日間輻射冷卻技術：從超穎材料到環境因子與能源收成

Study on Daytime Radiative Cooling Technologies: Metamaterials Design, Environmental Factors, and Energy Harvesting

張思偉(Sih-Wei Chang)

指導教授 ： 陳學禮

國立臺灣大學/工學院/材料科學與工程學系/博士(2022年)

https://doi.org/10.6342/NTU202203974

摘要

在人類科技發展的過程中，各式各樣的廢熱及汙染被排放到環境中，廢熱造成了全球溫度的上升，環境汙染也進一步造成生態的傷害；為了減輕全球暖化，減少能源的使用，建立高效率環境能源方式，以及減少污染的排放，成了一項重要的課題。輻射冷卻技術是近年興起的一種具備零耗能降溫的技術，利用大氣光窗將熱能輻射至外太空，地球的溫度能得到有效的降低。在本論文中，我們建立了具備輻射冷卻特性的超穎材料，並應用於環境能源回收上，除此之外，在最佳化日間輻射冷卻材料的過程中，我們製作了具備雙功能性的光學標準品，可用於漫反射校正白片與黑體的超穎材料，除此之外，我們分析了環境汙染排放對於輻射冷卻效應的影響。
在本論文第一部分中，我們利用六方氮化硼材料與氧化鋅製備了具有高導熱係數的日間輻射冷卻材料，在本研究中，我們發現導熱係數對日間輻射冷卻有重大的影響，特別是應用於近室溫廢熱回收上，高導熱的輻射冷卻材料具有傑出的散熱特性，結合商業用的熱電晶片，我們能有效地從近室溫的廢熱源回收環境能源，並從40 oC的廢熱源中分別於日間及夜間擷取了225.3 mW/m2及412.3 mW/m2，在日間與夜間分別造成了1030%及190%的輸出功率增益，除此之外，我們在60 oC的廢熱源中擷取出了2.352 W/m2的熱電功率，日間輻射冷卻結合熱電晶片展現了在低溫廢熱回收上的潛力。
在本論文第二部分中，利用氟化鈣、二氧化矽及六方氮化硼製作了應用於漫散射與熱輻射的雙功能性光學標準品。藉由控制整個材料於紫外光到近紅外光的散射效率，成功製造出具有可匹敵商業用漫反射校正白片能力的超穎材料，與商用光學校正片相比，其於紫外光到近紅外光平均有100.6%的相對全散射反射率，並且於2 μm新光通訊波段有優於商用校正片的相對散射能力達100.3%。除此之外，利用極性材料的互補性吸收特性，我們成功使超穎材料兼具於紅外光波段96.7%平均放射率及97.6%平均吸收率。我們製作的超穎材料具有優異的溫度穩定性，在大氣環境下於700 oC持溫三小時，對於整體光譜性質在中紅外光幾乎沒有造成變化，相對變化程度僅於中紅外光波段約0.64%。同時，因超穎材料具備的優異的散射、耐溫及熱輻射能力，其能承受雷射能量密度達96.8 J/cm2之雷射照射。
在本論文的第三部分，利用米氏理論及蒙地卡羅法分析了氣懸塑膠對於大氣輻射強迫的影響，隨著人類塑膠的使用及排放，海洋及大氣中的微奈米塑膠已經逐漸被報導，在人體及動物身體中發現塑膠的報導更是層出不窮。因為微奈米塑膠的低密度、小顆粒、化學穩定性，氣懸塑膠在大氣中徘徊的時間相當長，對於大氣光窗的穿透率有重要的影響。而我們發現，在過去的研究中主要針對微米等級塑膠對於大氣輻射強迫的影響進行探討，但隨著微米等級塑膠在傳播過程中受到風化作用及紫外光劣化的影響，環境中已經存在許多奈米等級的氣懸塑膠，尚未被前人詳盡探討，根據前人研究的缺失及驗證，我們以分析奈米及微米氣懸塑膠為主，更詳盡的分析了其對於大氣輻射強迫的影響，並發現小顆粒的微奈米塑膠對於溫室效應有較強作用，在質量濃度為10 μg/m3時，具有奈米等級精細顆粒分布的氣懸塑膠具有約1.1W/m2之輻射強迫，約為微米等級粗顆粒塑膠之10倍。

關鍵字

日間輻射冷卻 ； 低溫廢熱回收 ； 六方氮化硼 ； 超穎材料 ； 氣懸塑膠 ； 表面聲子極化子 ； 剩餘射線帶 ； 大氣光窗 ； 輻射強迫

並列摘要

During industrial technology development, people emit various pollutants and waste heat into the environment. The waste heat emission heats the Earth and raises the global temperature. Pollutants and waste heat would result in damage to the environment. Energy saving and reducing pollutant emissions are critical to minimizing global warming. Daytime radiative cooling (DRC) has recently been a popular cooling technology with zero-energy consumption. The global temperature could be reduced by transporting thermal radiation to outer space via the infrared atmospheric window (IRW). In this thesis, we developed the DRC metamaterials and applied them to environmental energy harvesting. After optimizing the optical properties of DRC metamaterials, we developed bifunctional optical standards for both diffuse reflectance and blackbody radiation.
In the first part of this thesis, we used hexagonal boron nitride (hBN) and zinc oxide (ZnO) to develop thermal conductive DRC materials. We found that thermal conductivity has a crucial impact on the performance of DRC materials. The thermal conductive DRC materials perform an outstanding heat dissipation during near-room temperature waste heat recovery. By cooperating with thermal conductive DRC materials and a commercial thermoelectric generator, we successfully harvested energy from a waste heat source with a temperature of 40 oC in both daytime and nighttime. The harvested thermoelectric powers are 225.3 and 412.3 mW/m2 in daytime and nighttime, respectively. The enhancement factors compared to the bare thermoelectric generator in daytime and nighttime are 1030% and 190%, respectively. Besides, we have recovered energy from a waste heat source with a temperature of 60 oC and generated a thermoelectric power of 2.352 W/m2. The DRC-assisted TEG shows potential for low-grade waste heat recovery.
In the second part of this thesis, we used calcium fluoride (CaF2), silicon dioxide (SiO2), and hBN to fabricate a bifunctional optical standard for diffuse reflectance and thermal emission. We successfully developed metamaterials with a diffuse reflectance comparable to the commercial reference plate by controlling the scattering efficiency from the ultraviolet (UV) to near-infrared (NIR) region. The average relative total diffuse reflectance is 100.6% in the UV-Vis-NIR region. For the new telecommunication wavelength of 2 μm, the CaF2 metamaterial performed a relative average total diffuse reflectance of 100.3%, which is superior to the commercial reference plate. Furthermore, by utilizing the complementary absorption of polar materials, the CaF2 metamaterials also have an average emissivity of 96.7% and the absorptance of 97.6% in the MIR region. In addition, the CaF2 metamaterials perform outstanding temperature stability. After heat treatment at a temperature of 700 oC for 3 hours in the atmosphere, the spectra properties only show a slight difference. The average relative difference in MIR region is 0.64%. Due to the excellent scattering ability, high-temperature stability, and high thermal radiation, our CaF2 metamaterials are stable under laser illumination with a laser energy density of 96.8 J/cm2.
In the third part of this thesis, we analyze the impact of airborne plastics (APs) on effective radiative forcing (ERF) by using the Mie scattering and Monte-Carlo methods. While humans produce and emit waste plastics into the environment, micro/nano plastics discovered in the atmosphere and marine have been reported recently. Due to the low density, small size, and chemical stability, APs can transport in the atmosphere for a long time. APs would be a crucial impact on the transmittance of IRW. A previous study focused on the ERF induced by plastics with micro-scale size. However, during the transportation of airborne microplastics, the weathering effect and UV irradiation would age and downsize the microplastics into nanoplastics. It has been proved that the existence of airborne nanoplastics in the environment. However, the thermal impact of nanoplastics has not been reported in detail. We analyzed and compared the ERF of APs based on previous studies. We found that fine particles (nanoplastics) have a much more significant greenhouse effect than coarse particles (microplastics). The ERF of fine particles with nanoplastics is estimated as 1.1 W/m2 at the mass concentration of 10 μg/m3. The estimated ERF of fine particles is about tenfold that of coarse particles with microplastics.

並列關鍵字

daytime radiative cooling ； low-grade waste heat recovery ； hBN ； metamaterial ； airborne plastics ； surface phonon polariton ； atmospheric window ； radiative forcing

參考文獻

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