Thermodynamics and thermal-fluid transport of a dual-stage sodium thermal electrochemical converter (Na-TEC)
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The sodium thermal electrochemical converter (Na-TEC) is a heat engine that generates electricity through the isothermal expansion of sodium ions within a β″-alumina solid-electrolyte. The Na-TEC can thermodynamically achieve conversion efficiencies above 45% when operating between thermal reservoirs at 1150 K and 550 K. However, thermal management limitations have constrained previous single-stage devices to thermal efficiencies below 20%. To mitigate these limitations, the isothermal expansion can be divided into two stages: one at the evaporator temperature and another at an intermediate temperature (650 K – 1050 K). This dual-stage Na-TEC takes advantage of regeneration and reheating, and could be amenable to improved thermal management through a reduction of parasitic losses. This dissertation investigates the thermodynamic operating limits of a dual-stage Na-TEC. The dual-stage device is shown to improve the thermal efficiency by up to 7.5% points over the best performing single-stage device. An application regime map for the single- and dual-stage Na-TEC in terms of the power density and the total parasitic loss is also established. Finally, a reduced-order thermal model is used to analyze the performance of a quasi-axisymmetric dual-stage Na-TEC design with a maximum thermal efficiency of 29%. This dissertation also explores the liquid-pumping subcomponent of the Na-TEC, which consists of a capillary wick in the high-temperature evaporator used to generate the driving force. Sodium corrosion at high temperature leads to several degradation mechanisms that reduce the long-term performance of the Na-TEC. To enable low-temperature pumping solutions, a unique sodium capillary pump for the Na-TEC is explored, whereby low-pressure sodium vapor is condensed within a non-wetting stainless steel porous structure. A net force at the solid-liquid-vapor interface effectively pushes liquid sodium towards a high-pressure bulk liquid region, in contrast to traditional evaporator wicks. First, a breakthrough pressure experiment is used to measure the temperature-dependent interfacial pressure of liquid sodium on the stainless steel porous structure. These results quantify the maximum pressure that can be sustained by the interface at various temperatures, and the wetting transition temperature can be linearly extrapolated from the data. A separate experiment is then conducted to study the performance (mass flowrate vs. pressure head) of this capillary pumping mechanism. To guide the operation of this experiment, a conjugate transport model is developed to characterize sodium vapor diffusion within the porous structure. Results demonstrating the potential for liquid sodium pumping with this mechanism are discussed.