Distributed Production and Deblending of Green Hydrogen

Abstract

Hydrogen is the ultimate clean energy. Despite being the most abundant element in the universe, hydrogen exists on the earth mainly in compounds like water. H2 produced by water electrolysis using renewable energy, namely, the green hydrogen, represents the most promising energy carrier of the low-carbon economy [1]. H2 can also be used as a medium of energy storage for intermittent energies such as solar, wind, and tidal [2-4]. The deployment of water electrolyser is geographically constrained by the availability of freshwater, which, however, can be a scarce commodity. More than one-third of the earth's land surface is arid or semi-arid, supporting 20% of the world's population, where freshwater is extremely difficult to access for daily life, let alone electrolysis [5]. On the other hand, areas rich in renewable energies are commonly short in water supply [6] and far from major industrial activities. In this talk, we demonstrate how high purity green hydrogen can be produced from air through a so-called direct air electrolysis (DAE) process where moisture is directly absorbed by the hygroscopic electrolyte and split in situ into H2 and O2. Such DAE unit can work under low humidity conditions comparable to that in a desert and reach a Faradaic efficiency of around 95% and for 12 consecutive days [7]. On the other hand, we envisage that apart from local use, excessive green hydrogen produced from the DAE can be injected to the natural gas pipeline network which largely runs across some of the driest but solar abundant areas on earth. At the terminal of the pipeline, hydrogen can be recovered from its blended mixture with different concentration of natural gases (70-95%). We demonstrate such deblending process using a six bed pressure swing adsorption (PSA) unit to produce a high purity hydrogen (>99%) with a recovery of more than 85%. We also compared the PSA system against an electrolyser generating hydrogen onsite [8].