ナノマテリアルによる10μ付近の放射冷却が
知る人ぞ知る冷却法として興味を引かれているが。
その構造などは知られいない。
探すと一つまともな論文が見つかった。
Metamaterial-Based Radiative Cooling: Towards Energy-Free All-Day Cooling
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情報が多くなりすぎて。処理が難しくなってきて
出るのはため息ばかりとなる。
Abstract
In the light of the ever increasing dangers of global warming, the efforts to reduce energy consumption by radiative cooling techniques have been designed, but are inefficient under strong sunlight during the daytime. With the advent of metamaterials and their selective control over optical properties, radiative cooling under direct sunlight is now possible. The key principles of metamaterial-based radiative cooling are: almost perfect reflection in the visible and near-infrared spectrum (0.3–3 µm) and high thermal emission in the infrared atmospheric window region (8–13 µm). Based on these two basic principles, studies have been conducted using various materials and structures to find the most efficient radiative cooling system. In this review, we analyze the materials and structures being used for radiative cooling, and suggest the future perspectives as a substitute in the current cooling industry.
Keywords:
metamaterial; daytime radiative cooling; infrared atmospheric window; selective reflection; selective emission
1. Introduction
As the awareness of the dangers of global warming grows, many novel ideas are being put forward to prevent the problem from developing further. Lots of research has been performed to find the method of cooling that minimizes the amount of air pollution and energy consumption in buildings, vehicles, and clothing [1,2,3]. Recently developed approaches are focused on radiative cooling, a technique requiring no energy to lower the ambient temperature, thus considered as a key to help to slow down the disastrous effects of global warming. Radiative cooling is a way of radiating heat into the universe using the transparency window of the Earth’s atmosphere (8–13 µm).
Various nighttime radiative cooling research has been reported by constructing selective emitters in the atmospheric transparency window [4,5,6] but the same techniques are inefficient during the daytime under strong sunlight. Daytime radiative cooling requires the additional condition of reflecting the entire solar spectrum (0.3–3 µm), while maintaining enough thermal radiation in the atmospheric window (Figure 1a) [7,8,9]. The biggest hurdle is that the absorption of solar energy usually far exceeds the possible thermal radiative power [9,10,11,12,13,14,15,16,17]. Metamaterial-based radiative cooling achieves sufficient daytime cooling by being designed to satisfy those exact conditions. Metamaterials are artificial structures which can realize various optical properties that do not exist in nature and have been developed with the advancement of high-level nanofabrication methods to create perfect absorbers, reflectors and spectral filters [18,19,20,21,22,23]. Metamaterial-based radiative cooling entered a new phase after influential research by Raman et al. [14,24,25] optimized the structure design to have optical properties that reflect the solar spectrum and emit thermal energy in the infrared (IR) atmospheric window [26,27]. Daytime radiative cooling was demonstrated with a decrease of 4.9 °C below the ambient temperature (cooling power: 40.1 W/m2) [28]. Follow-up studies have begun to research various shapes and structures of metamaterial-based radiative cooling methods [28,29,30], including biomimetics and the painting of metamaterials directly onto a surface [31,32]. Radiative cooling using phase change materials has also shown the additional function of temperature-dependent smart switching [33].
Figure 1. Schematic of the main concepts of metamaterial-based radiative cooling. (a) Schematic of the basic principles of radiative cooling. (b) Ideal reflection and emission properties for radiative cooling.
In this review, we discuss recent theoretical and experimental developments of radiative cooling techniques as an arising tool for energy-free all-day cooling, focusing on the structures and materials. We briefly describe the basic concepts and mechanisms needed to realize radiative cooling. Then we provide details of various investigations based on their experimental results from each structure and material. Finally, we provide the future perspectives and a discussion of radiative cooling. We expect this review to deliver insight into the concepts of radiative cooling and explore the advanced functionalities and applications which are useful for the cooling industry, such as in buildings, vehicles, and the functional clothing industry.