Simple Paint Can Passively Cool Buildings

There is nothing new about radiative cooling; the Persians used it to make ice at night 2,000 years ago. Engineer Robert Bean has explained that "We cool off at night as our buildings radiate long wave radiation out to the coldness of space. Our buildings do this in the daytime too, but the effect is overwhelmed by the incoming short wave infrared from the sun."

Now Adam Vaugan of New Scientist points to a new paint that is so reflective that it can reflect enough incoming short wave infrared that it can cool a surface 3.06 F (1.7 C) in the middle of the day. We have shown fancy films that promised this, but this is basically paint.

Most infrared radiation is blocked or absorbed by carbon dioxide or water molecules in the atmosphere, but there is a "sky window" or "atmospheric window" where infrared radiation with wavelengths of between 8-13 micrometers (8,000-13,000 nm) can escape.

The new research, published under the title "Full Daytime Sub-ambient Radiative Cooling in Commercial-like Paints with High Figure of Merit," describes a paint which radiates long wave radiation through that sky window right into space, which acts as an infinite heat sink. "If the thermal emission of the surface through the sky window exceeds its absorption of the sunlight, then the surface can be cooled below the ambient temperature under direct sunlight" – as we noted earlier, it's radiating the long wave through the sky window while reflecting the short wave that would otherwise heat the building.

Purdue professor Xiulin Ruan is quoted in the Purdue press release: “It’s very counterintuitive for a surface in direct sunlight to be cooler than the temperature your local weather station reports for that area, but we’ve shown this to be possible,”

But the remarkable thing here is that it's just a mix of calcium carbonate (CaCO3) – which is basically limestone or marble or oyster shells or calcite – mixed with an acrylic base. The trick is to get a mix of particle sizes at a concentration of 60%. "In this work, we experimentally demonstrate high solar reflectance, high normal emissivity in the sky window, and full daytime sub-ambient radiative cooling in single-layer particle-matrix paints with strong performance."

As the sketch with the patent application shows, the incoming solar radiation from the exterior bounces around and is then reflected back, while the long. wave radiation from the interior goes right through and up to space. And it worked, reflecting 95.5% of the short wave solar radiation, in a side-by-side comparison to regular white paint Dutch Boy exterior acrylic paint, which reflected 87.2%. The researchers note:

"Compared to conventional air conditioners that consume electricity and only move heat from the inside of the space to the outdoors, passive radiative cooling not only saves power but it also combats global warming since the heat is directly lost to the deep space."

We have been excited by the idea of Passive Daytime Radiant Cooling (PDRC) for years, showing plastic wraps and cooling systems and even paints, albeit more complicated than this one. Physicist Alison Bailes explained it for us, and we have quoted Robert Bean about their promise:

"There will come a time when we won’t use compressors for the cooling of people and buildings. It is simply not necessary. The heat sinks we need to reject heat to, or absorb heat from, are literally within our reach and there are some very smart people who will show us how to get very good at accessing them."

It is not a complete panacea; it won't work on cloudy days, and the surface that is getting cooler has to face that "sky window" to radiate the heat into space. But these very smart people are showing us what we should do on every roof. Air conditioning has been called the blind spot for climate and sustainable development, and anything that reduces demand for it is an important step.