Investigation of new nanofluids based on molten salts and their corrosion mechanisms for thermal energy storage applications.

Abstract

Thermal energy storage (TES) is considered as a vital component for concentrated solar power (CSP) technology, which leads to cost-effective dispatchable electricity production. The commercial TES technology in CSP plants is based on a two-tank solution of molten nitrate salts. The binary inorganic salt which consists of a mixture of 60 wt% of NaNO3 and 40 wt% of KNO3, so-called ¿Solar salt¿, is used due to its suitable operating temperature range, high density, low vapor pressure, and relatively high heat capacity. While, its poor thermal conductivity is considered as the main drawback that requires a complicated heat exchanger design for a better heat transfer efficiency during the charge and discharge processes. Thus, it contributes to the increment of the levelized cost of electricity (LCOE) in CSP plants. However, for further reduction of the CSP costs, the efficiency of the whole system can be improved by enhancing the thermophysical properties of the molten salt as the specific heat capacity and thermal conductivity.Recently, the development of nanofluids based on molten salt with anomalous enhancements of those properties was identified as a promising approach. Even some enhancements were already reported in the literature, a big discrepancy of the results was observed due to several factors such as the enhancement mechanisms are still unknown, the complexity of the nanofluid preparation method, the determination of those properties, the initial state of the nanoparticles dispersion and its evolution with time.In this work, we present an optimized simple dry method for the preparation of nanofluids to obtain a homogeneous dispersion of nanoparticles within the molten salt. In addition, to reveal the mechanisms of thermophysical properties and improve the dispersion stability, different type, shape, size and composition of the nanoparticles were used. For that purpose, monodispersed SiO2 and SiO2@Al2O3 core-shellnanoparticles were developed for the first time to prepare the molten salt-based nanofluids with improved stability compared to the commercial nanoparticles based nanofluids.Finally, the effect of adding nanoparticles in the molten salt on its corrosivity with the tank material is still needed to be explored. In the present work, we discovered new corrosion mechanisms related to the addition of the nanoparticles in the molten salts. In addition, a simple graphitization method was proposed as an anticorrosion method for molten binary nitrate salt with carbon steel.