Study of the heating capacity enhancement of solar salts by the addition os silica nanoparticles

Abstract

Over the last years, solar thermal salts have been studied as natural solution for storage and heat transfer in Concentrating Solar Power Plants (CSP). This new kind of renewable energy has become one of the alternatives of fossil fuels. The CSP plants transform the solar energy into electricity by the Thermal Energy Storage (TES) properties of this kind of material, which allows to store large amounts of heat. These types of salts provide a high thermal stability, low material costs, high density of the salt fluid, high heat storage capacity, non-flammability and low vapor pressure. In addition, compared with organic salts these inorganic salts present a higher melting point. [1] A large quantity of inorganic salts have been used as solar salt systems in CSP plants, mixtures of carbonates (Li2CO3 and K2CO3), mixtures of chlorides (BaCl2, NaCl and CaCl2) and finally, the most commonly used, a mixture of sodium and potassium nitrates (NaNO3:KNO3). [2] An interest in the enhancement of the heat capacity in solar salts with the addition of nanoparticles has been grown in the last years. In 2011, Shin and Banerjee [3] reported an unusual specific heat enhancement when silica nanoparticles were added to carbonate and chloride salts mixtures. These results were obtained when the solar salts were doped with 1-2 %wt of nanoparticles. The increase of the specific heat of the nanofluid seems to be against the mixture rule, which states that the specific heat of a nanofluid should decrease if the added nanoparticles have a lower specific heat than the fluid itself. This theory applies to molecular liquids, such as alcohols, thermal oils, ethylene glycol or water. [...]