Colourful bilayer paint can cool buildings by reflecting sunlight – Physics World


Bilayer coating
Cool colours: the hotter square on the left is coated in a single layer of black paint while the cooler square in the right is a black bilayer. (Courtesy: Jyotirmoy Mandal)

A bilayer paint that can dramatically cool sunlit surfaces yet comes in a range of colours has been developed by scientists in the US and China. The researchers say that the solar-scattering underlayer of their coating can cool surfaces in strong sunlight by almost 16 °C, compared with conventional paint. Such coatings could potentially be applied to buildings, cars and even textiles to increase radiative cooling in the summer, the team claims.

According to the US Energy Information Administration, around 15% of household electricity usage in the US is for cooling indoor spaces. As well as being energy intensive, cooling generally requires the use of coolants that can damage the environment. As a result, there is a lot of interest in developing low energy, eco-friendly alternative approaches to cooling buildings.

One area that has had a lot of attention is using surfaces that are very good at reflecting sunlight. “If you can enhance the reflectance of the wall or roof you can reduce the solar heating, and reduce the electricity consumption,” explains Yuan Yang, a materials scientist at Columbia University in New York City.

A glaring problem

Achieving this can be as simple as painting surfaces white or covering them in mirrors or highly reflective metallic surfaces. Researchers have also looked at other man-made and natural photonic structures that are highly reflective. The problem with these ideas is that they reflect all or most of the sunlight, making the buildings glaringly bright. And they offer limited options for adding colour to buildings.

To tackle these issues, Yang and his colleagues focused on reflecting infrared light in the 0.74-2.5 micron wavelength range (near-to-short wavelengths). This light accounts for about half of all solar energy, so reflecting this part of the spectrum reduces solar heating considerably. To achieve this, the researchers developed a coating that consists of two layers. The outer layer is commercial paint that provides colour and the inner layer reflects near-to-short infrared wavelengths to reduce solar heating.

Blue, red, yellow and black

In tests, blue, red, yellow and black hues of the paint reflected radiation more effectively than commercial, single layer paints of the same colour. Under a midday summer Sun, the black version kept an object 15.6 °C cooler than a standard black paint.

Yang told Physics World that while the overall effect on the internal temperatures of a building depends on many factors, such as building size and the ratio of wall to windows, the coating can reduce solar heating on a wall by around 10-20%.

While the outer layer absorbs visible light to produce the desired colour, the inner layer of the coating is made of a porous polymer that scatters and reflects near-to-short infrared light that passes through the outer layer. The researchers say that this strategy allows them to create coatings with near-identical colours and visible reflectance to standard commercial paints, but with significantly higher reflectance of infrared light.

“Our strategy is to enhance the reflectance in the infrared light so we can keep the same colour and also reflect more sunlight, to keep the building cool,” Yang explains. He adds that the pore size of the bottom layer is optimized to be in the range of infrared light, which makes it highly effective at reflecting those wavelengths.

Different shades

In addition to using different colour paints, the researchers found that by changing the thickness of the top layer of the coating they could achieve different shades of the same colour. For example, a thin top layer of Sudan blue produced a whitish blue, while a thick coat created a darker blue. They also demonstrated the stability and durability of the coating, with no change in the colour or reflectance of the bilayer after being placed outdoors or in an oven at 60 °C for 30 days.

According to the researchers, the bilayer design increases the versatility of the coating, making it suitable for many applications. The top layer can be changed to achieve specific attributes – such as weather proofing, while the infrared reflectance of the bottom layer is maintained. This could be particularly useful for the automotive industry, where a range of photonic designs and surface finishes are required, they add.

And they even claim that fibres can be dipped and coloured with the coating to produce fabrics and clothes with high solar reflectance, to help keep you cool in the summer.

The new bilayer is described in Science Advances.

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