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

Abstract

Concentrating solar power (CSP) technology can become a very valuable contributor to the transformation and decarbonization of our energy landscape, but for this technology to overcome the barrier towards market deployment, significant enhancements in the solar-to-thermal-toelectric energy conversion efficiency are needed. Here, an in-depth experimental analysis of the optical and transport properties of Pd-containing aromatic oil-based nanofluids is presented, with promising results for their prospective use as volumetric absorbers and heat transfer fluids in next-generation parabolic-trough CSP plants. A 0.030 wt.% concentration of Pd nanoplates increases sunlight extinction by 90% after 20 mm propagation length and thermal conductivity by 23.5% at 373 K, which is enough to increase the overall system efficiency up to 45.3% and to reduce pumping requirements by 20%, with minimum increases in the collector length. In addition to that, molecular dynamics simulations are used to gain atomistic-level insights about the heat and momentum transfer in these nanofluids, with a focus on the role played by the solidliquid interface in these phenomena. Molecules chemisorbed at the interface behave as a shelterlike boundary that hinders heat conduction, as a high thermal resistance path, and minimizes the impact of the solid on dynamic viscosity, as it weakens the interactions between the nanoplate and the surrounding non-adsorbed fluid molecules.

Interface chemistry effects in nanofluids: Experimental and computational study of oil-based nanofluids with gold nanoplates

“…The increasing thermal conductivity of the nanofluid with temperature is associated to the sensitivity to temperature of the heat transfer process in solid-liquid interfaces (compared to the base fluid bulk region), as proven by Petravic and Harrowell [72]. As it was previously indicated, according to both our experimental and theoretical results, the Au nanoplate-containing nanofluid model is particularly more sensitive to temperature than the Pd nanoplate-containing nanofluid model we studied in a previous work [26]. The presence of chemisorbed species in the case of Pd was proven to distort the nanoplate surface and to act as a high thermal resistance shelter that hinders heat conduction.…”

“…Increasing mass fraction of the nanomaterial affects the behaviour for increasing temperatures, even though the base fluid is still the majority component. This behaviour was also observed with Pd nanoplates in the same fluid [26]. Enhancements in thermal conductivity are likely expected because the thermal conductivity of Au nanoplates (and that of any other elemental metal) is much higher than that of this base fluid.…”

“…The above reported experimental results reveal a significant increment in thermal conductivity accompanied by a small gain in specific heat in Au nanoplate-containing nanofluids with respect to their base fluid. Interactions in solid-liquid interfaces are known to be responsible of specific heat increases in nanofluids if chemisorption of base fluid molecules at the solid-liquid interface occurs, as it was previously reported for nanofluids with Pd nanoplates in the fluid mixture of BP and DPO [24][25][26]. It seems not be the case for Au nanoplate-containing nanofluids.…”

“…Our research proposal relies on this premise. In previously published research [24][25][26], we prepared and characterised Pd nanoplate-containing nanofluids and reported increments in specific heat and thermal conductivity with mass fractions in the order of 10 À2 wt%. It was proven by DFT and molecular dynamics (MD) calculations that chemisorption of BP molecules on Pd surfaces is thermodynamically and kinetically favoured, and that the formation of a layer of chemisorbed molecules is directly responsible of the large increase in specific heat and modest increase in thermal conductivity.…”

Are Nanofluids Suitable for Volumetric Absorption in PTC-CSP Plants? An Exemplified, Realistic Assessment with Characterized Metal–Oil Nanofluids

“…CSP plants with parabolic-trough collectors (PTC), which is the most mature and widely used collector geometry, typically use the eutectic and azeotropic mixture of biphenyl and diphenyl oxide (commercially available as dowtherm A or Therminol VP-1) as the receiver working fluid . The development of nanofluids based on this organic thermal oil has been recently addressed with notable contributions devoted to their preparation, stability analysis, and characterization of the thermal and rheological properties affecting heat transfer − and, less frequently, numerical analysis of collector performance. − …”