Alkali-metal thermal to electric converter – Wikipedia, the free encyclopedia

The alkali metal thermoelectric converter (AMTEC) is a thermally regenerative electrochemical device for the direct conversion of heat to electrical energy. It is characterized by high potential efficiencies and no moving parts, which make it a candidate for space power applications.

This device accepts a heat input at 900 K–1300 K and produces direct current with predicted device efficiencies of 15-40%. In this device sodium is driven around a closed thermodynamic cycle between two heat reservoirs at different temperatures. The unique feature of the AMTEC cycle is the isothermal expansion of sodium vapour through a solid electrolyte which causes sodium atoms to separate into sodium ions and electrons. The AMTEC thus converts the work of isothermal expansion of sodium vapour directly into electric power.

The converter is based on the electrolyte used in the sodium-sulfur battery, sodium beta-alumina. The device is a sodium concentration cell which uses a ceramic, polycrystalline beta-alumina solid electrolyte (BASE), as a separator between a high pressure region containing sodium vapor at 900–1300 K and a low pressure region containing a condenser for liquid sodium at 400–700 K. For the single cell, the open voltage of 1.37 V and maximum power density of 0.40 W/cm² at temperature of 1350 °C (1080 K) have been obtained.

Efficiency of AMTEC cells has reached 16% in the laboratory…

AMTEC requires energy input at modest temperatures, and not at a specific wavelength, it is easily adapted to any heat source, including radioisotope, concentrated solar, external combustion, or nuclear reactor. A solar thermal power conversion system based on an AMTEC has advantages over other technologies (including photovoltaic systems) in terms of the total power that can be achieved with such a system and the simplicity of the system (which includes the collector, energy storage (thermal storage with phase change material) and power conversion in a compact unit). The overall system could achieve as high as 14 W/kg with present collector technology and future AMTEC conversion efficiencies. The energy storage system outperforms batteries, and the temperatures at which the system operates allows long life and reduced radiator size (heat reject temperature of 600 K)…

While space power systems are of intrinsic interest, terrestrial applications will offer large scale applications for AMTEC systems… The potential to scavenge waste heat may allow for integration of this technology into general residential and commercial cogeneration schemes although costs per kilowatt-hour would have to drop substantially from current projections.

via Alkali-metal thermal to electric converter – Wikipedia, the free encyclopedia. [my emphasis]

If you are going to store energy from solar thermal power stations in molten salts before putting them through heat engines, and then maybe also if you might consider storing electricity in sodium-sulfur batteries, why not cut out the middle step of using a heat engine that converts the heat into rotational energy and then into electricity in a generator.  If it were possible to use the thermal energy from the solar power station to generate electricity directly you might make things simpler. The AMTEC operating temperatures are very high though.