Analysis and improvement of the hydrogen liquefaction process’ flexibility

Nowadays the use of hydrogen as an energy carrier is rising a large interest, thanks to the development of the renewable energies. Indeed, hydrogen has been enlightened as one of the main actors of the incoming energetic transition, thanks to the possibility to produce it via water electrolysis fed by renewable energies, thus stocking the energy produced. The development of liquid hydrogen allows to resolve some of the main issues of the commonly used compressed gaseous hydrogen, such as its low energetic density and the high compression costs. Nevertheless, the liquefaction of the green hydrogen can have some major issues, when its production come from renewable energies having an intermittent nature, such as solar and wind energy. Indeed, this would cause non-constant working conditions in the liquefaction process, which would cause inefficiencies and possible damages to the different units. The objective of this work is to determine the range of operability of the hydrogen liquefaction process, by studying the behavior of the main units concerned in the process, compressors and heat exchangers. The off-design behavior and performances of compressors and heat exchangers have been evaluated. A correlation between the compressor efficiency and the load has been obtained. The use of a variable speed compressor showed performances able to assure an efficiency between 60 and 80% at almost all part-load conditions. For what concerns the heat exchangers, the evolution of the outlet temperatures with a variable load has been evaluated. Moreover, the implication of this condition in the liquid hydrogen process, in particular the Linde Leuna model, has been investigated on Aspen Hysys®. With the goal of maintaining a functioning plant at different load conditions, a correlation between the feed flowrate and the refrigerants’ flowrate has been established, showing a linear evolution of the liquid nitrogen flowrate against the feed variation but higher relative needs in the refrigerant hydrogen The obtained correlations have been used to perform simulations of the reference process at different load conditions, showing an increase of the SEC (Specific Energy Consumption) value of 30.5% when the load is at 50% of its nominal value. A limited value of +20% of variation from the nominal load has been assumed as acceptable in terms of SEC and SLC (Specific Liquefaction Cost). Two different solutions to improve the process flexibility have been evaluated: the implementation of a compressed hydrogen storage system, in order to mitigate the variations, and a liquid hydrogen re-vaporization system to allow closer-to-nominal working conditions in under-production periods. The former solution resulted in an unacceptable increase in the CAPEX of the project. The latter solution, on the other hand, allows the process to work closer to nominal conditions within an acceptable increase of the energetic costs. An economic evaluation was performed, taking into account both the off design modified performances and the re-vaporization system implementation. Compared with a constant production profile, the considered variable case led to an increase of the SLC. Without adaptation of the process, SLC rises by +41%. Mitigation options studied in this report allowed to limit the SLC increase to 6.5%, showing a relative economic feasibility of a flexible liquefier.