Mie-Metamaterials-Based Thermal Emitter for Near-Field Thermophotovoltaic Systems

A Review on Thermophotovoltaic Cell and Its Applications in Energy Conversion: Issues and Recommendations

Generally, waste heat is redundantly released into the surrounding by anthropogenic activities without strategized planning. Consequently, urban heat islands and global warming chronically increases over time. Thermophotovoltaic (TPV) systems can be potentially deployed to harvest waste heat and recuperate energy to tackle this global issue with supplementary generation of electrical energy. This paper presents a critical review on two dominant types of semiconductor materials, namely gallium antimonide (GaSb) and indium gallium arsenide (InGaAs), as the potential candidates for TPV cells. The advantages and drawbacks of non-epitaxy and epitaxy growth methods are well-discussed based on different semiconductor materials. In addition, this paper critically examines and summarizes the electrical cell performance of TPV cells made of GaSb, InGaAs and other narrow bandgap semiconductor materials. The cell conversion efficiency improvement in terms of structural design and architectural optimization are also comprehensively analyzed and discussed. Lastly, the practical applications, current issues and challenges of TPV cells are critically reviewed and concluded with recommendations for future research. The highlighted insights of this review will contribute to the increase in effort towards development of future TPV systems with improved cell conversion efficiency.

Spectral selectivity of multiple nanoparticles doped thin films

Microscopic thin film doped with different species of nanoparticles displays a unique wavelength selectivity in the context of micro/nanoscale radiative heat transfer. We propose a methodology to shift, broaden, and suppress the thermal radiative selectivity in the desired wavelength ranges. Measured transmittance spectra of potassium bromide pellet doped with a single species of nanoparticles are compared with the theoretical prediction using refractive indices that are extracted by refitting transmittance spectra curve according to the Lorentz-Drude model. For a media doped with more than two species of nanoparticles, a successive effective dielectric function using the refitted complex refractive indices and Maxwell Garnett theory is used to evaluate the thermal radiative selectivity of the composites. It has been confirmed theoretically and experimentally that the wavelength selectivity in the transmittance spectra can be influenced by choosing proper species of materials and varying volume fractions of multiple nanoparticles. This work has shed light on the design and fabrication of novel composites doped with multiple particles for applications such as thermophotovoltaics, radiative cooling, and biosensing.

Tunable wavelength selectivity of photonic metamaterials-based thermal devices

Wavelength-selective thermal devices have great applications in concentrating solar power systems, high-temperature thermoelectric systems, and solar thermophotovoltaics (STPVs). Lack of high-temperature stability and spectrally selective emissivity in different wavelength regions limits their efficiency. We propose a one-dimensional HfO₂/Al₂O₃-W nanocomposites/W/Al₂O₃/W multilayered photonic structure as potential wavelength selective thermal devices, and theoretically investigate the emission properties of the proposed Mie-resonance metamaterials from visible (VIS) to midinfrared (MIR) region. HfO₂ thin layer is introduced to serve as an antireflection coating film and W layer acts as an IR reflection layer that enhances the absorptivity/emissivity in VIS and near-infrared (NIR) region while reducing the MIR emission simultaneously. Effects of geometric parameters are discussed, such as different radii and volume fractions of W nanoparticles, the thickness of Al₂O₃-W nanocomposites, and HfO₂ thin film. The proposed thermal absorber and emitter exhibit nearly unity absorptance in both VIS and NIR regions, while the emittance approaches zero in the MIR region. The selective absorption/emission window is tunable by varying geometric parameters. The proposed solar thermal devices have great potentials in engineering applications such as STPVs and solar thermoelectric generator due to flexibility of geometric parameters and ease of fabrication.

Near-infrared optics of nanoparticles embedded silica thin films

This work investigates experimentally the near-infrared optical properties of SiO2 thin film embedded with tungsten (W) nanoparticles at varying volume fractions. The samples are prepared by using the technique of magnetron sputtering. The formation and distribution of W nanoparticles are characterized using transmission electron microscopy, and the volume fraction of W nanoparticles is validated by Auger electron spectroscopy. Near- and mid-infrared diffuse reflectance measurements are conducted using Fourier transform infrared spectroscopy. The samples exhibit wavelength selective optical response in the near-infrared region and are suitable for applications involving selective thermal emitters/absorbers. Measured reflectance data is utilized to estimate the effective dielectric function of the nano-composites. Calculated reflectance spectra in different samples are compared to the measured spectra using the experimentally measured dielectric function of these samples in the near-infrared region. Reflectance spectra after thermal annealing at different temperature are compared to show how the thermal treatment affects the optical properties of samples. Optimized structures are proposed for thermal emitters and absorbers with different volume fractions of W nanoparticles.

A Review of Tunable Wavelength Selectivity of Metamaterials in Near-Field and Far-Field Radiative Thermal Transport

Radiative thermal transport of metamaterials has begun to play a significant role in thermal science and has great engineering applications. When the key features of structures become comparable to the thermal wavelength at a particular temperature, a narrowband or wideband of wavelengths can be created or shifted in both the emission and reflection spectrum of nanoscale metamaterials. Due to the near-field effect, the phenomena of radiative wavelength selectivity become significant. These effects show strong promise for applications in thermophotovoltaic energy harvesting, nanoscale biosensing, and increased energy efficiency through radiative cooling in the near future. This review paper summarizes the recent progress and outlook of both near-field and far-field radiative heat transfer, different design structures of metamaterials, applications of unique thermal and optical properties, and focuses especially on exploration of the tunable radiative wavelength selectivity of nano-metamaterials.