385102 Transient Studies of a Sodium-Sulfur Cell
Modern grids are expected to be highly interacting due to multiple consumers and energy providers that include traditional fossil-fueled power generation facilities as well as renewable energy sources. Most renewable energy sources are intermittent and variable and thus introduce a very challenging situation with regard to grid stability and reliability. This is forcing the traditionally base-loaded fossil-fueled power generation facilities to cycle their load leading to a significant decrease in efficiency and an increase in environmental emissions. Furthermore, equipment life can be reduced by several years as a result of load-following. One option is to store the electrical energy in battery when the power production is in excess and use it when required. However, this requires massive energy storage facilities. The sodium-sulfur battery has high potential for electrical storage at the grid level due to its high energy density, low cost of the reactants, and high open-circuit voltage. However, the use of sodium-sulfur batteries at the grid level requires high current density operation that can cause cell deterioration, leading to lower sulfur utilization and lower energy efficiency. In addition, it can result in undesired thermal runaway leading to potentially hazardous situations. A rigorous, dynamic model of a sodium-sulfur battery can be used to study these phenomena, design the battery for optimal transient performance, and develop control strategies for maximizing efficiency while minimizing the cell deterioration and avoiding unsafe conditions.
The existing literature on the sodium-sulfur batteries is focused on the dynamics of the sulfur electrode (a sodium-polysulfide melt) alone. However, consideration of the dynamics of the entire cell is important considering frequent charging-discharging characteristics in a grid-connected system especially under high current-density operations. With this motivation, a first-principles dynamic model of a sodium-sulfur cell (with) that includes the sodium electrode, beta”-alumina electrolyte, and sulfur electrode has been developed.
The state of discharge (SOD) of a sodium-sulfur cell significantly affects the heat generation rate, rates of electrochemical reactions, and internal resistance. To capture these phenomena correctly, a fully coupled thermo-electrochemical model has been developed considering the operation of the cell in 0-85% state of discharge. The thermal model considers heat generation due to Ohmic loss, Peltier heat, and heat due to the entropy change. Species conservation equations are written in the sulfur electrode for the chemical and ionic species by considering the phase transition and change in the composition depending on the SOD. The electrochemical reactions are modeled by using Arrhenius-type rate equations with temperature-dependent terms and varying species concentration depending on the SOD. Species conservation equations are written in the beta”-alumina electrolyte for the ionic species by considering the effect of diffusion and migration. In addition, the potential distribution, cell resistance, and energy conservation has been modeled. A thermo-electrochemical model has also been developed for the sodium electrode. The physicochemical properties are considered to be temperature-dependent.
The PDE-based model is solved in Aspen Custom Modeler by using method of lines. Our work shows interesting temperature dynamics under high energy-density operations and suggests that an appropriate thermal management strategy is absolutely essential for these cells, especially in the case of high penetration of the renewable energy into the grid.
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