Increasing fuel efficiency and reducing CO2 emissions are not entirely dependent on the long-term development of alternative technologies. Automotive decision makers may be surprised to discover that existing technologies can increase fuel efficiency and reduce emissions today.
Infrared (IR) reflective film laminated into automotive glass is an ancillary load-reduction technology being used in more than 20 million vehicles. Of the 7 billion gal (26 billion L) of gasoline used each year to power air-conditioning, IR reflective glass, by reflecting the sun’s heat and maintaining a cooler cabin temperature, reduces A/C power consumption up to 20% and increases overall mpg by up to 5% while lowering emissions.
According to the U.S. Department of Energy (DOE), some 230 million vehicles in the U.S. consume those 7 billion gal of fuel annually to power A/C. This is equivalent to 5.5% of the total domestic light-duty vehicle petroleum consumption. Furthermore, A/C use can increase fuel consumption of high-efficiency vehicles by as much as 35%.
Though IR reflective glass has been offered since 1993 as an OEM option by Maserati, Mercedes-Benz, Volvo, Volkswagen, and others, it has been marketed to improve passenger comfort, not increase vehicle fuel efficiency. The focus is changing.
In a parked vehicle, the solar energy transmitted through windows represents 50% to 75% of the thermal energy entering the passenger compartment which is absorbed by the car’s interior mass. IR reflective glass used in DOE-tested vehicles reduced the cabin and seat temperatures by 34% and 35%, respectively, reducing the A/C usage required to reach a comfortable cabin temperature. In a moving vehicle, IR reflective glass also maintained a lower cabin temperature, similarly reducing the need for A/C.
A/C load reduction can decrease fuel consumption because the systems are designed to maximize capacity, not efficiency. Testing a Cadillac STS, DOE determined that by using IR reflective glass, solar-reflective paint, and solar-powered ventilation, the vehicle’s cooling capacity of 5.7 kW could be reduced by 30% to 4.0 kW while maintaining the same cool-down performance of 30 min. Simulation showed that reducing the A/C load by 30% also decreased A/C fuel consumption by 26%.
IR reflective glass and smaller A/C systems can benefit not only passenger vehicles but also large trucks and buses.
There are two approaches for making automotive glass that is IR reflective. The first approach requires that the glass manufacturer bond an IR reflective coating directly onto the glass itself. This technology, called sputter coating, involves processing glass through a vacuum deposition chamber, which creates high-density plasma of various heat-reflective metals. Atoms are deposited on the glass in thin transparent layers only 5 to 30 nm (0.2 to 1.2 µin) thick, or about 1/10,000 of the diameter of a human hair. Due to high capital and operational costs, automotive glass direct-coating capacity in the U.S. is limited, and differences in the performance, reliability, and price of IR reflective glass are significant.
As an alternative to conventional direct-coat IR reflective glass, Southwall Technologies’ sputters an IR reflective coating directly to clear polyethylene terephthalate (PET) film, eliminating the need to coat glass. This XIR coated film, which consists of transparent layers of indium, tin oxides, and noble metals, is available off-the-shelf to automotive glass manufacturers, who laminate it with uncoated glass to create IR reflective glass for OEM and aftermarket products.
Because IR reflective film is laminated into uncoated glass to meet a range of heat-rejection and visibility transmission standards, any automotive glass manufacturer can produce IR reflective glass without the need for expensive glass coating equipment. Coated film makes IR reflective glass cost-effective for low- to mid-volume model runs and for aftermarket replacement glass that otherwise could never justify the fixed costs associated with glass coating production.
Michael Johnson, Manager of Automotive Sales–Americas for Palo Alto, CA-based Southwall Technologies Inc., wrote this article for SAE Magazines.