show that reducing the A/C load by 30% 

results in a 26% reduction in A/C fuel use. 

CADILLAC STS VENTILATED SEATS 

Improving the delivery methods for conditioned air in an 

automobile is an effective way to increase thermal 

comfort with little energy cost. This reduces A/C needs 

and thus fuel use. Automotive seats are well suited for 

effective delivery of conditioned air due to their large 

contact area with and close proximity to the occupants. 

Normally a seat acts as a thermal insulator, increasing 

skin temperatures and reducing evaporative cooling of 

sweat. Ventilating a seat has low energy costs and 

eliminates this insulating effect while increasing 

evaporative cooling. W.E.T. Automotive Systems 

manufactures a ventilated seat used in the Cadillac STS 

that pulls air through the seat cushion and back. We 

assessed one of these seats using ADAM, NREL’s 

thermal comfort manikin7

. 

The Vehicle Climate Control Lab at NREL was 

developed to simulate the soak and cooldown of a 

vehicle passenger compartment8

. The passenger 

compartment from a compact car, A to C pillar, was heat 

soaked using a 963 W/m2

 ± 23% full-spectrum solar 

simulator for 3.5 hours. During this time, the average 

room environment was controlled to 31.6°C ± 0.4°C and 

30% ± 5% RH. ADAM and the subjects were 

conditioned in an office environment. The subject 

entered the heat-soaked room, stood for 30 seconds, 

and then did step exercises for one minute to simulate 

walking to the car. The subject entered the heat-soaked 

car, and took a pre-cool-down thermal comfort and 

sensation vote. The A/C system was started 45 seconds 

after the subject entered the vehicle, at which time the 

first cool-down vote was taken. Thermal comfort and 

sensation votes followed every two minutes for the 

duration of the test. ADAM predicted warmer overall 

thermal sensations than the subjects, but the trends 

were similar. Both showed a cooler sensation due to the 

operation of the ventilated seat. 

Figure 11 shows an operating ventilated seat increased 

the heat loss from ADAM’s back and bottom by ~ 60 

W/m2

 (25-35 minutes into the cool-down) compared to 

no ventilation (baseline). The seat contact temperature 

was reduced by ~ 4.7oC resulting in an overall thermal 

sensation improvement of 0.28 (on a +4 to –4 scale) 

shown in Figure 12. We determined that if the A/C 

system capacity was reduced by 7%, and the ventilated 

seat was used, the same thermal sensation and comfort 

as the baseline seat would result. Using NREL’s A/C 

fuel-use model1

, an estimated 522 million gal/year or 

7.5% reduction in U.S. A/C fuel use could be achieved. 

0

20

40

60

80

100

120

140

0 5 10 15 20 25 30 35

Cooldown Time (min)

Heat Flux Out [w/m2

]

Seat Off

Seat On

Figure 11. Heat Loss from ADAM’s Back and Bottom 

7
ACKNOWLEDGMENTS 

0

0.5

1

1.5

2

2.5

3

-15 -10 -5 0 5 10 15 20 25 30 35

Cooldown Time (min)

Thermal Sensation Vote

Seat Off

Seat On

The authors gratefully acknowledge the DOE Office of 

FreedomCAR and Vehicle Technologies (OFCVT) for 

supporting this work. The authors appreciate the support 

of Lee Slezak, DOE Program Manager; Terry Penney, 

NREL’s OFCVT Technology Manager; and Barbara 

Goodman, Director of the NREL Center for 

Transportation Technologies and Systems. The authors 

also thank Aaron Oakley and Sean Gooding at NREL; 

Stephen Andersen at the EPA; Greg Major and Bill Hill at 

GM; Reinhard Tatschl at AVL; Craig Makens at 

ThermoAnalytics, Inc.; George Galaitsis at BETA CAE 

Systems; Mike Nelson at 3M; the I-MAC Team 3; and the 

Figure 12. ADAM Thermal Sensation entire I-MAC core team. 

REFERENCES 

CONCLUSION 1. Rugh, J.; Hovland, V.; and Andersen, S. (2004) 

Significant Fuel Savings and Emission Reductions 

by Improving Vehicle Air Conditioning, Mobile Air 

Conditioning Summit, Washington, D.C., April 14-15, 

2004. 

NREL completed testing of technologies to reduce the 

thermal load on a Cadillac STS as part of the I-MAC 

Cooperative Research Program. Among the 

technologies tested were PPG Sungate EP solarreflective glass, PPG prototype solar reflective paint, and 

a Webasto solar-powered parked car ventilation system. 

A combination of these technologies reduced breath air 

temperature by 12°C (22°F), seat temperatures by 11°C 

(20°F), windshield temperature by 20.4°C (37°F), and the 

instrument panel surface temperature by 16.8°C (30°F). 

2. Wards 2001 Automotive Yearbook. 

3. U.S. Department of Energy’s Energy Information 

Administration, http://www.iea.doe.gov/ 

cabs/usa.html, accessed April 2001. 

4. Hoffpauir, E. (July-August 2005) “I-MAC 30/50:Year 

One.” ACtion – For The Automotive Air Conditioning 

And Cooling System Professional; pp. 60-62. 

5. Johnson, V. (2002) “Fuel Used for Vehicle Air 

Conditioning: A State-by-State Thermal ComfortBased Approach” Proceedings of Future Car 

Congress 2002, Paper # 2002-01-1957, Society of 

Automotive Engineers, Arlington, VA, June 2002. 

NREL also completed an analysis to estimate the impact 

of reducing the thermal load on A/C system capacity of a 

Cadillac STS. The integrated modeling process started 

with CAD data of the STS. The CAD data was cleaned 

up, and then meshed for RadTherm and CFD analysis. 

The models were benchmarked against test data and 

showed excellent agreement. A transient cool-down 

simulation of both the baseline and reduced load 

vehicles was performed. By maintaining an equivalent 

cool-down time in the baseline as compared to the 

reduced load vehicle, results show that the A/C load can 

be reduced by over 30%. Vehicle simulations show that 

the 30% reduction in thermal load results in a 26% 

reduction in fuel used for A/C. 

6. Wards 2005 Automotive Yearbook. 

7. Rugh, J.; and Bharathan, D. (2005) “Predicting 

Human Thermal Comfort in Automobiles,” 

Proceedings of the 7th Vehicle Thermal 

Management Systems Conference, Paper # 2005-

01-2008, May 10-12, 2005, Toronto, Canada, 

Society of Automotive Engineers. 

8. Lustbader, J. (2005) “Evaluation of Advanced 

Automotive Seats to Improve Thermal Comfort and 

Fuel Economy,” Proceedings of the 7th Vehicle 

Thermal Management Systems Conference, Paper # 

2005-01-2056, May 10-12, 2005, Toronto, Canada, 

Society of Automotive Engineers. 

The load-reduction technologies we tested, as well as 

climate-control seating, are available for today’s vehicles 

and are representative of techniques to reduce the 

thermal load on the passenger compartment. Other 

technologies could also be used to reduce the thermal 

loads and improve thermal comfort with similar results. If 

incorporated in conjunction with a smaller A/C system or 

if the A/C is used less often, the reduced A/C fuel use 

will save drivers money at the pump, and the reduced 

temperatures will improve thermal comfort immediately 

after a hot soak. The reduced fuel use translates directly 

to reduced CO2 emissions, which is good for the 

environment. 

CONTACT 

john_rugh@nrel.gov 

http://www.nrel.gov/vehiclesandfuels/ancillary_loads/ 

8