Toyota’s Lime Green Paint May Be Ugly, But It Can Cut Gas Bills
In the escalating war to squeeze every last mile from every last drop of gas, automakers are attacking on every front, going well beyond the cratered fields of aerodynamics and optimized engine controls. Among the many frontiers of this fight, one may surprise you: temperature control.
Running the air conditioner in very hot weather can cut fuel economy by five to 25 percent, according to the Department of Energy. For electric cars, cranking up the cool can be equally debilitating, draining the electricity you need to actually go places. In either case, the cooler you can make the car without pulling from the battery, the better.
One of the most promising weapons in this battle for temperature control goes to work before you even turn the key: Solar reflective paint aims to minimize the amount of heat a car’s exterior paint absorbs, keeping the cabin cooler and reducing the need for the air conditioner.
Which brings us to this 2017 Toyota Prius that customers in Japan can get in a seriously questionable lime green paint job. “Thermo-Tect Lime Green” (hey, there are worse names) represents the auto industry’s first production use of solar reflective pain. The paint, for now available only in Japan as a $350 option, is packed with tiny reflective titanium oxide particles and doesn’t contain carbon black, a common ingredient in paint that tends to absorb lots of heat.
Of course, you can achieve the same effect with regular old white paint, which reflects about 70 percent of the sun’s rays. Even silver paints are between 50 and 55 percent reflective, thanks to the mica flakes that make them sparkle. Toyota’s innovation is getting that same reflective quality from other colors. The automaker’s designers can now repel infrared heat while offering the exact hue of green they think customers want—as questionable as that desire might be.
“We expect heat increase control of around 5 degrees Celsius [9 degrees Fahrenheit] when comparing vehicle body surface temperature with and without thermal barrier function under the scorching sun in summer,” says Toyota spokesman Takashi Ogawa. Toyota won’t say how much the paint job might save you on gas, but even if it takes a while to break even on the extra $350, you get the advantage of having a car that’s cooler from the moment you step inside.
Independent research indicates this is more than a marketing stunt. A 2011 study in the journal Applied Energy found that a silver car with a solar reflective coating could reduce a car’s “thermal load” by up to 11 degrees Fahrenheit, compared to an otherwise identical black car. The silver car required 13 percent less air conditioning to cool the cabin to a baseline 77 degrees. Another study estimated that putting this kind of paint on every passenger car in Japan could cut CO2 emissions by 210,000 tons a year (the country produces about 1.4 billion tons of emissions annually).
Pair the paint with related tech like infrared-reflecting windows, and the effects are amplified. When the DOE tested a Cadillac STS with infrared-reflective glass (offered by automakers including Mercedes, Volkswagen, and Volvo) and solar reflective paint, it found the car’s cooling demands dropped by 30 percent (from 5.7 to 4.0 kW).
The idea of solar reflective paint has been around for a while. In the 1950s, the US Navy started painting its ships “battleship grey” to blend in with the ocean. But the color heated the vessels too much, so the Navy adopted solar reflective paint to reduce the vessels’ thermal signature. NASA used reflective paint on the parts of the space shuttle subject to the highest heat load during re-entry. It’s so popular in building applications, the FTC had to shutdown a cottage industry of manufacturers peddling fake insulating paints.
So why aren’t more automakers using the stuff? Toyota is evasive when asked why its new paint is only available in Japan (and only just now), but says one problem with the stuff is balancing its thermal properties with durability. Making the titanium oxide particles bigger enhances heat resistance, Ogawa says, but also makes it harder for the paint to adhere onto the base coat.
It doesn’t help that automakers are very, very touchy about their paint, both in hue and quality. They want to offer a broad palette of colors, and consumers will be damned if they accept a paint job that chips or fades. But in the 21st century, neither should be a problem.
“Automakers like to use carbon black for color adjustment, but if you’re trying to make a cool surface, a little carbon black goes a long way to affecting its solar reflectance,” says Dr. Ronnen Levinson. A staff scientist with the Heat Island Group, Levinson studies the effect of heat control on vehicles, and helped develop a pigment database for the Berkeley Lab that gauges the solar reflectance of hundreds of colors. It turns out even a tiny amount of carbon black can dramatically degrade the reflectance of a red paint by 15 to 40 percent, for instance.
The mere fact that Toyota has been able to develop a titanium oxide-infused paint without significantly affecting its durability means other automakers should be able to as well. Even if consumers aren’t clamoring for the stuff, there’s reason to try it out: the California Air Resources Board offers carbon credits to automakers who use solar reflective paints and other methods of heat control.
So if Toyota has struck a satisfactory balance of cost and quality to offer customers in Japan, maybe it and its competitors will find a way to bring this feature to American shores. Let’s just hope it’s not lime green.
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