With increasing global temperatures, more and more energy is being used to cool buildings with air conditioning (AC). This has led to carbon dioxide emissions from cooling doubling over the past 30 years, further exacerbating global heating. As well as using energy, AC uses refrigerant chemicals that are harmful to the environment if leaked during use or when scrapped.

New ways to help cool buildings and reduce AC use are therefore needed. Now, a research team, led by Dr Edwin Tso from the the School of Energy and Environment at City University of Hong Kong, has created a ceramic for building facades that can decrease indoor temperatures by 1-2°C without using energy or refrigerants.

Details of the material and results of real-world experiments were published in the journal Science.

Passive radiative cooling

The ceramic tiles use the principle of passive radiative cooling, where the energy emitted from a material is exceeded by the energy it absorbs.

To reduce energy absorbed and stop the material from warming up in the first place, the material was created with near-perfect solar reflectivity. This was achieved by creating an ultra-white material inspired by the scales of the Cyphochilus beetle, the whitest known insect on Earth, which is native to Southeast Asia.

By inspecting the beetle’s exoskeleton with powerful microscopes, the team discovered the ultra-white areas were densely covered with around 15,000 scales per square centimetre. These teardrop-shaped scales were connected by a random network of chitin material, which along with air pockets helped to scatter the incoming energy and reflect it away.

The team copied aspects of this structure in a ceramic made of aluminium oxide, an inexpensive and abundant material that can be easily manufactured. The material requires no special equipment and was achieved a record solar reflectance of 99.6%

The reflect sunlight this effectively, the material had to be particularly good at scattering light in the 0.25-2.5µm wavelength range. The other half of the material’s effectiveness comes from its ability to emit in the mid-infrared wavelength range of 8-13µm, which is the range needed to escape Earth’s atmosphere without being reflected back. The team’s ceramic was able to achieve 96.5% emission in this range.

Real-world testing

The combined effects of high reflectance in the solar range and high emissivity in the mid-infrared range mean the tiles can reach temperatures 4-6°C lower than ambient temperatures. They only reflect sunlight during the day, but at night the emissivity keeps them cool. However, their effectiveness does depend on the local climate conditions.

Being made of ceramic though does mean they are far more resilient to the elements than other potential radiative cooling materials. Polymer-based materials, for example, degrade under the UV component of sunlight, rapidly reducing performance. The team’s ceramic tiles experienced a solar reflectivity drop of only 1-2% after a year outside, still leaving a solar reflectance above 97%.

To test the ceramic further in real-world conditions, the team coated two huts with tiles: one with traditional white ceramic tiles and the other with their new tiles. When measuring during the heat of July 2022, the roofs showed at a peak a 5°C difference, translating to an interior cooler by 2.5°C for the new ceramic tiled building.

Each hut was then fitted with an AC unit, with their set points matched at 25°, 23° and 20°C, and their output measured. The cooling ceramic-tiled hut consumed less electricity, with energy savings of 26.8, 22.6, and 19.6%, respectively. While a whole, real house would have more complex energy needs and architecture, the team estimate this could translate to a more than 10% annual energy saving, particularly in tropical locations.

They were also able to estimate the cooling power of the tiles based on these measurements, showing they could generate the equivalent of around 130 Watts per metre squared in the Hong Kong environment.

Tso said: “The cooling power of the tiles would not be enough to replace air conditioning in most cases, but they are a remarkably simple way to save energy that can be added to old buildings or included in the construction of new ones.”

Expanded applications

Not all countries need cooling all year around. Many countries in northern Europe, for example, are experiencing record summer temperatures but still require heating in the winter. This is one area Tso would like to explore in further work with the tiles; whether there can be a tuning that reduces the solar reflectance down to 80%, akin to normal building materials.

Another area for expansion is colour. The team showed that the new tiles can be coloured while still achieving higher solar reflectivity than current commercially available building ceramics. However, the solar reflectance is lower than the white, since colour by definition absorbs certain wavelengths of light. One option, says Tso, may be to integrate quantum dots or carbon dots that absorb in one colour but can re-emit at other wavelengths, keeping overall solar reflectance high.

These are nice-to-haves, but Tso says only one issue needs to be sorted before commercialisation: brittleness. The team can create 20x20cm tiles easily with basic equipment, but larger, hotter furnaces will be needed to create less brittle tiles. Finding a partner to scale up manufacturing will be crucial to this, and Tso thinks the tiles could be on the market within a year.

This is possible because Tso has already commercialised a previous cooling invention: a paint that uses the same principle of passive radiative cooling, this time inspired by the Saharan silver ant. i2Cool was cofounded by Tso two years ago, based on years of research form him and his team, and has now sold its iPaint to customers in more than 30 countries.

The tiles would give people choice, says Tso: a concrete surface can be painted, or tiles can be integrated into new building facades. Tso recently attended the COP28 environment summit in Dubai and says there was lots of interest in the tiles.

He stresses, though, that buildings are not the only applications for cooling paints and tiles. Solar panels can overheat, reducing their energy conversion power. When the iPaint was applied to the frame and the base of solar panels in one experiment, the local cooling effect was enough to increase power conversion efficiency by 4-8%.

Tso says: “We can integrate technologies to maximize the solar panels’ efficiency: for example, if a building already has solar panels our paint can be added to further enhance power conversion efficiency. This is just an example of what we can do with passive radiative cooling technologies.”