Nearly all cars sold in California have air conditioners. Cars painted with reflective coatings stay cooler in the sun and are easier to air condition to a comfortable temperature, according to a recent study by researchers in Berkeley Lab's Environmental Energy Technologies Division.

"Solar reflective paints can decrease the 'soak' temperature of the air in a car that has been parked in the sun. This could improve the vehicle's fuel economy by letting the manufacturer install a smaller air conditioner that draws less power from the engine," says Ronnen Levinson, scientist in the Heat Island Group, and lead author of the study. The research was published in Applied Energy.

White, silver, and other light colors are coolest, reflecting about 60% of sunlight. However, dark "cool colors" that reflect primarily in the invisible "near infrared" part of the solar spectrum can also stay cooler than traditional dark colors.

Soaking in the sun
In the experimental portion of the study, researchers parked two nearly identical Honda Civic four-door sedans, loaned by the California Department of General Services, in a lot in Sacramento, California. One was silver, one black. The shells (opaque elements) of the black and silver cars had solar reflectances of 0.05 and 0.58, respectively (on a scale of 0 to 1). Higher solar reflectance keeps a surface cooler in the sun.

During the course of a sunny summer day, the cars were run through five identical cycles of soaking in the sun. Each cycle consisted of an hour with the air conditioners off, followed by a half hour of cooling with the air conditioners running at maximum. The researchers continuously measured the roof, ceiling, dashboard, windshield, seat, door, vent air and cabin air temperatures in each car, as well as the external weather conditions in the lot.

At the peak of the soak phases of testing, the roof on the silver car (high reflectance) was as much as 25 degrees C (45 degrees F) cooler than the roof on the black car (low reflectance). The study found that increasing the solar reflectance of the car's shell by about 0.5 lowered the "soak" temperature of cabin air by about 5 to 6 degrees C (9 to 11 degrees F).

Downsizing the air conditioner
The researchers developed a thermal model that predicted the air conditioning capacity (rate of heat removal) required to cool each vehicle to a comfortable final temperature of 25 degrees C (77 degrees F) within 30 minutes.

(This is an industry standard for vehicle air conditioner performance.) Based on the experimental measurements, the analysis predicted that the capacity required to cool the cabin air in the silver car is 13% less than that required in the black car.

Using a vehicle simulation tool called ADVISOR, the research team modeled the effect of air conditioner capacity on fuel consumption and pollutant emissions in each of several standard driving cycles.

These include an urban cycle, a highway cycle, and a transient driving cycle with numerous increases and decreases in speed. They combined these simulations with the output of the thermal model to estimate the potential energy savings and emission reductions from using solar-reflective car paint to downsize the air conditioner.

Their results suggest that replacing a traditional black paint, which has a solar reflectance of 0.05, with a typical cool-colored paint that has a solar reflectance of 0.35 would increase fuel economy by 0.24 mpg (1.1 percent). This benefit would result from using a smaller air conditioner that draws less power from the car's engine.

The change would also decrease carbon dioxide emissions by 1.1 percent, and reduce other automotive emissions, including nitrogen oxides, carbon monoxide and hydrocarbons, by about 0.5%.

Using a white or silver paint (solar reflectance = 0.60) instead of a black paint would raise fuel economy by 0.44 mpg (2.0 percent). It would also decrease carbon dioxide emissions by 1.9%, and reduce other automotive emissions by about 1%.

A roughly one- to two-percent improvement in fuel economy, when scaled to the fleet of light-duty vehicles in the U.S., represents an eventual potential savings of billions of gallons of gasoline, if these design changes are adopted by the automotive industry.

"Potential benefits of solar reflective car shells: Cooler cabins, fuel savings and emission reductions." Ronnen Levinson, Heng Pan, George Ban-Weiss, Pablo Rosado, Riccardo Paolini, and Hashem Akbari, was recently published in the journal Applied Energy (volume 88, pp. 4343-4357).