Homeowners may soon be able to opt for a dark roof while maintaining the cooling and environmental benefits of a white roof, following a research breakthrough by the US Lawrence Berkeley National Laboratory.
The research found that by using fluorescence or photoluminescence – the re-emission of absorbed light – dark-coloured roofs could stay as cool as their light counterparts.
The news means that the benefits of white roofs could soon be shared by those who prefer the aesthetic of dark roofs or who are hamstrung by restrictive councils.
Fluorescence is known to transport radiant heat, however its application in reducing object temperature is new. The researchers tested the novel concept by colouring roof coatings with ruby (aluminium oxide doped with chromium), which is known to fluoresce.
First the researchers found white paint overlaid with a layer of synthetic ruby crystals stayed as cool as white paint on its own. Next they synthesised a ruby pigment to mix into paint and found similar results at small concentrations, though they noted work was needed to get a similarly dark ruby colour.
In follow up research, blue materials that also fluoresced were identified, which could be combined with other colours to yield green and even black materials that stay cool.
Pigments have commonly been added to paints to reflect “near-infrared” light so dark surfaces stay cooler, however these materials struggle to stay as cool as a white roof. The new work shows that fluorescent cooling could help boost this performance, because when light hits a fluorescent material, the material actively emits energy in response, rather than passively reflecting the energy.
“People understand that materials that fluoresce are emitting energy,” lead researcher and Berkeley Lab scientist Paul Berdahl said. “What’s new here is the use of the fluorescence process to keep buildings cooler.”
It promises to be good news for the environment, as while white roofs have shown to both lessen the need for airconditioning and tackle the urban heat island effect, they have failed to take off due mostly to aesthetic concerns.
“We’ve heard many times (from roofing materials manufacturers), ‘We can’t sell white or pastel roofs; our customers want dark green, dark brown,’ and so on,” Dr Berdahl said.
The research also opens the door to a range of products using fluorescent pigments for cooling, including cars, ships, PVC piping and storage tanks.
“We do think cars will be a likely application,” Dr Berdahl said. “And it’s not just a matter of comfort or saving energy by avoiding AC use. We learned from colleagues that with electric vehicles, the battery lifetime is degraded by higher temperatures, so if you can keep the automobile cooler with use of a suitable coating then it extends the life of the battery.”
Dr Berdahl said the pigment paint could be produced cheaply.
“Rubies have a reputation for being expensive, but they’re mostly aluminium oxide, which sells for about 70 cents [AU$0.92] per kilogram,” he said.
Its durability is also expected to be similar to other coatings.