Challenges

As with all new technologies, innovators must overcome some key challenges if radiative cooling solutions are to reach the mainstream. The various applications outlined in this briefing note are all different from one another, and each faces specific hurdles. However, there are some higher-level issues that cut across all (or many) of the different areas of development.

Sensitivity to environment and location

With radiative cooling, where you are in the world really matters.

This is, in part, because environmental conditions have a significant impact on the effectiveness of a radiative cooling system. According to the National Science Review article, “radiative cooling technology performance is limited by the environment, especially under high-humidity conditions.” This is because water vapour eats into the atmospheric window by blocking some of the wavelengths that can typically escape the atmosphere when humidity is low.

“One of the issues to note is that radiative cooling doesn't work too well in humid climates as water vapour does capture a lot of those infrared emissions,” explains Sandra Go. Emissiv is therefore considering exactly how radiative cooling should be applied in hotter climates.

At the same time, with most radiative cooling solutions, once they are applied, they are applied year-round. This is fine near the equator where the demand for cooling is relatively consistent across the year, but it creates a risk of overcooling in seasonal climates.

“If you were to apply a radiative cooling coating on a building in the UK in the summer, it would have massive benefits,” explains Andreas Kafizas. “But in the winter, there could be drawbacks because it's reflecting that solar energy away, when actually it's a time when you want that heat in the building.” Because of this, when considering whether to apply a radiative cooling coating to a specific building, “you’d need to do the cost-benefit analysis to see if it’s worthwhile,” explains Kafizas. For buildings like data centres that produce excess heat all year, this is not a consideration, but for domestic and office building applications overcooling is a major factor.

Durability

For radiative cooling materials to work in the real world, they need to be able to withstand the elements without onerous maintenance.

“One of the biggest challenges is durability in real-world conditions,” explains Oliver Higbee. “Many materials that show excellent radiative cooling performance in the lab degrade rapidly outdoors due to UV exposure, moisture, abrasion, or contamination.” Similarly, Sandra Go notes that all the potential customers for Emissiv’s paints “think about durability because they don't want to just be coating it for one summer.”

Higbee further highlights that durability “often comes at the cost of photonic performance – the more durable the coating, the harder it becomes to maintain the ideal balance of reflectivity and emissivity.” This is particularly challenging for AssetCool’s chosen application of overhead lines due to their high operating temperatures.

For radiative cooling innovators, one of the most important success factors is therefore the ability to strike the right balance between performance and durability. “Finding that sweet spot is a unique materials science challenge,” explains Higbee.

Aesthetics

According to the Advanced Science review article, “while the concept of radiative cooling is typically to control the radiative thermal load of outdoor structures on buildings, automobiles, and clothing, the visual aesthetic is also a significant aspect that cannot be ignored.”

“These coatings are typically quite white,” Andreas Kafizas explains. This is fine if you’re happy with a white building, but what if you want to add a splash of colour?

Radiative cooling innovation comes from manipulating the optical properties of a material. Colour (and transparency) are themselves optical properties, so they have an impact on the material’s cooling performance. Despite this, Kafizas highlights that it is possible to develop radiative cooling coatings that have different colours and levels of transparency – there is just a trade-off between those aesthetic considerations and the extent of the cooling effect.

“If, for example, you have very fancy brickwork on your building and you don’t want to lose that aesthetic, you could maybe use a semi-transparent radiative cooling coating,” he explains. “The building could still have that aesthetic whilst also having some ability to radiatively cool.”

A company that has created radiative cooling paints in multiple colours is Ceracool. The startup’s red formulation, for example, has solar reflectance of 93 per cent, which is not only much higher than other red paints, but also conventional white paint (85 per cent).

Even though it is possible to create radiative cooling materials and coatings that are more colourful or transparent, colouration and transparency are yet further considerations that innovators must factor into the equation.

Scalability and sustainability

As with any innovation – and as touched on above – the ability to sustainably scale production is mission-critical for innovators looking to commercialise radiative cooling technologies.

“Producing coatings with carefully tuned optical properties – high solar reflectance and high thermal emissivity – can involve complex materials or application processes that are not always easy to scale cost-effectively,” explains Oliver Higbee.

Sandra Go echoes this by pointing out that some of the research in this area uses “expensive nanoparticles, or complicated methods that work at a small scale, but would be hard to scale up.”

According to the authors of the Advanced Science review article: “Although simple and scalable manufacturing methods have been evaluated, some are only compatible with specific materials and structures.” The article also points to some of the environment impacts of radiative cooling materials: “Some cause environmental problems, such as use of hazardous chemicals and emission of volatile organic compounds.”

Sandra Go adds that: “One issue with some of the other radiative cooling paints out there is that they use PFAS [so-called ‘forever chemicals’].” Emissiv, by contrast, is designing its paints “such that we do not need to use a PFAS-based polymer.”