Diffusion of hydrogen in solids is an intriguing intellectual problem. Permeation of hydrogen generated in combustion into gas fired Alkali Metal Thermal to Electric Converter (AMTEC) systems can be detrimental to AMTEC performance for various reasons. Potential effects include depriming of the AMTEC cell arteries, blockage of the condenser and hydride formation. Numerous papers and reports have been published concerning hydrogen diffusion in solids (Birnbaum and Wert, 1972; Garber, 1975; Strehlow and Savage, 1974). Many of these papers concern the embrittling effects of hydrogen and many concern the diffusion process itself. Hydrogen permeation and permeation resisting strategies have been examined extensively in connection with other energy conversion technologies such as Stirling engines (Alger, 1988; Khalili etal., 1989) and high temperature heat pipes (Anderson et al., 1995; North and Anderson, 1997).
Due to the different boundary conditions and materials involved, it was necessary to conduct experiments specifically designed for AMTEC to evaluate hydrogen permeation rates into AMTEC cells, to understand the effect of this permeation on AMTEC performance and finally, if hydrogen permeation turns out to be a cause for concern, to identify strategies to reduce permeation, and to design an AMTEC cell which will perform with adequate lifetime.
This paper describes experiments and their results for AMTEC cells operated in a partial pressure of hydrogen. Experiments in which the hydrogen permeation rate across different thickness cell walls were measured with and without barrier coatings are also described. Due to greater sensitivity, the change in pressure on the vacuum side of the specimen was observed as hydrogen permeated the metal.
Depending on the operating conditions and the operating life requirements of specific AMTEC systems, hydrogen permeation barriers may need to be included in the cell design. Some of the solution approaches that were implemented and their effectiveness in minimizing AMTEC degradation are also elucidated.