Applied Materials Today
Specific phase modulation and infrared photon confinement in solar selective absorbers
Graphical abstract
Without new strategies implemented in the material preparation or/and theoretical framework developed, it is a great challenge to manufacture high-temperature-endurable (>600 °C) solar absorber coatings with excellent optical properties. Here, a combination strategy via incorporation of alloy nanoparticle, specific phase-created reflective layer, and infrared photon dissipation confinement effect, was reported to realize a target coating stable at 650 °C, with an ultralow emissivity of 7.9 %@500 °C and a solar absorptance of 93 % after 300 h vacuum annealing.
Introduction
Along with the urgent utilization of green solar energy, solar thermal conversion serving as another major approach for harvesting solar energy has been intensively investigated [[1], [2], [3], [4], [5], [6]]. It gives rise to a vast array of applications including concentrating solar power, heating/cooling, seawater desalination, oil chemical refining, and so on [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. For efficient conversion from sunlight to heat, photothermal materials generally possess both intense solar absorption and subdued thermal loss simultaneously, desirably working at high temperatures. Currently, lots of options, including metallic plasmonic metamaterials, carbon-based microstructures or nanocermet (metal-ceramic nanocomposites), have been used to improve solar thermal absorbers [[14], [15], [16], [17]]. In terms of technology maturity, scalability and reliability, multilayer cermet-stacked solar absorber coating is the preferred scheme [[17], [18], [19]].
So far, for advanced parabolic trough solar collectors with improved overall efficiency, solar absorber coatings should work at high operating temperatures more than 600 °C [1,[20], [21], [22]]. Especially, it is worth noting that high operating temperatures generally bring about exponential increase of the thermal losses, according to the Stefan-Boltzmann law [23]. So, solar absorber selective coatings should have, simultaneously, excellent long-term thermal stability at temperatures more than 600 °C and low thermal emissivity with high solar absorptance.
Due to intrinsic absorption, scattering and plasmon extinction of metallic nanoparticles (NPs) and interlayer interference effect, nanocermet-based solar absorber coatings exhibit strong absorption covering a broad region ranging from 0.25 to 2.5 μm [[17], [18], [19],24,25]. Meanwhile, a metal infrared (IR) reflector intensively reflects the incident light in the IR regime from 2.5–25 μm, endowing the coating with low infrared emissivity (infrared absorptance). Certainly, the different layers all together should have optical impedance matching. So far, multilayer cermet coatings have their own limitations, still suffering from thermal aging instability, relatively high infrared radiation loss, etc.
From the material section point of view, refractory transition metals including Mo, W, Ta, Zr, Nb, etc., have been explored and used in the cermets and/or IR reflectors to build high-temperature (>400 °C) solar selective absorbers, ascribing to their strong absorption in visible light region and high refractive index within IR range. Also, these metals present superior diffusion barrier capability at high temperatures more than 500 °C [[26], [27], [28], [29]].
In the past years, classical double cermet layer-stacked configuration [17,19,21,[30], [31], [32], [33], [34]] has been intensively explored. Binary compound or alloyed nanoparticles embedded oxide or nitride hosts (FeSi2-SiO2, [32] NiSi(TiSi2)-SiOx, [35] MoSi2-Si3N4, [21,33] WNi-Al2O3, [26] WTi-Al2O3, [27] etc.) are reported, exhibiting fair anti-oxidation ability [21,26]. For ultra-low infrared emissivity, Ag-SiNx composite sandwiched between two W barrier layers had been attempted [32]. In our previous studies [27,36], binary alloy NPs-embedded cermet solar thermal absorbers can lead to remarkable enhancement in thermal stability up to 600 °C, with self-passivation effect involved. However, a relatively high infrared emissivity of 10.3 % at 500 °C after annealing at 600 °C was observed. From the above, one can find out that it still remains a great challenge to design high-temperature (>600 °C) solar absorber coatings with sound optical properties (infrared emissivity, solar absorptance) and longtime thermal stability together.
Thus, as expected, without new strategies implemented in the material preparation or/and theoretical framework developed, it is hard to break through the bottleneck of nanocermet-based selective solar absorbers, unable to keep pace with the industry demand. To address the issue, herein, we propose a ‘boxing combination’ concept, i.e., the hybrid strategies of alloyed nanoparticle, specific phase-created reflective layer, and infrared photon dissipation confinement effect usage in the solar absorber coatings, in order to boost the optical performance (in particular, radically reduced the infrared emissivity) and high-temperature stability. The resultant absorber coating capped on stainless steel (SS) exhibits a desirable thermal stability at 650 °C for a long period of time, with an ultralow emissivity of 7.9 % at 500 °C and a solar absorptance of 93 %. Our creative solution would open up a new path for developing high-temperature solar absorber coatings to fuel the photothermal conversion efficiency of the solar thermal collector system.
Section snippets
Samples preparation
All the deposition were performed onto various substrates (quartzs, silicon (100) and SS) by a multi-target magnetron sputtering system (Jsputter 8000, manufactured by ULVAC Co., Ltd.), as described in the previous report [27]. The commercially available high-purity targets (Al2O3, W and Ti with 99.99 % in purity and 2 inch in diameter) were utilized as sputter sources. The substrate was kept rotating at 20 rpm to guarantee film uniformity. The detailed sputtering parameters of all the samples
α-W infrared reflective layer
In the stack structure of solar absorber coatings, the metal infrared reflective layer plays a vital role in obtaining low thermal emissivity. The frequently used metallic W has many advantages over copper, nickel and molybdenum, in that its high melting point and a good diffusion barrier functionality [26,27,34]. However, as reported previously [37,38], tungsten films with different crystalline phases (the stable α phase with body centered cubic (bcc) structure and the metastable β phase (A15
Conclusion
Overall, we have successfully designed and prepared high-performance solar selective absorbers with the following structure: Al2O3 anti-reflection layer/W-Al2O3 (cermet 1)/WTi-Al2O3 (cermet 2)/WTi-Al2O3 (cermet 3)/Al2O3 barrier layer/ɑ-W dominated infrared reflective layer. The resultant coatings exhibit a desirable thermal stability at 650 °C in vacuum for a long period of time and an ultralow emissivity of 7.9 % at 500 °C with a solar absorptance of 93 %, which is very competitive in the
Author contributions
Junhua Gao and Hongtao Cao supervised the whole project. Xiaoyu Wang and Haibo Hu conducted the Matlab simulation. Junhua Gao conducted the FDTD simulation. Xiaoyu Wang and Haibo Hu fabricated and optimized the W metal layers and coatings. Xiaoyu Wang and Junhua Gao performed the characterizations and data analysis. Zhenyu Wang assisted the annealing experiments. Xiaoyun Li participated in the GISAXS measurement. Lingyan Liang, Hongliang Zahng and Fei Zhuge provided helpful discussions. Xiaoyu
Acknowledgements
We thank Dr. Yongxin Liu for useful discussions. The authors acknowledge support from the Ten Thousand Talents Plan of Zhejiang Province-Science and Technology Innovation Leader Project (Grant No. 2018R52006), Zhejiang Provincial Natural Science Foundation of China (Grant No. LY17E020012), the Key Research and Development Plan of Jiangsu Province (Grant No. BE2015049), and the program for Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2016B10005).
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