Elsevier

Energy

Volume 215, Part A, 15 January 2021, 119111
Energy

Facile preparation of core-shell Ag@SiO2 nanoparticles and their application in spectrally splitting PV/T systems

https://doi.org/10.1016/j.energy.2020.119111Get rights and content

Highlights

  • A facile process for achieving silica-coated silver (Ag) nanoparticles is proposed.

  • Ag@SiO2 particles prepared at optimized conditions shows absorption peak at 474 nm.

  • Adding Ag@SiO2 particles & CoSO4 to PG causes increase in the filtering efficiency.

  • Ag@SiO2/CoSO4-PG nanofluid (25.4 mg/L) gives good spectral match with silicon cell.

  • Using 25.4 mg/L nanofluid yields total η of 63.3% & market value increase of 67.8%.

Abstract

Several researchers have demonstrated that plasmonic nanofluids based filters can potentially enhance hybrid solar photovoltaic/thermal (PV/T) systems performance. In this work, we report a facile process for achieving silica-coated silver (Ag) nanoparticles using dimethylamine (DMA) as a basic solvent to induce tetraethyl orthosilicate (TEOS) hydrolysis. Then, the prepared Ag@SiO2 nanoparticles which have a controllable silica shell thickness are suspended in propylene glycol-CoSO4 hybrid fluid. Finally, the characteristics of the PV/T system filtered by the Ag@SiO2/CoSO4-PG nanofluids are evaluated based on the indoor test and photo-thermal conversion model. The results show that the nanoparticles prepared under optimized conditions, i.e., pH value of 8–9, water: ethanol volume ratio of 1:4, temperature of 25 °C, TEOS volume of 0.05 mL, and reaction time of 12 h, exhibit an absorption peak at 474 nm. Further, this study reveals that Ag@SiO2/CoSO4-PG nanofluid filter with concentration of 25.4 mg/L gives a reasonable spectral match with silicon concentrator solar cell according to the measured optical transmittance and the calculated filtering efficiency of 39.3%. The use of 25.4 mg/L Ag@SiO2/CoSO4-PG nanofluid filter produces a higher total efficiency of 63.3% and yields economic value enhancement of 67.8% compared to the PV only system when worth factor (w) is 3.

Introduction

Energy and environmental crises have motivated the development of solar energy technologies. At present, solar photovoltaic/thermal (PV/T) systems are one of the most successful technologies for converting sunlight into useable energy [[1], [2], [3]]. PV/T technologies combine the generation of electrical energy and the collection of thermal energy to achieve a full exploitation of the solar energy. However, in traditional PV/T systems, PV module with a thermal absorber mounted on the bottom suffers from the limited thermal efficiency by the low fluid temperature required to prevent thermal degradation of the PV cells [4]. In order to solve this issue, spectral beam splitting technology, which refers to lateral spectral separation of solar spectrum using optical splitters, was proposed for the PV/T system to maximize system performance [5].

Various approaches for spectral separation of sunlight for PV/T systems have been reviewed by Ju et al. [6]. Among them, selective absorbers employing common heat transfer liquids or nanofluids could be a more affordable approach of spectral beam splitting for PV/T system [6,7]. Further, nanofluids are considered to be more promising spectral beam splitter for the PV/T system because of their tunable optical properties and suitable thermal properties [8]. The nanofluid filters are fixed between the light source and the PV cell, acting both as the spectral beam splitter and the heat transfer medium. Cui and Zhu [9] explored the characteristics of the silicon cell filtered by 2 mm MgO/water nanofluid and they revealed the total efficiency of the PV/T collector was over 60%. Liang et al. [10] experimentally analyzed the applicability of glycol-ZnO nanofluid optical filter for concentrating PV/T system indicating it has considerable potential for this application. An et al. [11] developed Cu9S5/oleylamine nanofluid for spectral splitting PV/T system with the mono-crystalline silicon cell and obtained a moderate-temperature output of over 100 °C. In recent years, nanofluid filters which incorporate Au or Ag nanoparticles are receiving increased attentions due to Au and Ag show stronger localized surface plasmon resonance (LSPR) over a very broad spectrum of light (300–1200 nm) [[12], [13], [14], [15]]. Saroha et al. [16] compared the filtering performance of Au/water nanofluid to that of Ag/water nanofluid through an indoor PV/T test. Even though Ag/water nanofluid filter showed a higher total efficiency than Au/water nanofluid filter, the Ag/water nanofluid filter is not suitable for high temperature applications due to the Ag nanoparticles were not protected by a layer of stable oxides to get rid of the thermal and UV degradation under sunlight. He’s group [17,18] developed an approach to synthesize Ag@TiO2 nanoparticles and explored the system performance filtered by the nanoparticles dispersed in ethylene glycol/water. In fact, we found the optical transmittance of Ag@TiO2/water nanofluid shown in Ref. [18] deviated from that of the ideal optical filter for their selected PV cells, especially in the ‘PV window’ of the cell. Further, the TEM images of the Ag@TiO2 nanoparticles indicated that the core-shell nanostructure was not clear and the particles dispersion was not good [18]. To protect Ag cores from the solution environment for excellent stability, several researchers used the silica to coat Ag nanoparticles, but these Ag@SiO2 nanoparticles were prepared for enhancing the luminescence of lanthanide complexes or improving physic-mechanical and antibacterial properties of natural rubber [19,20]. Taylor’s group proposed the nanofluid contained Ag@SiO2 nanodiscs for spectral splitting PV/T receiver [21,22]. The results also demonstrated that silica is the suitable coating oxide for Ag particles owing to its excellent optical transparency. Compared to water filter, using Ag@SiO2 nanofluids filter (0.026 wt%) improved total efficiency by 30% [21]. However, the TEM images of the Ag@SiO2 nanoparticles showed that the nanostructures of Ag@SiO2 were not sharp and numerous coreless silica particles were observed [21]. Therefore, it is necessary to improve the synthesis method for the Ag@SiO2 nanoparticles.

Sto¨ber method is widely used to synthesize Ag@SiO2 core-shell nanoparticles. This process involves the ammonia catalyzed hydrolysis and condensation of hydrophobic silanes [23]. Tetraethyl orthosilicate (TEOS) is the most frequently used as the silica precursor in the method. Tang et al. [24] adopted this method to prepare Ag@SiO2 nanoparticles in methanol, ethanol and isopropanol, respectively. The well-dispersed case of Ag@SiO2 nanoparticles was observed only in ethanol. In addition, previous study showed that the soluble [Ag(NH3)2]+ complex would be formed when coexistence of Ag nanoparticles and ammonia occurred [25,26]. To avoid the Ag cores etched by ammonia, Li et al. [27] proposed an ammonia-free approach and developed a multistep procedure to prepare Ag@SiO2 nanoparticles. They used NaOH + Na2CO3/NaHCO3 (9:1) mixture and trisodium citrate solution as the base solution to adjust the pH value of the reaction system, followed by the silica shell growth in an alcoholic solvent. In fact, this procedure is quite complicated because it involved many catalysts and tedious operation. Recently, Selim et al. [28] prepared Ag@SiO2 core–shell clusters via a modified Sto¨ber method involving an ammonia-catalyzed reaction. However, TEM analysis of the prepared Ag@SiO2 clusters indicated that nanoparticles tend to aggregate. Lee et al. [29] synthesized Ag@SiO2 and Au@SiO2 core-shell nanoparticles through the similar approach. The results showed that numerous multinuclear metal@SiO2 core/shell nanoparticles were found in TEM images. Therefore, more work should be conducted to improve the silica coating process for protecting Ag cores against the oxidation when applied in PV/T systems.

The objective of this study is to fill the research gap in Ag@SiO2 nanofluid-based optical filters for spectral beam splitting PV/T system mentioned above and develop a new synthesis route of Ag@SiO2 nanoparticles dispersed into propylene glycol-CoSO4 hybrid fluid with the suitable optical properties as the optical filter. In this work, dimethylamine (DMA) is selected as a convenient basic solvent to induce TEOS hydrolysis and a facile process is developed for achieving silica-coated Ag nanoparticles. This synthesis process can overcome the issues described above. Then, the core-shell Ag@SiO2 nanoparticles are dispersed into propylene glycol-CoSO4 hybrid fluid to act as spectral beam splitter for PV/T systems. Transmitting electron microscopy, optical absorption spectroscopy and full spectral transmittance are utilized to characterize the nanofluid performance. Furthermore, an investigation on the silicon concentrator cell performance filtered by the Ag@SiO2/CoSO4-PG nanofluid is carried out to assess the filtering performance of the prepared nanofluid. Finally, the energy outputs of the PV/T system filtered by the nanofluid are compared with the PV only system and the typical nanofluid based PV/T systems to assess the relative performance of our developed Ag@SiO2/CoSO4-PG nanofluids filter.

Section snippets

Materials

Materials required for the preparation of core-shell Ag@SiO2 nanoparticles were: silver nitrate (AgNO3, A.R.), aqueous ammonia (NH4OH, 25–28%),trisodium citrate dihydrate (C6H5Na3O7·2H2O, A.R.), polyvinylpyrrolidone (PVP, K30), d(+)-glucose (C6H12O6), ethanol (C2H6O, A.R.), dimethylamine (DMA, C2H7N, 40 wt%), tetraethyl orthosilicate (TEOS, A.R.), deionized water. These materials were provided by Sinopharm, China and used without further purification.

Synthesis of core-shell Ag@SiO2 nanoparticles

The synthesis route of core-shell Ag@SiO2

The formation of core-shell Ag@SiO2 nanoparticles

Based on a series of experiments, the optimized conditions for synthesizing core-shell Ag@SiO2 nanoparticles were determined: the TEOS hydrolysis was discovered to be moderate at 25 °C and the pH value of the solution was regulated to be 8–9 by DMA instead of aqueous ammonia. Because we found that aqueous ammonia would react with silver ions to form soluble complexes in the synthesis process.

Fig. 4 shows the formation mechanism of core-shell Ag@SiO2 nanoparticles, including the formation

Conclusions

From this study, we can make the following conclusions regarding the preparation of core-shell Ag@SiO2 nanoparticles and Ag@SiO2/CoSO4-PG nanofluid as well as their use in the spectral splitting PV/T systems:

  • (1)

    The core-shell Ag@SiO2 nanoparticles with homogeneous and dense silica shells are prepared under optimized conditions (i.e., pH value of 8–9, water: ethanol volume ratio of 1:4, temperature of 25 °C, TEOS volume of 0.05 mL, and reaction time of 12 h) using dimethylamine (DMA) as a

Credit author statement

Ju Huang: Methodology, Investigation, Software, Writing - original draft. Xinyue Han: Conceptualization, Visualization, Writing - review & editing, Funding acquisition, Supervision. Xiaobo Zhao: Software, Investigation, Data curation. Chunfeng Meng: Resources, Software.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No.51776091), China Postdoctoral Science Foundation (2019M661741).

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