Experimental investigation on stability of thermal performances of solar salt based nanocomposite

Highlights

• Two-step method modified for preparing solar salt based nanocomposite.

• Stability of solar salt based nanocomposite investigated under thermal shock circulation.

• Reduction in thermal properties of nanocomposite found by two heat treatments.

• Mechanism of nanoparticle agglomeration to explain reduction of thermal properties.

Abstract

It is recognized that the thermal performances of the molten salt based nanocomposite can be significantly enhanced. However, there were rare investigations regarding its stability under practical operating thermal environments. In this study, both of the specific heat capacity and thermal diffusivity of the molten salt based nanocomposite were experimentally evaluated under various operating conditions, including that of exposure to constant high temperature and low-high temperature circulation. The typical solar salt based nanofluid was prepared via the modified two-step method. The selected nanoparticle was SiO₂ with an average diameter of 30 nm. Experimental results indicated that both the heat treatments could significantly decrease the thermophysical properties, which would result in poor thermal stability. In comparison with the samples exposing to constant high temperature, the thermal cycled samples showed a more stable thermal property. Further investigations on the SiO₂ particle morphology transition in the process of the heat treatments revealed that the impact of both temperature field and natural convection on particle agglomeration should be responsible for the decrease in thermophysical properties of nanofluid.

Introduction

With the adverse effects of harmful gas and CO₂ emission by the fossil fuels, it is therefore imperative to seek renewable and clean sources of energy. Among various types of renewables technologies, the generation of electricity with solar energy has attracted wide attention. Currently, there are two typical methods of converting solar energy into electricity, namely; photovoltaic (PV) and concentrated solar thermal power (CSP) [1]. For large scale production of electricity, CSP systems is more cost effective option since they concentrate solar energy as thermal energy source to be used in a general thermodynamic cycle, whilst also integrating with thermal energy storage (TES) [2,3].

For CSP, the cycle efficiency can be enhanced by increasing the working temperature of heat transfer fluids (HTF). In comparison with the conventional HTF such as mineral oil, it is noticed that molten salt can raise the working temperature to a temperature as high as about 560 °C [4]. Thus, investigations on molten salts have drawn more and more researchers’ attentions. However, the thermophysical properties of molten salt, such as its specific heat capacity, thermal conductivity, etc. are relatively low, which can lead to dramatic increase in size and cost of HTF/TES.

Over the past decades, many active and passive techniques had been used for heat transfer enhancement. Among these techniques, nanofluid, a dilute suspension of nanometer-size particles and fibers dispersed in a liquid, was reported to be able to enhance both the specific heat capacity and thermal conductivity, which was identified as an effective heat transfer fluid with enhanced thermal transport characteristics [5]. Recently, many research efforts has been devoted to high temperature molten salts based nanofluids. In the previous works, metal-oxide nanoparticles, CuO, TiO₂, ZnO, SiO₂, CNT, graphene, etc. [6], were proved to be the most applicable ones and they are used in various base fluids. Among these nanoparticles, SiO₂ were investigated more extensively due to its low cost, high chemical stability, easily synthesis procedure. The first study in this area was carried out by Shin and his team in 2011. Their research work was to put SiO₂ nanoparticles into various types of molten salts to seek any possible enhancement on the thermal properties [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. Andreu-Cabedo et al. investigated the influences of the mass fraction of SiO₂ nanoparticle on the specific heat capacity of binary nitrate also named Solar Salt (NaNO₃: KNO₃ = 60: 40 wt %). Their measurement suggested the best enhancement of SiO₂ can attain to 25% with 1 wt% addition [11]. Later on, Tiznobaik and Shin. Investigated the particle size effects of SiO₂ enhancement with different nanoparticle sizes of 5 nm, 10 nm, 30 nm, 60 nm. Accordingly, it was found that the specific heat capacity can be enhanced by 24%, 26%, 23% and 26%, respectively, which suggested that the SiO₂ particles size seems to have very limited effects on the specific heat capacity [15]. Besides, Song et al. studied the thermal enhancement of SiO₂ on the low-melting-point eutectic quaternary nitrate salt composed of Ca(NO₃)₂·4H₂O, KNO₃, NaNO₃, LiNO₃ and reported an enhancement of 17% on specific heat capacity [20].

It is recognized that by addition of nanoparticles can not only enhance the specific heat capacity, but also improve the heat transfer performance of base salt. Myers Jr et al. [21] investigated the heat transfer performance of nitrate salts (pure NaNO₃, pure KNO₃, and their eutectic) doped with 2% wt. % CuO nanoparticles. Their measurement suggested that the enhancement of thermal conducivity could attain to about 40%, 20%, 50% for KNO₃, NaNO₃ and their eutectic, respectively. Meanwhile, Madathil et al. [22] studied the thermal conductivity of MoS₂, CuO nanoparticle-ternary nitrate nanocomposite which included KNO₃, Ca(NO₃)₃ and LiNO₃. Their experimental results demonstrated that their thermal conductivity can be enhanced by 5% for both MoS₂ and CuO sample. Shin and Banerjee [23] investigated the binary carbonate doped with SiO₂, their measurement results indicated that the average enhancement on thermal conductivity was 40% with 1 wt% SiO₂ nanoparticle. However, the research on thermal conductivity of nitrate salt doped with SiO₂ nanoparticles were not found in recent years.

For CSP application, it is well known that HTF of TES usually maintains to work at high temperature during daytime. It will be firstly heated with enough sunlight as it circulates through the receiver, and then, returns to series of heat exchangers in power block, where the fluid is used to generate high-pressure superheated steam at overcast or night. It is often exposed to both constant high temperature and high-low temperature thermal shock. As such, its thermo-physical stability of HTF, especially nanoparticles doped HTF, becomes crucial to its further commercial applications. To the best of the authors’ knowledge, researches in this area were quite limited and there are still much room can be enhanced further.

The aim of this study is to investigate the possible influences of both constant high temperature condition and high-low temperature circulation condition on thermal properties of molten salt based SiO₂ nanofluids. For this purpose, the molten salt based SiO₂ nanofluid is prepared with binary nitrate salt (NaNO₃–KNO₃ (60:40) wt. %) as the base salts. The SiO₂ nanoparticle size is about 30 nm and the added amount is set as 1 wt %, which is reported to be the most efficient addition for improving the thermophysical properties [11]. The enhancement of thermophysical properties will be investigated experimentally. In addition, the variations of the thermophysical properties of the Solar Salt based nanocomposite under both high temperature conditions and high-low temperature circulation conditions will be revealed. The possible mechanism for the stability of the solar salt based nanocomposite is also discussed in detail.