Thermal energy storage using molten salts containing nanoparticles

The growing interest in energy efficient and sustainable technologies has created significant demand for novel heat transfer and thermal energy storage materials, such as nanofluids. This research is therefore aimed at developing novel types of hybrid nanofluids that can be used as heat transfer fluid and thermal energy storage medium for solar power plants. The importance of nanoscience cannot be underestimated here, since the motivation for the manipulation, through nanoparticle addition, of the properties of existing thermal fluids (e.g. water and molten salt) arises from their poor thermal properties which represents a major limitation to the development of more energy-efficient processes. Special consideration is given to assess the stability of embedded nanoparticles in the base-fluid under different operating conditions, using appropriate ultrasonic diagnostic techniques (acoustic methods). This innovative approach allowed in situ high temperature ultrasonic probes to be used to measure the changes of thermophysical properties of nanofluids via speed of sound variations. The work has also focused on combining the novel experimental development with the existing computational power to investigate the role of particle dynamic forces, mass and heat transfer of nanofluids in three-dimensional flows, using an advanced computational modelling approach (i.e. direct numerical simulation coupled to Lagrangian particle tracking). The advantage of the model developed is its ability to study in detail phenomena such as interparticle collisions, agglomeration, turbophoresis and thermophoresis, with the approach also of value in investigations of the long-term thermal stability of nanoparticle dispersions which as yet has not been considered in detail. The model has been applied to a stagnant nanoparticle dispersed fluid, as well as a turbulent channel flow with both non-isothermal and isothermal conditions. The results of both computational and experimental investigations indicate that these effects are found to play a key role in the thermal behaviour of the nanofluid at various particle concentrations, with predictions in agreement with theoretical and experimental results obtained in similar studies. The outcome of this study allowed conclusions to be reached regarding the stability and heat transfer characteristics of nanofluids and their implications for thermal energy storage systems.