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Optimal optical absorption properties are imperative for the utilization of nanofluids in solar energy applications. Excellent optical absorption facilitates the augmentation of solar energy utilization efficiency. The base fluid contributes significantly to the optical properties of the nanofluid. However, the effect of base fluid on the nanofluid optical properties has yet to be explored. This work investigates the effect of three base fluids, deionized water, ethylene glycol, and 1,2-propanediol, on the optical absorption properties of titanium nitride (TiN) nanofluids. The results show differences in the optical properties of the TiN nanofluids prepared for different sonication times. With the absence of surfactant addition, the stability of TiN nanofluids based on ethylene glycol-based and 1,2-Propanediol-based surpasses that of deionized water-based TiN nanofluids. However, in comparison to ethylene glycol-based and 1,2-Propanediol-based, deionized water-based TiN nanofluids exhibit commendable light absorption capacity. Finally, the photothermal conversion efficiency is calculated for different base fluids. The photothermal conversion efficiency of the deionized water-based TiN nanofluid is 50.82%, which is 13.91% and 15.98% higher than that of the ethylene glycol-based and 1,2-propanediol-based TiN nanofluids under the same conditions, respectively. Thus, deionized water-based TiN shows excellent optical absorption properties, essential for enhanced solar energy thermal utilization. This work will help guide the application of TiN nanofluids. [ABSTRACT FROM AUTHOR]

Solar absorber, which effectively collects solar energy and converts it into thermal energy, is the essential component of a solar thermal system such as concentrated solar power, thermophotovoltaic systems. However, most highly efficient solar absorbers require expensive lithographic techniques to implement. Therefore, in this work, we propose a lithography-free multilayer W/SiO2 solar absorber for efficient photothermal conversion. The solar absorption characteristics of the absorber in the full spectral range are investigated concerning the incidence, polarization angle, geometry parameters, and the number of cells. The results show that the absorber can achieve a total solar absorption efficiency of 97.17%, exhibiting polarization insensitivity and wide-angle characteristics. The absorption mechanism can be explained by superimposed Fabry–Perot resonance, confirmed by the electric field and normalized power dissipation density distributions. In addition, the absorber can achieve a photothermal conversion of up to 90.02% at 100℃ without considering the solar concentration factor. Finally, we investigated the variation of photothermal conversion efficiency with temperature for different solar concentration factors. The research shows that increasing the solar concentration factor is beneficial for improving the photothermal conversion of the absorber in high-temperature environments. This work may provide a valuable guide for the design of lithography-free solar absorbers. [ABSTRACT FROM AUTHOR]

Plasmonic nanofluids, which have superior optical properties, provide a novel way to achieve efficient solar thermal utilization. Here, the optical properties and photothermal conversion performance of hollow nanopillar structures based on four plasmonic materials (Ag, Au, Cu, TiN) were investigated and compared with the corresponding solid nanopillar structures. Numerical results show that the most significant increase in photothermal conversion efficiency is the plasmonic nanofluid based on hollow Au nanopillars, with an increase of 34.14%. The underlying physical mechanism is that the local and propagating surface plasmon resonances are further promoted in the hollow structure. In contrast, the increase of the photothermal conversion efficiency is only 1.41% for the hollow TiN nanopillars. This is due to the fact that the solid TiN nanopillars already have a high photothermal conversion efficiency of 97.19%. Thus, we can consider the enhancement of the photothermal properties of the plasmonic nanofluid by the hollow structure, especially for precious metal materials with narrow-band absorption, such as Au and Ag. For plasmonic materials with intrinsic broad-spectrum absorption (TiN, Cu), the enhancement effect is not significant. It is believed that this work will provide theoretical guidance for high-performance direct absorption solar collectors. • Optical properties of hollow nanopillars based on Ag, Au, Cu, and TiN are compared. • The most significant increase (Au) in photothermal conversion efficiency is 34.14%. • Local and propagating surface plasmon resonances are promoted in hollow structure. • This work will guide designing high-performance direct absorption solar collectors. [ABSTRACT FROM AUTHOR]

The triangular pyramid nanostructure (TPN) raises a perfect prerequisite for the aggregation of surface plasmon resonances (SPRs). However, the application of TPN in thermoplasmonics has not yet been developed. The aim of this study is to explore the development potential of TPNs for enhanced noble metal photothermal applications. We investigated the optical absorption properties, photothermal conversion efficiency, and photothermal response under continuous wave irradiation of TPNs based on Ag and Au nanomaterials. The results show that TPN helps to enhance the SPR excitation of Ag and Au in the near-infrared. TPN based on noble metal nanomaterials shows good light absorption capacity and photothermal response. The photothermal conversion efficiencies of the TPN nanoparticle suspensions increased by 56.95% (Ag) and 49.57% (Au), respectively, compared to the same volume of noble metal nanospheres. This work is intended to researchers better understand the development potential of TPN in thermoplasmonics applications. [ABSTRACT FROM AUTHOR]

• The effect of slotting on the optical properties of silver nanopillars is studied. • Slotting can enhance the resonance absorption peaks of silver nanopillars. • The best slotting method offers a 13 % improvement compared to nanopillars. • This work shows slotting has a promising prospect in solar thermal utilization. Metal nanoparticles (MNPs) have attracted enormous research interest due to their excellent plasma properties. However, in the thermal utilization of solar energy, the narrowband absorption of MNPs limits their application. The investigation of MNP absorption through adjustment in the broadband spectrum holds significant scientific and practical implications. In this work, we study the effect of slotting on the light absorption and photothermal properties of silver nanopillars. The results show that the increase in the number of slots is beneficial to reduce scattering and improving the resonance absorption of silver nanopillar. Through the calculation of photothermal conversion efficiency, it is found that different slotting methods have different levels of improvement compared with silver nanopillars. The utilization of a combination of transverse and longitudinal slotting techniques leads to a 13 % enhancement in the photothermal conversion efficiency of silver nanopillars. The electric field distribution and temperature distribution at resonant wavelengths shows that the excellent light absorption ability of the slot nanopillars is caused by the "lightning rod" effect at the tip of the slit and the excitation of multiple plasmon resonances. The existence of slots is beneficial to compensate for the photothermal response of silver nanopillars in the near-infrared region. Our study proves that slotting helps enhance the application of silver nanopillars in solar thermal utilization. [ABSTRACT FROM AUTHOR]

The heat exchange medium moderately determines the heat utilization efficiency of solar energy. Nanofluids, a type of working fluids with high thermal conductivity and strong light absorption, have been studied and applied to improve solar energy utilization. In this study, TiN@SiCw binary composite nanoparticles were prepared by a coupling agent method. The optical coupling absorption properties of TiN and SiCw nanoparticles were numerically simulated by the finite-difference time-domain method. Results showed the TiN@SiCw nanostructure can improve the width and intensity of spectral absorption. Then TiN@SiCw and SiCw nanofluids based on ethylene glycol were prepared by a two-step method and studied experimentally in terms of thermal conductivity and optical absorption. The photothermal conversion efficiency of the nanofluids was measured by a special flow and photothermal coupling model (side radiation). Experiments showed the thermal conductivity and light absorption of TiN@SiCw composite nanofluids were stronger than those of SiCw single-component nanofluids and the base fluid. Specifically, the energy absorption fraction and the photo-thermal efficiency of TiN@SiCw nanofluids at the highest concentration of 140 ppm were 96.2% and 90.5%, respectively, which were 12.1% and 21.7% higher respectively compared with SiCw nanofluids. Hence, the TiN@SiCw nanofluids are suitable for enhancing the efficiency of solar collectors. • TiN grafted SiC whisker nanostructures were prepared. • The width and intensity of solar spectral absorption can be improved by TiN@SiCw nanostructure. • The plasmonic nanofluid can realize both outstanding thermophysical properties and stability. • A photo-thermal efficiency of 90.5% is achievable for 140 ppm TiN@SiCw nanofluid. [ABSTRACT FROM AUTHOR]

It has been reported that plasma nanofluids are beneficial to improve the performance of a direct absorption solar collector (DASC). For this purpose, the solar absorption of gold sphere nanoparticles (NPs) in the near-infrared spectral region and gold nanorods in the visible spectral region needs to be focused. Based on this issue, we propose a core-shell composite nanorod and investigate its optical properties to illustrate its potential for improving the solar absorption efficiency of the DASC. The results show that the core-shell composite nanorod improves the absorption efficiency both in the near-infrared and visible spectral region. Further study reveals that the absorption mechanism of the composite nanorods can be attributed to localized surface plasmon resonance (LSPR) and propagating surface plasmon resonance (PSPR). In addition, in view of the discussion about the geometrical size of composite nanorods and the calculation of composite nanorod suspension, the solar absorption efficiency can be as high as 95.3%. Notably, the imperfect shell during the experimental synthesis will greatly reduce the absorption capacity of the composite nanorods. The results show that the core-shell composite nanorod has great potential in the DASC applications. •A core-shell composite nanoparticle is designed and investigated. •Two absorption peaks are excited in the composite nanorod. •The solar absorption efficiency of composite nanorod suspension is 95.3%. •The core-shell composite nanorod can be a candidate material for DASC applications. [ABSTRACT FROM AUTHOR]

• The effect of different hollow structures on the optical absorption properties of Au nanodiscs has been studied for the first time. • The continuous sharpening of hollow negatively affects the photothermal conversion properties of Au nanodiscs. • The square hollow structure demonstrates outstanding optical absorption and efficient photothermal conversion. • This work provides valuable guidance for enhancing plasmon nanoparticles. Plasmonic nanoparticles (PNPs) with hollow structures have shown great potential for enhancing optical absorption. While spheres or cylinders are commonly used for PNPs, the study of hollow PNPs for direct absorption solar collectors is limited. This research investigates the impact of three different hollow structures (cylindrical – Case 1, square – Case 2, and trigonal – Case 3) on the optical absorption properties of Au nanodiscs. The findings demonstrate that the presence of hollow structures significantly enhances the photothermal conversion efficiency of Au nanodiscs. Specifically, Case 1, Case 2, and Case 3 exhibit photothermal conversion efficiencies that are 17.9%, 22.0%, and 16.8% higher, respectively, compared to standard Au nanodiscs. Notably, Case 2 demonstrates the highest optical absorption capacity and photothermal conversion efficiency among the three hollow structures. Through optimization, Case 2 achieves a remarkable photothermal conversion efficiency of 97.6%. Meanwhile, we also investigated the effect of continuously sharpening the hollow edges on the optical absorption properties of the nanodiscs. The continuous sharpening of the hollow core hampers both the enhancement of optical absorption properties and photothermal conversion properties in Au nanodiscs. Therefore, this research provides valuable theoretical insights for utilizing plasmonic nanoparticles with hollow structures to enhance photothermal conversion. [ABSTRACT FROM AUTHOR]

Novel nanofluids (NFs) based on bio-nanotechnology have enormous potential for solar thermal utilization. The biomimetic nanoparticles (NPs) nanostructures provide new ideas for the design of solar collector medium. Inspired by the biological properties of waxberry, we proposed waxberry-shaped biomimetic nanoparticles (WSBNPs) for solar thermal conversion in this work. Numerical results show TiN WSBNPs exhibit good optical absorption properties compared to Ag- and Au-based WSBNPs. When the volume fraction f v = 5 × 10−6 and the depth h = 15 mm, the photothermal conversion efficiency η abs can reach 93.9%. The mechanistic analysis shows that the high absorption performance of WSBNPs can be attributed to the presence of the "lightning rod" effect on the surface of NPs and the mutual coupling of surface plasmon resonance. The η abs of TiN WSBNPs can reach 90% at a lower penetration h = 11 mm and f v = 5 × 10−6. Meanwhile, the η abs of WSBNPs is proportional to the number of protrusions on the exterior surface. It has been shown that TiN WSBNPs with a raised exterior surface have excellent potential for efficient solar energy utilization. [ABSTRACT FROM AUTHOR]

Solar absorber, which is widely used in concentrated solar thermal systems and solar photovoltaics, has attracted great attention in recent years. Concentrated solar irradiation not only saves production costs, but also effectively improves the photothermal conversion efficiency of the system. However, the huge radiative heat flux will cause the system temperature to rise rapidly, which poses significant challenges for designing solar absorbers. Here, a metasurface absorber based on cylinder array structure is proposed for efficient solar energy harvesting, using the high-temperature resistant materials TiN and SiO 2. The total solar absorption of the absorber is up to 0.94 at wavelengths of 300–2500 nm. The high absorption performance of the metasurface absorber can be explained as the coupling effect of surface plasmon resonance and Fabry-Pérot resonance, which is confirmed by the electric field distribution. Moreover, the effect of geometric parameters on absorption performance is analyzed. Finally, we discuss the influence of incident angle on the solar absorber. We believe this work could deepen the understanding of coupling resonance mode and guide the design of high-temperature solar absorbers. [ABSTRACT FROM AUTHOR]

Nanofluids are one of the keys to improving solar energy utilization efficiency. Metal nanoparticles (MNPs) have narrow-band absorption limitations in photothermal conversion. Recent studies have shown that it can overcome narrow-band absorption by adding slots on the nanoparticles (NPs) to improve photothermal conversion efficiency. In this work, we investigated the effect of curved slots on the optical properties of Ag nanorods by the finite element method. The results show that the presence of the curved slot enables the Ag nanorods to excite multiple resonance absorption peaks in the wavelength range of 300–1100 nm. The curved slotting reduces the overall scattering of the Ag nanorods and enhances its spectral absorption in the visible and near-infrared regions. In addition, we also calculated and compared the photothermal conversion efficiency of nanorods and nanorods with curved slots. At f v = 1 × 10−5 and h = 20 mm, the photothermal conversion efficiency of the curved slot Ag nanorods is increased by 6.8% over that of the Ag nanorods. In the measured volume fraction and fluid depth range, when the photothermal conversion efficiency reaches about 90%, the h and f v required for nanorods with curved slots are about 0.64 times that of nanorods. Nanorods with curved slots only need a meagre volume fraction and fluid depth to achieve high photothermal conversion efficiency. We believe that the proposed curved slot metal nanorods can provide a feasible solution for solving the narrow-band absorption of nanorods and improving the efficiency of solar energy utilization. [ABSTRACT FROM AUTHOR]

Due to the unique geometric effects and excellent thermophysical properties, nanoparticles (NPs) have attracted significant attention in recent years. Compared to composite NPs, the hollow structures are expected to reduce the complexity of the synthesis process and improve the long-term stability of the nanofluids. In this work, we propose a direct absorption solar collector (DASC) based on hollow TiN NPs for efficient solar harvesting. The optical properties of the hollow NPs are investigated using the finite element method. Numerical results show that the photothermal conversion efficiency of hollow TiN can reach 92.64%, which is 7.18% higher than that of solid TiN with the same particle size. The potential physical mechanism can be explained by the fact that surface plasmon resonances are excited on the inner and outer surfaces of the hollow TiN NPs. Further, we investigate the effect of geometrical parameters of hollow TiN NPs on the performance of DASC. Our study demonstrates that hollow TiN NPs have a promising prospect in efficient solar thermal utilization. • A hollow TiN nanoparticle for efficient solar thermal utilization is proposed. • Compared to solid TiN, hollow TiN can excite double plasma resonances. • The photothermal conversion efficiency can be increased by 7.18%. • Hollow Plasma-NPs have great potential for photothermal conversion applications. [ABSTRACT FROM AUTHOR]

The development of high-efficiency, low-cost and pollution-free new energy has become the focus of scientific research in the 21st century. As for this aspect, exploring more efficient nanoparticles (NPs) for direct absorption solar collectors (DASCs) has become a key issue in solar collector applications. However, the common NPs are usually composed of precious metal gold (Au) or silver (Ag). In this work, we propose a core-shell nanorod surrounded by a Cu shell with a C core and study its absorption properties in solar absorption spectroscopy. The aim is to explore its capability in DASC solar absorption efficiency. The research results show that this nanorod can reduce the application cost while maintaining high solar absorption efficiency. And the possible effects of nanoparticle shell defects during nanoparticle synthesis are considered. The results are as expected, which significantly impacts solar energy absorption. In addition, the C@Cu core-shell nanorods have a wider absorption band and higher resonance absorption peak compared with single Cu NPs. The results indicate that the core-shell composite nanorods proposed in this work have great potential in the application of DASC and are expected to be one of the candidate materials. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to give an in-depth study on the thermos-physical properties and photothermal conversion characteristics of carbon quantum dot (CQDs). In this study, CQDs nanofluids were synthesized by the simple one-step method. A series of stability analysis experiments were conducted to determine the excellent dispersibility and stability of CQDs nanofluids. The thermal conductivity of the nanofluids was measured by KD-2 Pro, and the results showed an enhancement up to 11 % at 50 °C. And the viscosity of CQDs nanofluids decreased with the concentration and temperature. In addition, it was found that the maximum photothermal conversion efficiency was up to 71.09 %. All the results indicated that the prepared CQDs nanofluids had great potential in the applications of direct absorption solar collectors (DASC). [Display omitted] • Nanofluids containing carbon quantum dots based on glycerol and polyethylene glycol-200 were prepared. • The maximum photothermal conversion efficiency of nanofluids is 69.89 %. • The thermal-physical properties of carbon quantum dots nanofluids were tested systematically. [ABSTRACT FROM AUTHOR]

Plasmonic nanofluids have excellent optical properties in solar energy absorption and have been widely studied in solar thermal conversion technology. The absorption of the visible region of solar energy by ordinary metal nanoparticles is usually limited to a narrow resonance band, so it is necessary to enhance the coupling effect of nanoparticles in the visible spectrum region to improve absorption efficiency. However, it is still a difficult task to improve solar energy absorption by adjusting the structure and performance of nanoparticles. In this paper, a plasma dimer Ag nanoparticle is proposed to excite localized surface plasmon resonance (LSPR). Compared with an ordinary Ag nanoparticle in the visible region, the plasmonic Ag dimer nanoparticle produces more absorption peaks and broader absorption bands, which can broaden solar energy absorption. By analyzing the electromagnetic field of the nanoparticle, the resonance mode of the plasma dimer is discussed. The effects of the geometric dimensions of the nanoparticle and the embedding of two spheres on the optical properties are studied. In addition, the effects of a trimer and its special structure on the optical properties are also analyzed. The results show that the proposed plasma dimer Ag nanoparticle has broad prospects for application in solar thermal conversion technology. [ABSTRACT FROM AUTHOR]