Liquid hydrogen (LH2) offers the highest storage density compared to other forms of storage, without requiring a chemical reaction. However, it requires the hydrogen be cooled to 20 K using an energy-intensive refrigeration process. LH2 storage is associated with the unavoidable evaporation of a fraction of the LH2, known as “boil-off”, which results in process inefficiency and energy losses. To ensure proper deployment and increased competitiveness of LH2 storage, evaporation should be minimized as much as possible or, alternatively, recovered in liquid or gaseous form. This review covers, for the first time in a single document, the key elements including definition, current challenges, and state of the art of the hydrogen storage in the form of LH2 and the three different recovery strategies to minimize hydrogen evaporation or recover boil-off hydrogen, namely: zero boil-off (ZBO), hydrogen reliquefaction, and commercial solutions for boil-off hydrogen compression. The expertise gained on ZBO over decades in combining active and passive insulation technologies for cryogenic storage can be beneficial for other applications beyond the spacecraft industry. H2 reliquefaction requires considering its cost-effectiveness, while effectively integrating reliquefaction systems with cryogenic LH2 tanks to harness the low temperature of boil-off hydrogen. Ultimately, the development of non-mechanical compressors capable of operating at the temperature of boil-off hydrogen is an attractive option for advancing the logistics of LH2. 

Strategies to recover and minimize boil-off losses during liquid hydrogen storage

Gas adsorption has been suggested as a means to drive a J-T device for cryogenic cooling1. The system has the advantages of being heat powered, involving no sealing, having no mechanical moving parts and no active control. A multi-stage concept2 was proposed to meet the cooling temperatures requirement ranging from 4 K to 80 R, Preliminary thermodynamic analyses3 of such a system indicated that the power requirement and the system mass were manageable in a spacecraft environment. As an on-going effort in the development of the advanced cooler at JPL, the kinetics of the gas adsorption compressor upon heating and cooling were examined both experimentally and analytically. The performance of the refrigeration system depends to a large extent on the compressor kinetics, for example: how fast can the compressor be heated to liberate enough gas for the J-T cooling? In this paper, a two-phase model4 is used to analyze the transients of a gas adsorption compressor. The modeling of the adsorption process is based on complete thermal and mechanical equilibrium between the gaseous phase and the adsorbed gas phase. In certain aspects, the adsorbed gas behaves as if it were liquid.

C. K. Chan

Papers

Kinetics of a Gas Adsorption Compressor