Abstract

Concentrated solar power (CSP) plants are systems that transform solar radiation energy to thermal energy. Nowadays, one of the problems that appears when producing energy with CSP is how to store the energy that is not used to generate electricity. Thermal energy storage (TES) systems allow storing the energy excess for a later use. CSP technologies can supply electricity on demand through TES materials, relying on the energy storage while solar radiation is abundant, and later, when there is a high demand, releasing the energy stored in TES materials and helping to switch the peak demand. Usually, solid particles can act as heat collector, heat transfer, and heat energy storage media, because of that they are used as TES material. The actual TES materials used are molten salts, but these materials have big issues to work with corrosion problems with containers and they do not allow working at temperatures higher than 500 ºC. Thereby, solid particle are being research in order to substitute molten salts. One kind of solid particle material is silica sand. Silica sand advantages outweigh molten salts in terms of transport, thermal stability, abundance, cost and availability. As silica sand has been proposed as the storage medium of direct receiver of solar radiation but the main problem it has is its clear colour. Therefore, this study aims to black-colour the sand by adding carbon black (CB) nanoparticles. In previous works, some properties as changes produced in the internal structure of the coloured-sand due to the colouring were measured as well as the absorption spectrum to compare the enhancement of the absorption before and after colouring it and, the fluidisation of the coloured sand through air to evaluate the stability of the coat formed. After these earlier tests, some additional measurements in terms of thermal stability, to ensure the suitability of the coloured sand as TES for the CSP, are included in this work. Thermal conductivity was measured by hot wire using a thermal properties analyser (KD2 Pro). The aim of this measure is to observe the difference of thermal conductivity between both sands, coloured and non-coloured. Thermal gravimetric analysis (TGA) was carried out to evaluate the mass variation due to thermal decomposition. Differential Scanning Calorimetry (DSC) was done to measure the specific heat capacity (Cp) at different temperature and observe possible differences between coloured sand and raw sand, and also to study the influence of the temperature on the Cp. A 90% increase of absorption has been achieved without increasing the Cp.