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Hong Kong team says new cool white ceramic can reflect nearly all sunlight and cut air conditioning costs

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Researchers in Hong Kong say they have developed a cooling ceramic with near-perfect solar reflectivity to reduce the energy consumption of indoor air conditioning and help create sustainable buildings.

When applied to roofs, a house covered with the novel white material could be up to 2.5 degrees Celsius (4.5 degrees Fahrenheit) cooler indoors than one fitted with ordinary white tiles found on the retail market, saving more than 20 per cent of air conditioning energy to reach the same indoor temperature, the team said.

According to the scientists, the material adds to the toolkit of technologies available to enhance energy efficiency and combat climate change.
Application of the ceramic is shown in a building envelope, with the white cooling ceramic applied on the roof. Photo: City University of Hong Kong

“With high weather resistance, high mechanical strength, favourable recyclability, notable Leidenfrost depression and optional colour features, the cooling ceramic can readily be applied to different scenarios and outdoor infrastructures on a large scale,” the team from City University of Hong Kong and Hong Kong Polytechnic University wrote in an article published in the peer-reviewed journal Science this month.

Carbon emissions from space cooling have nearly tripled since 1990 to 1 billion tonnes (1.1 billion tons) in 2022, according to the International Energy Agency. Since 2000, energy demand for space cooling has grown around an average 4 per cent a year.

The agency said the use of air conditioners was on track to become a top driver of global electricity demand in the next three decades.

As our planet warms under climate change, scientists are exploring solutions to beat the heat. The carbon footprint of air conditioners creates a vicious cycle that exacerbates the need for air conditioning.

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In the new study, researchers in Hong Kong advanced their research in passive radiative cooling, a technology where surfaces use the “cryogenic universe” between space and the Earth’s surface as a natural heat sink for heat removal, to create energy-free and refrigerant-free cooling ceramic.

The design was inspired by the whitest known insect, the Cyphochilus beetle native to Southeast Asia.

In mimicking its bio-whiteness, the team created a ceramic with a porous structure capable of efficiently scattering almost all the wavelengths of sunlight, resulting in a record high solar reflectivity of 99.6 per cent, which outperforms existing products.

The material made of aluminium oxide is highly resistant to UV degradation and fire, showing a 1.3 per cent drop in its solar reflectivity after being exposed to sunlight outdoors for a year and withstanding temperatures exceeding 1,000 degrees Celsius.

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It could be made curved and in other colours. Although colouring slightly compromises its reflectivity, the cooling ceramic in vivid colours similar to commercial tiles is still cooler when exposed to direct sunlight at midday, according to their experiments.

It could also be manufactured to repel water, enabling it to be self-cleaning when water comes in contact with dust on its surface, or attract water. It can depress the Leidenfrost effect, which prevents water from evaporating from heated surfaces, and achieve faster cooling effects in case of fire, according to the authors.

The team conducted field tests in Beijing, as well as Boston, Philadelphia and Yellowstone National Park in the United States, and found the material worked to reduce temperature under different climates.

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Lead author Edwin Tso Chi-yan, an associate professor at the school of energy and environment at CityU, said the product was set for market launch late next year via his start-up.

“Our simulation shows that places closer to the equator would benefit most from using the cooling ceramic as an external envelope material on the walls and roof,” Tso said.

The study found that more than 10 per cent of annual indoor air conditioning costs could be saved per year in tropical regions.

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Tso co-founded the company i2Cool last year, which has sold cooling paint and glass film invented by his lab to 39 countries and regions around the world, including China, Southeast Asia, the Middle East, Africa, Europe and North America.

“As researchers, we drill into the technicalities to improve product performance. When it comes to running a business, the team focuses on their real-world applications. We learn along the way where and how cooling technologies are best used and which regions to market them to,” he said.

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