Nanofluids using nanoencapsulated Phase Change Materials (nePCM) allow increments in both the thermal conductivity and heat capacity of the base fluid. Incremented heat capacity is produced by the melting enthalpy of the nanoparticles core. In this work two important advances in this nanofluid type are proposed and experimentally tested. It is firstly shown that metal and metal alloy nanoparticles can be used as self-encapsulated nePCM using the metal oxide layer that forms naturally in most commercial synthesis processes as encapsulation. In line with this, Sn/SnOx nanoparticles morphology, size and thermal properties were studied by testing the suitability and performance of encapsulation at high temperatures and thermal cycling using a commercial thermal oil (Therminol 66) as the base fluid. Secondly, a mechanism to control the supercooling effect of this nePCM type based on non-eutectic alloys was developed.

The availability of Thermal Energy Storage systems in Concentrated Solar Power plants makes them suitable to handle the gap between energy supply and power demand. Increasing the total thermal energy storage capacity of the Thermal Energy Storage materials used is of interest to improve their efficiency. In this work the thermal energy storage of the so called solar salt (60% NaNO3-40% KNO3) was improved by adding a phase change material composed of Al-Cu alloy nanoencapsulated with an aluminium oxide layer naturally formed when exposed to oxygen. The resistance of the oxide shell to thermal cycling up to 570ºC and its compatibility with the molten salt were proved. The specific heat and the total thermal energy storage were evaluated at different solid mass loads. Although the specific heat and thus the sensible heat storage decreases with solid content, the contribution of the phase change enthalpy and the latent heat storage can increase the total thermal energy storage up to a 17.8% at constant volume basis comparison. Besides, the thermal conductivity of the nanofluid was increased when adding the nanoparticles improving its heat transfer performance under some particular conditions.

Nanoparticles have been used in thermal applications to increase the specific heat of the molten salts used in Concentrated Solar Power plants for thermal energy storage. Although several mechanisms for abnormal enhancement have been proposed, they are still being investigated and more research is necessary. However, this nanoparticle-salt interaction can also be found in chemical applications in which nanoparticles have proved suitable to be used as an adsorbent for nitrate removal given their high specific surface, reactivity and ionic exchange capacity. In this work, the ionic exchange capacity mechanism for the nanoparticles functionalization phenomenon was evaluated. The ionic exchange capacity of silica and alumina nanoparticles dispersed in lithium, sodium and potassium nitrates was measured. Fourier-transform infrared spectroscopy tests confirmed the adsorption of nitrate ions on the nanoparticle surface. A relationship between the ionic exchange capacity of nanoparticles and the specific heat enhancement of doped molten salts was proposed for the first time.

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