California Institute for Energy and Environment (CIEE)

Air-conditioning in cars and small trucks lowers fuel economy by an estimated 22% and significantly increases tailpipe emissions. The design of vehicle air conditioners is based on the maximum cabin (soak) temperature attained when the vehicle is parked on a hot, sunny summer day. Cool colored paints reflect most of the sun’s energy in the near-infrared band (0.7 – 2.5 microns) while offering choice of color in the visible band (0.4 – 0.7 microns). Painting vehicle shells with these cool colors can reduce the soak temperature and thus increase fuel economy by decreasing the vehicle's ancillary load and permitting the use of smaller air conditioners. In this report we investigate cool colored paints (a.k.a. “cool coatings”) for cars. This was carried out by(1) establishing a 21-member collaborative research team representing 13 organizations including government, industry and other research institutions; (2) estimating fuel savings and emission reductions attainable with cool coatings; (3) developing an energy RD&D framework (roadmap) addressing energy efficiency measures that have potential for improving the air conditioning performance of cars; (4) initiating development of a database of cool colored coatings for cars with measurements of solar spectral reflectance and thermal emittance of over 180 car coatings. An experimental comparison of otherwise identical black and silver compact sedans indicated that increasing the solar reflectance () of the car's shell by about 0.5 lowered soak temperature by about 5-6°C. Thermal analysis predicts that the air conditioning capacity required to cool the cabin air in the silver car to 25°C within 30 minutes is 13% less than that required in the black car. Assuming that potential reductions in AC capacity and engine ancillary load scale linearly with increase in shell solar reflectance, ADVISOR simulations of the SC03 urban/highway driving cycle indicate that substituting a typical cool-colored shell (= 0.35) for a black shell (= 0.05) would reduce fuel consumption by 0.12 L per 100 km (1.1%), increasing fuel economy by 0.10 km L-1 [0.24 mpg] (1.1%). It would also decrease CO2 emissions by 2.7 g km-1 (1.1%), NOx emissions by 0.0054 g km-1 (0.44%), CO emissions by 0.017 g km-1 (0.43%), and HC emissions by 0.0041 g km-1 (0.37%). Selecting a typical white or silver shell (= 0.60) instead of a black shell would lower fuel consumption by 0.21 L per 100 km (1.9%), raising fuel economy by 0.19 km L-1 [0.44 mpg] (2.0%). It would also decrease CO2emissions by 4.9 g km-1 (1.9%), NOx emissions by 0.0099 g km-1 (0.80%), CO emissions by 0.31 g km-1 (0.79%), and HC emissions by 0.0074 g km-1 (0.67%).