A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

An Overview of Vehicle Test and 

Analysis from NREL’s A/C Fuel 

Use Reduction Research 

D. Bharathan, L. Chaney, R.B. Farrington, 

J. Lustbader, M. Keyser, and J. Rugh

Presented at the Vehicle Thermal Management Systems 

Conference & Exhibition (VTMS-8) 

Nottingham, England 

May 20-24, 2007 

Conference Paper 

NREL/CP-540-41155 

June 2007 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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An Overview of Vehicle Test and Analysis 

Results from NREL’s A/C Fuel Use 

Reduction Research 

Desikan Bharathan 

Larry Chaney 

Robert B. Farrington 

Jason Lustbader 

Matthew Keyser 

John P. Rugh 

National Renewable Energy Laboratory, Center for Transportation Technologies and 

Systems, United States 

Abstract 

The primary mission of the National Renewable Energy Laboratory’s Vehicle Ancillary 

Load Reduction Task is to develop and evaluate technologies that reduce the fuel use for 

automobile air conditioning. Using thermal comfort-based logic, we project that the 

United States uses 7 billion gallons of fuel per year for air conditioning light-duty 

vehicles. This paper summarizes results of air-conditioning fuel use reduction 

technologies and techniques evaluated over the last 10 years.

1 INTRODUCTION 

It takes energy and therefore fuel to provide climate control for vehicle occupants as 

well as energy for defrosting. In the past, air conditioning has been provided by a 

compressor driven directly by engine power while heating was provided by engine waste 

heat via engine coolant or in severe climates supplemented by a fuel-fired heater or an 

electric heater. Little thought has gone into significantly reducing fuel use for vehicle 

climate control. However, recent times with higher fuel prices, stricter emission 

requirements, down-sized engines with less waste heat, and advanced powertrain control 

strategies, such as idle-off controls (where there are times with little waste heat and no 

engine output power), have lead to a concerted effort to develop new, effective climate 

control systems. Likewise, plug-in hybrid electric vehicles may have long periods 

without engine operation. 

Fuel used for vehicle climate control significantly affects the United States energy 

security by decreasing the fuel economy of the 229.8 million light-duty conventional 

vehicles(1). Using the vehicle simulation software ADVISOR(2), a National 

1
Renewable Energy 

Laboratory (NREL) 

analysis shows air 

conditioning (A/C) 

increases fuel 

consumption 

between 1.8 and 2 

l/100 km for a 

conventional 

gasoline and diesel 

powered mediumsized sedan over 

the Federal Test 

Procedure drive 

cycle (Figure 1). 

The A/C system is a baseline R134a configuration controlled to a 5oC evaporator 

temperature with ambient conditions of 25oC and 65% relative humidity. The baseline 

fuel use for the gasoline and diesel vehicles is 10.2 and 8.6 l/100km (23.1 and 27.2 mpg) 

respectively. 

0.0

0.5

1.0

1.5

2.0

2.5

ICE, FTP Diesel, FTP Hybrid - 7 Vehicle Average,

J1634 (4)

Conventional - 5 Vehicle

Average (5)

A/C fuel Use (liters/100 km)

0.000

0.213

0.425

0.638

0.850

1.063

1.275

A/C fuel Use (gallons/100 miles)

Figure 1. Vehicle A/C fuel use 

A/C can also reduce the fuel economy of advanced vehicles by as much as 35%. 

Testing of a 2000 Honda Insight and 2001 Toyota Prius over the SC03 drive cycle show 

a 1.9 l/100km increase in fuel consumption due to the operation of the A/C(3). Test 

results from more recent hybrid vehicles assessed by the U.S. Department of Energy’s 

Idaho National Laboratory are also plotted in Figure 1 and show a similar 1.8 l/100km 

increase in fuel use over the SAE J1634 drive cycle(4). Data from the U.S 

Environmental Protection Agency shows similar magnitude of fuel use over the SC03 

drive cycle(5) 

We have calculated the U.S. national and state-by-state fuel use impact of air 

conditioning in light-duty vehicles(6,7,8). Our analysis used a bottom-up approach to 

estimate the fuel used in vehicles for air conditioning per year. A thermal comfort model 

determined the percentage of time that a driver used the air conditioning based on the 

premise that people dissatisfied with their thermal environment will turn on the air 

conditioning. Environmental conditions were an important input to the comfort model. 

The thermal comfort results were then combined with statistics on when people drive 

(time of day), where they live (climate including cloud cover), and how far they drive in 

a year. Finally, vehicle simulations determined the fuel use penalty of using the air 

conditioning in cars and trucks. This algorithm determined the fuel used for air 

conditioning in light-duty vehicles. 

The analysis showed that the United States uses approximately 7.0 billion gallons of 

gasoline every year for air conditioning vehicles(8), equivalent to 5.5% of domestic 

light-duty vehicle petroleum consumption(9). It would take 9.5% of U.S. imported 

crude oil to produce this much gasoline(10). We expanded the analysis to cover Europe 

and Japan in 2003 and more recently investigated India, Mexico City, and Australia. 

2
There is currently a great opportunity to reduce A/C fuel use and emissions because the 

world’s attention is focused on mobile air conditioning (MAC) like never before. The 

European Commission has passed a regulation prohibiting MAC refrigerants with a 

GWP > 150, which bans the currently used HFC-134a in new cars. Although the 

regulation does not specifically address MAC fuel use, the Association des 

Constructeurs Europeens d’Automobile (ACEA) and its members have a voluntary 

agreement to reduce CO2 emissions of vehicles sold in the European Union. Reducing 

the A/C fuel use helps meet the aggressive targets. The California Air Resources Board 

is rewarding the incorporation of improved efficiency A/C compressors in its 2004 

greenhouse gas emissions regulation. In the 2006 proposed U.S. Environmental 

Protection Agency fuel economy window sticker update, the fuel use from MACs will 

be incorporated into the new test/calculation methodology for determining miles per 

gallon. 

2 APPROACH TO REDUCE MOBILE AIR-CONDITIONING FUEL USE 

While most emphasis is on efficient equipment because suppliers manufacture 

hardware, the emphasis should begin with reducing the thermal load, such that 

equipment size is as small as possible. Reducing the cooling thermal load may include 

reflecting the infrared portion of the solar spectrum through reflective glazing, shades, 

and/or paint, minimizing conduction into the cabin via body insulation, or rejecting heat 

to the ambient using active or passive ventilation of the vehicle cabin or heat pipes 

while it is parked in the sun. In the winter, reducing the cabin heat loss will become 

increasingly important in advanced vehicles where waste heat is not readily available 

such as a plug-in hybrid vehicle operating in all-electric mode. Options include 

increased body insulation and advanced glazings with low-emissivity coatings or 

multiple layers. 

After the load is reduced, then the next logical step is to remove energy as effectively as 

possible from the occupants. Conditioning the entire cabin can be inefficient when there 

is only one occupant. Technologies might include occupant sensor, body coolers, and 

conditioned seats. 

The final step is to design efficient equipment. Waste heat should be used when 

possible, even for cooling using techniques such as thermoacoustic systems, absorption 

systems, or adsorption systems (such as metal-hydride). Climate-control equipment 

must be capable of operating without the engine and should minimize fuel cost. Electric 

energy can be used to operate an electric air-conditioning compressor, perhaps even 

operating in a heat pump mode to provide heating. However, electric heating can be 

expensive but might be an effective technique for direct delivery, such as heated seats or 

heated windshields. 

The options are many and include initial as well as operating costs. The barriers are 

high because an effective solution will involve multiple disciplines (such as seating, 

glazing, body, engine, etc.) that normally do not collaborate. However, the potential to 

reduce fuel use for A/C and increase occupant comfort is great. 

3
3 REDUCING THE THERMAL LOAD 

3.1 Solar-Reflective Glazing 

When a vehicle is parked, typically 50 to 75% of the thermal energy entering the 

passenger compartment is from the transmitted and absorbed solar energy at the 

glazings(11). Transmitted energy is primarily absorbed directly by the interior mass. 

The absorbed energy at the glazing is transferred to the interior by convection and reradiation in the thermal infrared wavelength range. Reflecting the incident solar 

radiation at the vehicle glazings is a critical step to making significant reductions in the 

thermal loads. With lower thermal loads, there is potential to reduce the capacity of the 

A/C system. 

A 3M nonmetallic solar-reflective film was soak tested in two minivans (tan Dodge 

Grand Caravans) and two sport utility vehicles (black Ford Explorers)(12). The 

configuration with solar-reflective film on all glazings had the best thermal performance 

with a 1.8°C (3.2°F) reduction in maximum breath temperature. The maximum 

instrument panel temperature was reduced by 3.4°C (6.1°F) in this case. These results 

were not as good as other solar-reflective technologies, but an advantage of the 

nonmetallic construction is that it does not attenuate electromagnetic 

transmission/reception from cell phones and other communication devices. 

NREL and PPG Industries conducted a test program with Sungate laminated solarreflective glass installed in a Ford Explorer to quantify improvements in fuel economy 

and reductions in tailpipe emissions(13). Sungate is a solar-reflective coating consisting 

of a double stack of silver and dielectric layers. The vehicle with the solar-reflective 

glazing at all locations had a maximum breath temperature 2.7°C (4.9°F) lower than the 

baseline vehicle, and the instrument panel was 7.6°C (13.7°F) 

More recently, NREL tested a new type of solar-reflective glass that improves reflection 

of the near-infrared portion of the solar spectrum on a 2005 Cadillac STS as part of the 

Improved Mobile Air Conditioning Cooperative Research Program (I-MAC)(14). The 

Sungate EP allows only 3% of the infrared energy to be transmitted through the glass. 

Figure 2 shows this technology at all glazing locations reduced the average air 

temperature by 7.1°C 

(12.8°F), seat 

temperatures by 8.7°C 

(15.7°F), windshield 

temperature by 19.3°C 

(24.7°F), and the 

instrument panel 

surface temperature by 

14.6°C (26.3°F). 

Another way to assess 

these data is in terms 

of the maximum 

possible temperature 

reduction. For the 

solar-reflective glass Figure 2. Cadillac STS – reduction in temperature 

0

5

15

20

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

Seat

Front Pass

Seat 

Windshield

Re n (o

10

25

duction i Temperature C)

Solar Reflective Glass-all locations Solar Reflective Paint-all tests

Solar Reflective Windshield-Backlite

Solar Reflective Windshield

4
in all locations, the average air temperature is reduced 34% of the maximum possible 

while the seat temperature is reduced 35%. 

Solar-reflective glass would also reduce the steady state thermal loads when the vehicle 

is being driven with the A/C on. In cold environments, solar-reflective glass would also 

reduce the temperature of the interior compared to traditional glazings. In today’s 

vehicles, drivers have grown accustom to a warmer than ambient passenger 

compartment if the sun is out and it is cold outside. With solar-reflective glazings, the 

drivers will have to adjust their expectations to a cooler interior in the winter also, but 

interior temperatures would not be lower than ambient, and it would be no different 

from entering the car after the sun goes down. 

3.2 Parked Car Ventilation 

When the solar energy does enter the vehicle, it heats 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. NREL tested various natural and forced 

parked car ventilation techniques on a 2000 Jeep Grand Cherokee. The results show 

natural convection induced flow can be effective with strategically located air inlets and 

was as effective as forced convection ventilation using the HVAC blower on medium 

speed. Figure 3 shows the sunroof open 6 cm and floor inlets reducing the cabin air 

temperature 5.7oC, 38% of the maximum possible temperature reduction, while the 

HVAC blower on 

medium reduces the 

breath air 

temperature 6.9oC. 

While air entering 

the vehicle at the 

foot level enhances 

the natural 

convection flow, 

implementation 

challenges include 

preventing the 

entrance of moisture 

and contamination 

including exhaust 

products, dirt, and animals. 

Figure 3. Parked car ventilation in a Jeep Cherokee 

30

32

34

36

38

40

42

44

46

Low flow / panel

vents

Medium flow / panel

vents

Sunroof open 6cm Sidelites open 2cm Sunroof 6cm/double

floorvents

Sunroof 6cm/double

floorvents/3 fans

Ventilation Technique

 Cabin Air Temperature (C)

Ambient Temperature = 30C

Baseline Cabin Air Temperature = 45C, Ambient Temperature = 30C

1.9C 

13%

6.9C

46%

2.0C

13% 3.5C

23% 5.7C

38%

7.5C

50%

4.1 watts 13 watts 81 watts

As part of the I-MAC thermal soak testing on the Cadillac STS, we tested a solarpowered parked car ventilation system(14). With air being pulled out of the vehicle, the 

average air temperature was reduced 5.6oC and the seat temperatures were reduced 5-

6oC. This was 26% and 21% of the maximum possible temperature reduction 

respectively. Air was being pulled in through the HVAC heater/defroster ducts and the 

natural vehicle body leakage areas. Even though they don’t reduce temperatures as 

much as solar-reflective glass, parked car ventilation technologies can be used in 

conjunction with solar-reflective technologies to significantly reduce interior 

temperatures. This combination in the Cadillac STS reduced the average air 

temperature 46% and the seat temperature 44% of the maximum possible(14). 

5
3.3 Solar-Reflective Opaque Surface Coatings 

If the exterior vehicle skin temperature is warmer than the interior surfaces, the resulting 

conduction heat transfer will warm the vehicle interior. This is more likely to occur 

when solar-reflective glazings and/or parked car ventilation are used and the interior is 

cooler. Using a 2001 Lincoln Navigator, NREL investigated the impact of solarreflective coatings on interior cabin temperatures on a parked vehicle. We used two 

identical vehicles: one baseline and the other modified. A 3M visibly reflecting roof 

film reduced the breath air temperature 12% of the maximum possible temperature 

reduction, which is defined as the difference between the baseline vehicle breath air 

temperature and ambient. The exterior surfaces were covered with aluminium foil to 

determine the largest potential reduction. The breath air temperature was reduced 28% 

relative to the maximum possible reduction for this case. 

As part of the I-MAC project, we soak tested a solar-reflective film on the roof of a 

stationary Cadillac STS in 2005. The film was an infrared reflecting product (CI-100T) 

manufactured by 3M. The baseline gray paint had a solar absorptance of 0.78, and the 

paint with film had a solar absorptance of 0.55. The 6.7oC cooler roof temperature 

resulted in a less than 1oC reduction in the breath air temperature (Figure 4). During the 

summer of 2006, we tested a prototype solar-reflective paint on the Cadillac STS. The 

baseline paint had a solar absorptance of 0.89 and the solar-reflective paint had a solar 

absorptance of 0.82. The exterior surface temperature was 6.0oC cooler than the 

baseline. The 

impact of paint by 

itself was not 

tested, but we 

estimate the results 

would have been 

similar to the 2005 

data. Ford found 

that reducing the 

vehicle soak 

temperature with 

solar reflective 

coatings can reduce 

the AC system 

Figure 4. Cadillac STS – reflective roof requirements(15). 

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)

While skin temperatures can be significantly reduced with these technologies, the 

impact on interior temperatures is not that large because 1) the majority of thermal load 

is entering though the windows, 2) the roof is typically well insulated, and 3) wind 

reduces the advantage of the lower temperature skin by cooling the hot skin. If the 

vehicle is moving, the increased convective heat loss further reduces the thermal impact 

of the solar-reflective coatings. 

3.4 Insulation 

The benefit of headliner insulation depends on the exterior surface and interior 

temperatures. If the interior is warmer than the exterior (e.g., white paint with 

traditional glazings), then increased insulation in the headliner will make the car 

6
warmer. However if solar-reflective glazings and/or parked car ventilation are used to 

cool the car, then insulation may reduce the heat gain of a parked car. A computational 

fluid dynamics (CFD) analysis of a Jeep Grand Cherokee resulted in only a 0.2oC 

reduction in average cabin air temperature due to insulation of the roof. As discussed in 

Section 3.3, advanced technology insulation on the roof of the Lincoln Navigator 

reduced the cabin breath air temperature 5% of the maximum possible. In a Cadillac 

STS, the addition of current technology roof insulation had little impact on the interior 

soak temperatures. 

For reducing the parked car thermal loads in conventional vehicles, insulation was not 

very effective. If the thermal loads are reduced with solar-reflective glass or ventilation, 

insulation may become more important. When the vehicle is in use, the case where the 

interior is cooler (A/C on in a warm environment) is common. In this case, insulation 

would reduce the thermal loads. Also in winter when the interior is warm and the 

environment is cold, insulation would help to reduce the heating load. In the United 

States, cabin heating is not an issue with today’s vehicles due to the available waste 

heat, but will be a challenge for advanced vehicles and small diesels that have less (or 

no) available waste heat for cabin warming. 

3.5 Instrument Panel Heat Pipe Cooling 

When a vehicle is parked, the instrument panel can absorb a large amount of heat, attain 

temperatures approaching 100oC, and impact driver thermal comfort and cool-down 

performance. A mock-up of a passenger compartment and instrument panel was built to 

assess the impact of cooling the instrument panel with heat pipes. Outdoor test results 

show that heat pipes cooled the instrument panel by nearly 20°C and the air temperature 

by 9°C to 12°C while maintaining a uniform temperature across the instrument panel 

during the day. Beside lower surface temperatures, the electronic components in the 

instrument panel would benefit from the lower temperatures as well as the reduced heat 

pickup of the A/C ducts. Barriers to incorporating heat pipes in the instrument panel 

include where to locate of the condensing section outside the cabin and the increased 

mass/volume of the heat pipes. 

4 EFFICIENT DELIVERY OF CONDITIONED AIR 

The goal of efficient delivery is to more effectively deliver climate control to each 

occupant, increasing thermal comfort with little energy cost, thereby reducing fuel use. 

Automotive seats have potential to improve delivery efficiency because of their large 

contact area with and close proximity to the occupants. Standard automotive seats 

thermally insulate the occupant and reduce evaporative cooling of sweat, increasing the 

occupant/seat contact temperature. Both actively ventilated seats and meshed seats are 

potentially low-energy approaches to improving thermal comfort and achieving efficient 

delivery of climate control. 

The NREL Vehicle Climate Control Laboratory was developed to simulate the soak and 

cooldown of a vehicle passenger compartment(16). A compact car passenger 

compartment was thermally soaked for 3.5 hours using a 963 W/m2

 ± 23% full spectrum 

solar simulator. The average room environment was controlled to 31.6 °C ± 0.4 °C and 

7
30% ± 5% RH. Both the ADvanced Automotive Thermal Manikin (ADAM) and the 

subjects were preconditioned in an office environment. ADAM and a subject entered 

the heat-soaked room, and the subject stood for 30 seconds before doing step exercises 

for 1 minute. The cooldown was started 45 seconds after cabin entry, allowing time for 

pre-cooldown votes to be taken. 

Ventilated seats provided by W.E.T. Automotive Systems were tested(14). ADAM 

measured a seat contact temperature reduction of ~ 4.7 °C and an increase in heat loss 

from the back and bottom of ~ 60 

W/m2 between seat on-and-off 

cases. This resulted in an overall 

thermal sensation improvement of 

0.28 (on a +4 to -4 scale). The 

average juror thermal comfort 

over the cooldown is shown in 

Figure 5. We estimate that the 

A/C system capacity could be 

reduced by 7%. Using NREL’s 

A/C fuel use model(6,7,8), an 

estimated 522 million gal/year or 

7.5% reduction in U.S. A/C fuel 

use could be achieved. 

A prototype meshed backseat was tested and provided encouraging results. With a 

reduced, 96% heat capacity A/C system, an average seat back and bottom temperature 

reduction of 4°C and 0.6 °C respectively were observed(16). Assuming that a meshed 

bottom would provide similar results to the meshed back, much of the thermal benefit 

gained through ventilation could be achieved using low-mass meshed seats while 

avoiding energy costs. A lack of thermal control, physical ergonomics, and safety is a 

significant challenge for mesh seats. 

5 EFFICIENT EQUIPMENT 

5.1 Improved Mobile Air Conditioning Cooperative Research Program 

Efficiency Team 

As part of the I-MAC project, a team of engineers from automobile manufacturers, 

suppliers, and the U.S. government have been investigating techniques to improve the 

efficiency of MACs. The goal of efficiency team is to demonstrate an improvement of 

30% in system coefficient of performance over the R134a System used in Phase II of the 

Alternate Refrigerant Cooperative Research Program. Results of the test and analysis 

will be available after the completion of the program. 

5.2 Waste Heat Utilization - Thermoacoustics 

Thermoacoustics is an innovative technology that uses sound to cool the interior of the 

vehicle or produce electricity. Thermoacoustic effects, which convert heat energy to 

sound, have been understood for over a hundred years. However, only over the past two 

decades has substantial improvements been made to the design of thermoacoustic 

-3

-2

-1

0

1

2

3

-5 0 5 10 15 20 25 30 35 40

Cooldown Time (min)

Thermal Comfort

Seat Off

Seat On

Increasing Comfort Decreasing Comfort

Seat on, improved comfort

A/C Start

Figure 5. Ventilated seat improved comfort 

8
engines and refrigeration cycles. Recent thermoacoustic refrigerators have flown on the 

space shuttle, cooled electronics in a U.S. Navy destroyer, and cooled Ben and Jerry’s 

ice cream. 

Thermoacoustics has many potential advantages over a conventional A/C system. It 

uses waste heat, is reliable and inexpensive, does not entail the use of an extra energy 

load on the engine, relies on gases that are environmentally benign, has no moving parts 

(and thus should have a long lifetime), and requires no lubrication. The downside is, 

because of its low energy density, the device could take up significant volume. If we 

can overcome that barrier, it could be one of the cooling technologies in your nextgeneration car. 

During the last quarter of 2004, NREL designed and developed a standing- wave 

thermoacoustic device that pumps heat using a standing sound wave to take the working 

fluid (helium) through a thermodynamic cycle. We rely on the heating and cooling that 

accompany the compression and expansion of a gas in a sound wave to produce the 

cooling for the interior of a vehicle. The device is modular and allows for different 

stack designs and heat exchangers to be used in order to assess the most cost-efficient 

and best-performing components. Modeling efforts show that a thermoacoustic 

standing- wave engine/heat pump has a heat efficiency of approximately 15% and a heat 

pump coefficient of performance of approximately 1. The NREL team completed 

testing of the standing- wave thermoacoustic engine and heat pump during the early part 

of 2005. The thermoacoustic engine performed within 10% of modeled results. 

However, the heat pump only provided 20 watts of cooling. The poor performance of 

the heat pump was attributed to combining the room temperature heat exchanger for 

both the engine and heat pump into a single unit. It was determined that the pressure 

wave from the engine needs to be fully developed before it can be utilized for cooling. 

This problem can be easily rectified by separating the heat engine from the heat pump 

and utilizing two ambient heat exchangers. Although the standing- wave 

thermoacoustic system works, the size necessary to generate sufficient cooling power 

for a light-duty vehicle preludes its use. Therefore NREL researchers concentrated their 

efforts on developing a smaller travelling-wave thermoacoustic system. 

During 2006, the NREL team 

assembled and tested a travellingwave thermoacoustic engine 

(Figure 6). Like the standing- wave 

prototype, the travelling-wave 

system was modular and allowed 

for different components (heat 

exchangers, regenerators, etc.) to be 

tested. In particular, we tested a 

micro-channel regenerator and a 

copper foam heat exchanger. It was 

determined from NREL’s modeling 

and test results that a travellingwave thermoacoustic system for 

direct vehicle cooling will not be 

Figure 6. Travelling-wave thermoacoustic 

prototype 

9
viable for light-duty vehicles—the resonant cavity will be too large. In order to decrease 

the size of the resonant cavity, a new combination of gases, working fluid, will need to 

be found that will lower the resonant frequency of the device without lowering the 

power per unit volume. One way to decrease the length of the resonant cavity is to 

combine a heavier gas with a lighter gas. Unfortunately, combining gases precipitously 

reduces the power per unit volume, resulting in an unrealistically large diameter for the 

device in order to provide the necessary cooling for the vehicle. However, it was 

determined that a travelling-wave thermoacoustic system could still be used in lightduty vehicles for electricity generation. Electrical power could be produced through the 

use of a flexible membrane, which would also determine the resonant frequency of the 

thermoacoustic system and therefore its size. Future work should be concentrated on 

reducing the cyclical fatigue of such a membrane for eventual integration into light-duty 

vehicles. 

6 TOOLS 

6.1 Vehicle Solar Load Estimator 

In order to fully understand solar-reflective glazings and their impact on heat load of 

passenger compartments, a Vehicle Solar Load Estimator was developed(17). The 

Vehicle Solar Load Estimator or VSOLE is GUI-driven and programmed in the 

MATLAB environment. The model calculates solar radiation transmitted, absorbed, and 

reflected by glazings as a function of the optical properties of the glazing, glazing 

location, vehicle geometry, vehicle orientation, time, and radiation source. The model 

accounts for the angular dependence of the optical properties for glass. The program can 

also display glazing properties and comparisons of different glazings under the same 

solar load and vehicle orientation as a function of time. This software is available to the 

public upon contacting the authors. 

6.2 Integrated Climate-Control Modeling Process 

A numerical model was developed to assess the impact of reducing the thermal load on 

A/C system capacity and vehicle fuel used for A/C. The numerical model of a Cadillac 

STS vehicle with solarreflective glass, solarpowered parked car 

ventilation, and solarreflective paint was 

developed using Fluent 

CFD software and 

RadTherm thermal 

analysis software. 

RadTherm is used to 

model the solar load on 

the vehicle, convection 

losses on the interior and 

exterior surfaces, and 

conduction through the 

surfaces. Fluent CFD Figure 7. Baseline soak results comparison to 

test data

30

40

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

50

60

10
software was used to model the convective heat transfer and fluid flow in the cabin. 

The model was compared to quasi steady-state soak conditions, and Figure 7 shows 

excellent agreement to the 

data. 

It was found that excessive 

run times were needed to 

achieve a cool-down 

simulation using the full 

CFD/RadTherm model. 

Therefore, a simplified 

model was created for the 

cool-down simulations, 

which used only 

RadTherm. This 

simplified RadTherm 

model was compared to 

STS cool-down data with 

the vehicle at idle 

conditions. The baseline 

model showed agreement with the data when A/C cooling capacity was 5.7 kW. The 

reduced thermal load model was then run with decreasing A/C capacity until the two 

models had equal cool-down at 30 minutes (Figure 8). The model showed that the A/C 

capacity could be decreased to 4.0 kW, a 30% reduction in A/C system capacity. 

These results are consistent with data from Visteon where they tested a mid-sized sedan 

with insulated solar-reflected glazings and body insulation(18). An average temperature 

reduction of 3-4oC during soak resulted in a 4-16% reduction in A/C power. This 

compares to the 11oC reduction in mass average temperature for the Cadillac STS and 

the 30% reduction in A/C power. 

Next the STS was simulated using the ADVISOR vehicle simulator to determine the 

effect of the reduced A/C load on fuel use. The A/C fuel use calculation followed the 

procedure used in reference 7. For the baseline case, we use a compressor power vs. rpm 

curve for the STS. For the reduced thermal load case, we reduce the compressor power 

by the same % reduction as the calculated % reduction in capacity. We are assuming the 

reduced thermal load A/C system has same COP as the baseline case. The vehicle 

simulation showed that reducing the A/C load by 30% resulted in a 26% reduction in 

A/C fuel use. 

6.3 Thermal Comfort Tools 

Designed to assess human thermal comfort 

in a transient nonhomogeneous environment, 

NREL’s thermal comfort portfolio includes a 

manikin or ADAM, a Human Thermal 

Physiological Model, and a Human Thermal 

Comfort Empirical Model. The 126 porous 

metal segments of the manikin allow 

Figure 9. ADvanced Automotive Manikin 

Figure 8. Mass average temperature 

30

Mass A erage Te ratu

35

40

45

50

55

60

0 5 10 15 20 25 30

Minutes

v mpe re (oC)

Baseline (5.7kW)

Reduced Thermal Load (4.0 kW)

11
independent control of surface temperature and sweat rate, and measurement of transient 

heat flux. The manikin, shown in Figure 9, can be clothed, and is designed for either 

tethered or wireless operation with an internal battery pack and radio transceiver(19,20). 

The Human Thermal Physiological Model is a three-dimensional, finite-element model 

of the human thermal physiological and thermoregulatory systems. The model consists 

of bone, muscle, fat, and skin layers, as well as blood circulation. The thermoregulatory 

system physiological responses of vasodilatation/constriction, sweating, shivering, and 

variable metabolic or cardiac rates are simulated(21). 

The Human Thermal Physiological Model controls the manikin, so the segments have 

human-like skin temperatures. Validation testing was performed to show the 

manikin/model respond like a human(22). The Human Thermal Comfort Empirical 

Model uses temperature data from the physiological model to predict local and global 

thermal comfort as a function of local skin and core temperatures and their rates of 

change(23). 

7 BARRIERS 

A critical barrier to the incorporation of advanced climate-control technologies is the 

increased vehicle cost. If there were a consumer demand, automobile manufacturers 

would have a financial incentive to incorporate the technologies discussed in this paper. 

In many cases, consumers simply do not know the technologies exist. Reduced fuel 

consumption has not been an important factor in the past; although, this can change 

quickly. It is clear that additional benefits, such as improved comfort and safety, are 

critical to the incorporation of advanced climate-control systems 

Another barrier is that automobile manufacturers are sometimes hesitant to change. The 

risk of recalls is an important consideration. Thermal management crosses multiple 

disciplines at automotive original equipment manufacturers which sometimes makes 

system-level decisions difficult. 

Since the A/C system is not operated during corporate average fuel economy tests, there 

has not been a fuel economy motivation to reduce the fuel use for MACs. Some have 

suggested adding an A/C on test to the CAFE tests, but the potential added complexity 

of a test that accounts for thermal load reduction is a barrier. 

Barriers to utilizing waste heat for cooling include: relatively low and variable 

temperatures, particularly in fuel cell vehicle applications; weight; volume; corrosion; 

and engine back-pressure requirements. 

8 CONCLUSION 

NREL is pursuing a variety of avenues in its efforts to improve vehicle efficiency and 

fuel economy by controlling the climate in the vehicle, while still keeping the 

passengers "comfortable." Because climate control loads can significantly impact the 

fuel economy and tailpipe emissions of conventional and hybrid electric automobiles, 

12
NREL is working closely with industry to develop techniques to reduce the auxiliary 

loads, such as climate control, in a vehicle. 

Our research shows that technologies, such as solar-reflective glass and parked car 

ventilation, can significantly reduce the thermal loads and fuel use. Solar-reflective 

coatings and insulation had less of an impact on interior temperature, but could become 

increasing important as cabin thermal loads and as engine waste decreases in advanced 

vehicles. A critical benefit of reducing the soak thermal loads is that the occupants 

experience lower temperatures upon entering a hot-soaked vehicle and feel less 

discomfort. Additional energy savings can be realized by cooling the occupants through 

ventilated, cooled, and low-mass seats. Using waste heat to generate cooling with 

thermoacoustics was not feasible for light-duty vehicles, but there is potential for using 

a more compact thermoacoutic system to generate electricity. 

REFERENCE LIST 

 

1) Ward’s 2006 Automotive Yearbook. Binder, A.K., ed. Southfield, MI: Ward’s 

Communications, 2006.

2) Wipke, K.; Cuddy, M.; Bharathan, D.; Burch, S.; Johnson, V.; Markel, A.; Sprik, 

S. ADVISOR 2.0: A Second-Generation Advanced Vehicle Simulatory for 

Systems Analysis. NREL/TP-540-25928 Golden, CO: NREL, 1999. 

3) Kelly, K.J.; Rajagopalan, A. Benchmarking of OEM Hybrid Electric Vehicles at 

NREL. NREL/TP-540-31086. Golden, CO: NREL, 2001. 

4) Hybrid Electric Vehicles. Idaho National Laboratory, 

http://avt.inl.gov/hev.shtml. Accessed February 12, 2007. 

5) Fuel Economy Labeling of Motor Vehicles: Revisions to Improve Calculation of 

Fuel Economy Estimates. U.S. Environmental Protection Agency. EPA420-D06-009, http://www.epa.gov/fueleconomy/420d06002.pdf. Accessed February 

12, 2007. 

6) Farrington, R.; Johnson, V. “The Impact of Vehicle Air-Conditioning Fuel Use 

and What Can Be Done to Reduce It,” Earth Technologies Forum, Washington, 

D.C., 2002. 

7) Johnson, V. “Fuel Used for Vehicle Air Conditioning: A State-by-State Thermal 

Comfort-Based Approach,” Proceedings of Future Car Congress, Arlington, VA, 

June 2002. Paper # 2002-01-1957. Society of Automotive Engineers. 

8) Rugh, J.; Hovland, V.; Andersen, S. “Significant Fuel Savings and Emission 

Reductions by Improving Vehicle Air Conditioning,” Mobile Air Conditioning 

Summit, Washington, D.C., April 14-15, 2004. 

9) “Total annual light-duty vehicle fuel use = 125.9 billion gal,” Ward’s 2001 

Automotive Yearbook. Southfield, MI: Ward’s Communications, 2001.

10) “Imported oil = 73 billion gal,” U.S. Department of Energy’s Energy Information 

Administration, http://www.iea.doe.gov/ cabs/usa.html. Accessed April 2001, 

11) Shimizu, S.; Hara, H.; Asakawa, F. “Analysis of air-conditioning heat load of a 

passenger vehicle,” International Journal of Vehicle Design (4:3); 1983; pp.292-

311. 

13
12) Rugh, J.; Farrington, R.; Boettcher, J. “The Impact of Metal-Free Solar 

Reflective Film on Vehicle Climate Control,” Proceedings of the 5th Vehicle 

Thermal Management Systems Conference, Nashville, TN, May 2001. SAE 

Paper # 2001-01-1721. Society of Automotive Engineers. 

13) Rugh J.; Hendricks, T.; Koram K. “Effect of Solar Reflective Glazing on Ford 

Explorer Climate Control, Fuel Economy, and Emissions,” Proceedings of the 

International Body Engineering Conference, 2001. SAE Paper # 2001-01-3077. 

Society of Automotive Engineers. 

14) Rugh, J.; Chaney, L.; Lustbader, J.; Meyer, J.; Rustagi, M.; Olson, K.; Kogler, R. 

(2007) “Reduction in Vehicle Temperatures and Fuel Use from Cabin 

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

Proceedings of 2007 SAE Congress, Detroit, MI. Paper # 2007-01-1194. Society 

of Automotive Engineers 

15) Hoke, P.; Greiner, C., “Vehicle Paint Radiation Properties and Affect on Vehicle 

Soak Temperature Climate Control System Load, and Fuel Economy,” 

Proceedings from the 2005 SAE Congress, Detroit, MI. March 2005. Paper # 

2005-01-1880. Society of Automotive Engineers. 

16) Lustbader, J. “Evaluation of Advanced Automotive Seats to Improve Thermal 

Comfort and Fuel Economy,” Proceedings of the 7th Vehicle Thermal 

Management Systems Conference, May 10-12, 2005, Toronto, Canada. Paper # 

2005-01-2056. Society of Automotive Engineers. 

17) Rugh, J.P. “Integrated Numerical Modeling Process for Evaluating Automobile 

Climate Control Systems,” Proceedings of Future Car Congress, Arlington, VA, 

June 2002. Paper # 2002-01-1956. Society of Automotive Engineers, 

18) Meyer, J; Southwood, J.; Chan, M. “Energy efficient thermal vehicle for reduced 

climate system load,” Proceedings of the 6th Vehicle Thermal Management 

Systems Conference, May 18-21, 2003, Brighton, United Kingdom. Paper # 

C599/094/2003. 

19) Rugh, J.; Farrington R.; Bharathan, D.; Vlahinos, A.; Burke, R.; Huizenga, C.; 

Zhang, H. “Predicting Human Thermal Comfort in a Transient Non-Uniform 

Thermal Environment,” European Journal of Applied Physiology (92:6); pp. 

721-727. 

20) Burke, R.; Rugh, J.; Farrington R. “ADAM – the Advanced Automotive 

Manikin.” 5th International Meeting on Thermal Manikins and Modeling, 

Strasbourg, France, 2003. 

21) Bharathan, D.; Rugh, J.; King, C.; Cheney, L.; Vlahinos, A. Thermal Model of 

Human Physiology for Air-Conditioning Load Reduction in Vehicles. NREL/TP540-40718 . Golden, CO: NREL, to be published. 

22) Rugh, J.; Bharathan, D. “Predicting Human Thermal Comfort in Automobiles,” 

Proceedings of the 7th Vehicle Thermal Management Systems Conference. May 

10-12, 2005, Toronto, Canada. Paper # 2005-01-2008. Society of Automotive 

Engineers. 

23) Zhang, H.; Huizenga, C.; Arens E.; Wang, D. “Thermal Sensation and Comfort 

in Transient Non-Uniform Thermal Environments.” European Journal of 

Applied Physiology (92:6); pp. 728-733. 

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