Abstract

The development and improvement of technologies for Thermal Energy Storage (TES) systems in Concentrated Solar Power (CSP) plants is crucial as it aims to a better handling of the gap between energy supply and power demand. The efficiency of these systems can be improved by increasing their thermal energy storage capacity. The most commonly used fluids for this purpose in CSPs are molten salts and, among them, the mixture of sodium and potassium nitrate (60%-40% wt. respectively) known as Solar Salt. The melting temperature around 220C also allows for the Solar Salt to be used as a Heat Transfer Fluid (HTF) in certain configurations. This mixture of nitrates, however, presents rather poor thermal properties such as thermal conductivity or specific heat (~0.5W/m·K and ~1.5J/kg·K). A way to improve these properties is the addition of nanoparticles to the molten salts in the creation of a nanofluid. Moreover, a different way to improve the thermal storage characteristics of a system is the use of Nanoencapsulated Phase Change Materials (nePCMs), nanoparticles with a core-shell structure in which the PCM is in the nuclei, encapsulated by a high melting point coating that prevents them from leaking when in liquid phase. The present work deals with the characterization of a nanofluid based in Solar Salt and nePCMs. The nanoparticles used were composed by an Al-Cu alloy (80%-20% wt.) encapsulated by the oxidation that naturally occurs when exposed to air. This encapsulation has been proven to be suitable for their use in the nanofluid working conditions, and the nanoparticles have been fully characterized in terms of size, morphology and thermal properties such as phase-change enthalpy and low supercooling. The nanofluid has also been characterized in terms of chemical particle-fluid compatibility and thermal properties such as specific heat, latent heat contribution and total thermal energy storage. Stability of the nanofluid has also been tested by means of a high-temperature Dynamic Light Scattering system. The nanofluid has been proven to be durable and resistant to thermal cycling. Increases in the total thermal energy storage up to 17.8% have been registered for nanofluids with a 10% mass loading of nePCMs with respect to the base fluid. Although agglomeration of the particles takes place, they have been proven to be easily redispersed up to conditions very similar to the initial stage.