It’s time to brace for record-breaking heat. Last year was the hottest on record and 2024 is shaping up to be even more extreme, with the mercury soaring close to 50 °C on days in Nevada, Egypt and Australia. June marked the 13th month in a row of chart-topping temperatures globally. And four consecutive days in July were the hottest in recorded history for the entire planet.
Scorching temperatures spur water shortages, damage crops, strain electricity grids and trigger heat stress and mass mortality — killing close to 500,000 people each year, according to one estimate1. So scientists are working hard to develop innovative ways to cool cities and slash electricity use in the warming world. Advances range from high-efficiency air conditioners to special materials that keep surfaces colder than their surroundings without using electricity.
In most air conditioners and refrigerators, a fluid is compressed to transfer heat from inside the room or appliance to outside. But this process emits greenhouse gases and guzzles energy. Globally, air conditioners and electric fans consume about 20% of the electricity used in buildings, according to the International Energy Agency. And the agency predicts that the amount of energy required for air conditioning around the globe will surge threefold by 2050.
With that in mind, many researchers are working to reduce the amount of energy that air conditioners consume. One potential solution emerged last year, when a team of researchers developed a technology that might make the appliances work much more efficiently2. And it has the added benefit of not relying on environmentally damaging liquid coolants.
Emmanuel Defay, a researcher at the Luxembourg Institute of Science and Technology in Belvaux, and his colleagues built a device that relies on ‘electrocaloric’ cooling. In this process, an electric field is applied to change the position of atoms in an insulating ceramic. Because the field constrains the atoms’ movements, their vibrations increase and are converted into heat, raising the temperature of the material. Fluid carries that heat away to the outside. Once the heat has been removed, the field is shut off and the atoms in the ceramic can move more freely. That causes their vibrations to decrease and the ceramic’s temperature drops, a change that can be used for cooling purposes.
The device was designed in collaboration with the Japanese manufacturing company Murata in Nagaokakyo, which already produces these kinds of ceramics for mobile phones, computers and other hardware. That will help to make the technology scalable, says Defay. But he warns that getting it into products might take time. He hopes that he and his team can work on the first niche cases, such as cooling down batteries in electric cars, within five years. Then, perhaps, they can tackle air conditioning in the next decade.
Other components — known as supercool materials — might be able to lower temperatures below ambient conditions without power.
All materials reflect some portion of the sunlight that hits them, and all emit energy as heat. But supercool materials do both extremely well — reflecting most of their incident solar radiation and emitting a lot of their thermal radiation. That makes them cooler than their surrounding temperature.
“These materials are potentially a game-changer,” says David Sailor, director of the School of Geographical Sciences and Urban Planning at Arizona State University in Tempe, who doesn’t develop these technologies but studies how they could be used in urban environments. Not only can they help to cool a building — thus reducing the demand for air conditioning — but they can also cool the outdoor air. “If a surface is always cooler than the air, then that surface is always taking heat out of the air as the air flows over it,” Sailor says. “So it’s actively cooling the urban atmosphere.”
The first supercool material was designed in 2014 when Aaswath Raman, a materials scientist now at the University of California, Los Angeles, was conducting research at Stanford University, also in California. He and his colleagues created a cooling surface that was highly reflective at the visible wavelengths where the Sun’s radiation peaks, and emissive in the mid-infrared3. The latter was key. The atmosphere traps most of the infrared radiation emitted as heat by objects on Earth’s surface. But a specific band of infrared, with wavelengths of 8–13 micrometres, passes straight through the atmosphere and disappears into space. Supercool materials exploit that infrared window.
Mounted on a roof, Raman’s technology — which was made of seven alternating layers of silicon dioxide and hafnium dioxide — stayed 5 °C cooler than the ambient air temperature.
Since then, the field has exploded. In the laboratory, supercool materials have been built in the form of plastics, metals, paints and even wood.
And scientists are continuing to push them further. In July, researchers at Sichuan University in Chengdu, China, reported that they had freeze-dried a solution of commercially obtained salmon sperm DNA and gelatin to create a supercool aerogel4. When placed outside, the aerogel cooled surfaces to up to 16 °C below the ambient air temperature.
Xianhu Liu, a materials scientist at Zhengzhou University in China, who was not involved in the study, is particularly excited that the material uses less energy and reduces pollutants compared with other supercool materials that use additives, such as metal oxide nanoparticles. The fact that this work used biomass materials also means it’s degradable. “It’s a rare combination of sustainability and energy efficiency,” Liu says.
But Raman doubts the accuracy of the study’s results. The team measured an air temperature 20–30 °C higher than that reported by weather stations for that day and location. “It seems likely that their temperature sensor was exposed to the Sun and heated up,” Raman says.
However, Jian-Wen Ma, the study’s lead researcher, says that he and his team measured the air temperature inside a closed cooling test box, not the external atmospheric temperature4. “There is ongoing debate about cooling test methods,” he says. “The characterization of radiative cooling remains complex and imperfect, and we hope for a unified standard in the future.”
