The physical exergy in hydrogen – maximising the utility of hydrogen as an aviation fuel

Hydrogen is a promising fuel to decarbonise aviation. Storage in liquid form is favoured for long-haul aircraft; storage as a high-pressure gas is preferred otherwise. The exergy expended during the compression or liquefaction process is stored as physical exergy in the fuel. Most discussions around hydrogen-fuelled aviation ignore this very significant exergy content. When combusted in an engine, the chemical energy of hydrogen can produce around 60 MJ of work per kg. The work that can be extracted from the physical exergy depends strongly on the method used. This paper presents an exergy analysis considering a range of storage conditions, operating conditions and work-extraction methods. For reasonable gas-turbine operating conditions, upwards of 16 MJ/kg might be extracted from compressed hydrogen (at 700 bar), and 30 MJ/kg from LH 2 . This additional work, representing 25–50 % of the shaft work produced by combustion has been by and large neglected. • Around 60 MJ can be extracted by simply oxidizing 1 kg of hydrogen in an engine or fuel cell. • Due to its compression or liquefaction, the fuel contains significant physical exergy. • Extracting the physical exergy from 700 bar hydrogen can add 5–20 MJ of work per kg of H 2 . • Extracting the physical exergy of liquid hydrogen can add 15–43 MJ of work per kg of H 2 .