Abstract
HITEC salt (NaNO2-NaO3-KNO3) and solar salt (NaO3-KNO3) are typical molten salts used in concentrated solar power. Adding nanoparticles is an effective method to improve the thermo-physical properties of pure salt. It is indispensable to experimentally study the rheological behaviours of salt seeded with nanoparticles, which can increase the specific heat capacity of pure salt. In this work, the viscosities of HITEC salt were measured with different shear rates in the temperature range of 200 °C to 450 °C firstly, while those of solar salt were measured in the temperature range of 250 °C to 500 °C. The experimental data showed reasonable agreement with the literature correlations, which verify the Newtonian behaviours of pure salts. The evolutions of the viscosities of nanocomposites in the same temperature range were measured and analysed, where the nanocomposites were synthesized with 1 wt.% or 2 wt.% Al2O3 nanopowder and graphene, respectively. Results showed that the addition of Al2O3 nanopowder had relatively little effect on viscosity, and the variations were about −35.4%~8.1% for the HITEC salt nanocomposites and −9.2%~68.1% for the solar salt nanocomposites. While graphene would apparently increase the viscosities of HITEC salt and solar salt, HITEC salt with the addition of graphene showed slight non-Newtonian fluid behaviour.
Highlights
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]
The salt is generally compatible with graphene, while several graphene are agglomerated on the surface of the samples, as shown in Energies 2019, good combination and dispersion are of great importance as only a small amount
HITEC salt and solar salt were used as pure materials, and the nanocomposites were synthesized with the salt and 1 wt. % or 2 wt.% Al2 O3 nanopowder and graphene by liquid dissolution method
Summary
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]. 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 [9,10]. 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. 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. The experimental results were verified with the correlations from the literature, and the influence of nanoparticles on flow characteristics were discussed according to the rheological behavioural results
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