The proliferation of air conditioning has problems of cost, materials, maintenance, lifespan, and producing more heat than cold. Passive cooling means no power is needed. It is widely deployed as translucent sky-facing multilayer polymers and convective chimneys in traditional African buildings. However, they have their limits. We must welcome two new passive options that use readily available non-toxic substances – metamaterials and an unprecedented white paint that reflects maximum sunlight away from a building.
The whitest paint in the world was created in an American laboratory. It is now in the Guinness World Records book as the whitest ever. Scientists claim it’s so white that it could possibly reduce or even eliminate the need for air conditioning. However, analysts at IDTechEx warn that, not being adjustable, it is best seen as part of the toolbox.
“When we started this project about seven years ago, we had energy conservation and tackling climate change in mind,” said Xiulin Ruan, professor of mechanical engineering at Purdue University.
Making it really reflective also made it look really white. The paint reflects 98.1% of solar radiation while emitting infrared heat. It absorbs less heat from the sun than it emits, so a surface is cooled below room temperature without consuming energy.
A roof area of around 100 square meters could add 10 kilowatts of cooling power, which is more powerful than air conditioners used by most homes but localized. Typical commercial white paint gets hotter than cold. Paints on the market designed to reject heat reflect only 80-90% of sunlight and cannot make surfaces cooler than their surroundings.
Two characteristics make this paint ultra-white: a very high concentration of a common benign chemical compound called barium sulfate – also used in photo paper and cosmetics – and different particle sizes of barium sulfate in the paint, said scientists at Purdue. Purdue researchers have partnered with a company to bring this ultra-white paint to market.
Metamaterials are composites that contain repeating patterns adapted to manipulate electromagnetic and other emissions in ways previously impossible. For the upcoming commercialization of electromagnetic versions, see the IDTechEx report, “Metamaterial and Metasurface Markets Electromagnetic 2022-2042”.
Many are transparent and one of them has been researched that can help solve the problem of silicon and some other PVs that require cooling to maintain their efficiency. The combination of high haze, low visible absorption, and high thermal emissivity makes these nanocellulose metamaterials attractive for use as coatings for solar cells, where the combined set of properties can enhance the performance. light absorption of the device, producing more electricity while improving the lifespan and efficiency in passive radiative cooling. Different colors are possible, and for buildings there is the prospect of better solar coating and perhaps passive cooling of windows coated with metamaterials.
To be precise, passive radiative cooling pulls heat from surfaces and diffuses it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the weak flux of infrared radiation from a surface near room temperature requires materials that emit heat energy strongly and barely absorb sunlight. The researchers integrated randomly resonant polar dielectric microspheres into a polymer matrix, resulting in a metamaterial that was 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 microspheres of silicon dioxide SiO2 distributed at random. For those interested in the impending $ 40 billion transparent electronics market, see the IDTechEx report “Transparent Electronics Materials, Applications, Markets 2021-2041”.
When covered with a silver coating, the metamaterial displays a midday radiative cooling power of 93 watts per square meter in direct sunlight. Additionally, the developers demonstrated cost-effective, high-throughput roll-to-roll manufacturing – vital to promoting radiative cooling as a viable energy technology.