Session: 11-10-04: Radiative Heat Transfer Across Scales

Paper Number: 150045

150045 - Bioinspried Radiative Cooling Material 

Although greenhouse gas reduction technology has been continuously developed, it has not been replaced by an eco-friendly system due to the limitations of the existing cooling system mechanism. Recently, the demand for the development of a new concept cooling mechanism system is increasing through the use of eco-friendly energy. Passive daytime radiative cooling radiates heat to the cold outer space. We present bioinspired radiative cooling material that mimic nano pore structure of butterfly wing in simple method for fabricating porous Poly(vinylidene fluoride-co-hexafluoropropylene). High solar reflectance 92.6%, and long wave infrared (LWIR) emittance 96.7% allows for sub-ambient temperature drops of 6.9℃ under solar intensities of 700 Wm-2.

Over the past 50 years, increased use of air conditioners has led to more than 6% of cooling energy consumption, and increased carbon dioxide and greenhouse gas emissions have accelerated global warming.1) Greenhouse gas reduction technology has been continuously developed, but has not been replaced by eco-friendly systems due to limitations of existing cooling system mechanisms.2) Recently, there is an increasing demand for the development of a new concept cooling mechanism system through the use of eco-friendly energy. In order to meet the emerging needs from this global warming problem, this study proposes a radiant cooling material technology that can be cooled without energy consumption. Radiative cooling is a technology that lowers the temperature by itself without using separate energy by dissipating heat in the form of electromagnetic waves through radiation, one of the heat transfer mechanisms.3) This technology may sound unfamiliar or think that cooling without using energy violates the laws of thermodynamics, but it is always happening around us and follows the laws of nature as follows The reason why Earth's temperature does not continue to rise is because of radiation cooling, which radiates infrared electromagnetic waves out of the cryogenic atmosphere of 3 K (270°C below zero) through the atmosphere. Radiative cooling of the Earth is carried out through a specific electromagnetic wavelength region (8-13 μm), which is called an atmospheric window. (Figure 1) 4) Infrared electromagnetic wavelengths in the window area of the atmosphere exhibit very high transmittance. At wavelengths except for the window area of the atmosphere, gases in the atmosphere, such as water vapor and carbon dioxide, absorb infrared rays, which are absorbed by the atmosphere, are emitted back to the surface by Kirchhoff's law and prevent the Earth's temperature from falling.3) This phenomenon is called the Greenhouse Effect. The total energy received on Earth's surface can be expressed as the sum of solar energy, energy coming down from the atmosphere, and energy going out of the atmosphere. Analysis of the total energy of (Fig2) shows a large difference between visible and near-infrared regions, which are solar energy, and infrared regions from the atmosphere, meaning that the absorption rate of visible and near-infrared regions must be lowered (increased reflectance) and the infrared region must be emitted through the window of the atmosphere to develop high-performance radiant cooling materials.(Fig 3)5) Radiative cooling materials can also be found in natural organisms. For example, the nanostructure hair (Hair) of Sahara Desert ants has a relatively low solar absorption rate (<40%) and high infrared radiation. (Fig. 4A)6) These structures can survive in hot deserts because they can regulate heat from the body while minimizing the impact of solar radiation. Nanostructures can also be found in butterflies. Butterflies regulate body temperature by dissipating heat generated from the body through wings due to long-term flight. If you look closely at the wing structure, it consists of a number of nanostructure pores and grids. (Fig. 4B) 7) The size and density of the pores and the spacing of the grids differ for each type of butterfly, and the infrared emissivity varies accordingly. In particular, the part of the butterfly's wings where many pores are distributed has a high infrared radiation rate. 

In addition, the regular lattice structure of butterfly wings also shows high emissivity. (Figure 5) Using these specific structures and materials, radiant cooling materials with high reflectance in the visible light region and radiance in the infrared region can be made, and if the wavelength is controlled according to the system environment, the system can be cooled below the outside temperature without additional energy.
In this study, a radiant cooling material that simulates the pore structure of butterfly wings was manufactured using a poly (PVDF-HFP) polymer. (Fig 6) The nano-sized pore structure of the butterfly wing generated a light scattering effect, increasing the reflectance of the visible light area and forming a difference in the refractive index between the air in the pores and the polymer material, thereby increasing the emissivity of the infrared area. Finally, through the external temperature evaluation of the radiant cooling manufacturing material, the average surface temperature of the material dropped by 6.9℃ compared to the outside temperature and the difference in the internal temperature of the box occurred by 43℃ when radiant cooling was applied, confirming a new concept of non-power cooling technology.

Presenting Author: Min Jae Lee Hyundai Motro Company

Presenting Author Biography: Min Jae Lee received his B.S. degree in mechanical engineering and M.S. degree in material science from UC, San Diego in 2015 and 2016 respectively. He joined Hyundai Motor Group since 2016. He is currently Ph.D. candidate under Professor SeungHawn, Ko in mechanical engineering at Seoul National University. His research focuses on radiative cooling, solar cell, and energy saving and harvesting material.

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