As cities become ever hotter, hopes are pinned on a slew of materials that promise to make them safer and more habitable. Anthony King reports.
Summer 2023 saw nonstop record-breaking temperatures making headlines across the globe. In Texas and Oklahoma and other southern US states, mercury levels tipped 48°C. Palermo in Sicily sizzled under 47°C. This is bad news.
‘We have over 1,300 cities worldwide that experience overheating, and 2bn people who live in conditions of severe overheating,’ says Mattheos Santamouris, Professor of high-performance architecture at the University of New South Wales, Australia. Scorching temperatures not only result in increased heat stress and mortality, but also in water shortages and drought, damaged crops and stressed electricity grids.
Economic productivity also suffers. As part of a European project, researchers at Loughborough University, UK, reported a 35% decrease in productivity during a work shift at temperatures of 35°C/50% relative humidity, plummeting by 76% at 40°C/70% relative humidity (Int. J. Biometeorol., doi: 10.1007/s00484-022-02370-7). And climate modellers warn these blistering extremes are no blip. By 2050, the planet’s average temperature is predicted to be 1.1 to 5.4°C warmer than today.
Heat-mitigating materials could help to at least offset some of the effects. Ten years ago, it was possible to turn down the thermometer in cities by 1.5°C, but improved mitigation materials mean 4°C is now possible, says Santamouris. ‘This can make cities more liveable and deliver health gains. Energy consumption for cooling can be halved and there is a 30 to 35% reduction in heat-related mortality and illness.’ Reducing urban temperatures also lowers ground ozone levels, another health benefit, especially for the elderly and those with respiratory conditions.
Painting surfaces white is one of the most common methods to reflect sunlight. However, most white paint contains titanium dioxide, which while reflective also absorbs violet and UV light. In 2020, researchers at the University of California, Los Angeles, described how to make paint that reflects 98% of incoming heat from the sun by replacing the metal oxide with ingredients such as barite, a pigment used in art, and Teflon.
But white paint is not the only answer. ‘There are a lot of opportunities to create surfaces that are highly reflective to the energy of the Sun, without necessarily being bright white,’ says David Sailor, an engineer and urban planning professor at Arizona State University, US. Sailor notes that over the last two to three decades, several US states have adopted cool roof strategies, whereby heat reflecting surfaces, paints or coatings are applied to roofs to reduce temperatures within buildings. ‘Coatings that reject short wave solar radiation and effectively emit long wave thermal energy have the potential to provide local cooling and even contribute to global cooling,’ he explains. Sailor’s group collaborates with materials company 3M on developing and testing potential cool coatings.
Another emerging type of supercool material can send heat directly from Earth into space. Mostly, our atmosphere insulates the planet, absorbing energy and trapping heat from the Sun. With one significant exception: energy in the IR range between 8 and 13µm wavelength is not captured by the atmosphere and instead exits directly out to space. ‘This gives you a channel to dissipate energy into the sky and out to the universe,’ says Zhengmao Lu, an energy transport scientist at the Swiss Federal Institute of Technology in Lausanne, Switzerland.
An everyday example is when there is a clear night sky, sometimes water will freeze even though the ambient temperature remains above freezing. The chilling of the water is due to heat exchange in the midIR between the sky and the water surface. This passive radiative cooling requires little or no energy input.
The phenomenon was long known, but with no direct application. ‘You need a surface that can reject solar heating, but also energy in the midIR,’ says Lu. A breakthrough came in 2014 when a group at Stanford University in California, US, reported a material that stayed 5°C cooler than its surroundings even in direct sunlight.
The group, led by Shanhui Fan, described a solar reflector and heat emitter made of alternating layers of hafnium dioxide and silicon dioxide on top of a silver layer, all placed on a thin silicon wafer. This reflected 97% of sunlight, yet also emitted strongly in the mid-IR. The Stanford group set up a company, SkyCool Systems, which sells a cooling material to improve the efficiency of aircon, save money and reduce CO2 emissions. Such radiative cooling systems could be game-changers in terms of cooling buildings and reducing their energy use by reflecting a broad spectrum of light, but also absorbing and then emitting in the mid-IR window.
UK startup Pirta was founded in the garage of a father-son team in Yorkshire during Covid lockdown. Read their story in C&I here. Image: Pirta
Other scientists have tried manipulating various commonplace materials to generate super-cool structures. At Colorado State University in the US, Xiaobo Yin and colleagues demonstrated daytime and night time radiative cooling with a hybrid glass-polymer made of a visibly transparent polymer encapsulating randomly distributed silicon dioxide microspheres. The cheap material emitted strongly in the mid-IR window and could be made by continuous roll-to-roll manufacturing. A team at the University of Maryland, US, also made a radiative cooling material from wood. First, they removed lignin to create a denser material eight times stronger than natural wood. The remaining cellulose was then engineered as cellulose nanofibers which backscattered sunlight and emitted strongly in the mid-IR.
At Purdue University, US, meanwhile, Xiulin Ruan and colleagues recently developed an extremely thin ultra-white acrylic paint with hexagonal boron nitride nanoparticles for daytime radiative cooling.
This achieved solar reflectance of 98% and high emissivity. During field tests, cooling of 5 to 6°C below ambient temperature on average was achieved during daylight hours. The material’s slimness and light weight – 150µm thick and 0.029g/cm2 – could make this suitable for use in buildings, cars, fabrics and even aircraft.
Other researchers are focusing on combining chemistry and technology to reduce heating and electricity bills. Phase change materials (PCM) are well known. The most obvious example is water, which releases or absorbs energy as it shifts phase from liquid to solid. However, new PCMs are now in use or under development in buildings, electronics and packaging. They work due to shifts in crystal structure, whereby at a certain temperature the material shifts from being insulating and allowing IR through, to being conductive and blocking IR radiation.
‘[This type of] passive cooling is basically energy-free climate control,’ says Mohammad Taha, an engineer at the University of Melbourne, Australia. Early in 2023, Taha’s group reported on ‘phase change inks’ for passively cooling homes and buildings. Taha sees big potential for the inks as a window coating. ‘If you look at the temperature profile of a house or building, most energy loss happens at the windows,’ says Taha.
The inks use nanoparticles to adjust the amount of radiation that can pass through them. Credit: University of Melbourne
To create the inks, Taha’s group uses vanadium oxide nanoparticles. Vanadium oxide is already tipped as promising for coatings that reflect heat above 68°C, but this is not especially helpful for use in buildings. By using nanoparticles, Taha’s group was able to make the phase shift happen at nearer room temperature.
The crystal structure of the material is all important. At lower temperatures, the nanoparticles adopt an insulating zig-zag structure that allows IR energy to pass through; once heat is applied, the structure becomes more linear and begins to conduct but also block IR. Taha was able to reduce the phase change temperature by dispersing large water molecules into the vanadium oxide crystal structure and encapsulating the material in a glass sphere. This creates a strain on the structure, which causes it to shift structures or ‘melt’ at lower temperatures. With greater molecular strain, Taha says the inks could shift at 30°C. The group is now talking to commercial partners about potential applications and manufacturing phase-change inks.
Emerging super-cool materials and phase-changing inks promise more tools for urban planners and building designs. Yet existing heat reduction technologies have often not gained traction, says Santamouris. ‘We now have the means to decrease the temperature of cities. But many of those responsible for cities such as local government or councils have not taken benefits from what we have developed.’
A lot of cities in the southern US, for example, still have heat-absorbing dark roofs. Sailor’s group has flown in a helicopter across Phoenix, Arizona, with an onboard IR camera. Many residential buildings have dark rooftops that become extremely hot, requiring significant aircon energy demand – warming neighbourhoods unnecessarily. ‘We’ve seen mixed results in terms of implementation of cool roofs,’ says Sailor. There is no mandate in Arizona for such roofs, whereas California and cities such as Los Angeles have increasingly mandated certain requirements for reflecting materials on roofs of commercial and even residential buildings.
There are, however, some downsides for such mitigation technologies. Passive cooling systems could reduce temperatures in winter, when you want to warm a building. The use of water for evapotranspiration could be problematic in hot, arid regions. Nonetheless, as cities break temperature records, there is at least some technological progress.
‘During the last five years, there’s been tremendous development with regards to new materials, such as reflective materials, but also super-cool materials,’ says Santamouris. ‘While coatings and materials surfaces for cooling might not be as noticeable to the public as photovoltaics or wind turbines, advances here nevertheless can reduce energy consumption while improving liveability.’ By implementing such new technologies in Riyadh, Saudi Arabia, Santamouris expects his group will be able to reduce city temperatures by 4.5°C in 2024. The group also has projects in Australia, Malaysia, India and the US.
Meanwhile, Sailor acknowledges he is ‘a proponent of using multiple technologies together or in combinations – so having many arrows in the quiver’. Those arrows will be needed as more cities move into the red zone and residents and businesses struggle with record-breaking temperatures.
- Joule; Vol 4, Issue 7, 1350-1356, 2020; doi: 10.1016/j.joule.2020.04.010
- Nature; 515, 540–544, 2014; doi: 10.1038/nature13883
- Science; Vol 355, Issue 6329, 2017; doi: 10.1126/science.aai7899
- Science; Vol 364, Issue 6442, 2019; doi: 10.1126/science.aau9101
- Cell Rep. Phys. Sci.; Volume 3, Issue 10, 2022; doi: 10.1016/j.xcrp.2022.101058