Sulfur heat transfer behavior in vertically-oriented isochoric thermal energy storage systems

Elemental sulfur is a promising medium for moderate to high-temperature thermal energy storage (TES) systems due to its low cost and excellent chemical stability up to very high temperatures (1200 degrees C). Previous studies show that vertically-oriented tubes of isochorically contained thermal storage media (i.e., supercritical CO2) can exhibit higher heat transfer rates than horizontal tubes. Storing thermal storage media in vertical tubes in a TES system also has some potential system-level advantages related to exergy capacity, operation and maintenance, and cost. This paper investigates the heat transfer behavior and performance of sulfur contained in vertically-oriented tubes between room temperature (25 degrees C) and 600 degrees C. Experimental and computational analyses show that the natural convection heat transfer behavior for sulfur in a vertically-oriented tube is strongly dependent on the sulfur viscosity, which varies greatly over the range of temperatures used in this study. Validated Nusselt number correlations for vertical tubes of lengths between 0.5 and 3 m and diameters between 5.5 and 21.2 cm are developed for use in parametric studies and designs. In comparison to the horizontally-oriented tube, the vertical tube can have better heat transfer performance with some ranges of tube length and diameter. Therefore, the selection of the tube orientation strongly depends on the tube dimensions and application needs. The results from the current study provide important quantitative and qualitative design bases for sulfur-based TES (SulfurTES) systems.