An extremely white paint that reflects 98.1 per cent of sunlight can cool itself by radiating heat into deep space. It could help keep buildings cool, potentially replacing energy-intensive air conditioners.
Xiulin Ruan at Purdue University in the US, and his colleagues previously developed an ultra-reflective paint using calcium carbonate particles that reflected 95.5 per cent of sunlight. They have now bested that by using barium sulfate particles in a paint that reflects 98.1 per cent of sunlight.
This new ultra-white paint absorbs less than half the energy from the sun as the previous paint. Standard commercial white paint absorbs between 10 and 20 per cent of sunlight energy.
The amount of sunlight absorbed by the new paint is now lower than the amount of energy it radiates through our atmosphere and into deep space, so the material actually becomes cooler than its surroundings. The team will be carrying out experiments with painted tubes carrying water and hope to create an electricity-free refrigeration effect.
The team hopes that the paint can lower global carbon emissions as houses coated in the paint would need less air conditioning. If the paint is used on a 930 square metre roof the cooling effect could be as high as 10 kilowatts, which the team says is more powerful than a standard air conditioner.
Ruan says there is a double-pronged positive effect because the paint sends energy away from our planet. “We send the heat to space, we’re not leaving the heat on Earth,” he says. “Traditional air conditioners leave the heat on earth’s surface, it’s just moved from the inside of your house to the outside.”
The team calculated that if 0.5 per cent to 1 per cent of the Earth’s surface was covered in this paint, for instance by coating roofs with it, the total effect would reverse global heating to date.
The painted surfaces will need to be kept clean of dust and dirt to retain their reflective properties but the team are working on ways to make it shed particulates.
Ruan is now working on an even more reflective material but says that there may be diminishing returns. “Pushing to 100 per cent is hard. You will get 19 watts per square metre more cooling benefit, so practically it may not be that attractive given the cost,” he says.
Journal reference: ACS Applied Materials & Interfaces, DOI: 10.1021/acsami.1c02368
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