Nanoencapsulated phase change materials (nePCMs) are nowadays under research for thermal energy storage purposes. NePCMs are composed of a phase change core surrounded by a shell that confines the core when molten. One of the main concerns of nePCMs when subjected to thermal processes is the mechanical failure of the passivation shell initially present in commercial metallic nanoparticles. In order to overcome this issue, multi-coated nePCMs, based on the synthesis of an additional coating by atomic layer deposition, appear to be as a candidate solution. With the objective of studying the influence of the composition and thickness of the additional nePCM shells on their probability of failure, a numerical tool combining a thermomechanical finite element model with phase change and Monte Carlo algorithms is developed. This tool also allows including the uncertainty of material and geometrical properties into the numerical analysis to account for their influence in the mechanical performance of nePCMs. In the present work, the mechanical reliability of SiO₂ and Al₂O₃ coatings on Sn@SnO_x nanoparticles is assessed by considering both deterministic and probabilistic failure criteria and Al₂O₃ coatings appear to have a better mechanical performance than their SiO₂ counterparts.

Nanoencapsulated phase-change materials (nePCMs) are investigated for enhancing thermal energy storage. However, the shell of these nanocapsules may fail due to stress developed during thermal processes, leading to melting enthalpy loss. To overcome this problem, SiO₂ and Al₂O₃ coatings on Sn nanoparticles are synthesized by atomic layer deposition (ALD). To study the influence of shell thickness and composition on the probability of failure (POF) of nePCM shells in single- and multicoated nePCMs, a probabilistic numerical tool combining Monte Carlo techniques and a thermomechanical finite-element model with phase change are used. The uncertainties of the material and geometrical properties of nePCMs are included in the analysis. Both deterministic and probabilistic failure criteria are taken into account to consider the effect of dispersion on tensile strength. The results indicate that multicoated nePCMs enhance thermomechanical performance in relation to their single-coated counterparts. Both the numerical simulations and experiments confirm that the POF of nePCM shells and melting enthalpy loss in multicoated nePCMs lower with shell thickness. The results after 50 ALD cycles indicate that Al₂O₃ coatings exhibit better performance because a POF of 1.66% is obtained with 1.1% enthalpy loss, while the POF for SiO₂ is 72.38% with 3.5% enthalpy loss.

The development of advanced heat transfer fluids (HTF) with enhanced heat transfer properties has been identified as a key target to increase the efficiency of industrial processes. In this work, heat transfer performance improvements of a novel nanofluid, consisting of metallic nanoparticles dispersed in a commercial thermal oil, were investigated. Nanofluids combining tin nanoparticles (1 mass %) with Therminol 66 (TH66) were synthesised using the two step-method and experimentally analysed. The effectiveness of biosurfactant addition and nanoparticle polyethylene terephthalate (PET) nanocoating for high temperature nanofluid stabilisation were independently investigated. The PET nanoscale coatings were grown by molecular layer deposition, which has been used for the first time in this field. The thermal conductivity, dynamic viscosity and specific heat capacity of the stable, oil-based nanofluids were characterised at high temperatures, and the results were compared and in good agreement with models found in the relevant literature. Finally, the heat transfer performance of the nanofluids with respect to their base fluids was evaluated, employing empirical values for the thermophysical properties of the involved materials. In this way, increments of the heat transfer coefficients up to 9.3% at 140 °C, relevant to industrial applications were obtained.

Renewable energy has become of great interest over the past years in order to mitigate Global Warming. One of the actions gaining attention is the enhancement of the thermal energy storage capacity of Concentrated Solar Power plants. The addition of nanoencapsulated phase change materials (core-shell nanoparticles) to the already used materials has been proposed for that purpose, due to the possibility of increasing thermal storage through the contribution of both core latent heat and sensible heat. In this work, Atomic Layer Deposition has been used to synthesise SiO₂ and Al₂O₃ nanoscale coatings on tin nanoparticles. The multi-encapsulated phase change materials have been characterised in terms of chemical composition, crystalline structure, particle size, thermal stability and thermal storage capacity. Sn@Al₂O₃ nanoparticles present the best thermal behaviour as they show the lowest reduction in the phase change enthalpy over 100 cycles due to the oxidation barrier of the coating. Moreover, the specific heat of both nanoparticles and solar salt-based nanofluids is increased, making the nanoencapsulated phase change material suitable for thermal energy storage applications.