Carbon Nanomaterial-Based Nanofluids for Direct Thermal Solar Absorption

Considering the Performance Study of ZnO Nanofluid at Different Concentrations for the Full-Spectrum Utilization System

Single photovoltaic (PV) and photothermal (PT) technologies in solar energy applications are limited to the conversion of visible light and high-quality infrared spectra, respectively; this limitation results in relatively low energy utilization efficiency. In contrast, liquid spectrum-splitting technology enables the separation and conversion of various spectral bands, with the composition of the medium playing a pivotal role in the efficient utilization of the full spectrum. Compared to previous static spectral-splitting systems, this study introduces a dynamic nanofluid concentration control mechanism, which actively balances PV and PT contributions based on real-time solar conditions, achieving higher adaptability and efficiency. This study proposes a concentrated photovoltaic-thermal (CPVT) system based on the variable concentration of spectrum-splitting media, employing a concave-bottom, hollow pipeline structure. By introducing nanofluids with varying concentrations, we measured and analyzed spot uniformity and transmittance, comparing the system's absorption properties for infrared light to its transmittance properties for visible light. A tunable model for photoelectric and photothermal-electric conversion was constructed, enabling the evaluation of differences in thermal and electrical performance. The results indicate that, within the designed spectrum-splitting pipeline, increases in nanofluid concentration correlate with improvements in both temperature and thermal efficiency. However, the photovoltaic efficiency decreased at higher concentrations. When the concentration reached approximately 280 ppm, the system achieved a peak comprehensive efficiency of 50.63%, demonstrating its superiority in adaptability and full-spectrum utilization compared to previous CPVT systems. As the concentration increased to 420 ppm, light transmittance nearly approached zero, resulting in a peak in thermal efficiency. This variability in concentration endows the system with a flexible capacity to modulate both thermal and electrical outputs, offering significant potential for further development.

Thermophysical Profile of Industrial Graphene Water-Based Nanofluids

The exceptional properties of high-grade graphene make it an ideal candidate for thermal dissipation and heat exchange in energy applications and nanofluid development. Here, we present a comprehensive study of few-layer graphene (FLG) nanofluids prepared in an industrial context. FLG nanofluids were synthesized through an ultrasound-assisted mechanical exfoliation process of graphite in water with a green solvent. This method produces FLG of high structural quality and stable nanofluids, as demonstrated by electron microscope, dynamic light scattering and ζeta potential analyses. Thermal conductivity measurements of FLG-based nanofluids were conducted in the temperature range of 283.15 K to 313.15 K, with FLG concentrations ranging from 0.005 to 0.200% in wt. The thermal conductivity of FLG nanofluids is up to 20% higher than water. The modeling of nanofluid thermal conductivity reveals that this enhancement is supported by the influence of the thermal resistance at the FLG interface, and the content, average dimensions and flatness of FLG sheets; this latter varying with the FLG concentration in the nanofluid. Additionally, the density and heat capacity of FLG suspensions were measured and compared with theoretical models, and the rheological behavior of FLG nanofluids was evaluated. This behavior is mainly Newtonian, with a weak 5% viscosity increase.

Performance Prediction and Optimization of Nanofluid-Based PV/T Using Numerical Simulation and Response Surface Methodology

A numerical investigation was carried out in ANSYS Fluent® on a photovoltaic/thermal (PV/T) system with MXene/water nanofluid as heat transfer fluid (HTF). The interaction of different operating parameters (nanofluid mass fraction, mass flow rate, inlet temperature and incident radiation) on the output response of the system (thermal efficiency, electrical efficiency, thermal exergy efficiency, and electrical exergy efficiency) was studied using a predictive model generated using response surface methodology (RSM). The analysis of variance (ANOVA) method was used to evaluate the significance of input parameters affecting the energy and exergy efficiencies of the nanofluid-based PV/T system. The nanofluid mass flow rate was discovered to be having an impact on the thermal efficiency of the system. Electrical efficiency, thermal exergy efficiency, and electrical exergy efficiency were found to be greatly influenced by incident solar radiation. The percentage contribution of each factor on the output response was calculated. Input variables were optimized using the desirability function to maximize energy and exergy efficiency. The developed statistical model generated an optimum value for the mass flow rate (71.84 kgh-1), the mass fraction (0.2 wt%), incident radiation (581 Wm-2), and inlet temperature (20 °C). The highest overall energy and exergy efficiency predicted by the model were 81.67% and 18.6%, respectively.

Plasmonic Nanofluids: Enhancing Photothermal Gradients toward Liquid Robots

In situ energy generation in soft, flexible, autonomous devices is challenging due to the need for highly stretchable and fault-resistant components. Nanofluids with pyro-, tribo-, or thermoelectric properties have recently emerged as promising solutions for realizing liquid-based energy harvesters. Yet, large thermal gradients are required for the efficient performance of these systems. In this work, we show that oil-based plasmonic nanofluids uniquely combine high photothermal efficiency with strong heat localization. In particular, we report that oleic acid-based nanofluids containing TiN nanoclusters (0.3 wt %) exhibit 89% photothermal efficiency and can realize thermal gradients as large as 15.5 K/cm under solar irradiation. We experimentally and numerically investigate the photothermal behavior of the nanofluid as a function of solid fraction concentration and irradiation wavelength, clarifying the interplay of thermal and optical properties and demonstrating a dramatic improvement compared with water-based nanofluids. Overall, these results open unprecedented opportunities for the development of liquid-based energy generation systems for soft, stand-alone devices.

The Thermophysical and Physicochemical Properties of the Aqueous Dispersion of Graphene Oxide Dual-Beam Thermal Lens Spectrometry

Modern heat-conducting materials require special attention to analyze their thermophysical properties. Compared to classical methods, thermal lens spectrometry (TLS) has advantages due to its high sensitivity to physical and chemical composition. To avoid a systematic error in the analysis of complex systems, it is necessary to realize the limits of the applicability of the method. This study considers the features of thermal-diffusivity measurements by TLS in the stationary state for dispersed systems with absorbances up to 0.05. The limits of applicability of the method in analyzing heterogeneous systems are shown, and a mathematical apparatus is proposed for indicating a systematic error in finding thermal diffusivity that does not exceed 1%. Graphene oxide (GO), which has attractive physicochemical properties, was used as the object of analysis. GO belongs to 2D objects, the study of which requires highly sensitive methods and special attention when discussing the results. The thermophysical properties of aqueous dispersions of graphene oxide in a wide range of concentrations (up to 2 g/L) and lateral sizes (up to 4 µm) were studied by TLS. It has been found that with increasing nanophase concentration, the thermal diffusivity of graphene oxide dispersions passes through a minimum, which can be used in solving thermal insulation problems. It has been established that prolonged laser irradiation of the dispersion leads to a change in thermal diffusivity, which indicates the photochemical reduction of graphene oxide.

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

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.

Nanofluids for Direct-Absorption Solar Collectors-DASCs: A Review on Recent Progress and Future Perspectives

Owing to their superior optical and thermal properties over conventional fluids, nanofluids represent an innovative approach for use as working fluids in direct-absorption solar collectors for efficient solar-to-thermal energy conversion. The application of nanofluids in direct-absorption solar collectors demands high-performance solar thermal nanofluids that exhibit exceptional physical and chemical stability over long periods and under a variety of operating, fluid dynamics, and temperature conditions. In this review, we discuss recent developments in the field of nanofluids utilized in direct-absorption solar collectors in terms of their preparation techniques, optical behaviours, solar thermal energy conversion performance, as well as their physical and thermal stability, along with the experimental setups and calculation approaches used. We also highlight the challenges associated with the practical implementation of nanofluid-based direct-absorption solar collectors and offer suggestions and an outlook for the future.

Graphene-Based Nanofluids: Production Parameter Effects on Thermophysical Properties and Dispersion Stability

In this study, the thermophysical properties and dispersion stability of graphene-based nanofluids were investigated. This was conducted to determine the influence of fabrication temperature, nanomaterial concentration, and surfactant ratio on the suspension effective properties and stability condition. First, the nanopowder was characterized in terms of crystalline structure and size, morphology, and elemental content. Next, the suspensions were produced at 10 °C to 70 °C using different concentrations of surfactants and nanomaterials. Then, the thermophysical properties and physical stability of the nanofluids were determined. The density of the prepared nanofluids was found to be higher than their base fluid, but this property showed a decrease with the increase in fabrication temperature. Moreover, the specific heat capacity showed very high sensitivity toward the graphene and surfactant concentrations, where 28.12% reduction in the property was achieved. Furthermore, the preparation temperature was shown to be the primary parameter that effects the nanofluid viscosity and thermal conductivity, causing a maximum reduction of ~4.9% in viscosity and ~125.72% increase in thermal conductivity. As for the surfactant, using low concentration demonstrated a short-term stabilization capability, whereas a 1:1 weight ratio of graphene to surfactant and higher caused the dispersion to be physically stable for 45 consecutive days. The findings of this work are believed to be beneficial for further research investigations on thermal applications of moderate temperatures.

Carbon-Based Nanofluids and Their Advances towards Heat Transfer Applications-A Review

Nanofluids have opened the doors towards the enhancement of many of today's existing thermal applications performance. This is because these advanced working fluids exhibit exceptional thermophysical properties, and thus making them excellent candidates for replacing conventional working fluids. On the other hand, nanomaterials of carbon-base were proven throughout the literature to have the highest thermal conductivity among all other types of nanoscaled materials. Therefore, when these materials are homogeneously dispersed in a base fluid, the resulting suspension will theoretically attain orders of magnitude higher effective thermal conductivity than its counterpart. Despite this fact, there are still some challenges that are associated with these types of fluids. The main obstacle is the dispersion stability of the nanomaterials, which can lead the attractive properties of the nanofluid to degrade with time, up to the point where they lose their effectiveness. For such reason, this work has been devoted towards providing a systematic review on nanofluids of carbon-base, precisely; carbon nanotubes, graphene, and nanodiamonds, and their employment in thermal systems commonly used in the energy sectors. Firstly, this work reviews the synthesis approaches of the carbon-based feedstock. Then, it explains the different nanofluids fabrication methods. The dispersion stability is also discussed in terms of measuring techniques, enhancement methods, and its effect on the suspension thermophysical properties. The study summarizes the development in the correlations used to predict the thermophysical properties of the dispersion. Furthermore, it assesses the influence of these advanced working fluids on parabolic trough solar collectors, nuclear reactor systems, and air conditioning and refrigeration systems. Lastly, the current gap in scientific knowledge is provided to set up future research directions.

Effect of Toluene Addition in an Electric Arc on Morphology, Surface Modification, and Oxidation Behavior of Carbon Nanohorns and Their Sedimentation in Water

Carbon nanohorns (CNHs) are attractive for various applications, where a high specific surface area and long dispersion stability in water are important. In the present work, we study these parameters of CNHs prepared by arc evaporation of graphite depending on the conditions of the synthesis and subsequent oxidation in air. It is shown that the addition of toluene in the reactor during the arcing allows obtaining CNHs functionalized with −CH_x groups. Heating of CNHs in air at 400 °C leads to substitution of −CH_x groups for oxygen-containing groups. Moreover, the CNH endcaps are opened at 500 °C, and as a result, the specific surface area of CNHs increases 4 times. Aqueous suspensions with a concentration of oxidized CNHs of 100 µg/mL are stable for 8 months.

Fabrication and Stabilization of Oxidized Carbon Black Nanoparticle Dispersion in Aqueous Solution for Photothermal Conversion Enhancement

This study aims to explore oxidized carbon black nanoparticles (OCB-NPs) capped with an inorganic surfactant dispersed in water, as a carbon black water-based nanofluid, on photothermal conversion enhancement. We used ultraviolet-visible (UV-vis) absorption spectroscopy and zeta potential analyzers to identify the optimal concentration of sodium hexametaphosphate (SHMP) as an inorganic surfactant for OCB-NPs in order to determine the maximum value of UV-vis light absorption and absolute zeta potential. Then, the concentrations of 0.025-0.1 wt % OCB water-based nanofluid with SHMP were formulated by an ultrasonic bath for the examination of rheological behavior, thermal conductivity, and heating rate. The results indicated that the heating rate improvement of the water-based nanofluid involving 0.1 wt % OCB-capped with SHMP after irradiation by UV-vis light with wavelengths ranging from 220 to 380 nm, which is included in the solar spectrum, and an intensity of 205 W/m² increased by approximately 66%, compared to the base fluid in the cyclic flow system. Furthermore, after a 1 month storage period, the dispersion stabilization of water-based nanofluid including 0.1 wt % OCB-capped with SHMP reached 98%, as estimated by the UV-vis spectrophotometer.

Thermophysical Properties of Nanofluids Composed of Ethylene Glycol and Long Multi-Walled Carbon Nanotubes

In this work, thermal conductivity, viscosity, isobaric heat capacity, and density of stable carbon-based nanofluids are presented. The nanofluids under study are composed of 1,2-ethanediol (ethylene glycol, EG) and long multi-walled carbon nanotubes (MWCNTs), so-called ‘in-house 16h’ (synthesized in our laboratory via catalytic chemical vapor deposition during 16 h with a diameter of 60–80 nm and length of 770 μm). Poly(N-vinylpyrrolidone) (PVP) was used to increase the stability of nanofluids. The nanofluids were prepared via an ultrasonication-assisted, three-step method while their key thermophysical characteristics were obtained using the hot-wire technique and rotary viscometer. As a result, the addition of MWCNTs significantly improved the thermal conductivity of nanofluids by 31.5% for the highest 1.0 wt% (0.498 vol%) long MWCNT content, leaving the Newtonian character of the nanofluids practically intact.

Flexible Transparent Heat Mirror for Thermal Applications

Transparent heat mirrors have been attracting a great deal of interest in the last few decades due to their broad applications, which range from solar thermal convection to energy-saving. Here, we present a flexible Polyethylene terephthalate/Ag-doped Indium tin oxide/Polydimethylsiloxane (PAIP) thin film that exhibits high transmittance in visible range and low emissivity in the thermal infrared region. Experimental results show that the temperature of the sample can be as high as 108 °C, which is ~23 °C higher than that of a blackbody control sample under the same solar radiation. Without solar radiation, the temperature of the PAIP thin film is ~6 °C higher than that of ordinary fabric. The versatility of the large-area, low-radiation-loss, highly-transparent and flexible hydrophobic PAIP thin film suggest great potential for practical applications in thermal energy harvesting and manipulation.