Alkali-metal thermal to electric converter
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°K (1080°C) have been obtained.
Efficiency of AMTEC cells has reached 16% in the laboratory. High voltage multi-tube modules are predicted (using 2009 state-of-the-art computer simulations) to be 20% to 25% efficient, and power densities up to 0.2 kilowatts per liter appear to be achievable in the near future. Calculations show that replacing sodium with a potassium working fluid increases the peak efficiency from 28% to 31% at 1100 K with a 1 mm thick BASE tube. Further development are expected to raise the power densities substantially, and raise the efficiency into the 35% to 40% range.
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). Deep-space applications would use radioisotope thermoelectric generators; hybrid systems are in design.
While space power systems are of intrinsic interest, terrestrial applications will offer large scale applications for AMTEC systems. At the +25% efficiency projected for the device and projected costs of $350/kW, AMTEC is expected to prove useful for a very wide variety of distributed generation applications including self-powered fans for high efficiency furnaces and water heaters and recreational vehicle power supplies. Cathodic protection of pipelines, remote telemetry from oil well sites are other areas where this type of electrical generation might be used. 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.
