(5c) Transient Studies of a Sodium-Sulfur Cell

In the modern grid era, multiple fossil-fueled power generation facilities and renewable energy sources are expected to interact actively. Power produced by the renewable energy sources is intermittent and variable. Thus, a strong penetration of the renewable energy sources can create a challenging situation not only for grid stability and reliability but also from the perspective of the fossil-fueled power generation facilities that have to cycle their load. An electrical storage can be used to improve the grid stability and reliability by storing energy when the electricity price is low and discharging it when needed. 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 the 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 situation. 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 mitigation strategies.

With this motivation, a first-principles dynamic model of a sodium-sulfur cell 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 thermal-electrochemical model is 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. In addition, the potential distribution, and the cell resistance for this spatially distributed system has been modeled. The physicochemical properties are considered to be temperature-dependent.

The PDE-based model is solved in Aspen Custom Modeler by using method of lines. A number of disturbances as would be expected in a grid-connected system is simulated and the key transient profiles are studied. Our work shows that an appropriate thermal management strategy is necessary for high current-density operation, especially in the case of high penetration of the renewable energy into the grid.