SUMMARY

The sodium thermal electrochemical converter (Na-TEC) is a heat engine that generates electricity through the isothermal expansion of sodium ions within a beta”-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 some of these limitations, the isothermal expansion can be divided into two stages: one at the evaporator temperature (1150 K) and another at an intermediate temperature (550 K – 1050 K). This proposed 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 thesis describes the thermodynamics and thermal-fluid transport of a dual-stage Na-TEC. In the course of this research, a reduced-order thermal model is used to analyze the performance of a proposed, axisymmetric dual-stage device. A maximum thermal efficiency of ~29% is demonstrated for this design. Another focus of this thesis is the development of a thermally-driven sodium capillary pump for the Na-TEC, whereby sodium vapor is condensed within a non-wetting stainless steel porous structure to pump liquid sodium into the evaporator. A conjugate thermal-fluid transport model is developed to characterize the diffusion of sodium vapor through the porous structure. Two experiments are proposed: one to measure key thermodynamic properties of the two-phase interface, and another one to demonstrate and assess the performance of this capillary pumping mechanism. The analytical and computational predictions from the transport model will be validated using pressure, flowrate, and other thermodynamic properties measured with these experiments.