Nanofluids with Enhanced Heat Transfer Properties for Thermal Energy Storage

This chapter describes recent progress on the development of suspensions of nanometer-sized solid particles in base liquids (nanofluids) for thermal energy storage application. Among the various methods of energy storage, Latent Heat Thermal Energy Storage (LHTES) Systems using Phase Change Materials (PCMs) have been gaining importance in many fields like solar energy systems, heating and cooling systems and buildings due to their high energy storage density and their ability to provide heat at a constant temperature. The storage systems utilizing PCMs can be reduced in size respect to systems based on sensible heat. The increase of the thermal capacity of a storage media may allow several advantages for the thermal energy storage systems since a high quantity of heat can be stored in a small volume of material. In this way the thermal storage systems become more compact reducing the overall costs. Research on nanofluids specific heat has been, however, limited compared to that on thermal conductivity while the description and the analysis of the heat capacity of nanofluids are in fact crucial since its improvement can reduce the amount of storage material. The chapter starts with a comprehensive description of the principal methods of nanofluid preparation and then it reports the experimental results focusing on the thermal properties with particular attention on the heat capacity enhancement and thermal storage capability as well as the heat of fusion and the characteristic melting temperatures. Nanofluids based on salts (e.g. nitrates and carbonates) as phase change materials are considered since in the last years several works reported results of nanofluids properties based on mixed salts with different ratio with the addition of different nanoparticles (e.g. oxide nanoparticles, nanotubes). This chapter proposes an overview on the heat capacity enhancement of these nanofluids with the comparison of both theoretical models and experimental results. Finally, the challenges of using nanofluids in solar energy devices are discussed.