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Direct absorption-based solar collectors, which rely on volumetric absorption of sunlight by optical nanofluids, have emerged as a superior way to conventional surface absorption-based collectors for harvesting solar-thermal energy. In this short review, we summarize the recent progress on preparing medium-to-high-temperature solar-thermal nanofluids that can be potentially used in direct absorption-based solar collectors under concentrated solar fluxes. Emphasis is placed on discussing the challenge and strategies to achieve stable dispersion of the optical nanofluids. [ABSTRACT FROM AUTHOR]

Direct absorption of sunlight with carbon nanofluids is a facile way to efficiently harvest solar-thermal energy, but poor dispersion stability of nanofluids under elevated temperatures severely limits their applications. Herein, we report a general strategy to prepare stably-dispersed medium-temperature solar-thermal nanofluids by employing crumpled particles of ethanol-wetted graphene oxide sheets as the self-dispersible photothermal converters. The crumpled particles simultaneously possess intensively-deformed surface structure, a low density and a small particle size. Such features weaken the inter-particle van der Waals attraction and gravitational sedimentation of the crumpled particles, thereby enabling their long-term stable dispersion within commercial thermal storage oils. Uniform dispersion of nanofluids was maintained after continuous heating for 2 weeks under a heating temperature up to 200 °C. The homogenously dispersed nanofluids also achieved high solar absorptance with low particle loadings, and retained the large heat capacity and suitable viscosity. These combined advantageous thermophysical properties enabled consistent high-performance medium-temperature direct absorption-based nanofluidic solar-thermal energy harvesting under large-flux solar illumination. [Display omitted] [ABSTRACT FROM AUTHOR]

• We generate a stably self-dispersed crumpled graphene/water nanofluid at 100 °C. • The nanofluid enhances the heat transfer coefficient by 89.1% over deionized water. • The enhancement is due to the porous structure generated by self-assembly. Boiling can induce the self-assembly of nanomaterials in nanofluids to generate reliable micro/nanostructures for enhancing boiling heat transfer. The practical applications of most nanofluids in boiling, however, are limited by the poor thermal stability at the boiling point, and improving the thermal stability of nanofluids at such high temperature is challenging. Here, we demonstrate that the crumpled graphene can remain uniformly and stably dispersed within deionized water at the boiling point of 100 °C due to its highly wrinkled structure, while self-assembling to generate a porous structure on the substrate during boiling. Such porous structure can vary the surface micromorphology, wettability and roughness, which increases the nucleation sites and departure frequency of bubbles, thus effectively enhancing the critical heat flux and the maximum heat transfer coefficient by 80.7 and 89.1% compared to the pristine copper substrate, respectively. This in situ boiling-induced self-assembly provides a facile and low-cost strategy to achieve high-performance boiling heat transfer for a wide variety of nanomaterials. The crumpled graphene/water nanofluid used can be directly utilized as a phase change working medium in boiling systems to harvest thermal energy and generate clean steam. [Display omitted] [ABSTRACT FROM AUTHOR]

Direct absorption and conversion of sunlight by optical nanofluids into storable heat is an efficient and facile way to harness abundant clean solar energy, but achieving long-term stable dispersion of the nanofluids is challenging under elevated temperatures. Herein, we report the preparation of mesoporous crumpled graphene particles as self-dispersible solar absorbers that can achieve homogenous dispersion within medium-temperature silicone oil. The mesoporous crumpled graphene particles were synthesized by using an aerosol-assisted capillary compression process and a subsequent oxidative-etching reaction. By controlling the etching time, the introduced mesopores can tune the apparent density of the crumpled particles into a value comparable with the dispersing base fluid. The crumpled surface structure, small particle size and compatible density weaken the inter-particle van der Waals attraction and gravitational sedimentation tendency. The resultant silicone oil nanofluids have maintained their uniform dispersion under a heating temperature of 200 °C for 50 h. The stable uniform dispersion together with the high solar absorptance, the large specific heat capacity and the proper viscosity enabled the application of prepared nanofluids for consistent high-performance medium-temperature solar-thermal energy harvesting under concentrated solar irradiation. [Display omitted] • Mesopores are introduced to crumpled graphene particles to tailor the density. • Stable dispersion of mesoporous crumpled graphene particles in silicone oil is achieved. • The nanofluids have high solar absorptance, large heat capacity and suitable viscosity. • Medium-temperature nanofluidic solar-thermal energy harvesting is demonstrated. [ABSTRACT FROM AUTHOR]