Glass Coating Could Decrease Air-Conditioning Dependence

Anyone who has been frustrated with a bowl of hot soup cooling too quickly has had to battle passive cooling, the phenomenon by which heat seeps out into the ambient air. But researchers are using the same principle to accelerate the rate of cooling in buildings, which might help battle extreme heat as the planet warms.  

A team of scientists led by Professor Lianbing Hu at the University of Maryland has developed a microporous glass coating that, when applied to a building’s surface, can decrease dependence on air-conditioning. The coating can lower the temperature of the material underneath it by 3.5 °C (38 °F) at the sun’s noon peak.  

Reflecting waves

 

Xinpeng Zhao, lead researcher on the project, and assistant research scientist, said that in addition to carbon sequestration or reduction, reflecting heat back to the universe is effective at addressing global warming. One of the ways in which passive cooling works is to use the sky as a heat sink, as the earth’s atmosphere allows heat—certain wavelengths of infrared radiation (8-13 µm), including long-wave infrared radiation (LWIR)—to pass on through and escape into space.  

Zhao and his team found a thickness of around 500 micrometers was optimal for the glass-based coating, which reduces temperatures by reflecting up to 99 percent of solar radiation. This way the building does not absorb most of the heat in the first place. Second, it reflects heat in the form of LWIR from the buildings into space.  

The coating is a mix of glass and aluminum oxide in equal proportion, each of which brings its own strengths to the final product. Glass has several advantages, Zhao points out: It melts at high temperatures, making it an efficient binder. It also has a high emissivity in the atmospheric transparency window, areas with limited or no absorption by the atmosphere. This means glass can emit heat from buildings effectively. The challenges with glass however, are its transparency and low solar reflectance, which means it can’t reflect enough solar heat back into the sky.  

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To overcome these challenges, the researchers included aluminum oxide, which provides high solar reflectance and prevents the final result from becoming transparent. “We needed to optimize their ratios to make sure the final product has high solar reflectance, high infrared emittance, and whose strength is very good,” Zhao said. The team experimented with different sizes for the materials as well: The glass particles are 10 micrometers in size, while the aluminum oxides are in the nanometer range.  

Although coatings based on the same principles have been explored, they have previously been made with polymers that add to the plastics problem and are not very long-lasting. A glass-based composition is more environmentally stable for a longer period. Particulate matter in the environment can affect solar reflectance and degrade coatings as well. To decrease such incidences, the coating has a very thin layer of glass on top to make the surface less vulnerable to contamination. The surface is also easier to clean.  

Future applications 

 

Zhao said the team is exploring manufacturing and commercialization of the coatings. One method would be to get it pre-applied to building materials or to make them easy to apply using a spray gun. These coatings could be used on a variety of surfaces including metals, ceramics, bricks, and concrete—but not on plastic. The high temperature resistance of the coatings also enables them to be used in fire-resistant products. 

A rapidly warming world means increasing needs for air-conditioning. The “use of air conditioners and electric fans already accounts for about a fifth of the total electricity in buildings around the world—or 10 percent of all global electricity consumption,” according to the International Energy Agency. Using coatings like the one from the UMD team will make cooling more equitable around the world.  

“People in many areas of the developing world find it hard to have a nice living environment in the summer, there’s a lot of inequity so we’re hoping to develop a way to provide free—or at least low-cost—cooling,” Zhao said. 

Poornima Apte is a technology writer in Walpole, Mass.