Molten salt-based nanofluids as efficient heat transfer and storage materials at high temperatures. An overview of the literature

The research in the field of nanofluids has experienced considerable advances from their discovery two decades ago. These liquid mixtures with tiny quantities (< 10% in volume) of nanometric size solid particles (< 100 nm) in suspension have a great potential for thermal management applications due to their excellent thermophysical properties. The so-called traditional nanofluids (based on water or industrial oils) have been extensively studied so far with a special focus on the enhancement observed in their thermal conductivity. Experimental results, mechanisms and models regarding these materials have been published and reviewed on a large number of articles. A new kind of nanofluids based on inorganic salts has been developed in the last few years with the aim of storing and transferring thermal energy at high temperatures. These Molten Salt-Based Nanofluids (MSBNFs) are characterized by a considerable increase of their specific heat due to the presence of particles at the nanometric scale. On the contrary, the specific heat of the traditional nanofluids is lower compared to that of the base fluid. This surprising behaviour has caused an opened debate in the scientific community, which is currently dealing with these controversial results and the lack of theories and models for these materials. This article reviews the published scientific contributions on MSBNFs. The influence of several facts on the specific heat is deeply analysed, as well as the synthesis methods. Other important aspects related to the behaviour and development of the MSBNFs such as the stability of the NanoParticles (NPs) in the molten salt, their latent heat, viscosity and thermal conductivity, have also been reviewed in this article. Finally, the difficulties and challenges concerning the further development of these materials have been summarized and the main conclusions have been listed.