The Sodium Thermal Electrochemical Converter (Na-TEC) is an electrochemical, solid state converter that has high efficiency, high power density, and is easily scalable. The Na-TEC extracts power from the isothermal expansion of high pressure sodium through an ion conducting membrane . The β”-alumina solid electrolyte (BASE) allows the passage of sodium cations while impeding the transfer of electrons. The cycle begins with the heating of liquid sodium in an evaporator and the high pressure sodium is then oxidized at the anode/BASE interface. The resulting cations pass through the solid electrolyte due to the pressure gradient, while the electrons travel through an external load where electric power is extracted. The electrons and cations recombine at the cathode/BASE interface and diffuse to a condenser. The liquid sodium is then transported back to the evaporator with a wick in order to complete the cycle.
The Na-TEC is capable of reaching relatively high values of the second law efficiency. For a device operating between 1150K and 550K, the theoretical efficiency is above 45%. However, this technology is limited by certain physical parameters. These include resistive losses including ionic, contact, and sheet resistances, overpotential losses stemming from limited chemical kinetics at the cathode, and various thermal losses, most notably radiation to the low pressure plenum . Moreover, the hot operating temperatures and the high reactivity of sodium make the device materials a critical factor. The BASE will degrade over time due to loss of sodium oxide from the electrolyte, crack propagation, reactions with transition metals from the enclosure, formation of dendrites, etc. The electrode materials must be porous enough to not impede the flow significantly while simultaneously allowing for a large charge transfer efficiency. To date, the highest demonstrated efficiency has been 22.5%. 
 Cole, T., Thermoelectric Energy Conversion with Solid Electrolytes. Science, 1983. 221(4614): p. 915-920.  Wu, S.-Y., L. Xiao, and Y.-D. Cao, A review on advances in alkali metal thermal to electric converters (AMTECs). International Journal of Energy Research, 2009. 33(10): p. 868-892.  Tournier, J.-M. and M.S. El-Genk, Performance analysis of Pluto/Express, multitube AMTEC cells. Energy Conversion and Management, 1999. 40(2): p. 139-173.