19 May 2022

Passive cooling for buildings – white paint and metamaterials to the rescue!

A metamaterial pattern. Source: IDTechEx

Passive cooling means no power is needed. It is widely deployed in the form of translucent multilayer polymers facing the sky and convective chimneys in traditional African buildings. However, there are limitations. Now, two new passive options are available that use readily available, non-toxic substances – metamaterials and unprecedentedly white paint reflecting maximum sunlight away from a building.

With global warming kicking in and rising sea levels, there needs to be much more cooling of buildings. The proliferation of air conditioning has issues of cost, materials, maintenance, life, and making more heat than cold.

Whitest paint in the world

Now in the Guinness World Book of Records, the whitest paint in the world has been created in a US laboratory. The paint reflects 98.1% of solar radiation while also emitting infrared heat, whereas paints currently on the market that are designed to reject heat reflect only 80% to 90% of sunlight and cannot make surfaces cooler than their surroundings and get warmer rather than cooler.

The new paint absorbs less heat from the sun than it emits, so a surface is cooled below the surrounding temperature without consuming power. Scientists claim that it is so white that it could eventually reduce or even eliminate the need for air conditioning. However, analysts at IDTechEx caution that, not being adjustable, it is better regarded as part of the toolkit.

“When we started this project about seven years ago, we had saving energy and fighting climate change in mind,” said Xiulin Ruan, a professor of mechanical engineering at Purdue University.

A roof area of about 100m² could add cooling power of 10kW, more powerful than air conditioners used by most houses but localised.

Two features make this paint ultra-white: a very high concentration of a benign common chemical compound called barium sulfate – also used in photo paper and cosmetics – and different particle sizes of barium sulfate in the paint.


Metamaterials are composites that contain repetitive patterns tailored to manipulate electromagnetic and other emissions in a manner previously impossible. For the coming commercialization of electromagnetic versions, see the IDTechEx report, “Metamaterial and Metasurface Markets Electromagnetic 2022-2042”.

Many are transparent and one of these has been researched that may assist with the problem of silicon and some other photovoltaics needing cooling to maintain efficiency. The combination of high haze, low visible absorption, and high thermal emissivity makes these nanocellulose metamaterials interesting for use as coatings for solar cells, for which the combined set of properties may enhance device light absorption, making more electricity while also improving lifetime and efficiency by passive radiative cooling. Different colours are possible and, for buildings, there is the prospect of better solar cladding and maybe metamaterial-covered windows cooling passively.

Passive radiative cooling draws heat from surfaces and radiates it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the low infrared radiation flux from a near-ambient-temperature surface requires materials that strongly emit thermal energy and barely absorb sunlight. The researchers embedded resonant polar dielectric microspheres randomly in a polymeric matrix, resulting in a metamaterial that is fully transparent to the solar spectrum while having an infrared emissivity greater than 0.93 across the atmospheric window.

The metamaterial consists of a visibly transparent polymer encapsulating randomly distributed silicon dioxide SiO2 microspheres. When backed with a silver coating, the metamaterial shows a radiative cooling power at noon of 93W/m² under direct sunshine. Further, the developers have demonstrated high-throughput, economical roll-to-roll manufacturing of it – vital for promoting radiative cooling as a viable energy technology.

The metamaterial consists of a visibly transparent polymer encapsulating randomly distributed silicon dioxide SiO2 microspheres. For those interested in the impending $40 billion market for transparent electronics, see the IDTechEx report “Transparent Electronics Materials, Applications, Markets 2021-2041”.