Exergetic sustainability analysis of an unmanned air vehicle with a hydrogen fuelled turbofan engine

In this thesis instant and overall exergetic sustainability indicators of a high altitude long endurance unmanned air vehicle, having a high by-pass turbofan engine fuelled with dodecene and hydrogen with 0 to 100 percent storages by weight either with or without heat recovery by the fuel from the exhaust gasses throughout its flight profile ranging from 0 to 16 km are investigated. Hydrogen is assumed to be stored at 20 K, 1 bar as a liquid or as a gas or in materials at 35 K and 25 bar, 50 bar 100 bar; 80 K and 50 bar, 100 bar, 200 bar; 125 K and 100 bar, 200 bar, 350 bar and at environment temperatures under 200 bar, 350 bar and 700 bar pressures. Engine thermodynamic calculations are made by assuming the fuel to be entering the combustion chamber at environment temperatures while it is assumed to be entering the heat exchangers at storage temperatures in case of heat recovery. Fuel saving potentials with heat recovery and fuel use required for heating the fuel from storage temperatures to environment temperatures are calculated seperately. Use of physical exergy of the fuel is ignored in all cases. All the results are obtained from a special program developed by the author in MATLAB software. Hydrogen's exergetic performances vary according to storage pressures and temperatures and heat recovery. Best exergetic sustainability performances are obtained with low pressure – low temperature storages and heat recovery in general. Maximum overall exergetic sustainability index is obtained as 1.02 changing less than 0.05 throughout 43 to 52.7 % fuel depletion (=storage by weight) range for all storage temperatures which is much higher than the actual hydrogen storage capacities. It is concluded that actual hydrogen storage capacities are not suitable for UAV missions longer than 7 hours.