Radiative cooling coatings that can adapt to the seasons

Radiative cooling systems work by absorbing thermal radiation (i.e. heat) from a building and then emitting it to the sky. Because the atmosphere is transparent at these wavelengths, the heat escapes directly into space. Some systems use reflective surfaces like super-white paints to “bounce” sunlight and heat away, keeping the building cooler. Unfortunately, these systems emit thermal radiation even when the surface temperature is lower than desired, for example at night or in winter. This unwanted thermal radiative cooling will increase energy consumption for heating. 

 

Recently, scientists have been working on a material that could trap heat in cold weather, therefore acting as a temperature-adaptive radiative coating (TARC). The key to this technology is a compound called vanadium dioxide (VO₂), which has the surprising property of conducting electricity but not heat, under certain conditions.  

 

A team of engineers at the University of California, Berkeley designed 2 cm² (0.8-in²) thin-film TARC patches and compared them to samples of commercial dark and white roof materials. They found that their novel roof coating was able to absorb solar energy and emit up to 90% of its heat to the sky when the ambient temperature was above 30 °C (86 °F). When the temperature dropped below 15 °C (59 °F), the TARC emitted just 20 % of its heat. Regardless of the weather, their roof coating was able to reflect about 75% of sunlight on average. For further details, their research has been published here.

 

An independent team of researchers at Nanyang Technological University, Singapore (NTU Singapore), has developed a similar material to coat glass window panels. This self-adaptive glass was developed using layers of vanadium dioxide nanoparticles composite, poly(methyl methacrylate) (PMMA), and low-emissivity coating to form a unique structure. The newly developed glass, which has no electrical components, works by exploiting the spectrums of light responsible for heating and cooling. 

 

Windows are typically the least energy-efficient component in a building design. As a proof of concept, the scientists tested the energy-saving performance of their invention using simulated climate data from seven climate zones worldwide. Their results showed energy savings in both warm and cool seasons, with an overall energy saving performance of up to 9.5%, or about 330,000 kWh per year less than commercially available low-emissivity glass in a simulated medium-sized office building. For further details, their research has been published here.

 

 

Sources 

[1] Smart roof coating reflects heat in summer and traps it in winter. https://newatlas.com/materials/tarc-roof-coating-adaptive-heating-cooling/ 
[2] Tang, Kechao, et al. "Temperature-adaptive radiative coating for all-season household thermal regulation." Science 374.6574 (2021): 1504-1509. https://wu.mse.berkeley.edu/publications/Tang-Science2021.pdf 
[3] Scientists invent energy-saving glass that 'self-adapts' to heating and cooling demand. https://techxplore.com/news/2021-12-scientists-energy-saving-glass-self-adapts-cooling.html 

(image credit: Pr Junqiao Wu. TARC samples tested on a rooftop. https://newscenter.lbl.gov/2021/12/16/roof-year-round-energy-savings/)