Thermodynamic Well to Wheel analysis of using compressed SNG derived from biomass for an SOFC-GT powered UAV

Abstract

Unmanned aerial vehicles (UAV) for science missions are becoming increasingly popular in the present context. Science missions require high altitude operation and long endurance (HALE). With rising awareness of climate change phenomenon it becomes imperative to look into more efficient and more sustainable means of powering these missions. A 'well-to-wheel' study is a comprehensive way of comparing different operations for powering a vehicle. For this study , the HALE UAV is powered by a solid oxide fuel cell and gas turbine (SOFC-GT) hybrid. The HALE UAV design is based on concept that has been developed before by NASA. It is also essential that a suitable fuel is selected for powering the hybrid system. Previous studies which were based on liquid hydrogen (LH2 ) concluded that liquefication of hydrogen as an avenue for major exergy losses. For the current study, use of compressed synthetic natural gas (SNG) is analysed. Exergy analysis is carried out for fuel production and the hybrid system operation at cruise altitude. To begin with, syngas is synthesised from woody biomass through gasification. The syngas is then converted into SNG and compressed for storage. This SNG is utilised in the SOFC-GT hybrid. Design calculations are made for the take -off stage. Effect of pressure variation is studied to get the optimum system size. Then the performance of the system is analysed in the cruising stage to estimate the endurance of the flight. Exergy efficiency analysis and mission endurance calculations are made for the system during cruising conditions as a means to evaluate the system performance. The effect of position of air preheater is studied in two different configurations. The exergy efficiency for compressed SNG synthesis is 59.7% and the SOFC-GT operation at cruise is 69.6% effcient. A total 'well-to-wheel ' effciency of 41.6 % is attained. Based on the results of the analysis carried out thus far, the mass for the system components is estimated. Volumetric estimates are also made and the system is fitted in the nacelle volume as defined in NASA concept. The results are eventually compared to the system working on LH2 . The results from compressed SNG study show that specific energy density of the fuel is important. The mass estimates show that the infrastructure required to carry the SNG fuel reduces the specific energy of the fuel. liquid fuels such as methanol should be analysed as another option.