A new scalable metamaterial could be used as a cooling material that can operate under direct sunlight without the need for any external energy or continuous water withdrawal. The material reflects incident solar radiation but at the same time allows any surface to which it is applied to shed more of its heat as infrared thermal radiation.

A team at the University of Colorado Boulder, in the USA, describes the details of this cool new material in the journal Science [Zhai et al. Science (2017); DOI: 10.1126/science.aai7899]. They suggest that the metamaterial might ultimately be used as an environment friendly means of supplementary cooling for thermoelectric power plants. Such sites currently need vast quantities of water and electricity to maintain equipment operating temperatures.

The team based their metamaterial on a glass-polymer hybrid a mere 50 micrometers thick, which they say can be manufactured economically in a roll-to-roll process, making it a potentially viable large-scale technology for both residential and commercial applications.

"We feel that this low-cost manufacturing process will be transformative for the real-world application of radiative cooling technology," explains team leader Xiaobo Yin. The material exploits the phenomenon of passive radiative cooling, whereby an object naturally loses heat through infrared radiation when facing towards the sky. Such cooling is commonly only useful when the sun is not out, as even a small amount of solar energy absorption can cancel out the typical cooling gains from passive radiation.

The team therefore developed their metamaterial to reflect any incident light while greatly enhancing the emission of infrared from the surface. The final material consists of glass microspheres embedded in a polymer film, which together work to draw in and then radiate heat, but remain transparent to visible sunlight. A thin silver coating beneath this layer then flips this into full reflectance of the incident light.

A few patches of this material on the roof of a domestic dwelling would be sufficient to cool a family home in the summer without the need to fire up the air-conditioning unit, suggests team member Gang Tan. Of course, the same material could be used to cool power plants and other industrial installations on a larger scale. It might also be used to improve the efficiency - by a useful 1 to 2 percent - and life expectancy of solar panels by reducing their risk of overheating when the sun is shining directly overhead. "The key advantage of this technology is that it works 24/7 with no electricity or water usage," explains team member Ronggui Yang.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".