Developing measurements for passive cooling in direct sunlight

The technology is based on using materials and colors that reflect sunlight and emits thermal infrared radiation in a wavelength range that allows the heat to be dissipated through the atmosphere and out into the cold space. It is particularly effective in areas with a lot of sunlight and little cloudiness, such as deserts, where low temperatures can be achieved even in the middle of the day.

"The technology is particularly attractive in warmer countries, but even in Sweden we have problems with phenomena such as the heat island effect, which makes cities warm. Replacing black roofs with white ones can be one way to counteract that", says Anne Andersson, researcher at RISE, and continues:

"We also receive many enquiries from Swedish companies that are interested in developing their materials with these properties in mind."

Cools its surroundings

When two objects of different temperatures come into contact, heat is transferred between them. The hotter object gets colder, and the colder object gets hotter. In this way, buildings can be cooled when, for example, the roof gets colder. The effect can be observed on some mornings when frost forms on the car's windscreen, even if it is not freezing, when the surface has become colder than the surroundings. The more sunlight that is reflected and heat that is emitted, the more the material can cool its surroundings. Through measurements, a solar reflective index can be determined to quantify the effect of different materials and colors.

"All colors and materials reflect, but some more than others. White color that reflects a lot of sunlight and at the same time allows heat to be emitted is the best choice. At the same time, the material and color need to work for the building from other aspects as well, therefore it is important to characterize different materials to find the most optimal solutions", says Anne Andersson.

European project develops the measurements

Homes, office buildings, industries, shopping centers and data centers can all benefit from the technology as a cost-effective cooling system. And even the cooling effect only results in a few degrees colder, it can be an important piece of the puzzle to help combat climate change. However, despite the obvious advantages, there is still a lack of methods to evaluate and compare the performance of different materials and colors under real conditions. Anne Andersson participates in the European research project PaRaMetriC (Metrological framework for passive radiative cooling technologies), where she performs and develops measurements on these materials and colors.

"This is nothing new, it is no coincidence that buildings are white in warmer countries. But to develop the technology and be able to choose the most optimal materials and colors, we need methods to compare, which requires us to develop reliable measurements", says Anne Andersson.