K2CO3–Li2CO3 Molten Carbonate Mixtures and Their Nanofluids for Thermal Energy Storage: an Overview of the Literature

The research and development of new thermal energy storage materials with high working temperatures are key topics to increase the efficiency of thermal energy to electricity conversion. The use of molten salt combinations with a wide range of operating temperatures is one of the ways to fulfil this purpose, and among them, molten carbonates present several advantages, such as high thermal stability, moderate cost, and less corrosiveness, compared to other molten salt mixtures. The present work contains a state-of-the-art review of the most important thermophysical properties for the thermal energy storage capacity of binary mixtures of potassium and lithium carbonates (K2CO3–Li2CO3). The available literature on the properties that play a key role in the heat transfer rate (viscosity and thermal conductivity) and volumetric storage capacity (melting point, density, latent heat of fusion and specific heat) is reviewed and presented. This includes the works that deal with nanofluids based on these binary mixtures of molten carbonates by analysing the influence of nanoparticles on thermophysical properties. Special attention is paid to specific heat as abnormal increases are registered in molten salts when introducing nanoparticles. Although future research is necessary about the thermophysical properties enhancement of these materials, the advanced capacities they offer for high-temperature thermal energy storage are promising, and this work aims to compile the available data on them until the present day.