The main objective of the thesis was to develop concepts for hydrogen boil-off gas reliquefaction. Three different processes were simulated, analyzed, and compared: helium Brayton refrigeration cycle with two helium turbine expanders, hydrogen Claude refrigeration cycle with two hydrogen expanders, and Brayton refrigeration cycle with two expanders for the mixture of helium and neon called "nelium". Regarding assessed energy efficiency, the most efficient was the nelium Brayton refrigeration process, followed by the hydrogen Claude process, and the least efficient was the helium Brayton refrigeration cycle.
This research topic addresses an important aspect of using a relevant and promising energy source. This thesis provides a systematic approach to an energy efficiency evaluation of boil-off gas reliquefaction solutions as well as some general conclusions that can be made when comparing the options studied. Interestingly, one conclusion is that higher hydrogen feed pressure is not theoretically beneficial if compared to hydrogen liquefaction processes. Furthermore, findings suggest that the benefits of implementing the liquid hydrogen turbine expander instead of the Joule-Thomson valve do not justify implementation of a more complex, and more expensive, machinery. It was also concluded that the mixed refrigerant process with helium and neon would need to be significantly improved to be an attractive option for implementation. Adding hydrogen into the mix seems to be one of the most promising solutions.
This thesis provides general guidelines and insights regarding the handling of boil-off gas. Even though the focus of this master's thesis was on reliquefaction processes of the entire boil-off gas, other alternatives for handling boil-off gas are discussed and compared.