Thermal energy storage system with a high-temperature nanoparticle enhanced phase change material: Charging and discharging characteristics upon integration with process preheating

Nanoparticle enhanced Phase Change Materials (NePCMs) are modified forms of phase change materials containing nanoparticles such that the thermophysical properties are improved. Solar salt, a phase change material comprising a eutectic mixture of sodium nitrate and potassium nitrate has a thermal conductivity in the range of 0.5-1 W/m K and the addition of nanoparticles to solar salt (nanoparticle enhanced solar salt) has been demonstrated to improve its thermophysical properties. This work evaluates the charging and discharging characteristics of solar salt and nanoparticle enhanced solar salt, with copper nanoparticles (at 0.5 wt%) and reduced Graphene Oxide nanostructures (at 0.5 wt%) individually as additives. Experiments were performed simulating a thermal energy storage system (TES) for process preheating. Silicone oil and Therminol (R) 55 were used as heat transfer fluids during charging and discharging cycles respectively. The overall heat transfer coefficients of TES containing copper nanoparticle enhanced solar salt and reduced Graphene Oxide nanostructure enhanced solar salt as an energy storage medium were greater than that containing solar salt during both charging and discharging cycles. The enhancements in overall heat transfer coefficients were due to respective enhancements in PCM side heat transfer coefficients caused by thermal conductivity and specific heat increase brought about by the addition of copper nanoparticles/reduced Graphene Oxide nanostructures to solar salt. Considering >10 % enhancements in overall heat transfer coefficient in both charging and discharging cycles with these two NePCMs, latent heat thermal energy storage systems employing them as phase change material will be suitable for integration with a process preheating application.