Anomalous Increase in Specific Heat of Binary Molten Salt-Based Graphite Nanofluids for Thermal Energy Storage

Abstract

An anomalous increase of the specific heat was experimentally observed in molten salt nanofluids using a differential scanning calorimeter. Binary carbonate molten salt mixtures were used as a base fluid, and the base salts were doped with graphite nanoparticles. Specific heat measurements of the nanofluids were performed to examine the effects of the composition of two salts consisting of the base fluid. In addition, the effect of the nanoparticle concentration was investigated as the concentration of the graphite nanoparticles was varied from 0.025 to 1.0 wt %. Moreover, the dispersion homogeneity of the nanoparticles was explored by increasing amount of surfactant in the synthesis process of the molten salt nanofluids. The results showed that the specific heat of the nanofluid was enhanced by more than 30% in the liquid phase and by more than 36% in the solid phase at a nanoparticle concentration of 1 wt %. It was also observed that the concentration and the dispersion homogeneity of nanoparticles favorably affected the specific heat enhancement of the molten salt nanofluids. The dispersion status of graphite nanoparticles into the salt mixtures was visualized via scanning electron microscopy. The experimental results were explained according to the nanoparticle-induced compressed liquid layer structure of the molten salts.

Thermal energy storage characteristics of binary molten salt nanofluids: Specific heat and latent heat

“…Additionally, the specific heat enhancements were affected by the composition of the salt components comprising the base solvent of the nanofluids for both the carbonate and nitrate salts. The effects of the base solvent on the specific heat of binary‐salt‐based nanofluids have already been observed in our previous studies 14,15,17 . Based on the experimental results in this study and previous studies, it may be hypothesized that the thermophysical property changes in the solvent material, which are dependent on the composition, affect the specific heat of the nanofluids.…”

“…However, the redundant nanoparticles (2 wt%) unfavorably affected the specific heat of the salt mixture, decreasing it by 5.7%. 7 In addition, various molten salt mixtures, that is, a binary nitrate salt (NaNO 3 -KNO 3 ), [8][9][10][11][12][13] binary carbonate salt (Li 2 CO 3 -K 2 CO 3 ), [14][15][16][17] ternary nitrate salt (LiNO 3 -NaNO 3 -KNO 3 ), 18,19 ternary carbonate salt (Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 ), 20,21 quaternary nitrate salt (NaNO 3 -KNO 3 -LiNO 3 -Ca[NO 3 ] 2 ), 22 and binary chloride salt (NaCl-CaCl 2 ), 23 were employed to examine the effects of material, size, concentration, and dispersion homogeneity of nanoparticles on the specific heat behaviors. In most of these studies, the specific heat values of the molten salt nanofluids were increased irrespective of the nanoparticles used.…”

Synthesis and Characterization of Molten Salt Nanofluids for Thermal Energy Storage Application in Concentrated Solar Power Plants—Mechanistic Understanding of Specific Heat Capacity Enhancement

“…However, experimental results contradict this prediction where specific heat capacity values are enhanced on mixing with solid nanoparticles (e.g., in the form of stable colloidal suspension). Modified models were, therefore, developed to mitigate these discrepancies by accounting for the high surface energy of nanoparticles compared to the bulk property values of the solid phase [ 48 , 49 ] and recognizing the existence of a “compressed phase” or “compressed layer” that is induced on the surface of the nanoparticle by molecules of the solvent phase that are adsorbed on the surface [ 50 , 51 ]. In the compressed layer, the dominance of the adhesive forces over the cohesive forces causes the molecules of the solvent phase in the vicinity of the nanoparticle surface to be packed into smaller volumes which results in a higher mass density and a greater total potential energy.…”

Special Issue on Nanofluids and Their Applications

“…H. J. Kim and B. Jo [13] focused on the use of the nanofluid comprising the base fluid of a binary carbonate molten sate mixture and graphite nanoparticles for thermal energy storage application. It was observed that the specific heat of the nanofluid was significantly enhanced by the presence of the graphite nanoparticles.…”