Thermal performance and physicochemical stability of silver nanoprism-based nanofluids for direct solar absorption

Highlights

• Rapid modified Stöber procedure used to produce silica coated silver nanoprisms.

• Real-time temperature measurements in solar simulator.

• Enhancement in photo-conversion efficiency of three times water base-fluid.

• Stability testing under application relevant conditions vital.

• Stability tests indicate need for further optimisation.

Abstract

To utilize nanofluids for direct absorption solar collectors (DASCs), they need to maintain their performance and physicochemical stability with exposure to solar radiation. In the present studies, three water-based nanofluids were characterized under light exposure, including silver nanoprisms (AgNPrs), silica coated silver nanoprisms (SiO₂@AgNPr) and silica (SiO₂) nanoparticles. Their temperature profiles and stability were monitored using simulated sunlight (SSL) and natural sunlight exposure (NSL), quantified by UV-vis spectroscopy and, in the case of SSL, characterized by transmission electron microscopy (TEM). With SSL both silver nanofluids showed an increase in maximum temperature of approximately 40–45 °C, with a photo-conversion efficiency of about three times greater than the SiO₂ nanofluid and water base-fluid. Stability tests showed the SiO₂@AgNPr nanomaterial to be morphologically unstable, with the AgNPrs etching over a period of several hours. The AgNPrs showed a higher tendency to aggregation than SiO₂@AgNPr nanofluids when exposed to NSL sunlight over a two-week period. Contrarily, the latter exhibited notable changing in shape, consequently effecting the absorption band position. The results highlight strongly the need for stability trials under realistic conditions for the development of nanofluids for direct solar absorption.

Introduction

Direct absorption solar collectors (DASCs), which were developed during the 1970s, utilize a volume of liquid as the working fluid to collect solar thermal energy. At their most basic, a DASC consists of a flowing working fluid protected by a glass or glazed front face, which absorbs the solar radiation directly without the need for a conventional selective surface absorber. Compared to selective absorbing surface solar collectors (Gupta et al., 2015a), DASCs exhibit advantages such as simpler manufacturing, saving raw material (in particular copper) (Otanicar and Golden, 2009, Otanicar et al., 2010, Tyagi et al., 2009), improving heat transfer (Iyahraja and Rajadurai, 2015, Lee et al., 2016) and generating an even distribution of the rising temperature through the fluid system, which reduces overall heat loss (Luo et al., 2014, Xu et al., 2015). These factors have led to an overall increase in efficiency of ~10% (Nasrin et al., 2015, Turkyilmazoglu, 2016). Since water-based DASCs absorb only about 13% of the solar radiation (Otanicar et al., 2009), highly solar absorbing additives such as micro particulate carbon black or Indian ink were explored (Gorji and Ranjbar, 2016). However, this addition can cause operational issues such as pump blockage, erosion and sedimentation (Kazemi-Beydokhti et al., 2014, Khullar et al., 2014). In this regard, following the advanced development of nanotechnology, the addition of nanoparticles having notable absorption characteristics has led to the realization of working-nanofluids (i.e. nanoparticles suspended in base-fluid) providing great potential for the improvement of DASC performances.

A nanofluid is an engineered dilute colloid consisting of nanoparticles suspended in a suitable base-fluid (such as water or ethylene glycol) normally with the addition of various stabilizing agents (such as surfactants) to prevent aggregation and sedimentation of the nanoparticles. A wide range of nanomaterials have been adopted for such applications, including carbon based nanomaterials (e.g. graphite, graphene oxide, nanotubes) (Delfani et al., 2016, Gorji and Ranjbar, 2017b, Hordy et al., 2014, Karami et al., 2014, Khosrojerdi et al., 2017, Li et al., 2020, Luo et al., 2014, Otanicar et al., 2010, Shende and Ramaprabhu, 2016), metal oxides (e.g. Al, Ti, Cu, Fe, Zn, Ce and Si oxides) (Gorji and Ranjbar, 2017b, Gupta et al., 2015b, Hatami and Jing, 2017, Karami et al., 2015, Milanese et al., 2016, Turkyilmazoglu, 2016, Xu et al., 2015) and metallic nanoparticles. Nanofluids containing more than one type of nanomaterials, or composite nanomaterials, have been found to be more adaptable, offering potential performance improvements (Akilu et al., 2018, Dhinesh Kumar and Valan Arasu, 2018, Menbari et al., 2017, Ranga Babu et al., 2017, Yu and Xuan, 2018). For the metallic nanoparticle based additives, most work has been focused on gold (Gorji and Ranjbar, 2017a, Sharaf et al., 2019, Zeiny et al., 2018), copper (Luo et al., 2014), nickel (Gorji and Ranjbar, 2017a) and silver (Abdelrazik et al., 2019, Bandarra Filho et al., 2014, Chen et al., 2016, Gorji and Ranjbar, 2016, Gorji and Ranjbar, 2017b, Otanicar et al., 2010). In particular for silver, hybrids have also been studied, including carbon nanohorns (Sani et al., 2015), SiO₂ coating (Hjerrild et al., 2016, Hjerrild et al., 2018) and TiO₂ (Xuan et al., 2014).

Studies on DASCs containing silver nanoparticles (AgNPs) have shown efficiency improvements between 5% and 144% (Bandarra Filho et al., 2014, Otanicar et al., 2010, Walshe et al., 2019). Currently, most of AgNPs related studies have been focused on silver nanospheres, with limited attention paid to anisotropic silver nanomaterials for volumetric absorption (Crisostomo et al., 2017, Hjerrild et al., 2018, Taylor et al., 2018). In our series of research on silver nanoparticles (Carboni et al., 2013, Carboni et al., 2016, Mabey et al., 2019, Zmijan et al., 2014), silver nanoprisms (AgNPrs) that exhibited a strong absorbance of near infra-red (NIR) wavelengths attributed to surface plasmon resonance were used. These may be beneficial for DASC performance. In order to preserve the physical and mechanical properties of AgNPs, a coating strategy using silica as an inert yet optically transparent material has been further developed by our research group (Carboni et al., 2016) and others (Hjerrild et al., 2018). However, there still remains the challenge of maintaining their physicochemical stability as a working-nanofluid during operation of DASCs, due to the possibility of photo-induced chemical processes occurring on exposure to sunlight (Tang et al., 2013). These lead to changes in the absorption spectra of the nanofluid with time effecting the efficiency of the DASC.

To be suitable for DASCs, the nanofluid employed needs to be physically and chemically stable during thermal cycling (Goel et al., 2020), pumping and under sunlight exposure. This includes maintaining the same viscosity, thermal conductivity and freezing / boiling temperature with no particle aggregation, as well as showing no reactions between chemicals within the nanofluid or externally with any material used for the DASC. This is challenging for silver nanofluids due to photo conversion that can change the nanoparticle shape and size, hence the need for protection such as using a coating. Many parameters influence the physicochemical stability of the nanofluid. The base-fluid’s composition is crucial, and pH, contaminants, additives or presence of oxygen may lead to disintegration of the nanoparticles (at high pH) or aggregation (absence of solubilizing and stabilizing agents). In addition, the nature, size, shape and presence of coating or additional stabilizing agents can affect the colloidal or nanofluid stability. Currently there is a lack of knowledge on the stability of AgNPr nanofluids; only very few reports can be found on the investigation of related non-spherical silica coated silver nanodiscs (Hjerrild et al., 2018, Taylor et al., 2018).

Although thermophysical properties of nanofluids such as viscosity, thermal conductivity and specific heat are important for DASC applications, this study has been focused on optical properties. This is primarily because of the significant possibility of photo-induced chemical processes having a detrimental effect on the optical properties and hence performance and secondly because optical properties have traditionally received less attention in the literature (Khanafer and Vafai, 2018).

Here, we have investigated silver and SiO₂ coated silver nanoprism, water-based nanofluids for their suitability for DASC applications. By utilizing a solar simulator, real-time temperature profiles were obtained for these nanofluids which exhibited strong absorbance in the near-IR wavelength band (>750 nm). The thus obtained photo-conversion efficiency (PE) was compared to silicon-based nanofluids and to water itself. The stability of the optical properties of the nanofluids were evaluated under real-world application relevant conditions using a static DASC system exposed to SSL in the visible to near IR range, and under natural sunlight exposure. This is the first time that nanofluids containing silver nanoprisms with a maximum absorption peak at a wavelength >750 nm and SiO₂ coatings of about 50 nm thickness have been evaluated for optical and morphological stability in such a manner.

Although other SiO₂ coated non-spherical silver nanoparticle based nanofluids have been investigated by other researchers for their optical stability (Taylor et al., 2018), the results presented in this paper differ considerably, highlighting the importance of further research into stability for these and other nanofluids.