Understanding the Specific Heat Enhancement in Metal-Containing Nanofluids for Thermal Energy Storage: Experimental and Ab Initio Evidence for a Strong Interfacial Layering Effect

Abstract

Nanofluids have emerged as an addition for thermal management and energy conversion applications. The dispersion of a small amount of solid nanoparticles occasionally leads to an unexpected enhancement in the specific heat of the dispersant fluid. This effect has technical, economic, and social significance, and for that, it has received a lot of attention from applied research, but the associated physical and chemical phenomena explaining this phenomenon are yet to be described. We report here a combined experimental and theoretical investigation of nanofluids consisting of palladium nanoplates in a typical heat transfer oil used for concentrating solar power. Their specific heat per unit volume is found to be maximally enhanced at intermediate nanoparticle concentrations, at all temperatures. This is consistent with the phenomenological description provided by the mesolayer model. Density functional theory calculations of adsorption energies and diffusion/desorption activation barriers reveal a strong interaction between the base fluid molecules and palladium surfaces, leading to a nanofluid model where the metal particles are decorated by a static layer of organic molecules. Such layering is potentially responsible for the anomalous enhancement on the thermal properties of the nanofluid, such as the specific heat. Our contribution with this work is a first step toward a complete understanding on the structure and properties of nanofluids using ab initio molecular simulation techniques rather than phenomenological descriptions only.

Optical and Transport Properties of Metal–Oil Nanofluids for Thermal Solar Industry: Experimental Characterization, Performance Assessment, and Molecular Dynamics Insights

“…Such a unique behaviour was also observed in a previous analysis of the specific heat of these nanofluid samples, which was also found to be maximized at intermediate mass fractions. 19 A numerical model developed by Hentschke, 47 assuming the existence of interacting interfacial layers of fluid around nanoparticles, was found to successfully describe this behaviour of the specific heat. Yu and Choi created a numerical model 48 to calculate the effective thermal conductivity of a cubic arrangement of monodispersed spherical nanoparticles in a base fluid, in which solid-liquid interfaces exist and overlap.…”

“…An indepth description on the formulation procedure, together with the basic characterization of the nanomaterial and the stability of these nanofluid samples, is available elsewhere. 19 Optical measurements -Spectral optical transmittance in the 300-2600 nm range of nanofluid samples and also of the base fluid and the host fluid (base fluid with 1.0 wt.% of Triton X-100 surfactant), for reference, was measured using a double-beam UV-VIS spectrophotometer (PerkinElmer Inc., LAMBDA 900), holding the sample in a variable-length cell. 22,23 Samples with mass fractions of 0.060 wt.%, 0.030 wt.% and 0.006 wt.% in Pd nanoplates are considered for characterization.…”

“…Thermal conductivity (as a function of the mass fraction of nanoplates and temperature) and dynamic viscosity (as a function of the shear rate, the mass fraction of nanoplates and temperature) are also studied, to assess thermal and flow requirements for the application. Based on these results and on density and specific heat characterizations from previous work, 19 the overall efficiency of these nanofluids in both surface and volumetric parabolic-trough collectors (PTCs) and also in heat exchangers is assessed. We find that the characterized nanofluids, compared to their base fluid (the eutectic mixture of diphenyl oxide and biphenyl, commercially available as Dowtherm TM A), are superlative candidates for both volumetric absorption and heat transfer, as they exhibit full sunlight extinction at short path lengths and a notable increment in thermal conductivity, with negligible change in dynamic viscosity.…”

Interfacial molecular layering enhances specific heat of nanofluids: Evidence from molecular dynamics

“…The solid-liquid interface has been proposed to play a key role for the anomalous enhancement of specific heat in some nanofluids, for which experimental data exists [5][6][7][8][9]. Particular emphasis should be put on such anomaly, since the most widely used numerical models for the specific heat of mixtures fail to predict it [10].…”

Effects of interface layer on the thermophysical properties of solar salt‐SiO 2 nanofluids: A molecular dynamics simulation

“…Lee et al 32 investigated the enhancement of SHC and latent heat of the carbonate salts (Li 2 CO 3 + K 2 CO 3 ) and nitrate salts (NaNO 3 + KNO 3 ) based graphite nanofluids, and pointed out that the quantitative analysis of the interface layer was necessary to understand the influencing mechanisms in the SHC and latent heat of MSBNFs. Hentschke et al 33 studied the mechanism of the significant increase in SHC of MSBNFs, and proposed every nanoparticle was surrounded by an interface layer and the effect of nanoparticles on the base fluid had a long range (100 nm or more). Berdugo et al 34 proposed that the interface layer adsorbed around the nanoparticles was a static molecular structure with strong stability, and the interface layer affected both the SHC and TC of the MSBNFs.…”