Radiative cooling applications

The ability to passively cool objects without any external energy input is extremely valuable across a wide range of human activities. Indeed, humans have been using passive cooling for thousands of years, even if the underlying mechanism of radiative cooling wasn’t understood until relatively recently. “We were inspired by the past, where over two centuries ago people in the desert were able to make ice even though temperatures never fell below freezing,” explains Sandra Go. “We were pretty interested in how they were able to do something like that.”

Today, modern innovators are applying radiative cooling across a range of applications, and below we have put together a non-exhaustive list of some of these use cases.

Built environment

The operation of buildings accounts for 30 per cent of global final energy consumption and 26 per cent of global energy-related CO2 emissions. What is more, nearly 20 per cent of the electricity used in buildings goes to fans and air conditioning units. The built environment is therefore an obvious place to start when thinking about where to apply the principles of radiative cooling.

“There are just so many uses for radiative cooling materials, but the first thing that comes to mind is definitely buildings,” explains Go. There are different ways of applying radiative cooling in the built environment, but perhaps the simplest is the creation of paints containing materials that impart a radiative cooling effect. These can be applied to the exterior surfaces of an existing structure to reduce the demands placed on active cooling systems. This is one of the main focuses of Emissiv, but other startups are also working on cooling paints, such as Ceracool in the US, Pirta in the UK, and i2Cool in Hong Kong.

There are also other ways to bring the power of radiative cooling to buildings. One of the leading academic pioneers of radiative cooling is Associate Professor Aaswath Raman, currently of The University of California, Los Angeles (UCLA). The startup he co-founded, SkyCool Systems, creates cooling panels that act as an add-on to existing air conditioning and refrigeration systems. These panels cool refrigerant fluid pumped through them, delivering claimed efficiency savings of 10 to 40 per cent. In some conditions, the panels can even replace an air conditioning unit entirely.

Meanwhile, a team of researchers from the University of Buffalo, the University of Wisonsin-Madison, and King Abdullah University of Science and Technology (KAUST) have tested a system that combines radiative cooling with solar heating. This setup consists of two spectrally selective ‘mirrors’ of layered silver and silicon dioxide placed in a V-shape. Rather than scattering light in the visible and near-infrared range (as typically happens in radiative cooling systems), the mirrors absorb this solar energy so it can be used for water heating. At the same time, the mirrors reflect waves that are in the mid-infrared range to a vertical box between them called an ‘emitter’. This bounces the waves through the atmospheric window and out into space. The researchers aim to commercialise the technology through a spin-out company called Sunny Clean Water.

Homes and office buildings could benefit from radiative cooling, but there are also other, more specialist buildings that have a particular need for 24/7, year-round cooling, such as data centres. These applications are particularly promising, as aesthetics, which can be a challenge with radiative cooling coatings as we shall see below, are not a major consideration.

Vehicles

In addition to buildings, an article in the journal National Science Review highlights that “radiative coolers also present a passive strategy for various vehicles, lowering gas exhaust emissions.”

This is particularly true for vehicles involved in cold-chain logistics. “There are vehicles that need to be cold all the time because they are transporting food or medicine,” explains Andreas Kafizas. “Normally these vehicles are quite tall, so you can’t see the top of them. If you just coat the whole top of the vehicle with this white reflective paint, aesthetically it won't damage the appearance too much and it provides a massive benefit – especially on those really hot summer days,” he adds.

According to Sandra Go: “In grocery and pharmaceutical deliveries, we've had some interest from customers who see the benefit of coating their delivery vehicles to reduce the fuel need or increase the mileage if the vehicles are electric vehicles.”

Passenger vehicles can also benefit from radiative cooling. “You can reduce the temperature of vehicles if you make a transparent film that can be stuck onto vehicles during the summer, and then removed for the winter,” explains Go.

Containers, refrigerated truck compartments, tank trucks, and ships are among the applications for i2Cool’s solutions. Similarly, in addition to its cooling panels, SkyCool Systems produces a film that can be applied to refrigerated trucks or other vehicles.

Water

According to the National Science Review article: “Harvesting water from the atmosphere is another dramatic application for radiative cooling.”

Atmospheric water harvesting is a promising solution for tackling water scarcity, but it is challenging in areas with low relative humidity. By achieving sub-ambient temperatures both day and night, radiative cooling solutions can facilitate condensation, making it more feasible to harvest moisture even in extreme conditions. Australian company Dewpoint Innovations, for example, develops cooling paints that it claims can make a surface cool enough to form water droplets in certain conditions. When the droplets are big enough, they roll down and can be collected by a normal rooftop water collection system. KAUST researchers, meanwhile, have created a radiative-cooling-based water-harvesting device that addresses one of the key challenges with such systems – the tendency for the water droplets to stick to the surface. To tackle this, the researchers added a lubricating layer to make it easier for the water droplets to slide off under the force of gravity and be collected.

In another paper published in the journal ACS Photonics in 2020, researchers explored the possibility of using radiative cooling to harvest moisture from solar panels during the nighttime. By doing so, the research team envisaged that solar assets can be useful even when they are not producing energy after dark.

Energy assets

Another promising use for radiative cooling is for the thermal management of energy-related assets. AssetCool, for example, has developed a radiative cooling coating that cools overhead power lines, thereby increasing their capacity.

“Radiative cooling helps make the grid more efficient,” explains Oliver Higbee. “Cooler conductors mean lower resistive losses, so less energy is wasted. It also increases line capacity – meaning the same infrastructure can carry more electricity without overheating.”

Other energy assets can also be optimised through the application of radiative cooling principles. “Beyond power lines, there’s clear potential in grid infrastructure like transformers and substations, where passive cooling can improve reliability,” Higbee explains. “There's also interest in solar panels and battery systems, where even modest temperature reductions can improve efficiency and lifespan,” he adds.

“A transparent coating or film can be applied to solar panels, as by reducing solar panel temperatures, you can increase their efficiency,” explains Sandra Go. “It's the same for cables and all of these other energy assets that typically get really hot, and need to be cooled to operate efficiently.”

Personal thermal management

Away from infrastructure, radiative cooling can also help to keep individuals comfortable in hot conditions.

One way to do this is through textiles that have a radiative cooling effect. To this end, two PhD candidates from the University of Chicago have created a multi-layered textile that helps people stay cool even in tightly packed urban environments. In these conditions, individuals are subjected to reflected heat from pavements and buildings in addition to direct heat from the sun. To tackle this, the top layer of the researchers’ material selectively emits in the atmospheric window, while a middle silver nanowire layer rejects incoming thermal radiation from the built environment. A third wool bottom layer draws heat from the wearer’s skin and moves it into the middle layer. The result is a material that has a cooling effect, even in the midst of urban heat islands.

Clothing is not the only way that radiative cooling can be applied for personal thermal management. Researchers at China’s Tsinghua University, for example, have created a sunscreen formulation that cools the wearer’s skin in addition to protecting it from harmful UV light. This effect is achieved by adjusting the size of titanium dioxide nanoparticles – an ingredient already present in existing formulations.

According to Oliver Higbee, the ability to cool without energy is “a powerful tool in a world that’s getting hotter and more energy-hungry.” The list in this briefing therefore only touches on a few of the many potential applications of radiative cooling. However, it provides an overview of some of the areas that innovators are currently focusing on.