Researchers from Polytechnic University on Monday unveiled a solar-powered coating that can be applied to buildings to cool them down.

They said the coating, which can be put on with paint rollers, reduces the surface temperature of a building by up to 25 degrees Celsius, and the indoor temperature by two to three degrees.

Vivien Lu from the university's department of building environment and energy engineering said the coating is driven by carbon dots that convert solar power into light energy.

"For the traditional passive radiative cooling coated, in fact, they cannot be weather adaptive. That means they have a fixed cooling performance over the years," she said.

"Our coating, we are solar-driven. The stronger the solar radiance, our cooling performance will become better."

Lu said traditional cooling materials pose environmental risks, but PolyU's invention is biologically harmless as well as being highly durable.

"According to our estimation, it costs around HK$12 per square metre. We believe that with scalability, this price will decrease accordingly," she added.

The coating is set to be applied on a student hostel under construction in Kowloon Tong.