There are three key issues and choices that aerospace companies will be considering when it comes to the development and operation of hydrogen-powered commercial airliners:
+ Whether to use traditional jet engines adapted to run on hydrogen or a fuel-cell electric powertrain
+ Whether to use cryogenically cooled liquid hydrogen or pressurized hydrogen gas
+ What the source of the hydrogen should be, also referred to as its color.
The consequence of these decisions will impact the carbon footprint of operating a hydrogen airliner, the potential range of the airliner, and, therefore, its ability to replace jet fuel-powered routes. Making the wrong decisions at each stage could be the difference between a genuine impact on reducing GHG emissions from the aviation industry and hydrogen planes becoming a green-washing exercise.
Hydrogen internal combustion vs hydrogen fuel cell
Hydrogen internal combustion or H2ICE will likely be very appealing to existing engine suppliers like Pratt and Whitney, General Electric, and Rolls-Royce. It uses largely the same design and componentry as existing jet-turbine engines with relatively minor changes to the fuel system and injectors to make them suitable for hydrogen. It would also be appealing for Boeing and Airbus, as operation and maintenance will look very similar to the existing model.
The drawback is that jet engines won't be as efficient as a fuel cell electric powertrain. In fact, the fuel cell plane will get approximately 50% more range for the same amount of hydrogen. Another way of looking at it is that the H2ICE plane will generate 33% more carbon for the same journey length as the fuel-cell electric plane, assuming both are using a non-carbon-neutral hydrogen source. In fact, the efficiency difference is so great that a fuel-cell-powered plane would get about 50% more range than a H2ICE plane for the same amount of hydrogen. Another way of looking at it is that the fuel-cell plane will have a 33% lower carbon footprint if both are fuelled with a non-carbon-neutral hydrogen source.
Fuel-cell electric planes will also have their challenges, and if left unsolved, could make them unviable in the first place. One of the key challenges is the longevity of the fuel cell. IDTechEx's report "Sustainable Future Aviation 2025-2045: Trends, Technologies, Forecasts" finds that Proton Exchange Membrane Fuel Cells (PEMFCs) are the most likely flavor of fuel cell to be used in hydrogen planes.
Its main advantage is its power density over other fuel cell types. However, it also has a very short life expectancy compared to other technologies and compared to jet turbine engines. It is likely that the fuel cell could require replacing as often as every 18 months, increasing downtime and maintenance costs for the airlines. IDTechEx's full report goes into further detail on the balance and compromises between H2ICE and fuel-cell electric options, including total cost of ownership, potential ranges achievable, and carbon footprint differences.
Liquid hydrogen versus pressurized hydrogen
The biggest fundamental issue with hydrogen as a fuel is its volumetric energy density. Some people will get very excited about its gravimetric density since it weighs around three times less than jet fuel for the same energy content. The downside is that at room temperature and pressure, it takes up around 3,000 times the same space as jet fuel for the same energy. Therefore, it must be pressurized or liquified to get to a usable volumetric energy density.
At 700bar, hydrogen has around 1/6th the volumetric energy density of jet fuel, meaning if the same tank space is available, it will only have around 1/6th the range, maybe slightly more using a fuel cell. The best option for volumetric energy density is liquification. As a liquid, hydrogen occupies around four times the volume for the same energy. Combined with a fuel cell, this gives the best case for range, achieving around 37% of the range of a normal jet plane.
However, hydrogen needs to be kept at -250 C (-420 F), which will add complications to storage at the airport and the addition of cryogenic cooling systems on the plane. Not to mention, any system on the plane would likely need duplicating for redundancy. The full report from IDTechEx explains in more detail the trade-offs between pressurized and liquid hydrogen and the impact it would have on the range.
The color of hydrogen
Another common talking point around hydrogen and its potential use as a fuel is that it is the most abundant element in the universe. While this is true, it has a frustrating habit of becoming trapped in other compounds or simply escaping as a gas. Since this is the case, producing hydrogen can be quite difficult.
The majority of hydrogen today comes from grey and black sources. In grey, hydrogen is stripped from natural gas, leaving the remaining carbon to combine with oxygen and releasing CO2 into the atmosphere. In black, the hydrogen is stripped from coal, also leading to CO2 release into the atmosphere. Both of these are terrible solutions when it comes to reducing overall CO2 footprint. IDTechEx's report finds that grey hydrogen in a fuel cell results in a 27% carbon saving compared to jet fuel; all other combinations of grey, black, fuel cell, and H2ICE are worse. Black hydrogen used in H2ICE has twice the carbon footprint of jet fuel!
The only realistic options that ensure hydrogen is not more damaging to the environment than jet fuel are green and blue hydrogen. In green hydrogen, electrolysis of water from renewable sources is used to produce hydrogen with zero emissions (at the point of production). Blue is similar to grey, but the CO2 is captured during the production process, leading to a ~95% reduction in GHG impact compared to grey.
The downside of blue and green hydrogen is that they are significantly more expensive than grey and black. If this does not improve, then airlines will be looking at a difficult choice between retaining similar levels of overall GHG emissions, absorbing the high price of green and blue hydrogen, or having to put ticket prices up.
While hydrogen offers a realistic path to the decarbonization of air travel, offering a significant range improvement over battery electric options, the journey is going to be challenging. Aerospace companies, engine manufacturers, and airlines will need to make some tough choices around the propulsion technology, storage state, and hydrogen source.
IDTechEx's report "Sustainable Future Aviation 2025-2045: Trends, Technologies, Forecasts" provides a detailed technical analysis of the choices relating to hydrogen aviation. The report offers guidance in these areas, gives IDTechEx's opinion on where the hydrogen aviation industry is heading, and forecasts the uptake of these new technologies is available here.
Related Links
IDTechEx
Aerospace News at SpaceMart.com
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