The aircraft industry is talking increasingly about the use of hydrogen as the best step to get to zero carbon emissions no later than 2050. Airbus has put its full weight behind the development of a hydrogen airliner, which should become available around 2035. The next few years will be spent on research and development, as there are quite a few elements that will influence the design of a hydrogen airliner. We asked German Aerospace Center (DLR) and MTU Aero Engines about these potential problems. Hydrogen airliner still takes a lot of R&D. Hydrogen has three times the energy of kerosene without producing any carbon emissions, so it is an attractive alternative to traditional aviation fuels or sustainable aviation fuels (SAF’s). It creates water vapor and ice crystals at cruise altitude, which could contribute to the greenhouse effect. Initial studies show that the crystals are heavier and precipitate faster, so contrails are thinner. But hydrogen has also less density than jet fuel, up to four times. This means it needs more storage space on an airliner. Gaseous hydrogen is no option as it would require a huge tank, but in liquid form as LH2, hydrogen can be stored in a special tank within the fuselage. And in order to keep it liquid, LH2 must be stored at -253 Celsius, potentially requiring additional cooling systems. It’s no surprise that storage and temperature are two of the key elements that need to be studied. DLR and MTU have been involved in numerous studies and have vast knowledge, but still more study is needed. In July, DLR, Lufthansa Technik, Hamburg Airport, and the ZAL research center announced a two-year research program at Hamburg Airport to look at all the aspects of using hydrogen during storage, ground operations, and on an aircraft. Lufthansa will convert a phased out Airbus A321 and fit it will a hydrogen system, including tanks and piping. A Lufthansa Airbus A321ceo will be modified and fitted with a full hydrogen system to act as a testbed in the Hamburg tests. (Lufthansa) How does super-cold hydrogen behave in an airliner? DLR’s founding director Björn Nagel explains that one of the focal points of the test will be how super-cold liquid hydrogen behaves in an airliner: “The low volumetric density of hydrogen can be increased significantly by liquidation which is realized at temperatures of -253°C. Handling liquids at such extreme temperatures imposes a great challenge. On the one hand’s side, the temperature of the liquid has to be kept as good as possible low in order to prevent boil-off. On the other hand side, the structure of the aircraft has to be protected from extremely low temperatures and strong temperature gradients.” Nagel continues: “At the same time, the very small H2 molecules have a good ability to dissipate through materials. Thermal and dissipation loads are very high. In this context, the integration of H2 systems into aircraft is most critical at the interface between the cryogenic system and the airframe as well as interferences with other aircraft systems. Thus, the details of the design are of concern.” The behavior of hydrogen can be simulated in the computer, but only to a limited extent. That’s why DLR wants to see for real how the liquid behaves in the A321. “They are very difficult to compute due to the complexity of aircraft with all its interacting sub-systems. This is why we need a realistic LH2 system and aircraft environment to investigate how we can operate LH2 aircraft, to measure the loads, and to identify the impact on the system. In our focus is placed on Maintenance, Repair and Overhaul relevant knowledge on aeronautical LH2 systems which might have a severe impact on operational costs and should be considered in the design of new aircraft.” DLR will feed all the data into a database. “We are going to use the state-of-the-art digital methods to design the hydrogen system and to plan the manufacturing and assembly process. In this step we will use all knowledge we have today to design and to certify such systems”, explains Björn Nagel. “Then we will realize the system in the ‘real world’ and explore ground operations like refueling. We expect to observe some phenomena which were not predicted by the design models properly. This is normal since the digital models cannot cover the full complexity of a real aircraft and its environment. Furthermore, very little experience is available in the design and operation of aeronautical LH2 systems. Thus, in the experiments, we will make experiences to build knowledge beyond the state-of-the-art how to design and operate LH2 systems in aviation. This might be specific solutions to refueling devices and procedures or safety devices to detect fatigue in the specific system with extreme temperature gradients.” “Throughout the experiments, we will add the gained new knowledge to the digital design models we used for the initial design of our experiment. As a result, we will create a digital design environment, which can also consider aspects of ground operations and MRO. These new methods will in a later stage be applied to our studies on future climate-neutral aircraft. Our ambition is designing aircraft not only for flight performance (which would be very academic) but concurrently also for real-life aspects like efficiency in manufacturing and in operations.” The Hy-ShAir-concept from Bauhaus Luftfahrt is a doubledecker with split LH2-tanks at the front and aft of the fuselage, plus a tail-mounted engine that improves efficiency. (Bauhaus Luftfahrt) Two options how to store hydrogen MTU Aero Engines is also spending a lot of time and money on the research of new engine technologies. The German company is a partner on the Pratt & Whitney Geared Turbofan and is working on numerous projects for an improved version of the GTF. But it also has a look at hydrogen, in particular the requirements of storage and fuel tanks. With its higher volume compared to kerosene, hydrogen needs a storage tank within the fuselage. The first concepts of the Airbus ZEROe hydrogen planes confirm this. Both have a tank at the rear of the fuselage, with the turboprop variant converting hydrogen and oxygen into electricity via fuel cells to run the engines while the medium-haul airliner will likely directly burn the hydrogen in the turbofan engines. Especially the latter design has a sizeable tank, but the size depends on numerous factors. MTU's director of corporate communications Markus Woelfle explains that “in general, there are two options. The first is to use an LH2 tank that has both a liquid hydrogen (LH2) and a gaseous hydrogen (GH2) phase and operates at a pressure of around 10 bar. This system allows us to use the GH2 at adequate pressure for a fuel cell system without any additional compression system. However, the tank has to be sized to this pressure (plus safety factors). For smaller fuel systems this is the preferred way forward.” “The second option is especially interesting for larger fuel systems. With the active/passive concept, you have a storage LH2 tank at low pressure (e.g. 3 bar maximum) and pump the LH2 with a cryogenic pump into a much smaller high-pressure tank. This concept reduces the structural weight of the storage tank and allows for much higher operating pressure (e.g. for combustion) but increases system complexity. So there is a trade between weight, complexity, reliability, etc. and it depends highly on the aircraft requirements to which solution one leans.” The tank could be split and distributed within the fuselage, with one at the front and the other at the back, as is the option on the Hyliner 2.0 study from Bauhaus Luftfahrt. “Different layouts are possible. Depending on the aircraft multiple tanks can be mandatory to provide redundancy”, says Woelfle. Universal Hydrogen offers a 'hydrogen kit', consisting of a fuel tanks system that can be retrofitted to existing aircraft. It has announced agreements with Icelandair and Air Nostrum to modify their turboprops for use of hydrogen. The size of the tank also mandates the range of a hydrogen airliner. Airbus reckons a first hydrogen airliner could have a range of around 2.000 nautical miles/3.700 kilometers. MTU thinks the maximum range possible in a conventional aircraft design will be around 3.500 nautical miles. “The volumetric density of LH2 gives a larger size tank. This either means reducing payload (as the tank is located within the fuselage) or increasing drag (e.g. tank in double bubble configuration). Depending on payload, speed, power requirement, etc. there is a cross-over point where flying longer range with LH2 becomes less attractive (time of flight, number of pax, etc.) than e.g. an aircraft powered with sustainable aviation fuels”, says Markus Woelfle. Disruptive aircraft designs like a blended wing could have more range as there is space for bigger fuel tanks in there. Keeping the liquid hydrogen at -253 Celsius is another issue. An Airbus spokesperson tells Airinsight that it is studying the need for an active cooling system, which would add weight and complexity to the aircraft. MTU thinks there is no need for that, says Markus Woelfle: “We do not actively cool LH2. It is insulated and stays in the liquid phase for a defined time. After a defined duration there could be the theoretical need to ‘boil off’ some hydrogen (to prevent overpressure in the tank – RS), but this will typically hardly ever happen in operations.” Fuel cells versus direct combustion Another element of a hydrogen airliner that takes R&D is the use of hydrogen in fuel cells or direct combustion. The former requires additional systems, like the electrolyzer that converts the liquid hydrogen with oxygen into electricity. That needs to be stored in heavy batteries. Whereas burning off fuel or hydrogen during flight reduces the weight of an airliner at the time it lands, batteries retain the same volume and weight. At take-off or at landing, an electric airliner will have the same weight, which requires additional energy. During an interview with Airinsight last November, then-acting Airbus Chief Technical Officer Grazia Vittadini said she wasn’t sure what the preferred solution is. “When you have the hydrogen on board, why would you want to carry the weight of all the systems? That’s something we need to study with the engine makers.” DLR’s Björn Nagel confirms that this is an issue “is part of extensive studies on this topic which have not yet been completed.” There is a trade-off to be made between hydrogen and fuel cells versus the direct burning of hydrogen in the engines, as Nagel explains: “The propulsion train with the best energy efficiency would be based on batteries and electric motors. However, the poor gravimetric energy and power density of batteries only offer potential in the general aviation market segment. 6-19 passenger aircraft with a range beyond 200 km are feasible based on hybrid concepts combining batteries and a combustion motor with the generator. The majority of operations in this segment are less than 200 kilometers permitting full electric flight. For the longer ranges, the combustion motor & generator are used as a range extender (with synthetic kerosene).” “Hydrogen can be used with fuel cells which offer a better energy efficiency compared to combustion engines and due to the absence of open combustion, emissions are strictly water and oxygen. On the negative side, the mass of the power train, its complexity, and its costs are high. In our studies, we created aircraft with this propulsion concept which have the potential to offer the most climate compatible solutions in the class of regional aircraft, though also in this class it is a tough competition with the two following described technologies.” Universal Hydrogen offers a retrofit hydrogen kit for turboprops, with hydrogen capsules that can be easily exchanged for fresh ones. (Universal Hydrogen) It depends on the size and range of the airliner which solution works out best: “In the class of short and medium-range aircraft, it appears to be the better option to directly burn the LH2 in the turbines. The reduced mass clearly compensates for the slightly reduced energy efficiency of turbines compared to fuel cells. Next to water vapor and oxygen, also NOx is emitted due to the open combustion. Currently, in the short-/medium range segment, LH2 direct burn appears to be the best option. However, research is ongoing concerning the evaluation of the climate impact and the technical and economic challenges of LH2 on board an aircraft. In such aircraft, we assume that onboard electricity will not be produced by generators attached to the combustion engines but that a fuel cell will be on board which will be driven by hydrogen and which will consume all boil-off gas.” “For long-range aircraft, mass and volume for LH2 storage increase strongly. For this segment, synthetic kerosene (e-fuels) appears to be the best option. This is produced based on green (gaseous) hydrogen in combination with CO2. The emissions from combusting synthetic kerosene are like from combusting oil-based kerosene water vapor and CO2. To be climate neutral, the CO2 which is used to create syn. kerosene and which is emitted during combustion needs to be taken out of the atmosphere (direct air capture). Capturing CO2 from the air and processing a liquid fuel is quite energy demanding (significantly more than the liquidation of hydrogen). Nevertheless, synthetic kerosene is a drop-in solution that is inevitable for operating the existing fleet of aircraft in a climate-neutral way. Since no liquid hydrogen tank is required, this solution appears to be best for long-range aircraft. For new short-/medium range and regional aircraft, syn. kerosene is a viable option as well. But the overall energy efficiency for operating the aircraft and producing the energy carrier appears to be a bit better for liquid hydrogen.” Nagel stresses that “all this is subject of research. Difficulties arise from the broad spectrum of aspects to consider in the evaluation. Energy demand for the full chain and CO2 emissions are clear, but we are also heavily evaluating the climate impact including the non-CO2-effects, industrialization with technical development risks, and costs for the different stakeholders of aeronautics. Effort and cost of ground operations/MRO are some of the very relevant questions which are addressed in our project.” Boeing and Embraer are doubtful of a quick solution It is all up to studies in topics like these mentioned above that will have to be completed first, before any airframer can commit to a hydrogen airliner. Airbus CEO Guillaume Faury has said that four to five years will suffice to master these challenges and be ready for the next step in R&D, leading to a program launch around 2027 and entry into service around 2035. Airbus will update media in September on where it stands, exactly a year after the ZEROe program was unveiled. Other OEMs are skeptical about Airbus’ hydrogen roadmap. Boeing CEO David Calhoun and Mike Sinnett, Vice President for Product Development, have stated that far too much research needs to be done before hydrogen is applicable. This includes setting up the production and infrastructure, which is in its infant stages right now. SAF’s and e-fuels are what Boeing is focusing on as it expects that hydrogen will only become an alternative around 2050. It’s a view shared by Embraer. Although its new turboprop will be designed with hydrogen in mind, Vice President Marketing and Product Strategy Rodrigo Silva e Souza told Airinsight last week that the cold liquid fuel isn’t expected to play a significant role in aviation before 2050. A hydrogen airliner still takes a lot of R&D.
