Global S&T Development Trend Analysis Platform of Resources and Environment
| DOI | 10.1126/science.abe4476 |
| Photon-engineered radiative cooling textiles | |
| Po-Chun Hsu; Xiuqiang Li | |
| 2020-11-13 | |
| 发表期刊 | Science
 |
| 出版年 | 2020 |
| 英文摘要 | Take a few seconds to look around and list all the technologies that are indispensable for you. If your list does not include textiles, try to live a typical day without them. Textiles are arguably one of the earliest human inventions. Without textiles to cover the human body for warmth, our ancestors would not have been able to spread across the various climate zones of the Earth. Today, many textiles are made for social etiquette and aesthetic purposes, but the pressing threat of global warming has created demand for innovative textiles that help to better cool the person who wears them.
The rationale behind linking textiles and climate change is that wearing the cooler textiles for localized “personal thermal management” may reduce the demand for air conditioning. The impact of air conditioning is considerable, given that it is not only responsible for 10% of U.S. electricity consumption but that the refrigerants are also a source of high global-warming-potential gasses ([ 1 ][1], [ 2 ][2]). Considering an indoor setting distinguishes the new generation of cooling textiles from textiles oriented for sports. On average, the metabolic heat rate of indoor light activities is 60 to 80 W/m2, balanced by the heat flux from the skin to the environment. This flux occurs through all viable heat transfer pathways: conduction, convection, radiation, and evaporation. Because one of the criteria of thermal comfort is the absence of sensible perspiration, evaporation only accounts for ∼5 W/m2. Using the American Society of Heating, Refrigerating and Air-Conditioning Engineers Standard 55 as the reference and assuming the clothing insulation is 1 clo (0.155 m2K/W; clo is the industry unit for thermal insulation), then the heat transfer through textile conduction and natural convection contributes ∼40 W/m2, and radiation is responsible for ∼25 W/m2. This calculation demonstrates the substantial role of radiation in the human body heat balance. Unlike convective heat transfer, heat radiation is a surface property and does not require any media or moving part, making it a perfect tool for personal thermal management.
Regardless of skin pigmentation, the human skin is a nearly perfect black body that emits thermal radiation through Planck's law. This radiation transmits through the air gap and is absorbed by the textile, which contains various types of resonating molecular vibrational modes. The textile then reemits to the ambience. Almost all clothing materials are highly absorbing in the spectral region of human body radiation. Although Kirchhoff's radiation law indicates that they are also good emitters, their infrared (IR) opacity inevitably results in the radiation shielding effect. Therefore, rather than engineering the existing clothing materials, the key to radiative cooling is to re-invent the material so that it is transparent in mid-IR, allowing the thermal radiation from the hot human skin to bypass the textile and directly reach the ambience (see the figure). By contrast, traditional IR-absorbing textiles emit from the cold outer surface with a much lower radiation power that is proportional to the fourth power of temperature, according to Stefan-Boltzmann's law. This design principle leads to polyethylene (PE) textiles because of its simple chemical bonds and very few resonance peaks in mid-IR. In 2015, Tong et al. numerically predicted that radiative cooling fabric could be achieved by controlling the PE fiber diameter and light-scattering mechanisms to obtain both IR transparency and visible light opacity for wearability ([ 3 ][3]). One year later, Hsu et al. experimentally demonstrated nanoporous PE (nanoPE) radiative cooling fabric ([ 4 ][4]). The nanoporous structure was designed to achieve multifunctionality such as visible opacity, breathability, and softness. Several efforts to enhance the wearability were also conducted, including sweat-wicking and wind permeability. Built on the intrinsic IR transmittance and nanoscale photonic engineering, this concept was further developed into large-scale woven and knitted textiles ([ 5 ][5]), outdoor cooling textiles ([ 6 ][6]), and visibly colored cooling textiles ([ 7 ][7], [ 8 ][8]). Depending on the types, these radiative cooling textiles can have the cooling performance equivalent to more than a 2°C increase of indoor temperature setpoint. One should not underestimate this amount of setpoint increase because it applies to the entire building space, which has orders of magnitude more thermal inertia and heat loss to manage compared with the occupants. Estimates of energy savings suggests the potential for a 20% reduction of heating, ventilation, and air conditioning energy with this small setpoint increase ([ 9 ][9]).
Looking ahead from the promise of energy saving, challenges and opportunities exist for the radiative cooling textiles. Thousands of years of evolution have made the expectation for clothing design to be complicated and often subjective in terms of what is considered necessary. Even for simple energy-efficient thermal comfort, psychological and environmental factors are in play that go beyond what might be ideal from a physical science perspective. The choices of IR-transparent polymers should be broadened to accommodate the various needs for wearability, such as moisture transport, skin touch comfort, scratch durability, and laundering. A general rule is to search for materials with a high content of crystalline aliphatic segments, followed by quantitative measurement of absolute extinction coefficient. Also, textiles should be adaptive. For example, the textile woven from carbon nanotube–coated bimorph yarns can respond to sweat and modulate the mid-IR emissivity ([ 10 ][10]), which is energy free and effective for the outdoors, but needs to increase the trigger sensitivity for the indoor scenario. A different strategy is to use active cooling by devices such as Peltier coolers that are either directly wearable or connected with recirculating water ([ 11 ][11], [ 12 ][12]). These systems have superior cooling power, but the power consumption likely needs an energy storage or supply breakthrough to become part of daily clothing. A hybrid solution is variable passive thermoregulation, which uses energy to control the heat transfer coefficients rather than supply the thermal power. The tunable range is similar to the purely passive approach but can be actively changed according to user preference and potentially other signal inputs from the environment or the human body ([ 13 ][13], [ 14 ][14]). Ultimately, the radiative thermal engineering can be combined with other textile heat-management mechanisms to accomplish multimodal control. A radiative cooling textile with coupled dynamic evaporative cooling can perform human body cooling even when the ambient temperature is higher than the skin.
![Figure][15]
Designing more comfortable textiles
Heat transport through traditional textiles occurs by conduction or convection (blue paths), but infrared (IR) radiation is blocked (red paths). Cooling textiles improve radiative heat transfer between the skin and the environment. Adaptive textiles control the heat balance without working fluids or continuous energy input.
GRAPHIC: N. CARY/ SCIENCE
Like many other renewable energy technologies, radiative cooling textile was initially a materials science and nanophotonics effort for niche applications for energy-efficient buildings by accomplishing the localized personal cooling, and it will eventually need to find the proper market position to be economically sustainable. In particular, these textiles will be joined by both traditional textile engineering and the booming wearable technologies. Our ancestors invented textiles as the “secondary skin” for thermal regulation. As textiles become an indispensable part of our lives, they hopefully will move closer to being as smart, versatile, and natural as our actual skin.
1. [↵][16]U.S. Department of Energy, “ARPA-E DELTA Program Overview” (2013); |
| 领域 | 气候变化 ; 资源环境 |
| URL | 查看原文 |
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| 文献类型 | 期刊论文 |
| 条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/304067 |
| 专题 | 气候变化 资源环境科学 |
| 推荐引用方式 GB/T 7714 | Po-Chun Hsu,Xiuqiang Li. Photon-engineered radiative cooling textiles[J]. Science,2020. |
| APA | Po-Chun Hsu,&Xiuqiang Li.(2020).Photon-engineered radiative cooling textiles.Science. |
| MLA | Po-Chun Hsu,et al."Photon-engineered radiative cooling textiles".Science (2020). |
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