Bi-directional Solid Oxide Cells used as SOFC for Aircraft APU system and as SOEC to produce fuel at the airport

Abstract

Leaders of European union and G8 have set the target of 80% reduction in greenhouse gases emissions by 2050 by decarbonizing the power and transport sector. There are several pathways to achieve this, some of them being a) use of renewable power and biomass b) improvement in transport and building energy efficiency and c) replacement of fossil based fuel with sustainable fuel. This thesis is focussed on some of the measures which can be taken by Air Transportation Industry to reduce GHG emissions and carbon footprint. Although, there are many ways in which air transport industry can achieve this goal, this work mainly discusses a) increase in power generation efficiency of aircraft systems b) use of sustainable fuels for aircrafts and c) some ways for sustainable fuel production for aircrafts in an efficient and economic way. However, it is only starting point for discussion and can be further extended to include more efficient and cheaper technologies with reduced carbon foot-print. In this study, use of bi-directional Solid-Oxide Cells (SOC) as auxiliary power unit (APU) on-board commercial aircraft is explored. These bi-directional SOC APU units can be used in the airport energy network either to produce cleaner energy or to produce sustainable fuels depending on the energy demand. This work focusses on use of these bi-directional SOCs as fuel cells during flight operations and as electrolysers to produce sustainable fuel at the airport when the aircrafts are parked. Hence, complete fuel production plant is designed to be situated at the airport. The scope of this thesis is limited to production of fuel for aircraft APU use only. Further extension of this project can include use of these bi-directional SOCs for providing electricity to the airport and fuel for other purposes. For analysis, medium range aircrafts like A320 and B737 are considered. This system is designed and dimensioned for producing 500KW of electric power on-board aircarft. Jet fuel and ammonia are considered as fuel options for Solid Oxide Fuel Cell-Gas Turbine (SOFC-GT) based APU. Small scale airports like Eindhoven are studied to understand the flight frequency and parking duration for the aircrafts. These bi-directional SOCs are operational as Solid Oxide Electrolyzer Cell (SOEC) only when the aircraft is parked. Co-electrolysis is performed to produce syngas and steam electrolysis is done to produce hydrogen at the airport. Jet fuel is synthesized from syngas through Fischer Tropsch process and ammonia is synthesized from H2 and N2 through Haber Bosch process. Fuel synthesis plants are also designed as part of stationary fuel production plant at the airport. The fact that electrolyzer operates on excess available renewable electricity only, needs a special mention here. Intermittent nature of excess renewable electricity requires implementation of another source of sustainable syngas for fuel production so that sufficient capacity to supply all demand is ensured and robustness against delivery risks is achieved. Biomass gasification is one other method for generating fossil-free fuel. It uses biomass (birch wood) to produce syngas for sustainable fuel production at the aiport alongwith electrolysers. This leads to three cases of fuel production: - Case-1: Gasifier+fuel synthesis - Case-2: SOEC+fuel synthesis - Case-3: Gasifier+SOEC+Fuel synthesis ASPEN PLUS is used for modelling SOFC and stationary fuel production plant models with both jet fuel and ammonia. Modelling procedure for all the models is explained in detail with input parameters and process conditions. Thermodynamic analysis is carried out to compare the exergy efficiency of jet fuel and ammonia based SOFC-GT systems. It is observed that both jet fuel and ammonia based SOFC-GT systems give 62% and 58% exergy efficiency respectively which is higher than the conventional APU systems. Similarly, section by section exergy analysis is carried out for jet fuel and ammonia production plants to understand the exergy destructing processes. Comparison is made between exergy efficiency of jet fuel and ammonia production plants at the airport to understand thermodynamic behavior of both. Jet fuel synthesis produces significant amount of hydrogen and gasoline alongwith jet fuel as product. Therefore, two scenarios are analysed for exergy comparisons. a) Ammonia and jet fuel considered as product: Ammonia shows higher exergy efficiency than jet fuel production for all three cases enumerated above. b) Ammonia and (jet fuel, gasoline, hydrogen) are considered as useful products: For case-1 and case-3, jet fuel plant is exergetically more favorable than ammonia plant. However, for case-2, ammonia plant has higher exergy efficiency than jet fuel plant.