물 온도를 낮추는 폴리머 코팅 VIDEO: Polymer coating cools down buildings

Polymer coating cools down buildings

(Nanowerk News) With temperatures rising and heat-waves disrupting lives around the world, cooling solutions are becoming ever more essential. This is a critical issue especially in developing countries, where summer heat can be extreme and is projected to intensify. But common cooling methods such as air conditioners are expensive, consume significant amounts of energy, require ready access to electricity, and often require coolants that deplete ozone or have a strong greenhouse effect.

Glass skyscrapers such as London’s “Walkie-Talkie” are ill-adapted for climate change

Richard Baker/Getty


건물 온도를 낮추는 폴리머 코팅 

미 컬럼비아 대학, 아르곤 국립 연구소 개발

  미국 컬럼비아 대학(Columbia University)과 아르곤 국립 연구소(Argonne National Laboratory)의 연구진은 건물 온도를 낮출 수 있는 새로운 폴리머 코팅을 개발했다.

기온이 상승하면서 냉각 솔루션은 필수적인 요소가 되었다. 특히, 개발도상국에서 심각한 문제이고 여름이 될수록 더 큰 문제로 대두되고 있다. 그러나 에어컨과 같은 일반적인 냉각 방법은 상당한 에너지를 소비하며 종종 오존을 고갈시켜서 온실 효과를 발생시킬 수 있다.

에너지 집약적인 냉각 방법의 대안으로 PDRC(passive daytime radiative cooling)가 있는데, 이것은 햇빛을 반사하고 더 차가운 대기 중으로 열을 방출시켜서 표면을 자발적으로 냉각시키는 것이다.

실용적인 PDRC 디자인을 개발하는 것은 어려운 일이다. 최근의 많은 PDRC 디자인은 복잡하거나 비용이 많이 들며 모양과 질감이 건물에 적용할 수 없었다. 지금까지는 저렴하면서 적용이 쉬운 백색 도료가 가장 좋은 것으로 알려졌지만, 백색 도료는 자외선을 흡수하는 색소를 가지고 있어서 더 긴 태양 파장을 잘 반사하지 못하므로 성능이 매우 낮았다.

이번 연구진은 자발적인 공기 냉각기 역할을 하면서 옥상, 건물 외부, 수조, 자동차, 우주선까지 적용할 수 있는 고성능 PDRC 폴리머 코팅을 개발했다. 다공성 폴리머의 공기 공극은 공극과 주변 폴리머 사이의 굴절률 차이로 인해서 햇빛을 반사하고 산란시켰다.

이번 연구에서는 간단한 용액 기반의 방법인 상반전(phase-inversion)을 사용했다. 폴리머와 용매는 페인트에서 이미 사용되는 것이고 이것은 모든 파장의 태양빛을 반사시키는 공기 공극을 가지고 있다.

폴리머 코팅은 높은 태양광 반사율(R > 96%)과 높은 열 방출량(ε ~ ​​97 %)을 가지고 있으며 넓은 공간의 온도를 낮출 수 있는 능력을 가지고 있다. 미국 애리조나 주의 따뜻하고 건조한 사막에서는 6℃, 방글라데시의 안개가 자욱한 열대 환경에서는 3℃ 정도 더 차가운 조건을 만들 수 있다.

올해 북미, 유럽, 아시아, 호주 등에서 기록적인 온도 상승을 경험했다. 이런 기후 변화에 대한 해결책을 찾는 것이 필수인데, 이번 연구는 이것을 해결하는데 매우 유용하게 적용될 수 있을 것이다. 이 연구결과는 저널 Science에 “Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling” 라는 제목으로 게재되었다(DOI: 10.1126/science.aat9513).


edited by kcontents

An alternative to these energy-intensive cooling methods is passive daytime radiative cooling (PDRC), a phenomenon where a surface spontaneously cools by reflecting sunlight and radiating heat to the colder atmosphere. PDRC is most effective if a surface has a high solar reflectance (R) that minimizes solar heat gain, and a high, thermal emittance (ε) that maximizes radiative heat loss to the sky. If R and ε are sufficiently high, a net heat loss can occur, even under sunlight.

Developing practical PDRC designs has been challenging: many recent design proposals are complex or costly, and cannot be widely implemented or applied on rooftops and buildings, which have different shapes and textures. Up to now, white paints, which are inexpensive and easy to apply, have been the benchmark for PDRC. White paints, however, usually have pigments that absorb UV light, and do not reflect longer solar wavelengths very well, so their performance is only modest at best.

Researchers at Columbia Engineering have invented a high-performance exterior PDRC polymer coating with nano-to-microscale air voids that acts as a spontaneous air cooler and can be fabricated, dyed, and applied like paint on rooftops, buildings, water tanks, vehicles, even spacecraft—anything that can be painted. They used a solution-based phase-inversion technique that gives the polymer a porous foam-like structure. The air voids in the porous polymer scatter and reflect sunlight, due to the difference in the refractive index between the air voids and the surrounding polymer. The polymer turns white and thus avoids solar heating, while its intrinsic emittance causes it to efficiently lose heat to the sky. The study is published online today in Science ("Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling").


                                    When exposed to the sky, the porous polymer PDRC coating reflects 

                                    sunlight and emits heat to attain significantly cooler temperatures than 

                                    typical building materials or even the ambient air.

Passive daytime radiative cooling (PDRC) involves simultaneously reflecting sunlight and radiating heat into the cold sky to achieve a net heat loss. The process, which is spontaneous, can cool down structures to sub-ambient temperatures.

The team – Yuan Yang, assistant professor of materials science and engineering; Nanfang Yu, associate professor of applied physics; and Jyotirmoy Mandal, lead author of the study and a doctoral student in Yang’s group (all department of applied physics and applied mathematics) – built upon earlier work that demonstrated that simple plastics and polymers, including acrylic, silicone, and PET, are excellent heat radiators and could be used for PDRC. The challenges were how to get these normally transparent polymers to reflect sunlight without using silver mirrors as reflectors and how to make them easily deployable.

They decided to use phase-inversion because it is a simple, solution-based method for making light-scattering air-voids in polymers. Polymers and solvents are already used in paints, and the Columbia Engineering method essentially replaces the pigments in white paint with air voids that reflect all wavelengths of sunlight, from UV to infrared.

When exposed to the sky, the porous polymer PDRC coating reflects sunlight and emits heat to attain significantly cooler temperatures than typical building materials or even the ambient air.

“This simple but fundamental modification yields exceptional reflectance and emittance that equal or surpass those of state-of-the-art PDRC designs, but with a convenience that is almost paint-like,” says Mandal.

The researchers found their polymer coating’s high solar reflectance (R > 96%) and high thermal emittance (ε ∼ 97%) kept it significantly cooler than its environment under widely different skies, e.g. by 6°C in the warm, arid desert in Arizona and 3°C in the foggy, tropical environment of Bangladesh.

“The fact that cooling is achieved in both desert and tropical climates, without any thermal protection or shielding, demonstrates the utility of our design wherever cooling is required,” Yang notes.

The team also created colored polymer coatings with cooling capabilities by adding dyes. “Achieving a superior balance between color and cooling performance over current paints is one of the most important aspects of our work,” Yu notes. “For exterior coatings, the choice of color is often subjective, and paint manufacturers have been trying to make colored coatings, like those for roofs, for decades.”

The group took environmental and operational issues, such as recyclability, bio-compatibility, and high-temperature operability, into consideration, and showed that their technique can be generalized to a range of polymers to achieve these functionalities. “Polymers are an amazingly diverse class of materials, and because this technique is generic, additional desirable properties can be conveniently integrated into our PDRC coatings, if suitable polymers are available,” Mandal adds.

“Nature offers many ways for heating and cooling, some of which are extremely well known and widely studied and others that are poorly known. Radiative cooling—by using the sky as a heat sink—belongs to the latter group, and its potential has been strangely overlooked by materials scientists until a few years ago,” says Uppsala University Physics Professor Claes-Göran Granqvist, a pioneer in the field of radiative cooling, who was not involved with the study. “The publication by Mandal et al. highlights the importance of radiative cooling and represents an important breakthrough by demonstrating that hierarchically porous polymer coatings, which can be prepared cheaply and conveniently, give excellent cooling even in full sunlight.”


                                  Illustration showing how passive daytime radiative cooling (PDRC) involves 

                                  simultaneously reflecting sunlight and radiating heat into the cold sky to 

                                  achieve a net heat loss. The process, which is spontaneous, can cool down 

                                  structures to sub-ambient temperatures.

Yang, Yu, and Mandal are refining their design in terms of applicability, while exploring possibilities such as the use of completely biocompatible polymers and solvents. They are in talks with industry about next steps.

“Now is a critical time to develop promising solutions for sustainable humanity,” Yang notes. “This year, we witnessed heat waves and record-breaking temperatures in North America, Europe, Asia, and Australia. It is essential that we find solutions to this climate challenge, and we are very excited to be working on this new technology that addresses it.”

Yu adds that he used to think that white was the most unattainable color: “When I studied watercolor painting years ago, white paints were the most expensive. Cremnitz white or lead white was the choice of great masters, including Rembrandt and Lucian Freud. We have now demonstrated that white is in fact the most achievable color. It can be made using nothing more than properly sized air voids embedded in a transparent medium. Air voids are what make snow white and Saharan silver ants silvery.”

Source: By Holly Evarts, Columbia Engineering