A hydrogen-powered aircraft is an aeroplane that uses hydrogen fuel as a power source. Hydrogen can either be burned in a jet engine or another kind of internal combustion engine, or can be used to power a fuel cell to generate electricity to power an electric propulsor. It cannot be stored in a traditional wet wing, and hydrogen tanks have to be housed in the fuselage or be supported by the wing. Hydrogen, which can be produced from low-carbon power and can produce zero emissions, can reduce the environmental impact of aviation. Airbus plans to launch a first commercial hydrogen-powered aircraft by 2040–2045, while Boeing is less optimistic. McKinsey & Company forecast hydrogen aircraft entering the market in the late 2030s and scaling up through 2050, when they could account for a third of aviation's energy demand.
Hydrogen properties
Hydrogen has a specific energy of 119.9 MJ/kg, compared to ~43.5 MJ/kg for usual liquid fuels, 2.8 times higher. However, it has an energy density of 10.05 kJ/L at normal atmospheric pressure and temperature, compared to ~31,293 kJ/L for liquid fuels, 3,114 times lower. When pressurised to 690 bar (10,000 psi), it reaches 4,500 kJ/L, still 7 times lower than liquid fuels. Cooled at 20 K (−253 °C), liquid hydrogen has an energy density of 8,491 kJ/L, 3.7 times lower than liquid fuels.
Aircraft design The low volumetric energy density of hydrogen poses challenges when designing an aircraft, where weight and exposed surface area are critical. To reduce the size of the tanks, liquid hydrogen will be used, requiring cryogenic fuel tanks. Cylindrical tanks minimise surface for minimal thermal insulation weight, leading towards tanks in the fuselage rather than wet wings in conventional aircraft. Airplane volume and drag will be increased somewhat by larger fuel tanks. A larger fuselage adds more skin friction drag due to the extra wetted area. The extra tank weight is offset by dramatically lower liquid hydrogen fuel weight. Gaseous hydrogen may be used for short-haul aircraft. Liquid hydrogen might be needed for long-haul aircraft. Hydrogen's high specific energy means it would need less fuel weight for the same range, ignoring the repercussions of added volume and tank weight. As airliners have a fuel fraction of the Maximum Takeoff Weight MTOW between 26% for medium-haul to 45% for long-haul, maximum fuel weight could be reduced to 9% to 16% of the MTOW. Fuel cells make sense for general aviation and regional aircraft but their engine efficiency is less than large gas turbines. They are more efficient than modern 7 to 90-passenger turboprop airliners such as the DASH 8. The efficiency of a hydrogen-fueled aircraft is a trade-off of the larger wetted area, lower fuel weight, and added tank weight, varying with the aircraft size. Hydrogen is suited for short-range airliners; its use in longer-range aircraft will require new aircraft designs. Liquid hydrogen is one of the best coolants used in engineering, and precooled jet engines have been proposed to use this property for cooling the intake air of hypersonic aircraft, or even for cooling the aircraft's skin itself, particularly for scramjet-powered aircraft. A study in the UK, NAPKIN (New Aviation, Propulsion Knowledge and Innovation Network), with collaboration from Heathrow Airport, Rolls-Royce, GKN Aerospace, and Cranfield Aerospace solutions, has investigated the potential of new hydrogen-powered aircraft designs to reduce the environmental impact of aviation. The aircraft designers have proposed a range of hydrogen-fuelled aircraft concepts, ranging from 7 to 90 seats, exploring the use of hydrogen with fuel cells and gas turbines to replace conventional aircraft engines powered by fossil fuels. The findings suggest that in the UK hydrogen-powered aircraft could be commercially viable for short-haul and regional flights by the second half of the 2020s with airlines potentially able to replace the entire UK regional fleet with hydrogen aircraft by 2040. However, the report highlighted that national supply, and the price of green liquid hydrogen relative to fossil kerosene are critical factors in determining uptake of hydrogen aircraft by airline operators. Modeling showed that, if hydrogen prices approach $1/kg, hydrogen aircraft uptake could cover almost 100% of the UK domestic market.
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