Window coating takes a load off energy-intensive air conditioning

As average temperatures rise around the world, people are expected to start cranking up air-conditioners and buying new ones. That will add greenhouse gases to the atmosphere. But a new transparent window coating could lessen the need for air-conditioning, reducing these emissions and saving millions of dollars in cooling costs.

The coating, reported in the journal Joule, reflects up to 70 percent of the sun’s heat, keeping buildings cooler while letting in light, and cutting the load on air conditioners. The film stays see-through below 32 degrees Celsius, or 89 degrees Fahrenheit. Above this temperature, it looks like frosted glass and reflects heat. If every outside-facing window in a building were covered in this film, the building’s air conditioning costs could drop by 10 percent, its developers estimate.

Today’s air-conditioners are energy-hogs and they rely on chemicals that are potent greenhouse gases or deplete ozone. Air conditioners use about 6 percent of all the electricity produced in the United States, emitting 100 million tons of carbon dioxide a year. The number of air-conditioner units worldwide is expected to go up from 1.6 billion today to 5.6 billion by 2050, according to the International Energy Agency, and greenhouse gas emissions from powering these air conditioners will double.

Smart windows on the market today can help. But they are either “not very efficient in rejecting heat from the sun, or, like some electrochromic windows, they may need more power to drive them, so you would be paying to basically turn windows opaque,” said Nicholas Fang, a mechanical engineer at MIT, in a press release.

So he and his colleagues made a plastic film containing microscopic particles of a heat-sensitive color-changing material. The particles are essentially webby spheres filled with water. When the temperature reaches 32°C, the particles shrink, squeezing out water and forming bundles of fibers. This makes the film look translucent instead of transparent.

In an experiment, the researchers covered a small chamber with the film and shone light on it from a sun simulator. Without the film, the inside temperature reached 102° F (38.8 °C), but the coating reduced that to 93° F (33.8°C).

The team now plans to see if adjusting the film’s formula or applying it in different ways could boost its heat-reflecting properties.

Source: Xin-Hao Li et al. Broadband Light Management with Thermochromic Hydrogel Microparticles for Smart Windows. Joule, 2018.

Photo: MIT


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