Performance Investigation Of Fuel Cell Propulsion Systems On Unmanned Aerial Vehicles With Long Endurance

Unmanned aerial vehicles with long endurance can be operated at high altitudes for a long time and are paid attention to in the civil and military fields.Propulsion systems with low thrust specific fuel consumption are important to the unmanned aerial vehicle.The performance improvement of the traditional turbine engine is limited by the turbine inlet temperature.Therefore,solid oxide fuel cell gas turbine hybrid propulsion systems are proposed in this paper.In the system,compressors are powered by the fuel cell.Combustor exhaust directly enters into the nozzle.The coupling between the compressor and turbine does not exist for the novel system.However,the coupling between the fuel cell and compressor is complex in terms of pressure,temperature,power,and mass flow.So,the working principles and characteristics of the system are different from the traditional turbine engines’.In order to comprehend the performance and coupling of the hybrid propulsion systems,related work has been carried out and listed below:Firstly,fuel cell models with multi-fields,gas turbine modes,exergy models,and mass models were built.Important models are validated by the experiments.In addition,the compact fuel cell configuration scheme is designed and pressure loss caused by the fuel cell system is evaluated.Results show that the volume and pressure loss of the fuel cell leads to a small impact on the hybrid propulsion systems.The aircraft equipped with the hybrid system has advantages when the power density of the fuel cell is high.Secondly,the thermodynamic cycle analysis is completed.Effects of combustion temperature on the hybrid systems and turbine engines are different.With decreasing fuel cell working temperature,the fuel utilization of fuel cells increases,which leads to the increasing of the thermal efficiency of the hybrid system.At the combustion temperature of 1350 K,the efficiency of the system is high up to 82.2%,which is higher than the efficiency of Carnot cycles or fuel cells.The study results of the ideal hybrid cycle and cycles fed by different types of fuels both show that the hybrid system have obvious advantages over the traditional combustion engines in terms of specific thrust and specific impulse.From the point of the thermodynamic second law,the performance of nozzles and compressors should be improved at first,which will contribute to the improvement of the thermal efficiency of the hybrid system.Thirdly,coupling and match among the components are studied.The operating range of pressure ratios is wide for the hybrid systems integrated with anode exhaust recirculation.However,the performance of the hybrid system integrated with the partial oxidation oxygen reformer and cathode exhaust recirculation is limited by the compressor pressure ratio.Parameters affecting the performance of the fuel cells are analyzed.Results showed that the number of the fuel cell seriously affects the performance of hybrid systems and the endurance of aircraft equipped with the system.The efficiency of the fuel cell is high and its power density is also high when the hybrid systems are operated at the low current density of fuel cell and big equivalence ratio.Meanwhile,the specific thrust of the hybrid system is high and its weight is moderately lower.The hybrid systems integrated with air bypass are proposed.The thrust and specific impulse of the hybrid system are improved sharply by increasing bypass ratios.Importantly,the increased degree of thrust is higher than the degree of the weight due to the huge increase of propulsion efficiency.Therefore,the thrust weight ratio of the hybrid system will increase.Finally,the parameters related to the hybrid system are optimized under the maximum,or cruise thrust for a long endurance aircraft.As the pressure ratio of the bypass fan and the compressor and bypass ratios change,the weight of the hybrid system slowly changes.The parameters related to the fuel cell such as pressure loss,power density,and fuel utilization of the fuel cell seriously affect the specific fuel consumption and weight of the hybrid system.So,the endurance of the aircraft equipped with the hybrid system is also affected by them.The fuel cell area and power of the motor under the maximum thrust conditions are respectively higher than that under the cruised thrust conditions.Therefore,the parameters from the former are considered as the designed parameters for the system.Its weight is 260% weight of the original turbine engines.The specific fuel consumption and thrust for the system are 10.1 g/s/kN and 4kN,respectively.The operating pressure ratio and combustion temperature are decreased 75% and 26%.The design and manufacture difficulty for the compressors and combustion chambers is lowered.Meanwhile,the specific fuel consumption of the system is decreased 42%,compared with the original engine.The maximum endurance for the aircraft equipping the hybrid system is increased by 50%,compared with the original aircraft.The altitude and throttle characteristics are analyzed.The hybrid system has high thermal efficiency and propulsion efficiency at high altitudes.