Radiative cooling is a passive cooling technology without any energy consumption, compared to conventional cooling technologies that require power sources and dump waste heat into the surroundings. For decades, many radiative cooling studies have been introduced but its applications are mostly restricted to nighttime use only. Recently, the emergence of photonic technologies to achieves daytime radiative cooling overcome the performance limitations. For example, broadband and selective emissions in mid-IR and high reflectance in the solar spectral range have already been demonstrated. This review article discusses the fundamentals of thermodynamic heat transfer that motivates radiative cooling. Several photonic structures such as multilayer, periodical, random; derived from nature, and associated design procedures were thoroughly discussed. Photonic integration with new functionality significantly enhances the efficiency of radiative cooling technologies such as colored, transparent, and switchable radiative cooling applications has been developed. The commercial applications such as reducing cooling loads in vehicles, increasing the power generation of solar cells, generating electricity, saving water, and personal thermal regulation are also summarized. Lastly, perspectives on radiative cooling and emerging issues with potential solution strategies are discussed. 

/pdf/photonic-structures-in-radiative-cooling-22er185z.pdf

Photonic structures in radiative cooling

With the aggravation of global warming and extreme weather, cooling demand has witnessed a continuous increase and is expected to increase tenfold by 2050. Radiative sky cooling (RSC) without any pollution and energy consumption has drawn worldwide attention in the past few years. Large‐scale and scalable porous cooling materials have greatly advanced the progress of this technology. Herein, a critical review on porous cooling materials with the goal of advancing their commercial applications is presented. The detailed design principles of daytime radiative cooling are first discussed to clarify the critical factors of porous structures for great cooling performance. What follows are detailed discussions on porous cooling materials from pore‐forming methods. Subsequently, the recent progress of the promising particle‐embedding porous structures, mainly including white and colorful coolers for various applications, is outlined. Additionally, some special cooling materials are highlighted to further broaden the applications of RSC technology. Last but not the least, the remaining open challenges and the insights are presented for the further advance of the commercialization progress.

Micro‐Nano Porous Structure for Efficient Daytime Radiative Sky Cooling

Scalable multifunctional radiative cooling materials

Dynamic Radiation Regulations for Thermal Comfort

Although passive radiative cooling has received great attention recently because it can minimize the absorption of thermal energy and emits its own thermal energy to the cold outer space, the...

A self-adaptive film for passive radiative cooling and solar heating regulation

Thermal Homeostasis Enabled by Dynamically Regulating the Passive Radiative Cooling and Solar Heating Based on a Thermochromic Hydrogel


 Conventional static radiative coolers have an unadjustable cooling capacity, which often results in overcooling in low temperature environment. Therefore, there is a great need for an adaptive dynamic radiative cooler. However, such adaptive coolers usually require complex preparation processes. This paper proposes an adaptive radiative cooler based on a Fabry–Perot resonant cavity. By optimizing the structural parameters of the radiative cooler, this adaptive radiative cooler achieves a modulation rate of 0.909 in the atmospheric window band. The net radiative cooling performance difference between low and high temperatures is nearly eight times. Meanwhile, the device is easily prepared, has a high tolerance, and can effectively prevent W–VO2 oxidation. This study provides new insights into adaptive radiative cooling with potential for large-scale applications.

Theoretical study of a highly fault-tolerant and scalable adaptive radiative cooler

Abstract. Passive radiative cooling is an environmentally friendly and energy-free cooling method, but the practical application of radiative cooling materials is still limited by high costs and cumbersome preparation processes. Here, low-cost and chemically stable polyvinylidene fluoride (PVDF) was selected as the raw material and porous P-PVDF films for daytime radiative cooling were prepared by a simple phase separation method. A solar reflectivity of 95.6% and an atmospheric window emissivity of 99.1% were obtained, resulting in high-performance radiative cooling without a metal reflective layer. A cooling power of 69.43  W  ·  m  −  2 was achieved in direct sunlight, achieving sub-ambient cooling of 4.2°C. This work provides a unique solution for radiative cooling materials, which is expected to be implemented in practical applications of passive radiative cooling.

Low-cost and scalable sub-ambient radiative cooling porous films


 With the development of technology, people are demanding more comfort in their lives, leading to an increasing demand for space cooling. The traditional cooling process produces a great amount of energy consumption and carbon emission, which puts tremendous pressure on the environment. Direct evaporative cooling technology is a good choice for reducing energy consumption and carbon emissions. However, most of the current direct evaporative coolers offer excellent cooling performance at any temperature, whether the space needs to be cooled or heated. The unwanted evaporation cooling at low temperature will increase the heating energy consumption of air conditioners. Here, an adaptive evaporative cooling structure that can intelligently adjust the evaporative cooling ability according to the ambient temperature is proposed. The structure consists of temperature-sensitive hydrogel (prepared by chemical synthesis), perforated PET and silica coating (prepared by screen printing and hot pressing technology), and the evaporation rate of the cooler is regulated by both the hydrogel and the porous membrane. The cooler can maintain high-efficiency cooling performance during the hot time, while suppressing the cooler's overcooling behavior at low temperatures to achieve more efficient energy savings. When the ambient temperature is higher than 36°C, its cooling capacity can reach 10°C. And when the temperature is lower than 18°C, it can inhibit the temperature difference to less than 1°C. This cooler structure has important application prospects in these fields such as building, automobile and personal wearable device. It can substantially reduce the energy consumption and environmental pollution.

An adaptive direct evaporative cooler that regulates the sub-ambient temperature by controlling the evaporation rate

Radiative cooling is an effective technology with zero energy consumption to alleviate climate warming and combat the urban heat island effect. At present, researchers often use foam boxes to isolate non-radiant heat exchange between the cooler and the environment through experiments, so as to achieve maximum cooling power. In practice, however, there are challenges in setting up foam boxes on a large scale, resulting in coolers that can be cooled below ambient only under low convection conditions. Based on polymer materials and nano-zinc oxide (nano-ZnO, refractive index > 2, the peak equivalent spherical diameter 500 nm), the manufacturing process of heat pump film (HPF) was proposed. The HPF (4.1 mm thick) consists of polyethylene (PE) bubble film (heat transfer coefficient 0.04 W/m/K, 4 mm thick) and Ethylene-1-octene copolymer (POE) cured nano-ZnO (solar reflectance ≈94% at 0.075 mm thick). Covering with HPF, the object achieves 7.15 °C decreasing in normal natural environment and 3.68 °C even under certain circumstances with high surface convective heat transfer (56.9 W/m2/K). HPF has advantages of cooling the covered object, certain strength (1.45 Mpa), scalable manufacturing with low cost, hydrophobic characteristics (the water contact angle, 150.6°), and meeting the basic requirements of various application scenarios.

/pdf/a-scalable-heat-pump-film-with-zero-energy-consumption-1qc10djk.pdf

A Scalable Heat Pump Film with Zero Energy Consumption

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