Sulfur heat transfer behavior in vertically-oriented and nonuniformly‑heated isochoric thermal energy storage systems

Elemental sulfur thermal energy storage (SulfurTES) is a promising low-cost solution for many medium to high temperature (300–1200 °C) TES applications. Demonstrations of SulfurTES have shown that the heat transfer behavior of sulfur in isochoric tubes is critical to system thermal performance. Previous studies have elucidated and quantified the sulfur heat transfer rate for idealized uniform charge and discharge; however, nonuniform conditions are more likely to be encountered in practice and need to be understood. This paper uses experimental and computational efforts to investigate sulfur heat transfer as well as exergy and energy performance in vertically-oriented tubes for two nonuniform thermal charge scenarios: top-heating and bottom-heating. In comparison with uniform thermal charge, the top-heating causes significant thermal stratification of sulfur that helps the SulfurTES system achieve superior exergetic performance. In contrast, the bottom-heating causes rapid mixing between hot and cold sulfur resulting in high charge rates. Both nonuniform charge strategies could be utilized during the operation of the SulfurTES system to improve system performance as well as provide operational flexibility. Using the computational results, this article originally develops two simplified analytical procedures to estimate the energy and exergy performance of sulfur in tubes of different sizes under top- and bottom-heating. The current study provides significant qualitative and quantitative heat transfer descriptions and design bases for SulfurTES systems and encourages further investigations into the complicated thermal performance for other thermal storage applications.