## 3 Evaluation of the Energy Conversion Subsystem and Energy Storage Subsystem
### Trade Study  
A trade study is conducted because the choice of the fuel to use as energy storage is highly complex. To do so, an abstract representation of the system and underlying processes is made (cf. Appendix B for more details). The steps included are the harvesting of energy by a wind turbine, required conversions to chemical sources, storage on the SailREC System, followed by transport, and the final use of the energy in the form of heat and electricity. Similar chains of production, transport, and use of the different fuels are followed to make comparisons between the options possible. The flow chart for liquid hydrogen is presented in Figure 5. The hydrogen gas generated after electrolysis is liquefied and stored as a liquid on-board in Dewar tanks (cf. Appendix B.1 for further details on the storage tank) at 20.68 K (i.e., \(-252.47\)\({}^{\circ}\)C) at a pressure of 4 bar. The liquefier consumes 10 kWh for the liquefaction of 1 kg of hydrogen. Thus, the efficiency of the block is calculated as 70%. The flow charts for gaseous hydrogen, ammonia, and methanol can be found in the Appendix B.2. Overall, the trade study is conducted for a specific use case, as summarised in Table A2 in the Appendix B.3. The trade parameters used in the trade study, along with their values and weights (considering a range of 1-10), are listed in Table 1. As technology in the energy sector is very cost-driven, the trade parameter levelised cost of energy (LCOE) is given the highest  
Figure 5: Liquid hydrogen as the stored fuel. weight of 10. Having the main motivation of a SailREC System of delivering clean energy in mind, 7 is assigned as the weight for the trade parameter on environmental friendliness. Another trade parameter is the partial load capability, which is the ability to produce fuel with interruptions in input power. Fuels requiring chemical reactors and intermediate steps would be at a disadvantage in this assessment. The weight for the partial load capability is set higher than the remaining ones because power harvested in a SailREC System could be subject to periodic stopping. Considering the fact that the weight of the fuel itself and its storage containment needs to be carried by the SailREC System, the trade parameter displaced water volume--referring solely to the specific fuel and storage tank (in future studies, the weight of the production facilities, auxiliary systems, etc. of the fuel can also be taken into account)--is defined, since fuel which requires more displaced water volume may also require a more challenging design of the entire SailREC System. Further trade parameters are the technology readiness level (TRL) of the systems finding use in the Energy Conversion and Storage Subsystems as well as the safety of maintenance personnel. Finally, the transport infrastructure readiness level in 2032 is thought of as a measure of the maturity of infrastructure ten years from now, for transporting the fuel through pipelines, rail tracks, etc., and connecting ports (where SailREC Systems would offload) with points of use inland. Only for the trade parameters LCOE and displaced water volume are values calculated (cf. Appendix B.1 for further details on the calculation of LCOE), which, however, are not intended to represent exact values but rather serve as indicative measures that can be used for comparison within the trade study. For the remaining trade parameters, the values are quantified by ranking. This is achieved by distributing the values from 1 to 4, with 1 representing the worst-scoring fuel and 4 the best-scoring one. This ranking is based on literature research and discussions with, among others, experts from the Department Hydrogen Labs and Field Tests of the Fraunhofer Institute for Wind Energy Systems IWES. For the evaluation of the trade study, the values are normalised for each trade parameter. Subsequently, the normalised values are multiplied by their respective weights. Finally, the sum of all weighted values across the parameters reflects the score for each fuel.