Solar energy is one of the most abundant clean non-renewable energy resources globally. Existing studies have shown that its applicability in processes such as desalination, photocatalysis, sterilization and vapor generation among others, can be beneficial in two folds: economically and environmentally. Unfortunately, the rate at which it can be employed for the stated purposes is inhibited by the low harvesting efficiency of the incident solar energy. In the recent past, absorbers with high photothermal conversion efficiency, of which categorically display nearly 100% absorbance in the narrow band, have been proposed. Regardless, there is need to enhance this performance further. Recently published research papers have pointed out a game changing discovery whereby it has been noted that certain materials can exhibit natural hyperbolic behavior in the visible and near-infrared range which would be progress from the common mid-infrared/terahertz frequency region materials. To our dismay, application of such auspicious materials has not been implemented in the enhancement of solar absorption up to now.
Researchers led by Professor Ping Cheng from Shanghai Jiao Tong University in collaboration with Professor Zhuomin M. Zhang from Georgia Institute of Technology developed a perfect light absorption structure that utilizes an array of pyramidal nanostructures made of bismuth telluride (a natural hyperbolic material) over a thin substrate to absorb incident solar radiation. The researchers hoped that by using their proposed structure, they would achieve perfect broadband absorption by seamlessly combination of the slow-light effect and reflection suppression effect with gradient index in the type-II hyperbolic region. Their work has been published in the research journal, Solar Energy.
The research team initiated their experiments by numerically computing the absorbance of the developed bismuth telluride structure by fine-tuning the geometric parameters of the nanostructure as well as the substrate material and breadth in order to achieve tremendously high absorbance in the solar radiation spectral region. Next, they employed power dissipation density distribution function so as to elucidate on the underline mechanisms they had utilized during broadband absorption.
The authors observed that the underlying mechanisms contributed significantly to the combination of the slow-light effect and the gradient index effect. In addition, they realized that the optical properties of the absorbers were affected by the geometry of the nanostructure: that included the height of the nano-pyramid, the width of its top and bottom, the distance between two adjacent nano-pyramids, as well as the material and thickness of the substrate.
The study successfully presented a perfect absorber design which manipulates a periodic array of pyramidal nanostructures that are made of a natural hyperbolic material bismuth telluride on a metallic substrate. The results from the experimental procedure undertaken in this study have shown that the proposed structure can achieve absorptance values of almost 100% in the wavelength range of 300–2400 nm, upon which most of the solar radiation spectrum fall into. Altogether, the proposed metamaterial has great potential application and can lead to the effective harvesting of solar energy during photothermal conversion processes in water or aqueous solutions.
Zhaolong Wang, Zhuomin M. Zhang, Xiaojun Quan, Ping Cheng. A perfect absorber design using a natural hyperbolic material for harvesting solar energy. Solar Energy, volume 159 (2018) pages 329–336.
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