A Review on Recent Progress in Preparation of Medium-Temperature Solar-Thermal Nanofluids with Stable Dispersion

Abstract

Direct absorption of sunlight and conversion into heat by uniformly dispersed photothermal nanofluids has emerged as a facile way to efficiently harness abundant renewable solar-thermal energy for a variety of heating-related applications. As the key component of the direct absorption solar collectors, solar-thermal nanofluids, however, generally suffer from poor dispersion and tend to aggregate, and the aggregation and precipitation tendency becomes even stronger at elevated temperatures. In this review, we overview recent research efforts and progresses in preparing solar-thermal nanofluids that can be stably and homogeneously dispersed under medium temperatures. We provide detailed description on the dispersion challenges and the governing dispersion mechanisms, and introduce representative dispersion strategies that are applicable to ethylene glycol, oil, ionic liquid, and molten salt-based medium-temperature solar-thermal nanofluids. The applicability and advantages of four categories of stabilization strategies including hydrogen bonding, electrostatic stabilization, steric stabilization, and self-dispersion stabilization in improving the dispersion stability of different type of thermal storage fluids are discussed. Among them, recently emerged self-dispersible nanofluids hold the potential for practical medium-temperature direct absorption solar-thermal energy harvesting. In the end, the exciting research opportunities, on-going research need and possible future research directions are also discussed. It is anticipated that the overview of recent progress in improving dispersion stability of medium-temperature solar-thermal nanofluids can not only stimulate exploration of direct absorption solar-thermal energy harvesting applications, but also provide a promising means to solve the fundamental limiting issue for general nanofluid technologies.

Keywords: dispersion stability; medium-temperature nanofluid; solar collector; solar-thermal nanofluids; stabilization mechanism.

Figure 5

(a) Steric stabilization of Fe₃O₄ nanoparticles through surface grafting with phosphate-terminated PDMS ligands, which enable their stable uniform dispersion within silicone oil ((a) reprinted/adapted with permission from Ref. [95]. 2016, Royal Society of Chemistry). (b) Titanate coupling agent modified graphene oxide (T-GO) dispersion within hydraulic oil ((b) reprinted/adapted with permission from Ref. [98]. 2017, Elsevier). (c) Self-dispersible crumpled RGO particles stably dispersed within silicone oil due to weakened inter-particle van der Waals attraction and gravitational sedimentation ((c) reprinted/adapted with permission from Ref. [102]. 2022, Elsevier). (d) Mesoporous crumpled graphene particles as self-dispersible solar absorbers within silicone oil ((d) reprinted/adapted with permission from Ref. [103]. 2022, Elsevier).

Figure 6

(a) Surface modification of SiO₂ nanoparticles with fluorocarbon brushes to create steric hinderance stabilization and enable their stable dispersion within [C₄mim]BF₄ ionic fluids ((a) reprinted/adapted with permission from Ref. [108]. 2015, the American Chemical Society). (b) Surface modification of graphene by grafting the molecular chains similar to [HMIM]BF₄ iconic fluids enable stable dispersion of the resultant nanofluids under medium temperatures and cycled heating/cooling conditions ((b) reprinted/adapted with permission from Ref. [109]. 2017, Elsevier). (c) Homogenous dispersion of MXene (Ti₃C₂) within [EMIM][OSO₄] and diethylene glycol ((c) reprinted/adapted with permission from Ref. [111]. 2020, Multidisciplinary Digital Publishing Institute). (d) Electrostatically stablized SiC nanoparticles within [HMIM]BF₄ ionic fluids ((d) reprinted/adapted with permission from Ref. [112]. 2017, Elsevier).

Figure 7

(a) Homogeneous dispersion of colloidal nanoparticles within molten salts due to chemical affinity between the nanoparticle surfaces and the molten salt ions ((a) reprinted/adapted with permission from Ref. [115]. 2017, Springer Nature). (b) Core-shell SiO₂@Al₂O₃ nanoparticles dispersed within melted solar salts due to reduced sedimentation tendency through matching the density of particles and molten salts. ((b) reprinted/adapted with permission from Ref. [119]. 2019, Elsevier). (c) Dependence of nanofluid dispersion stability on the size of SiO₂ particles ((c) reprinted/adapted with permission from Ref. [120]. 2022, Elsevier).

Figure 1

Schematic showing converting concentrated sunlight into medium-temperature heat by surface absorption-based solar collectors and direct absorption-based solar collectors.

Figure 2

Representative thermal storage fluids and solar absorbers for preparing medium-temrature solar-thermal nanofluids.

Figure 3

Schematics showing different types of stabilization strategies for preparing medium-temperature solar-thermal nanofluids: (a) hydrogen bonding stabilization, (b) electrostatic stabilization, (c) steric stabilization, (d) self-dispersion stabilization.

Figure 4

(a) The hydrogen bonding between the hydroxyl groups on the surface of GQDs and the ethylene glycol molecules enables homogenous dispersion of nanofluids with different concentrations (0.05 mg/mL, 0.1 mg/mL, 0.2 mg/mL) after heating at 180 °C for 7 days. ((a) reprinted/adapted with permission from Ref. [91]. 2019, Royal Society of Chemistry). (b) Covalent surface modification of deaggregated nanodiamond particles via formation of poly(glycidol) polymer brush chains to achieve stable dispersion within ethylene glycol. ((b) reprinted/adapted with permission from Ref. [92]. 2013, the American Chemical Society). (c) Electrostatically stabilized single-layer Ti₃C₂T_x MXene homogeneously dispersed within ethylene glycol at room temperature for 30 days. ((c) reprinted/adapted with permission from Ref. [93]. 2021, Elsevier). (d) Uniform dispersion of RGO within ethylene glycol with the assistance of electrostatic stabilization by surfactants. ((d) reprinted/adapted with permission from Ref. [94]. 2020, Elsevier).