Combined effects of nanoparticles and ultrasonic field on thermal energy storage performance of phase change materials with metal foam

To further improve the performance of thermal energy storage (TES) system with phase change materials (PCMs), this paper proposed a novel method, i.e. combining the additions of TiO₂ nanoparticles, metal foam and the provision of ultrasonic field, investigated its synergetic effects in enhancing conduction and convection heat transfer. The thermal characteristics, including the TES time distributions and the energy consumption of the TES system, were discussed to evaluate the combined effects of TiO₂ nanoparticles and ultrasonic field on the TES rate and TES efficiency. The results showed that the latent TES time reduction index reached 46.50%, when the TiO₂ nanoparticles concentration was 5.0 wt% and the ultrasonic power was 100 W, while the TES efficiency dropped to 10.66%. Increasing TiO₂ nanoparticles concentration and ultrasonic power positively improved the TES rate due to conduction heat transfer enhanced by nanoparticles and convection heat transfer enhanced by the acoustic streaming effect and the cavitation effect of the ultrasonic field, but which negatively reduced the TES efficiency mainly due to the energy consumption of the ultrasonic field. Therefore, the effects of the ultrasonic field introduced at four action stages on the TES rate and TES efficiency were compared, and it confirmed that introducing ultrasonic field at the latent TES stage was better than that in the sensible TES stage. Additionally, the proposed novel combined method needed to consider the priority relationship between TES rate and TES efficiency for designing the TES system, favoring the potentials for further advances in TES applications.