In a new paper, we dissect the exergy balance of a large-scale hydrogen liquefaction process.

The paper entitled Dissecting the exergy balance of a hydrogen liquefier: Analysis of a scaled-up claude hydrogen liquefier with mixed refrigerant pre-cooling has just been accepted for publication in International Journal of Hydrogen energy. In the paper, we analyze in detail a hydrogen liquefier that is likely to be realisable without intermediate demonstration phases, and decomponse all irreversibilities to the component level. The overall aim is to identify the most promising routes for improving the process. The overall power requirement is found to be 7.09 kWh/kg, with stand-alone exergy efficiencies of the mixed-refrigerant pre-cooling cycle and the cryogenic hydrogen Claude cycle of 42.5% and 38.4%, respectively. About 90% of the irreversibilities are attributed to the Claude cycle while the remainder is caused by pre-cooling to 114 K. For a component group subdivision, the main contributions to irreversibilities are hydrogen compression and intercooling (39%), cryogenic heat exchangers (21%), hydrogen turbine brakes (15%) and hydrogen turbines (13%). Efficiency improvement measures become increasingly attractive with scale in general, and several options exist. An effective modification is to recover shaft power from the cryogenic turbines. 80% shaft-to-shaft power recovery will reduce the power requirement to 6.57 kWh/kg. Another potent modification is to replace the single mixed refrigerant pre-cooling cycle with a more advanced mixed-refrigerant cascade cycle. For substantial scaling-up in the long term, promising solutions can be cryogenic refrigeration cycles with refrigerant mixtures of helium/neon/hydrogen, enabling the use of efficient and well scalable centrifugal compressors.

The figure shows how the exergy destruction in the process is exactly balanced with the exergy input in terms of e.g. compressor duties. The work has been carried-out in collaboration with two of my colleagues at SINTEF, David Berstad and Geir Skaugen.