The role of hydrogen fuel in hard-to-decarbonise modes of transport : an energy systems perspective

To reduce the adverse effects of global warming, there is an international drive towards the rapid decarbonisation of the aviation and maritime sectors. Simultaneously, there is an opportunity for the maritime sector to assist with the decarbonation of the wider global energy system via the supply of low-carbon energy. There is potential to transport such energy from regions where natural renewable and low-carbon energy resources are in abundance to where they are scarce. In this thesis, utilisation of electricity grid derived liquid hydrogen (LH2) fuel for these purposes is investigated. Particular focus is dedicated to: (1) the geographical, situational and design niches for which the use of LH2 is favourable within aviation and shipping; (2) the ability of LH2 to power the global aviation and shipping fleets and the modifications required to accommodate the fuel; (3) the identification of technologies that are likely to significantly effect decarbonisation efforts; and (4) the influence of weather induced boil-off on LH2 ship design. Methods such as energy systems analysis, thermodynamic concepts and numerical methods (e.g., Monte Carlo simulations) are utilised to construct models of LH2 fuelled aircraft and ships. These models are applied to various scenarios to analyse the performance of LH2 from a component, vehicle, network and global energy systems perspective. Models of a single-fuel LH2 and a dual-fuel LH2- liquefied natural gas (LNG) aircraft are constructed from data on an existing Boeing 787-8. The aircraft are modelled to operate towards 2050, in which time: (1) LH2 technologies develop, (2) both the World-average and region-specific electricity grids used to produce the hydrogen are assumed to decarbonise and (3) the International Air Transportation Association emissions targets tighten and so aircraft emissions decrease accordingly. The model identifies certain niches within the 2050 aviation sector where use of LH2 is beneficial. Single-fuel LH2 aircraft can only travel further than LNG fuelled aircraft in North America, accounting for 17% of the global demand. However, under stricter emissions constraints, LH2 can also outperform LNG aircraft in Europe and Latin America which accounts for an additional 24% of the demand. These superior ranges are achieved at a reduced passenger number and an increased energy consumption. The use of the dual-fuel aircraft in place of single-fuel aircraft in all three of these regions leads to improved range, passenger number and energy consumption. The LH2 aircraft models based on the Boeing 787-8 are expanded to utilise a dataset of multiple, differently sized existing aircraft. The models are combined with an estimation of the global aviation passenger-range demand to represent a 2050 aircraft fleet that is fuelled by LH2. This process indicates that, to meet the global aviation demand in 2050 using LH2 fuel: (1) 1.76 times the internal volume of the largest existing fossil fuel aircraft is required to meet the maximum range, (2) different passenger and range combinations are required and (3) low-carbon energy is demanded in regions where such energy is scarce. Therefore, a combination of aircraft design, network and energy system changes are imperative. A Monte Carlo simulation is conducted to assess the uncertainty of the LH2 aircraft models and their sensitivity to the projected development of LH2 technologies. The analysis indicates that there is a 95% probability that the single-fuel LH2 fuelled Boeing 787-8 that employs electric propulsion and utilises World-average electricity in 2050 travels within - 44% and + 49% of the range predicted by the unperturbed model. The ability of LH2 aircraft to fill specific niches within the aviation industry and the design, network and energy system changes required to accommodate such aircraft are thus highly uncertain. Of all the technological parameters, the LH2 aircraft that employ electric propulsion are the most sensitive to the efficiency of the fuel cells and the ones that do not (e.g., the dual-fuel aircraft) are the most sensitive to the efficiency of electrolysis. It is thus recommended that fuel cells and electrolysers are the focus of future research and development. This will not only further the development of LH2 aircraft but will also further the development of transport systems with similar well-to-end-use pathways (e.g., LH2 fuelled ships). The First Law of Thermodynamics, ship motion equations and data on an existing LNG carrier are utilised to predict weather induced boil-off in an LH2 ship tank at different ship velocities, weather conditions and ship design configurations. It is found that weather induced boil-off heavily influences the design and operation of LH2 ships, so its estimation is essential for successful design. Firstly, the use of real time routing is essential to avoid significant boil-off at high Beaufort Number as, relative to LNG, LH2 boil-off is twice as sensitive to adverse weather conditions. Secondly, significant tank and ship redesign is required to accommodate LH2. The presence of a reliquefaction unit is essential: without it, 8 times the thickness of insulation is required relative to an LNG ship in calm weather. To deliver the same energy, an LH2 ship that is 1.7 times the volume of the existing LNG ship is required. Using this model of boil-off and low-carbon technology projections towards 2050, an energy systems model is constructed to compare grid derived LH2 with liquid ammonia (LNH3) for use in freight and low-carbon energy transportation. The analysis shows that, in terms of limiting well-to-end-use carbon dioxide emissions, LH2 is favourable for use in energy transportation and LNH3 is favourable for use in freight. The maritime transportation of LH2 can thus provide a means of conveying low-carbon energy to regions where such energy is in low supply. Among many other end-uses, the LH2 can thus be used directly as fuel for aviation or can be converted to LNH3 for maritime freight. However, the high energy demand of LH2 production means this process is reliant on a high availability of low-carbon energy at the source. Hence, the role of LH2 in both aviation and shipping is dependent on a complex interaction with the wider global energy system.