Energy intensive air conditioning could soon be replaced with colourful, cooling coatings. Here's how they work.
With summer heatwaves becoming more common, you might be weighing the benefits of installing air conditioning. Mylar Greenhouse
But these energy-intensive, chemical-leaking units take a devastating toll on our planet.
So is there a greener way to stay cool?
Scientists at Cambridge University in the UK are working on an eco-friendly alternative. Their invention consists of a plant-based film that stays cool when exposed to sunlight.
The material could someday be used to keep buildings and cars cool without the need for external power. Coming in a range of textures and bright iridescent colours, it’s aesthetically pleasing too.
For a material to stay cooler than the air around it during the daytime, there are two critical requirements. It must have high solar reflectance to reflect the warmth of the sun and not heat the air around it. It must also have a high emissivity in infrared bands to emit heat into outer space efficiently.
Only a few materials have these properties and scientists are already developing them into paints and films capable of what is known as ‘passive daytime radiative cooling’ (PDRC).
When applied to the surface of a car or building, it means that these materials create a cooling effect without consuming electricity or creating pollution.
Since they need to be solar reflective, PDRC materials are usually white or silver.
Adding colour would decrease their cooling performance. This is because coloured pigments selectively absorb specific wavelengths of light, only reflecting the colours we see. This extra light absorption creates a warming effect.
“These limited colours hinder the applications where visual appearance is a key consideration, such as for architecture, cars and clothes,” says project member, Dr Qingchen Shen.
To increase the desirability of these materials, colour is an important factor.
Along with the project’s lead investigator, Dr Silvia Vignolini, Dr Shen set out to research ways of achieving colour without the use of pigments.
They looked to structural colouration as a solution. This is where shapes and patterns reflect specific colours of light without the presence of pigmentation, as seen on soap bubbles and oil slicks.
Seeking a natural source of this phenomenon, the research team used cellulose nanocrystals (CNCs) - derived from the cellulose found in plants - to create iridescent, colourful films without any added pigment.
“We specifically use cellulose-based materials for our films because cellulose is the most abundant polymer in nature,” says Dr Shen.
It is also one of the few natural materials capable of promoting PDRC.
After experimenting with basic colours, the researchers are now working on glittery CNC-ethyl cellulose films. They are also developing different textures that could blend in with various wood finishes.
The researchers created layered cellulose films in vibrant blue, green and red colours and put them to the test.
When placed under sunlight, they were an average of nearly 4°C cooler than the surrounding air.
One square metre of the film generated over 120 watts of cooling power, rivalling many types of residential air conditioners.
As a general guideline, bedrooms require around 80 watts per square metre and living spaces 125 watts of air conditioning capacity.
The researchers hope to find new ways to leverage CNC-ethyl cellulose films. These include adding sensors to detect environmental pollutants or weather changes.
Greenhouse Stained Glass Ultimately, they hope the film coating could serve several purposes at once. It could be used to both cool buildings and to alert to changing levels of pollutants in congested areas, for example.