SulfurTES: Next-generation thermal energy storage

Renewable energy such as wind and solar are promising alternative energy sources for alleviating the global energy crisis and reducing the overdependence on fossil fuels which contributes to the highest percentage of carbon footprint. Energy storage system is a critical component for power generation facilities with intermit renewable energy by providing significant dispatchability and operation flexibility. Unfortunately, the high cost and limited stability for current energy storage devices, such as battery, have greatly hindered the development of relevant renewable energy technologies.

The use of thermal energy storage has recently been on the rise due to its attractive cost efficiency and long-term reliability. However, this technology is still under development and currently facing several challenges: limited operation temperature range for molten-salt thermal energy storage and poor thermal performance for the thermal storage with phase-change or solid materials. To this end, addressing these performance challenges and developing effective energy storage methods is highly desirable.

Professor Richard Wirz at UCLA proposed a novel high-temperature thermal storage solution (SulfurTES) by using elemental sulfur as storage medium. With low cost for the storage and containment materials, high thermal stability from room temperature to above 1000^°C, and high heat transfer rate at liquid stage, SulfurTES shows great potentials to be a competitive technology for the next-generation thermal storage in future commercial renewable power generation infrastructures. Based on the efforts by the UCLA researchers, including Prof. Wirz and his colleagues Dr. Kaiyuan Jin, Dr. Amey Barde, and Dr. Karthik Nithyanandam, configuration changes in the orientation of the sulfur-based thermal storage elements was found to significantly affect the heat transfer performance for the SulfurTES systems.

To investigate the heat transfer behavior of sulfur isochorically stored in vertically-oriented steel tube (as shown in the given figure) and provide quantitative design bases for selecting the tube configuration with vertical or horizontal orientations, these researchers conducted both experimental and computational studies and published their work in the journal, Applied Energy.

The authors experimented to investigate the effect of unique viscosity variation and solid-liquid phase change of sulfur on the heat transfer behavior of vertical sulfur tube from room temperature to 600^°C. Low heat transfer rates were observed in lower-temperature sulfur, i.e. from 25^°C to 275^°C attributed to low thermal conductivity of solid sulfur and high viscosity for liquid sulfur in this temperature range. However, a drop in the sulfur viscosity beyond 275^°C was found to significantly increase the heat transfer rate. A computational model was developed to analyze the sulfur heat transfer behavior between 200 to 600^°C and the Nusselt number correlations were proposed based on the computational results to quantify the sulfur heat transfer rate in isochoric storage tubes with various dimensions. Eventually, the thermal performance for vertical tube system configuration was assessed and compared to that reported for horizontal tube configuration. Low and high aspect ratio resulted in higher heat transfer coefficients of sulfur in vertically- and horizontally-oriented tubes, respectively, since the two tube configurations have distinctive characteristic length for natural convection.

In summary, the study presents qualitative and quantitative design approaches for investigating the heat transfer behavior of sulfur in the SulfurTES system with vertical storage elements. The authors provide vital information that will pave way for the development of frontier thermal energy storage and the advancement of relevant renewable energy technologies.