Metal- and Alloy-Based Core-Shell Particles in Nitrate Senary Salt with Low Thermal Hysteresis for Solar Thermal Energy Storage

Abstract

In this work, the microencapsulated phase change materials, Sn/amorphous-carbon (Sn/a-C), and SnBi/amorphous carbon (SnBi/a-C) microparticles (MPs) were successfully synthesized. The thermal stabilities of Sn/a-C and SnBi/a-C core-shell MPs were verified by cycling tests, and stable latent heats of 56 and 45.7 J/g were obtained for Sn/a-C and SnBi/a-C MPs, respectively. Compared to the high melting point of 231 °C and large thermal hysteresis (TH) of ∼106 °C for the Sn/a-C MPs, the SnBi/a-C MPs exhibited a lower melting point of 125 °C and a smaller TH of 20 °C. The nitrate senary salt with a lower melting point of ∼75 °C than that of the commercial HITEC salt (melting point of ∼142 °C) was also synthesized to enlarge the working temperature range of the working fluid in a solar thermal power plant and to demonstrate the latent heat-enhanced thermal energy storage using the SnBi/a-C MPs. The heat capacity can be enhanced by 200% by doping with 20 wt % Sn/a-C MPs into the HITEC salt, and it can be enhanced by 734% by doping with 20 wt % SnBi/a-C MPs into the senary salt. In addition, the viscosities of the HITEC salt and senary salt doped with the Sn/a-C and SnBi/a-C MPs were not appreciably raised by doping with the MPs. The various approaches accomplished in this work demonstrate (1) enhancing heat capacity of the working fluid by exploiting the latent heats of the embedded MPs; (2) lowering the TH of the MPs by using the alloy metal particles; and (3) extending the working temperature range by synthesizing the senary salt. These approaches could be applied for enhancing energy storage in solar thermal power plants and facilitating waste heat recovery.