Nowadays fossil fuel energy resources are severely reduced, which is causing an increasing demand for the development and application of suitable alternative energies, especially solar energy, which is a renewable energy [1
]. Concentrated solar thermal power (CSP) is one of the main technologies for solar energy utilization [2
], while the suitability of a solar collector should be chosen according to the testing conditions specified in the existing standards [6
]. A CSP system always uses heat transfer fluid (HTF) to send the intense heat to a thermal power plant which is able to provide power at night-time with the amount of stored thermal energy. Thus, a CSP system combined with a thermal energy storage system becomes the first choice to provide flexible and consistent power [7
]. The development of reliable thermal storage system is vital for the sustainable and efficient utilization of solar energy resource.
Because of their large specific heat capacity, low viscosity, wide temperature range, and good compatibility with containers, molten salts have been widely used as latent thermal storage media in solar power fields [9
]. Molten salts are also used as HTFs to store sensible heat, and they are considered as an alternative to oils because of their low cost and harmlessness to the environment [11
]. There are many studies conducted about molten salts, including their thermo-physical properties and system performances [12
]. Siegel et al. [13
] measured the thermo-physical properties of several molten salt mixtures, such as ternary eutectic salt (Na-K-Li, Na-K-Ca), quaternary eutectic salt (Na-K-Li-Ca), solar salt (Na-K), and HITEC salt. They found that all the data presented including density, viscosity, heat capacity, and thermal conductivity were dependent on temperature. The viscosity showed an Arrhenius relationship with temperature for the salt without calcium nitrate, while the Vogel-Fulcher-Tammann-Heese equation could possibly be a better fit for the viscosity of the salt with calcium nitrate. Ni et al. [14
] used a molecular dynamics simulation method to study the characteristics of solar salt with the addition of 5–25 mol.% Ca2+
. They pointed out that the viscosities of the mixtures increased and the self-diffusion coefficients of all the ions decreased, which was due to the formation of network restricting the motion of all the ions. However, the intrinsic limitations of the thermo-physical properties of pure salts hinder their widespread applications.
Nanoparticles have been used to enhance the thermo-physical properties of pure salt [15
], and the nanocomposites fabricated by salt and nanoparticles were extensively investigated both in their thermo-physical properties [16
] and system performances [19
]. The most important result is that the addition of nanoparticles can increase the specific heat capacities of pure salts [21
]. It is also indispensable to understand and characterize the viscosities of nanocomposites, which would provide basic information for the experimental and numerical researches on how nanoparticles affect the flow of liquid salt fluid. The viscosities of the nanocomposites in a fluid state can be affected by many parameters, such as volume fraction, temperature, nanoparticle size and shape, aggregation and type of base liquid [22
]. Jin [25
] measured the viscosities of ternary nitrates using the viscometer based on the rotating cylinder method, and it was found that the addition of lithium nitrate to eutectic salt affected the viscosity slightly, while calcium nitrate would increase the viscosity of eutectic salt to some extent, e.g., the viscosity of solar salt adding calcium nitrate increased by about 53.2% at 550 K. Chen et al. [26
] experimentally and theoretically studied the rheological behaviours of TiO2
seeded ethylene glycol nanofluids, and pointed out that the concentration and structure of nanoparticles would greatly influence the viscosities of the nanofluids. Lasfargues et al. [27
] measured the viscosities of solar salt and salt doped with 0.1% CuO, and it was found that the viscosities of the nanofluids increased by about 5.0~18.0% in the temperature range of 250~500 °C. Munoz-Sanchez et al. [28
] measured the viscosities of solar salt seeded with alumina and silica nanoparticles at various conditions, and found out the influences of salt purity, concentration of nanoparticles, and measuring configuration of rheometer on the viscosities of solar salt. The results indicated the Newtonian behaviour of the nanofluids in spite of different rheometer configurations, and the Arrhenius model was suitable to explain the relationship between the viscosity and temperature. Jo and Banerjee [29
] performed the viscosity measurements of nanofluids, which was fabricated with Li2
(62:38) and 1 wt.%, 2 wt.% or 5 wt.% multi-walled carbon nanotubes (MWCNTs). It was found that the viscosity increased by about 93.0% with the addition of 2 wt.% MWCNT, which showed good agreement with the Krieger-Dougherty model considering the effect of nanoparticle agglomeration.
Besides the common nanoparticles, graphene has drawn researchers’ attention recently and has been studied extensively [30
]. Liu et al. [34
] prepared and characterized two types of reduced graphene oxide (GO) combined with alkane, alcohol and carboxylic acid phase change materials (PCMs), and the thermal conductivity of the rGO/stearic acid composite could be increased to 3.21 (W/m·K). Kant et al. [35
] pointed out that emulsifying graphene nanoparticles in PCM resulted in a relative increase of the dynamic viscosity, and significantly degraded its natural convection heat transfer performance. Furthermore, the corrosion of carbon steel with the contact of molten nitrate salt is always an important issue, and it was found that graphite could form a protective carbonate layer to improve the resistance of carbon steel [36
]. Thus, adding graphene into the salt should positively affect the corrosion of container in the CSP system.
However, it can be seen that the influence of graphene on the rheological behaviour of pure salt received little attention and was seldom experimentally reported, which is an essential property affecting the flow and heat transfer characteristics of the CSP system. Moreover, the effects of the shear rates on the rheological behaviour have not been clarified and are relatively inadequate, and the data with different mass fractions of nanoparticles are still insufficient. Thus, it becomes an increasingly critical task to study the viscosities of the nanocomposites seeded with graphene and to discuss the side effects of viscosity change, which is related to the pump power of the CSP system as the nanocomposites work as HTFs. HITEC salt and solar salt were used as pure materials in the present study, aluminium oxide (Al2O3) nanopowder and graphene were considered as the nanoadditives. The nanocomposites were synthesized with the liquid dissolution method and were characterized with a scanning electron microscope (SEM) so as to check the combination of the nanocomposites. Then the viscosities of HITEC salt and its nanocomposites were measured using a rheometer in the temperature range of 200 °C to 450 °C, while those of solar salt were in the temperature range of 250 °C to 500 °C. The experimental results were verified with the correlations from the literature, and finally the influence of nanoparticles on flow characteristics were discussed according to the rheological behavioural results.