Reducing human reliance on energy-inefficient cooling methods such as air conditioning would have a large impact on the global energy landscape. By a process of complete delignification and densification of wood, we developed a structural material with a mechanical strength of 404.3 megapascals, more than eight times that of natural wood. The cellulose nanofibers in our engineered material backscatter solar radiation and emit strongly in mid-infrared wavelengths, resulting in continuous subambient cooling during both day and night. We model the potential impact of our cooling wood and find energy savings between 20 and 60%, which is most pronounced in hot and dry climates.

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A radiative cooling structural material.

Radiative cooling: A review of fundamentals, materials, applications, and prospects

Radiative sky cooling cools an object on the earth by emitting thermal infrared radiation to the cold universe through the atmospheric window (8–13 μm). It consumes no electricity and has great potential to be explored for cooling of buildings, vehicles, solar cells, and even thermal power plants. Radiative sky cooling has been explored in the past few decades but limited to nighttime use only. Very recently, owing to the progress in nanophotonics and metamaterials, daytime radiative sky cooling to achieve subambient temperatures under direct sunlight has been experimentally demonstrated. More excitingly, the manufacturing of the daytime radiative sky cooling material by the roll-to-roll process makes large-scale deployment of the technology possible. This work reviews the fundamental principles of radiative sky cooling as well as the recent advances, from both materials and systems point of view. Potential applications in different scenarios are reviewed with special attention to technology viability and benefits. As the energy situation and environmental issues become more and more severe in the 21st century, radiative sky cooling can be explored for energy saving in buildings and vehicles, mitigating the urban heat island effect, resolving water and environmental issues, achieving more efficient power generation, and even fighting against the global warming problem.

Radiative sky cooling: Fundamental principles, materials, and applications

Radiative cooling is a passive cooling strategy with zero consumption of electricity that can be used to radiate heat from buildings to reduce air-conditioning requirements. Although this technology can work well during optimal atmospheric conditions at night, it is essential to achieve efficient cooling during the daytime when peak cooling demand actually occurs. Here we report an inexpensive planar polydimethylsiloxane (PDMS)/metal thermal emitter thin film structure, which was fabricated using a fast solution coating process that is scalable for large-area manufacturing. By performing tests under different environmental conditions, temperature reductions of 9.5 °C and 11.0 °C were demonstrated in the laboratory and an outside environment, respectively, with an average cooling power of ~120 W m–2 for the thin film thermal emitter. In addition, a spectral-selective structure was designed and implemented to suppress the solar input and control the divergence of the thermal emission beam. This enhanced the directionality of the thermal emissions, so the emitter’s cooling performance was less dependent on the surrounding environment. Outside experiments were performed in Buffalo, New York, realizing continuous all-day cooling of ~2–9 °C on a typical clear sunny day at Northern United States latitudes. This practical strategy that cools without electricity input could have a significant impact on global energy consumption. Radiative cooling can reduce air-conditioning requirements. In this study, the authors demonstrated an inexpensive thermal emitter that provided continuous daytime cooling up to 9 °C outdoors on a clear, sunny New York day.

/pdf/a-polydimethylsiloxane-coated-metal-structure-for-all-day-49ygpp34j3.pdf

A polydimethylsiloxane-coated metal structure for all-day radiative cooling

Subambient Cooling of Water: Toward Real-World Applications of Daytime Radiative Cooling

We demonstrate that photonic media, when properly randomized to minimize the photon transport mean free path, can be used to coat a black substrate and reduce its temperature by radiative cooling Even under strong solar radiation, the substrate temperature could reach substantially below that of the ambient air Our random media that consist of silica microspheres considerably outperform commercially available solar-reflective white paint for daytime cooling We have achieved the outstanding cooling performance through a systematic study on light scattering, which reveals that the structural parameters of the random media for maximum scattering are significantly different from those of the commercial paint We have created the random media to maximize optical scattering in the solar spectrum and to enhance thermal emission in the atmospheric transparency window In contrast to previous studies, our random media do not require expensive processing steps or materials, such as silver, and can be applied to 

Effective Radiative Cooling by Paint-Format Microsphere-Based Photonic Random Media

We demonstrate that white coatings consisting of silicone embedded with randomly distributed microbubbles provide highly efficient daytime radiative cooling with inexpensive materials and fabrication processes. In our material system, sunlight is strongly scattered with minimal absorption, and heat is effectively removed through mid-infrared (IR) radiation. In our previous study, solid microsphere-based coatings outperformed commercial solar-rejection white paint in cooling efficiency, but their mechanical robustness needed improvement for practical applications. The material system in our work substantially enhances the mechanical robustness while providing superior cooling performance to commercial solar-rejection paint. For ease of processing, we use nonoptimized structures with reduced optical scattering strength. Strong solar rejection is yet achieved by increasing coating thickness. This strategy is desirable for practical rooftop applications where coating thickness is of minor importance in comparison to cooling performance and materials cost. In addition, silicone is stable in extraterrestrial environments and efficiently radiates heat over broad mid-IR spectrum. These material properties of our silicone coatings promise great potential for radiative cooling in space applications.

Radiative cooling by silicone-based coating with randomly distributed microbubble inclusions

Photonic structures in biological systems typically exhibit an appreciable level of disorder within their periodic framework. However, how such disorder within the ordered framework renders unique optical properties has not been fully understood. Toward the goal of improving this understanding, we have investigated Langmuir-Blodgett assembly of microspheres to controllably introduce randomness to photonic structures. We theoretically modeled the assembly process and determined a condition for surface pressure and substrate pulling speed that results in maximum structural order. For each surface pressure, there is an optimum pulling speed and vice versa. Along the trajectory defined by the optimum condition, however, the structural order decreases moderately as the pulling speed increases. This moderate decrease in structural order would be useful for controlled introduction of randomness into the periodic structures. Departing from the trajectory, our experiment reveals that a small change in pulling speed at a given surface pressure can significantly disrupt the structural order. For multilayer assembly, we find that, at a fixed pulling speed, the surface pressure should increase as the number of layers increases to achieve maximum structural order. In totality, we quantitatively present the optimum trajectories for the nth layer assembly relating surface pressure and pulling speed.

Control of Randomness in Microsphere-Based Photonic Crystals Assembled by Langmuir–Blodgett Process

Inclusion of microencapsulated phase change materials in waterborne paints to enhance radiative cooling performance

Radiative cooling paints offer a sustainable method for reducing energy demand in buildings. This work investigates the effects of particle volume concentration (PVC) of calcium carbonate (CaCO3) and hollow silicon dioxide (SiO2) microparticles in acrylic-based paints on cooling capability. Paint solar reflectance increases as the total PVC increases until a peak PVC is reached. The addition of CaCO3 improves solar reflectance due to an enhancement in near-infrared reflection but decreases thermal emission. The paint, with a total PVC of 0.45 and a CaCO3:SiO2 PVC ratio of 1:1, offers the right balance between solar reflection and thermal emissivity and achieves the best cooling performance. This work demonstrates that the CaCO3-SiO2 paint system allows tuning of the radiative cooling performance. 

https://www.tandfonline.com/doi/pdf/10.1080/14786451.2023.2221082?needAccess=true&role=button

Effects of pigment volume concentration on radiative cooling properties of acrylic-based paints with calcium carbonate and hollow silicon dioxide microparticles

Sarun Atiganyanun

Papers

Effective Radiative Cooling by Paint-Format Microsphere-Based Photonic Random Media

Radiative cooling by silicone-based coating with randomly distributed microbubble inclusions

Control of Randomness in Microsphere-Based Photonic Crystals Assembled by Langmuir–Blodgett Process

Inclusion of microencapsulated phase change materials in waterborne paints to enhance radiative cooling performance

Effects of pigment volume concentration on radiative cooling properties of acrylic-based paints with calcium carbonate and hollow silicon dioxide microparticles