Smart windows that regulate solar energy by changing optical characteristics have recently gained tremendous interest for energy-saving and indoor-comfort applications. Among them, thermochromic smart windows are promising because of their simplicity for industrial production and ease of implementation. Although significant advancements have been reported on thermochromic materials, both optical and transition properties remain unsatisfactory. This review focuses on the recent advancement of thermochromic materials for smart windows in terms of operation, performance, and potential for commercialization. It discusses the parameters typically used for gauging performance and provides a summary and comparison of various promising thermochromic materials, including vanadium dioxide, hydrogels, and perovskites. The article also points the challenges in the practical implementation of these materials and provides an outlook for future development.

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Planck's law predicts the distribution of radiation energy, color and intensity, emitted from a hot object at thermal equilibrium. The Law also sets the upper limit of radiation intensity, the blackbody limit. Recent experiments reveal that micro-structured tungsten can exhibit significant deviation from the blackbody spectrum. However, whether thermal radiation with weak non-equilibrium pumping can exceed the blackbody limit in the far field remains un-answered experimentally. Here, we compare thermal radiation from a micro-cavity/tungsten photonic crystal (W-PC) and a blackbody, which are both measured from the same sample and also in-situ. We show that thermal radiation can exceed the blackbody limit by >8 times at λ = 1.7 μm resonant wavelength in the far-field. Our observation is consistent with a recent calculation by Wang and John performed for a 2D W-PC filament. This finding is attributed to non-equilibrium excitation of localized surface plasmon resonances coupled to nonlinear oscillators and the propagation of the electromagnetic waves through non-linear Bloch waves of the W-PC structure. This discovery could help create super-intense narrow band thermal light sources and even an infrared emitter with a laser-like input-output characteristic.

Solar energy can alleviate our dependence on traditional energy sources like coal and petroleum. In this regard, the design and performance of solar absorbers are crucial for capturing energy from sunlight. Specifically, for applications relying on solar-thermal energy conversion, it is desirable to construct solar absorbers using scalable techniques that also allow a variation in optical properties. In this study, we demonstrate the ability to tune the spectral absorptance of nickel-infused nanoporous alumina using a scalable and inexpensive fabrication procedure. With simple variations in the geometry of the nanostructures, we enable broadband absorption with a net solar absorptance of 0.96 and thermal emittance of 0.98 and spectrally-selective absorption with a net solar absorptance of 0.83 and thermal emittance of 0.22. The simple manufacturing techniques presented in this study to generate nanoengineered surfaces can lead to further advancements in solar absorbers with well-controlled and application-specific optical properties.