Performance and degradation of gas-gap heat switches in hydride compressor beds

Abstract

A closed-cycle gas-gap heat switch (GGHS) can function as a variable vacuum dewar for metal hydride compressors to reduce nominal input power to each compressor by ∼80% over its lifetime without the use of any moving parts. The hydrogen gas pressure in each GGHS is varied with an independent actuator containing ZrNi hydride. As the GGHS must attain low H 2 gas pressure in its insulating “off” state to function reliably, any accumulation of additional gas in the GGHS can compromise the vacuum causing parasitic heat losses within the system. The effects of such accumulations have been found to be a combination of outgassing from metallic components and hydrogen permeation through the walls of the high-pressure compressor bed when it is heated between 290 K and 670 K. This accumulation has been volumetrically measured over a wide range of operating temperatures and pressures on different versions of hydride compressor beds to provide hydrogen accumulation rates in the GGHS. Correlations with literature values are given, addressing the performance of stainless steel in terms of permeation and outgassing of hydrogen gas.