Materials

Ultra-white ceramic cools buildings with record-high 99.6% reflectivity

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A prototype of the new cooling ceramic material on a rooftop
City University of Hong Kong
A prototype of the new cooling ceramic material on a rooftop
City University of Hong Kong
Samples of the cooling ceramic material, which reflect a record high of 99.6% of sunlight that hits them, cooling interiors of buildings
City University of Hong Kong

Scientists in Hong Kong have demonstrated a new ultra-white ceramic material that can drastically cool buildings by reflecting sunlight and heat at record highs. The beetle-inspired material gets its ability from its nanostructure, stays tough to the elements and should be relatively easy to scale up for production.

When our homes get too hot, the first solution for many people is to crank the air conditioning. It may be effective, but it’s not very energy efficient, as heating and cooling buildings accounts for a huge percentage of energy costs. So, scientists are investigating ways to passively manage interior temperatures, and one of the simplest is to just paint buildings and rooftops white.

Basic physics dictates that lighter colors absorb less light than darker ones, and therefore remain cooler. In recent years new ultra-white paints have been developed that reflect over 95% of the sunlight that hits them, doing a decent job of cooling buildings. But these coatings have their issues, including durability.

For the new study, scientists at City University of Hong Kong (CityU) developed a new cooling ceramic material that performs better than others. Rather than just being a very white paint, the material gets its high reflectivity from its nanostructure, which efficiently scatters almost the entire spectrum of sunlight – just like the Cyphochilus beetle which inspired it. This results in a solar reflectivity of 99.6%, which is a record high, along with an infrared thermal emission of 96.5%.

Samples of the cooling ceramic material, which reflect a record high of 99.6% of sunlight that hits them, cooling interiors of buildings
City University of Hong Kong

The material is made of alumina, which not only reduces solar absorption, but the team says it makes the cooling ceramic more durable in the face of weather. It resists degradation from UV light exposure, which is a weakness of other passive cooling materials and coatings, and boosts the rate of water evaporation from the surface, which adds the bonus effect of evaporative cooling. It even boasts fire resistance by withstanding temperatures of over 1,000 °C (1,832 °F).

““The beauty of the cooling ceramic is that it fulfills the requirements for both high-performance PRC and applications in real-life settings,” said Professor Edwin Tso Chi-yan, co-corresponding author of the study. “Our experiment found that applying the cooling ceramic on a house roof can achieve more than 20% electricity [reduction] for space cooling, which confirms the great potential of cooling ceramic in reducing people’s reliance on traditional active cooling strategies and provides a sustainable solution for avoiding electricity grid overload, greenhouse gas emissions and urban heat islands.”

Finally, the researchers also say that the material can be easily mass produced, using common materials like alumina and a two-step process of phase inversion and sintering. And if white is too boring for some houses, the material can apparently be produced in other colors and patterns by adding extra layers.

The research was published in the journal Science.

Source: City University of Hong Kong

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8 comments
TechGazer
White being "too boring" is just a cultural artifact. You need to have some influencer make white roofs popular, and dark roofs something to be ashamed of.

I don't understand how the material can be produced in other colours and maintain its high efficiency. Colours other than white means that the material is absorbing the other wavelengths, unless it's somehow reflecting it in a direction away from the viewer.
DaveWesely
You are correct, TechGazer. The color white is a reflection of the entire color spectrum. The pure color red, for example, is the reflection of the red portion and the absorption of the green and blue wavelengths. By absorbing just a little bit of blue, the color reflected would look a little yellow (the opposite of blue). It's really a matter of color density. The lighter the color, the more light is reflected.
Don Soards
The reflected heat would be an energy benefit during the summer, but wouldn't the reflected heat be an energy liability during the winter?
Trylon
Wasting perfectly good energy. I'd rather have a system that splits the solar spectrum, directing useful frequencies toward solar cells and less useful ones toward blackened absorption pipes to heat water. Why give up perfectly good heat when we need hot water in buildings anyway?
windykites
You could have white roller blinds on the roof. I agree with Trylon. Pipe the hot water into an insulated tank in the house for later use.
TechGazer
To Don Soards: roofs generally have good insulation below them, so a black roof in winter isn't going to warm the building interior much. Really good ventilation would reduce heat gain in summer, but most houses don't have good enough ventilation. Maybe if roofs had another layer, providing a (large) ventilation channel between the roof and the insulation, roof colour wouldn't be much of a issue. I have an arch-frame cabin with only 4" of insulation between the roof and the inner wall, and no ventilation space, so switching from brown asphalt shingles to white metal made a huge difference in indoor temperature in summer.
Niclas
Very interesting in hot and sunny areas, though:

- Does it need frequent cleaning to remain efficient?
Though I suppose also solar panels ned some cleaning.

- Whats the cost and durability compared to solar panels + AC per cooling effect?
Considering electricity is for free on sunny days when it’s needed.

- Would it also increase isolation inside in houses, heat storage, … or is it not efficient on relatively low temp IR?
ReservoirPup
You would have to pay for this, while PV would pay for itself or even earn $$$