This device accepts a heat input in a range 900–1300 K and produces direct current with predicted device efficiencies of 15–40%. In the AMTEC, sodium is driven around a closed thermodynamic cycle between a high-temperature heat reservoir and a cooler reservoir at the heat rejection temperature. The unique feature of the AMTEC cycle is that sodium ion conduction between a high-pressure or -activity region and a low-pressure or -activity region on either side of a highly ionically conducting refractory solid electrolyte is thermodynamically nearly equivalent to an isothermal expansion of sodium vapor between the same high and low pressures. Electrochemical oxidation of neutral sodium at the anode leads to sodium ions, which traverse the solid electrolyte, and electrons, which travel from the anode through an external circuit, where they perform electrical work, to the low-pressure cathode, where they recombine with the ions to produce low-pressure sodium gas. The sodium gas generated at the cathode then travels to a condenser at the heat-rejection temperature of perhaps 400–700 K, where liquid sodium reforms. The AMTEC thus is an electrochemical concentration cell, which converts the work generated by expansion of sodium vapor directly into electric power.