ABSTRACT

The objective of the study was to assess the impact of a

Saflex1 S Series solar control PVB (polyvinyl butyral)

windshield on conventional vehicle fuel economy and electric

vehicle (EV) range. The approach included outdoor vehicle

thermal soak testing, RadTherm cooldown analysis, and

vehicle simulations. Thermal soak tests were conducted at the

National Renewable Energy Laboratory's Vehicle Testing and

Integration Facility in Golden, Colorado. The test results

quantified interior temperature reductions and were used to

generate initial conditions for the RadTherm cooldown

analysis. The RadTherm model determined the potential

reduction in air-conditioning (A/C) capacity, which was used

to calculate the A/C load for the vehicle simulations. The

vehicle simulation tool identified the potential reduction in

fuel consumption or improvement in EV range between a

baseline and solar control PVB configurations for the city and

highway drive cycles. The thermal analysis determined a

potential 4.0% reduction in A/C power for the solar control

PVB configuration. The reduction in A/C power improved

the vehicle range of EVs and fuel economy of conventional

vehicles and plug-in hybrid electric vehicles.

INTRODUCTION

Continued volatile oil prices have automakers increasingly

focused on improving fuel consumption. Additionally, global

environmental regulations such as U.S. Corporate Average

Fuel Economy (CAFE) standards and European Directives

provide a regulatory framework, which continue to challenge

the automotive industry to meet improved fuel economy

and/or meet carbon dioxide (CO2) emissions standards. As

powertrains become electrified, climate control loads reduce

range or cause the gasoline engine to operate more

frequently. If the thermal loads of the vehicle can be reduced,

and correspondingly air-conditioning (A/C) usage and

compressor power, the electric vehicle (EV) range with A/C

on can be increased. If the battery is sized to account for the

energy to operate the A/C system, reducing the thermal loads

may decrease the battery size and cost and reduce vehicle

weight.

Automotive windshields provide occupant visibility yet also

enable the sun's energy to enter and build up inside the

vehicle-more than 40% of heat transmitted into a vehicle

enters via the windshield. Laminated windshields consist of

two pieces of glass held together by a polyvinyl butyral

(PVB) interlayer forming a strong bond that tends to stay

together even after breakage occurs. For decades, PVB usage

has been mandated in windshields by National Highway

Traffic and Safety Administration (NHTSA) and the

international community for safety reasons, however,

advanced laminated glazings have increasingly been utilized

by original equipment manufacturers (OEMs) to improve fuel

efficiency and reduce CO2 remissions while enhancing

occupant comfort. In addition to providing traditional safety

benefits of automotive laminated glazing, solar control

interlayers absorb infrared (IR) wavelengths that were

previously transmitted through the vehicle's glazings, which

reduce air-conditioning usage and improves vehicle fuel

economy and CO2 emissions.

Impact of Solar Control PVB Glass on Vehicle

Interior Temperatures, Air-Conditioning Capacity,

Fuel Consumption, and Vehicle Range

2013-01-0553

Published

04/08/2013

John Rugh, Larry Chaney, Laurie Ramroth and Travis Venson

National Renewable Energy Laboratory

Matthew Rose

Eastman Chemical

doi:10.4271/2013-01-0553

1

Saflex is a registered trademark owned by Eastman Chemical Inc.

NREL/CP-5400-57489. Posted with permission.

Presented at the SAE 2013 World Congress & Exhibition
Thermal load reductions due to advanced glazings have been

previously researched by the National Renewable Energy

Laboratory (NREL) [1, 2, 3]. The work discussed in this

paper adds to the previous research by testing the impacts of

absorbing PVB. Also a new vehicle simulation software

(FASTSim) was used to determine the impact of reduced A/C

compressor power on three types of vehicles (conventional,

PHEV, and EV). The thermal analysis process was improved

by the addition of a

• highly detailed thermal model,

• thermodynamic model to convert the convert change in vent

temperature to a change in A/C capacity including the impact

of humidity, and

• improved compressor power input to the vehicle simulation

software.

A common question regarding solar control glazings is, “Will

thermal comfort be impacted in the winter?” The design case

for cabin heating is driving at night (no solar load); therefore,

if the heating system is designed properly, thermal comfort

will not be impacted by glazing optical properties.

In a conventional vehicle, waste heat from the engine heats

the cabin. Glazing does not impact winter fuel economy. In

an EV, battery energy heats the cabin and reduces vehicle

range. The following characteristics of the environment

combine to minimize the solar load in the winter and the

impact of glazing properties on range:

1. low sun angle and low solar intensity,

2. less daylight hours, and

3. fewer sunny days.

APPROACH

The objective of the testing and analysis program was to

determine the impact of a solar control PVB windshield on

interior temperatures, A/C power, and vehicle performance

(fuel use and range).

An outdoor thermal soak test was performed on two vehicles

to determine the reduction (or increase) in temperatures due

to the solar control PVB windshield. NREL then performed a

thermal analysis to determine the impact on A/C system

capacity. The test results were used set up the steady-state

thermal soak simulations and initialize the cooldown

simulations. The resulting reduction in A/C capacity was

used to reduce the A/C power in the vehicle performance

model which provided fuel use and range impacts. An

overview of the approach is shown in Figure 1.

Figure 1. Overview of test and analysis approach

VEHICLE TESTING

A vehicle test program consisted of two vehicles: a control

vehicle and a test vehicle. Thermal soak tests were conducted

on each vehicle to quantify vehicle interior temperature

distributions resulting from solar loading and outdoor

ambient conditions. During the thermal soak test procedure,

the engines were off to simulate parked car conditions. The

windows were closed to replicate how people commonly park

their vehicles due to security, dust, and rain concerns. The

vehicles were oriented south and located away from any

shading structures to maximize solar loading. If the vehicles

were oriented in a different direction, the measured impact of

modifying the windshield would have been less. Since a

vehicle can be parked/driven in any orientation with respect

to the sun and absorbing treatments can be incorporated on

any window, this test and analysis determined the impact of

improving the optical properties of the window most likely to

impact driver thermal comfort at a sun position most likely to

impact the thermal load. Weather conditions were monitored,

and “repeatable test days” were defined as days with average

solar irradiance more than 700 W/m2 and average wind

speeds less than 3 m/s.

The control vehicle was simultaneously soaked alongside the

test vehicle to account for variances in weather conditions

between test days. Each vehicle's response to ambient

conditions was characterized across four baseline days. The

inherent temperature differences between the two vehicles

were calculated. With this information, we were able to

calculate what the interior temperatures of the test vehicle

would have been if we had not modified the windshield. Prior

to baseline testing, the OEM windshields in both vehicles

were replaced with green tint windshields of known optical

properties (Tts=62%). Once the baseline testing was

completed, the Saflex S Series solar control PVB windshield

(Tts=55%) was installed in the test vehicle.
Test Setup

Experiments were conducted at NREL's Vehicle Testing and

Integration Facility (VTIF), shown in Figure 2, during

September 2011. The facility is located in Golden, Colorado,

at an elevation of 5,997 feet at latitude 39.7 N and longitude

105.1 W. Table 1 summarizes the average weather conditions

between 12:30 and 13:30 MDT for each configuration. Given

the potential variation in weather conditions, the solar

irradiance and ambient temperature were remarkably close

for the two configurations.

Table 1. Summary of weather conditions for the vehicle

test

The test and control vehicles were 2011 Toyota Corollas,

both the same color to minimize variance in response

between vehicles due to solar loading. White vehicles were

selected to improve the ability of measuring an impact due to

a small change in windshield optical properties. Since

exterior color has a second order effect on interior

temperatures [3], the authors do not feel the choice of vehicle

color excessively influenced the results. The vehicles shown

in Figure 2 were separated by a distance of 15 feet to

minimize cross-shadowing and reflective effects. The

interiors were light grey with charcoal trim. Each was

instrumented with 32-type-k thermocouples. Each

thermocouple was calibrated in a National Institute of

Standards and Technology-traceable calibration bath with a

maximum measurement uncertainty, U95, of ±0.36°C

(calculated using ASME Standard PTC 19.1-2005). Surface

thermocouples were attached using OMEGABOND 101

thermally conductive epoxy. Accuracy of air temperature

measurements was maximized by shielding each air

thermocouple with radiation shields.

Figure 2. Test setup

The data acquisition system recorded data every minute

throughout the day. The difference in temperature (control

vehicle versus test vehicle) was then computed for each

location. The time period from 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. The interior air temperature above ambient was also

fairly constant during this time. The average reduction in

temperature for a given day is the time average from

12:30-13:30.

Test Results

After a series of baseline tests were conducted to determine

the inherent differences between the two vehicles, a solar

control PVB windshield was installed in the test vehicle, and

temperatures were measured for four test days. Repeatable

temperature reductions were observed as reported in Figure 3.

The variability in results between the test days was primarily

due to changes in ambient weather conditions. As expected,

the enhanced absorptive properties the solar control PVB

windshield caused the windshield temperature to increase

(negative reduction) after installation. This led to an average

steering wheel temperature reduction of 5.3°C and dashboard

reduction of 3.5°C. The air temperatures were reduced by

1.2°C at breath level and 0.9°C at the foot level.

Figure 3. Solar control PVB temperature reductions

The average vehicle temperature reductions are summarized

in Table 2. Average air temperatures decreased by 1.0°C for

solar control PVB. Interior surfaces with incident solar load,

such as the dashboard and steering wheel, had larger

reductions. A vehicle equipped with this technology will

reduce the cabin soak temperature on a hot, sunny day.

Cooler cabin temperatures can improve occupant comfort,

reduce cooldown time, and lead to less demanding operation

of the A/C system.
Table 2. Summary of average temperature reductions

THERMAL ANALYSIS

The purpose of the thermal analysis was to quantify the

reduction in A/C power that resulted from lowering vehicle

interior soak temperatures by using solar control PVB. The

approach taken was similar to that used in a previous analysis

that was presented by NREL at the 2007 SAE World

Congress [3]. Thermal analysis was used to determine the

change in A/C vent temperature in a modified vehicle that

would achieve the same cooldown time as the baseline

(unmodified) vehicle. The reductions in A/C capacity and

compressor power due to the change in A/C vent temperature

were calculated. The reduction in A/C compressor power was

then input into a vehicle simulation that calculates the change

in fuel use or electric range due to the reduction in A/C

compressor power.

RadTherm Analysis Methodology

The thermal model included a numerical representation of a

passenger compartment. The thermal analysis tool used for

this analysis was RadTherm from ThermoAnalytics, Inc. In

this type of analysis, the heat transfer between the interior

and environment is calculated. Inputs to the model include

vehicle geometry, material properties, including glass

properties, and environmental (weather) data.

NREL used an existing model of a small sedan that was

similar to this project's test vehicle. The model was validated

using data from previous extensive thermal soak testing

conducted at NREL. The glass properties used in the model

were provided by Eastman Chemical and are shown for

reference in Table 3. The environmental conditions (solar

irradiance, sun position, wind) were obtained from the NREL

weather station on 9/10/11. The dry-bulb temperature in the

analysis was maintained at 35°C to simulate a constant hot

ambient condition. Heat transfer coefficients on the interior

surfaces were computed during previously conducted

computational fluid dynamics (CFD) simulations and then

mapped to the RadTherm model. In the RadTherm model, the

cabin air was represented by a simplified 12-node fluid model

and the interior represented by 237 mass nodes. Energy was

transferred from the interior surfaces to the air nodes by

convection. Energy was also conducted between the interior

surfaces and exterior surfaces by conduction. The direction of

the heat transfer was determined by the interior and exterior

surface temperatures. To allow for a proper transient analysis,

the model included thermal mass and temperature distribution

within each layer. Flow between the air nodes was calculated

by previous CFD simulations and input to the RadTherm

model.

Table 3. Glass properties

Steady-State Simulation

Two types of RadTherm simulations were performed: steadystate and transient cooldown. The purpose of the steady-state

simulation was to initialize the passenger compartment

temperatures for the cooldown simulation. The steady-state

temperatures were compared to soak test data and minor

adjustments were made to model parameters to improve

correlation. Steady-state simulations were run for the baseline

and the solar control PVB configuration. Starting a drive

from a full thermal soak condition is a fairly common

situation.

Cooldown Simulation

Cooldown simulations were performed to determine the

allowable increase in vent temperature due to the solar

control PVB's reduction of thermal load. The primary

differences between the steady-state and cooldown

simulations are dashboard vent register flow, vehicle speed,

and time-based changes in incident solar. The cooldown

simulation consists of a 30-minute duration with the sun

position and intensity starting at 11:45 MST on 9/10/11.
The A/C vent mass-flow rate was set to 0.136 kg/s,

representing a high blower setting, and remained constant

throughout the 30-minute cooldown. The internal heat

transfer coefficients for the cooldown case were calculated by

a separate CFD simulation and were then mapped to the

RadTherm model. The heat transfer coefficients remained

constant during cooldown. The baseline case used the A/C

vent temperature profile shown in Figure 4.

The environmental parameters varied across the 30-minute

cooldown (e.g., solar direction, wind speed, etc.); however,

the ambient temperature was held constant at 35°C. In

addition, the simulation assumed that the vehicle was

travelling at a speed of 48 mph (77 kph) with a heading of

due south. This is the average speed of the U.S.

Environmental Protection Agency Highway Fuel Economy

drive cycle. These conditions were selected to provide a

representative hot case and exterior velocity. This is the type

of weather that drivers would use the A/C. If the ambient

temperature or solar were lower, the A/C might not be used.

The weather conditions selected are consistent with the vent

temperature profile in Figure 4 and A/C power profile in

Figure 9.

Figure 4. Baseline A/C vent temperature profile

The approach used in this analysis was to achieve an equal

performance metric for baseline and solar control PVB

configurations after 30 minutes of cooldown. Three metrics

were considered to quantify the impact of reducing the

vehicle interior temperatures and thermal load:

Volume average air temperature. A challenge of using the

volume average air temperature as the performance metric is

that the air temperature changes rapidly due to the inflow of

cool air from the A/C system and the low thermal mass of the

interior air. Plus when occupants enter a vehicle, some air

exchange occurs with the ambient. After 30 minutes of

cooldown, the impact on the air temperature is very small in

the analysis. This metric will underestimate the impact of

solar load reduction technologies since the interior mass of

the vehicle impacts thermal comfort and A/C use. In addition,

the simplified 12-node air model tends to smooth out the

temperatures and would not be as accurate as a full CFD

analysis.

Average interior mass temperature. The mass average

temperature (Tmass) is calculated by multiplying temperature

(Ti) by its mass (mi). The sum of the mass temperatures

(Timi) is then divided by the sum of the masses.

(1)

To achieve equal average interior mass temperatures for the

solar control PVB configuration, the A/C vent temperature

was increased by a uniform amount at each time point. The

amount that the vent temperature was raised was iterated

upon until the mass average temperatures at 30 minutes were

equal to the baseline case. The interior mass average

temperature fully accounts for the reduced interior

temperatures, but it tends to overestimate the effect of the

reduction that the occupant would feel.

Occupant thermal comfort. The ideal solution would be to

perform a thermal comfort analysis that would include both

the effects of the cool A/C air as well as the effects of hot

interior surfaces on the occupant. However, this more

involved analysis was beyond the scope of this project.

To reduce errors, volume average air temperature and

average interior mass temperature performance metrics were

combined to estimate the reduction in A/C capacity and

compressor power.

Increasing the vent temperature was the selected method of

decreasing A/C system capacity. It was assumed that an

automatic temperature control system would adjust the vent

temperature higher due to the lower initial temperature and

lower solar gain during the drive. Another way is to reduce

the air flow rate (blower setting). In a real-world case, some

combination of the two methods would be used, with the

driver reducing the blower setting or increasing the

temperature set point. For this analysis, increasing the vent

temperatures was much easier to achieve because reducing

the air flow would have required re-running the CFD case to

obtain updated interior heat transfer coefficients.

RadTherm Steady-State Results

The baseline glass configuration analysis temperatures in

Figure 5 compared favorably to the test data. The most

important locations (air, dashboard, windshield) matched

well.
Figure 5. Baseline comparison of analysis temperature

results to test data

The solar control PVB windshield model was run, and the

change in temperature between solar absorbing PVB and

baseline configurations was calculated. These temperature

reductions were then compared to the changes in temperature

measured during the test. Figure 6 shows the results for the

solar control PVB windshield.

Figure 6. Reduction in temperature for the solar control

PVB windshield

As expected, the windshield was warmer than baseline

(negative temperature reduction) because the solar control

PVB had higher absorptivity. All of the other results are

within 0.5°C of test data except for the front right seat.

The significance of these results is:

1. The steady-state simulation yielded accurate temperature

and resulted in initial temperatures for the cooldown model.

2. Changing the optical properties in the model resulted in

changes in temperatures that were very close to the test

results.

RadTherm Cooldown Results

Figure 7 shows the average interior mass temperature for the

two configurations. As can be seen in the figure, the initial

average interior mass temperature is 1.1°C lower for the

configuration with solar control PVB. To achieve equal mass

average interior mass temperature at 30 minutes, the A/C vent

temperature was increased by 2.3°C for the configuration

with the solar control PVB windshield.

As discussed in the Cooldown Simulation Section, using the

average interior mass temperature alone will overestimate the

impact of solar control PVB. Interior air temperature most

certainly impact occupant thermal comfort and the control of

the A/C. Therefore the vent temperature increase for the

volume average air temperature method (essentially 0°C) was

averaged with the 2.3°C vent temperature increase from the

average interior mass method to yield a 1.2°C increase in

vent air temperature.

A 30-minute cooldown was originally selected because the

average interior mass temperature was beginning to stabilize

at the end of the time period. To be consistent with the

duration of the compressor power profile and duration of the

vehicle simulations, a 20-minute duration should have been

used. This would have resulted in a larger increase in vent

temperature; therefore, the authors chose to use the more

conservative 30-minute duration.

Figure 7. Average interior mass temperature at 35°C

ambient temperature

A/C System Capacity Analysis

A thermal model was developed in Engineering Equation

Solver (EES) to quantify the impact of higher vent

temperatures on the A/C evaporator capacity. The analysis

accounts for latent and sensible cooling loads integrated

across the cooldown profile (Figure 4). Equation 2 is an

energy balance calculation of the air side of the evaporator
where is evaporator load, ṁa is the mass-flow rate of dry

air, h is the enthalpy, and ω is humidity ratio. A subscript of a

is used for air, v for vapor, l for liquid, 1 is the condition at

the inlet, and 2 is at the exit.

(2)

Next, the percent reduction in evaporator capacity (θ) was

determined for each configuration as shown in Equation 3.

(3)

The mass flow rate of the mixture into the evaporator was

assumed to be 0.136 kg/s, equivalent to the flow used during

RadTherm analysis. This is representative to A/C blower

speed of approximately 250 cfm. A 100% outside air blower

setting was also assumed for modeling purposes. The exit air

temperature profile for the baseline case, shown in Figure 4,

was increased 1.2°C at each time point for the solar control

PVB. The inlet air was assumed to be 35°C to be consistent

with the RadTherm analysis. The atmospheric pressure was

assumed to be at sea level, or 101.4 kPa, and dry air was

modeled as an ideal gas. Because the reduction in capacity is

fairly small, it is reasonable to assume the A/C system would

operate similarly in the baseline and solar control PVB case.

Therefore, it was assumed that the coefficient of performance

(COP) in all cases was the same and that the percent

reductions in A/C capacities and compressor powers were

equivalent.

Reduction in A/C capacity for the solar control PVB

configuration is shown in Figure 8. The results show that the

impact of the advanced solar control technologies on A/C

capacity decreases as relative humidity increases. In high

humidity environments, a larger portion of the evaporator

capacity goes to condensing moisture.

Figure 8. A/C model results - % reduction in A/C power

VEHICLE SIMULATION

A/C Power vs. Time Profile for the

Vehicle Simulation

A literature survey was conducted in reference [4] to develop

a representative A/C load profile for a vehicle simulation.

The climate control load was divided into two parts:

1. Transient - After a thermal soak, the climate control load

was characterized by a high initial value that decreased with

time. An example is entering a hot vehicle after it has been

parked in the sun, driving, and having the A/C on with

maximum blower airflow to cool the interior. Vehicles have

different transient times due to a variety of factors based on

manufacturer design choices. We selected 10 minutes as a

representative transient duration.

2. Steady State - During steady state, the impact of the

thermal soak was diminished. The climate control system

maintained the thermal conditions in the passenger

compartment. An example is driving down the interstate and

having moderate temperature and blower settings.

Table 4 shows the range of vehicle types, environments, and

A/C systems from a variety of sources that we considered.

Table 4. Cooling load data sources

The data from these sources were averaged to create an A/C

compressor power profile. The 10-minute transient load was

applied to the model as a linear decay from a peak power of

3.89 kW at the start of the drive to a 2.10-kW, steady-state

load. This equates to an average transient load of 2.99 kW for

the 10-minute period. Additionally, an electric condenser fan

was assumed to draw 150 W during the 10-minute transient

and 50 W during the steady-state period. A 150-W heating,

ventilating, and A/C (HVAC) blower load was assumed

constant for the vehicle simulation.

The composite A/C power profile, including the compressor,

condenser fan, and HVAC blower, is shown in Figure 9. The

intention of this profile is to define a reasonable hot

environment compressor power for the FASTSim model.

This profile was not a result of the RadTherm thermal model.

The RadTherm model and A/C system capacity analysis

results were used to define a potential increase in vent
temperature and respective decrease in A/C power due to the

thermal load reduction.

Figure 9. A/C power curve for the baseline vehicle

simulation

FASTSim Analysis

NREL used validated conventional vehicle, PHEV, and EV

models to determine the effect of A/C loads on fuel economy

and electric range. The impact is measured on both the city

and highway drive cycles. For each powertrain type, we

applied the baseline A/C power vs. time curve in Figure 9.

For vehicle simulations that extended beyond 20 minutes, the

steady-state A/C power was applied to the end of the

simulation. We then parametrically reduced the A/C

compressor power while keeping the condenser fan and

HVAC blower loads the same. For comparison purposes, we

have included a “no A/C” case where we assume only the

blower was operated.

The Future Automotive System Technology Simulator

(FASTSim) is an Excel, power-based model that uses

vehicle- and component-specific parameters to predict

performance. It back-calculates the power demanded from the

engine and/or battery to overcome drag, acceleration, ascent,

rolling resistance, and inertia for a specified drive cycle.

Battery, electric motor, and engine component models match

well with detailed component models included in vehicle

simulation software such as PSAT and ADVISOR. The

model includes energy management strategies and accounts

for regenerative braking as well as auxiliary loads.

Table 5 lists the essential parameters of the Toyota Camry,

Chevrolet Volt, and Nissan Leaf. These vehicles served as the

conventional vehicle, PHEV, and EV, respectively, used in

this analysis

Table 5. Vehicle- and component-specific parameters

Each FASTSim vehicle model was validated within 10% of

published performance metrics. Table 6 compares simulated

results to published/reported performance metrics. The

simulated energy use of the PHEV was validated in both

charge-depleting (CD) and charge-sustaining (CS) modes.

CD is the mode of vehicle operation associated with the

operation of an EV or the start up of a PHEV when fully

charged. During this mode, power is primarily sourced from

the battery, with the engine providing supplemental power

when necessary. When the battery of the PHEV depletes to

its minimum state of charge, it enters CS mode, where

regenerative braking and the power plant work to maintain

that state of charge as necessary.

Table 6. Validation of Toyota Camry, Chevrolet Volt, and

Nissan Leaf in FASTSim

Figure 10, Figure 11, and Figure 12 present the parametric

results for the conventional vehicle, PHEV, and EV. These

curves can be used to assess the impact of any technology

that reduces the thermal load and compressor power.

For the conventional vehicle, a reduction in A/C load resulted

in an increase in fuel economy. The blue curve represents

fuel economy results on the unadjusted city (UDDS) cycle

while the red curve represents fuel economy results on two
unadjusted highway (hwy) cycles. The duration of the UDDS

cycle was 22.9 minutes while the duration of the two

highway cycles was 25.5 minutes. These durations are close

to the average commute travel time of 22.85 minutes for the

United States [10]. An implication of the cycle durations is

that the analysis approximately equally weights the impact of

the transient and steady-state compressor powers.

Figure 10. Impact of A/C load reduction on fuel

economy - conventional vehicle

To simulate real world PHEV operation, we combined and

adjusted city/highway results. Similar to the conventional

vehicle, fuel economy increased with lower A/C loads. For

the EV, a reduction in A/C load resulted in increased range.

For both the PHEV and EV, multiple back-to-back cycles

were performed for charge balancing for the PHEV and

determining total range for the EV. This results in a heavier

weighting of the steady-state A/C load.

Figure 11. Impact of A/C load reduction on fuel

economy - PHEV

Figure 12. Impact of A/C load reduction on electric

range - EV

Combining the Solar Control PVB

Results with the FASTSim Results

In the RadTherm analysis that determined the increase in vent

temperature due to the solar control PVB configuration, the

ambient air temperature was 35°C. This parameter strongly

influenced the heat loss to the environment and the resulting

required A/C vent temperatures. A relative humidity of 60%

was also assumed to provide a hot/humid environment

representative of a reasonably severe operating condition.

Using this humidity in Figure 8, the solar control PVB

windshield resulted in a 4.0% reduction in A/C power. These

reductions in A/C power were then applied to the data in

Figure 10, Figure 11, Figure 12 to determine the impact on

vehicle performance.

If a solar control PVB windshield were installed in an EV, the

range could potentially increase 1.0 mile (1.5 km) on the city

drive cycle and 0.6 mile (1.0 km) on the highway cycle

during operation of the A/C system. This was calculated by

determining the range at 4.0% A/C reduction and subtracting

the range at 0% A/C reduction (baseline A/C load) in Figure

12.

For the conventional vehicle (Figure 10), the solar control

PVB windshield increased the city and highway fuel

economy by 0.3 mpg during the operation of the A/C system.

The PHEV combined adjusted fuel economy (Figure 11)

increased 0.8 mpg due to the solar control PVB windshield.

Summaries of the results are shown in Table 7, Table 8, and

Table 9.

Table 7. Impact of solar absorbing PVB - summary of

thermal results
Table 8. Impact of solar absorbing PVB - summary of

vehicle results (metric units)

Table 9. Impact of solar absorbing PVB - summary of

vehicle results (English units)

The percent increase in EV range and fuel economy are

shown in Table 10.

Table 10. Impact of solar absorbing PVB - percent

increase in EV range and fuel economy

Any technology that reduces air and surface temperatures

during a thermal soak or cooldown in a similar manner would

have similar A/C power reduction and vehicle performance

results.

SUMMARY/CONCLUSIONS

The objective of the project was to assess the impact of a

solar control PVB windshield on vehicle fuel economy and

EV range. The approach included outdoor vehicle thermal

soak testing, RadTherm cooldown analysis, and vehicle

simulations. Outdoor thermal soak tests were conducted at

NREL. Measureable reductions in interior temperatures were

observed due to the installation of a solar control PVB

windshield. The average interior air temperature dropped 1°C

while the dashboard surface temperature dropped 3.5°C.

Thermal analyses and vehicle simulations were performed to

determine the impact of the reduction in temperatures on A/C

capacity, A/C power, vehicle fuel use, and electric range. Due

to the incorporation of the solar control PVB windshield, the

A/C system capacity could be reduced 4% which improved

the conventional vehicle fuel economy by 0.7% to 1.1% and

EV range by 0.7% to 1.5% depending on the drive cycle.

Critical assumptions included ambient temperature, ambient

humidity, A/C control strategy, drive cycle, and vehicle type.

Although real-world fuel use and range will be different,

these analyses provide an estimate of the magnitude of the

impact of solar load reduction glass. The improvement in

occupant thermal comfort from lower interior temperatures

following a thermal soak would be realized in the real world

but was not quantified in this project. Advanced solar control

technologies that reduce the thermal loads during soak and

cooldown are part of the system solution to reduce the impact

of climate control on vehicle fuel economy and range.

REFERENCES

1. Farrington, R.; and Rugh, J. “Impact of Vehicle Air

Conditioning on Fuel Economy, Tailpipe Emissions, and

Electric Vehicle Range,” NREL paper # CP-540-28960,

Earth Technologies Forum, Washington, DC., October 2000.

2. Rugh, J.; Hendricks, T.; and Koram, K. “Effect of Solar

Reflective Glazing on Ford Explorer Climate Control, Fuel

Economy, and Emissions,” Proceedings of the International

Body Engineering Conference, Paper # 2001-01-3077,

Society of Automotive Engineers, Detroit, MI.

3. Rugh, J., Chaney, L., Lustbader, J., and Meyer, J.,

“Reduction in Vehicle Temperatures and Fuel Use from

Cabin Ventilation, Solar Reflective Paint and a new Solar

Reflective Glazing,” SAE Technical Paper 2007-01-1194,

2007, doi:104271/2007-01-1194.

4. Barnitt, R.; Brooker, A.; Ramroth, L.; Rugh, J.; Smith, K.

“Analysis of Off-Board Powered Thermal Preconditioning in

Electric Drive Vehicles.” 25th World Battery, Hybrid and

Fuel Cell Electric Vehicle Symposium & Exhibition,

Shenzen, China, Nov. 2010.

5. Society of Automotive Engineers. Cooperative Research

Program: Alternative Refrigerant Project Phase I. July 14,

2003. (Draft final report - unpublished).

6. Lohse-Busch, H.; “Small EV Testing and Analysis.”

Presentation to the U.S. Department of Energy. Argonne

National Laboratory November 17, 2009.

7. Rugh, J. “Impact of Sungate EP on PHEV Performance:

Results of a Simulated Solar Reflective Glass PHEV

Dynamometer Test.” NREL Report No. TP-540-45908.

Golden, CO: National Renewable Energy Laboratory, 2009;

32 pp.

8. Maranville, C. Personal email. Ford Fusion cooling load.

Ford Motor Company, June 4, 2010.

9. Meyer, J. A/C Cooling Load data. Visteon Corporation,

March 1, 2007.

10. Santos A.; McGuckin, N.; Nakamoto, H.Y.; Gray, D.;

and Liss, S. “Summary of Travel Trends: 2009 National

Household Travel Survey,” Report # FHWA-PL-ll-022, U.S.

Department of Transportation, June 2011.
CONTACT INFORMATION

John P. Rugh

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Co 80401

john.rugh@nrel.gov

Matt Rose

Eastman Chemical

575 Maryville Centre Drive

St. Louis, MO 63141

(314) 674-1248

mmrose@eastman.com

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support for NREL's

vehicle climate control load reduction work provided by Lee

Slezak, David Anderson, and Patrick Davis in the Vehicle

Technologies Program of the U.S. Department of Energy's

Office of Energy Efficiency and Renewable Energy.

The test program and application of existing NREL thermal

and vehicle models was funded by Eastman Chemical Inc.

who also provided the test materials and window property

data.

The Engineering Meetings Board has approved this paper for publication. It has

successfully completed SAE's peer review process under the supervision of the session

organizer. This process requires a minimum of three (3) reviews by industry experts.

ISSN 0148-7191

Positions and opinions advanced in this paper are those of the author(s) and not

necessarily those of SAE. The author is solely responsible for the content of the paper.

SAE Customer Service:

Tel: 877-606-7323 (inside USA and Canada)

Tel: 724-776-4970 (outside USA)

Fax: 724-776-0790

Email: CustomerService@sae.org

SAE Web Address: http://www.sae.org

Printed in USA