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Keeping our homes and offices at a comfortable temperature plays an important role in our modern lives. At present, this is usually achieved by using conventional air-conditioning units that have some drawbacks associated with them, including the need for electricity supply and the use of compressed environmentally non-friendly refrigerants.
A group of scientists from the USA has developed a simple and inexpensive polymer coating that can be painted on the exterior of a building and lower the interior temperature, even under direct sunlight, without the need for additional energy sources.
This is achieved through a process called passive daytime radiative cooling. The polymer coating reflects the sunlight, while at the same time, the accumulated heat from the building is radiated to the cold outer space through the atmosphere’s long-wave infrared transmission window.
Cooling homes, offices and factories in hot weather is an important requirement for maintaining a comfortable living and working environment. Employing conventional compressor-based air-conditioning has numerous drawbacks: it consumes a significant amount of energy, often requires refrigerants that are greenhouse gasses or deplete the ozone layer, and has a net positive heating effect on the environment.
The prospect of inexpensive, safe and energy-efficient (or even energy-independent) cooling approaches is very tempting but still seems to be beyond the reach of our technology. As it turns out, some recent developments in material science have come very close to fulfilling these conflicting requirements.
Radiating Heat to the Outer Space
A promising alternative approach is based on the so-called passive daytime radiative cooling. If an object is highly reflective for the visible light (wavelengths in the range 0.3 – 0.7 µm) and near-to-short–wavelength infrared light (0.7 – 2.5 µm), and at the same time radiates light in the long-wave infrared range (in the range 8 – 13 µm) – in what is known as the long-wave infrared transmission window of Earth’s atmosphere – then, such an object would spontaneously cool down by radiating the excess heat directly to the cold outer space beyond Earth’s atmosphere.
By tuning the surface properties of the object, a sufficient cooling effect can be achieved even under bright sunlight. It can be suggested that, if material with such properties is applied to the roof and the walls of a building, the cooling of the building would occur without any need for electricity, refrigerants and any associated machinery.
In the past 20 years, scientists have developed a wide range of materials capable of passive daytime radiative cooling, such as photonic structures, dielectrics, polymers, polymer-dielectric composites, metal mirrors, and many others.
Although capable of passive cooling, most of these designs have certain downsides such as high manufacturing cost, low corrosion resistance and being susceptible to weathering. In addition, the prefabricated composite materials often require a specially prepared surface to be applied on and cannot be easily coated onto existing roofs or walls. Because of that, paint-like compounds have also been developed for easier application on buildings, but their passive cooling capabilities are compromised by low solar reflectance and parasitic absorbance in the ultraviolet range.
Nano-Cavities Outperform Nano-Particles
Recently, researchers from Columbia University in New York and from Argonne National Laboratory succeeded in developing an inexpensive and scalable method for the fabrication of a new passive daytime radiative cooling material that can be applied by painting or spraying onto a wide range of surfaces such as plastics, metal and wood.
The hypothesis is that, instead of using dielectric reflective pigments (such as silicon dioxide, titanium oxide or zinc oxide) embedded in a polymer matrix, a porous material with far superior optical performance can be created.
By controlling the size of the voids in the material, the scattering and reflectance properties of the material can be tuned to the desired wavelength ranges. Furthermore, eliminating the pigments and replacing them with light-scattering air voids reduces manufacturing cost. This also eliminates the threats that the potential release of nanoparticles/dyes (upon the degradation of the conventional polymer-dye composite coatings) pose to the environment.
Cheap and Efficient Self-Assembly
To verify that hypothesis, the researchers chose fluorine-based co-polymer - poly(vinylidene fluoride-co-hexafluoropropene). This co-polymer is soluble in acetone but insoluble in water. The co-polymer/acetone/water suspension can be easily painted or sprayed onto a substrate. Then, upon drying in the air, the acetone rapidly evaporates and disturbs the equilibrium of the suspension. This, in turn, leads to the formation of small water droplets with sizes ranging from a few hundred nanometres up to several microns.
Since the co-polymer is insoluble in water, it crystallizes around the water droplets, producing a porous structure surrounding the water droplets. In the final stage, all the water evaporates, forming micro- and nano-cavities in the coating that efficiently backscatter the sunlight and enhance the thermal emittance. The resulting coating has a matte, white appearance, and reflects up to 96% of the incident sunlight and has an emittance of 97% in the long-wave infrared range.
Durability under the Sun
The researchers also tested the daytime cooling ability of the new coating under a wide range of conditions. Under the clear skies of Arizona (USA), the newly developed co-polymer coating managed to cool the underlying substrate to around 6 °C below the ambient air temperature.
In Bangladesh’s more humid environment, the coating achieved 3 °C of undercooling, despite the fact that fog and haze hinder the radiative heat dissipation through Earth’s atmosphere. It was also established that, after being exposed to the atmospheric elements for more than a month, the coating retains its optical properties as well as its passive cooling capability, without noticeable degradation. It is worth noting that the fluorine-containing co-polymer gives the coating an excellent chemical residence and hydrophobic self-cleaning properties (repelling the airborne contaminants).
Another bonus is the proposed method’s compatibility with a wide variety of commercially available polymers, including biocompatible materials such as ethyl cellulose.
References and Further Reading
Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling. J. Mandal et al., Science 2018: 362, 315-319. Available at: https://doi.org/10.1126/science.aat9513
Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling. Y. Zhai et al., Science 2017: 355, 1062 – 1066. Available at: https://doi.org/10.1126/science.aai7899
Burke, M., Hsiang, S. & Miguel, E. Global non-linear effect of temperature on economic production. Nature 527, 235–239 (2015). https://doi.org/10.1038/nature15725
J. Song et al., Solar Energy Materials and Solar Cells 2014: 130, 42 – 50. Available at: https://www.journals.elsevier.com/solar-energy-materials-and-solar-cells
Metal-based nanoparticles in soil: fate, behavior, and effects on soil invertebrates. P. S. Tourinho et al., Environmental Toxicology and Chemistry 2012: 31, 1679 – 1692. Available at: https://doi.org/10.1002/etc.1880
Radiative Heat Pumping from the Earth Using Surface Phonon Resonant Nanoparticles. A. R. Gentle and G. B. Smith, Nano Letters 2010 10 (2), 373-379 DOI: 10.1021/nl903271d
Surface coatings for radiative cooling applications: Silicon dioxide and silicon nitride made by reactive rf-sputtering. T. S. Eriksson et al., Solar Energy Materials 1985: 12, 319 – 325. Available at: https://doi.org/10.1016/0165-1633(85)90001-2