Thermofluidics of Sulfur-based Thermal Energy Storage
- Author(s): Jin, Kaiyuan
- Advisor(s): Wirz, Richard E.
- et al.
Thermal energy storage (TES) is crucial for future low-cost and large-scale (GWh) energy use and supply. Sulfur as a storage medium provides exceptional TES system cost efficiency and performance due to several characteristics, including low cost, high availability, excellent thermal stability up to 1200 deg C, high heat transfer rates, and impressive material compatibility when compared to existing and competing options. The sulfur-based TES (SulfurTES) systems analogous to shell-and-tube heat exchanger have been successfully demonstrated with attractive energy density and system stability. In such systems, the sulfur heat transfer behavior in isochoric containers play a critical role in performance prediction and optimization and need to be fully understood and quantified.
This dissertation discusses recent studies on sulfur’s thermofluidic behavior during thermal charge and discharge for several isochoric configurations. The first study compares experiments and computational analyses performed at UCLA of sulfur inside vertically oriented steel tubes undergoing 25 to 600 oC charge and discharge. The study developed and validated the Nusselt number correlations to show that low aspect ratio (length/diameter < 7) tubes, when uniformly heated, provide heat transfer performance superior to horizontal tubes.
In comparison to uniform-heating, nonuniform thermal charge including top-heating and bottom-heating are more likely to be encountered in practical SulfurTES with isochoric-tube configuration. In the second study of this dissertation, the combination of experimental and computational investigations elucidates the top-heated sulfur tube exhibits a stable thermal stratification, which leads to superior exergetic performance. On the other hand, bottom-heating scenarios cause rapid mixing between hot and cold sulfur, resulting in high charge rates. With the developed correlations, two analytical procedures are originally proposed and allow estimation of the energy and exergy performance of sulfur in tubes of different sizes with top-heating and bottom-heating, respectively.
In comparison to the isochoric-tube system wherein the sulfur is stored and sealed within multiple tubes, a “bath” configuration is proposed to potentially provide a simplified configuration and attractive thermal performance by maintaining the bulk of sulfur inside a large shell with an array of thermal charge tubes near the bottom and discharge tubes at the top. Validated computational models allow the characterization of sulfur heat transfer behavior dominated by multiple geometric parameters in the bath system. Nusselt number correlations based on Rayleigh number and tube pitch ratio have been developed and utilized for the system parametric design. The preliminary comparison between the tube- and bath-configuration systems proves the potential superiority of the bath system on thermal performance and cost for the industrial-scale thermal storage applications.
All the results in this dissertation provide important qualitative and quantitative heat transfer descriptions and design bases for SulfurTES systems. The observations from these studies and the investigation methods and tools can be utilized in future studies and will encourage further investigations for other novel thermal storage technologies.