MODEL THE COMPLEXITY
Hydrogen-powered aircraft for sustainable flight
Using a digital twin to reimagine aircraft design for a sustainable future
Aviation currently accounts for nearly 5 percent of global greenhouse gas emissions.1 The problem is compounded by the fact that there are currently around 500,000 people in the air at any given time2 and twice as many air travelers are expected in 2037 as there are today.3
There are many initiatives being taken to improve sustainability of flight, such as improving fuel efficiency by reducing weight, reducing single-use plastics, investing in carbon offsets, and more.
But, as with road transport, the real game-changer is to find an alternative, carbon-neutral propulsion system. A system capable of transporting passengers with the capacity, speed and range of kerosene-fueled jet engines – but none of the environmental impact.
One of the most promising options currently being explored is hydrogen.
Figure 2. The increased fuselage space of blended wing body aircraft can be used to store batteries, hydrogen or hydrogen and fuel cells without sacrificing passenger or cargo capacity.
Hydrogen as a sustainable fuel
Hydrogen offers many advantages as a power source for aviation, but generating it is not straightforward. Although it is abundant, it’s almost always found as part of another compound such as water (H2O) or methane (CH4) from which it must be separated.
There are several common ways to produce hydrogen,4 but to power a sustainable aircraft the most practical method to date is electrolysis. In electrolysis, an electric current is used to split water into hydrogen and oxygen. If the electricity is produced by renewable sources such as solar or wind, the resulting hydrogen is considered renewable.
Once produced, hydrogen can be stored in gaseous or liquid form. Storing gas typically requires high-pressure (5,000 to 10,000 pounds per square inch) tanks, while storage as a liquid requires cryogenic temperatures, because the boiling point of hydrogen at one atmosphere pressure is -252.8 Celsius (C°).5
Aerospace engineers developing hydrogen-based sustainable aircraft propulsion systems have three main options: electric motors powered by fuel cells; pure hydrogen-powered gas turbines; or hybrids involving both fuel cells and hydrogen-powered gas turbines.
In the case of a hydrogen-powered jet engine, which is a type of internal combustion engine, air is sucked into the inlet, compressed, mixed with the hydrogen, and ignited to generate a high-temperature flow.
Challenges of hydrogen-powered aircraft
Perhaps the most immediate challenge is that developing a hydrogen-powered aircraft is new territory for most engineers. And designing a burner for a hydrogen gas turbine requires special features and structures.
For example, because hydrogen burns much faster and hotter than kerosene, a hydrogen burner must be designed to prevent flashbacks. It’s also necessary to understand the fluid dynamics, along with any stresses that occur at thermal boundary conditions of the hydrogen and electric-powered propulsion systems – including the operational phenomena they encounter such as flashbacks, thermoacoustics, thermal gradients and embrittlement.6, 7, 8, 9
Figure 3. Using Simcenter, engineers can build a digital twin to accurately predict aircraft performance, optimize designs and innovate faster with greater confidence.
Re-imagining the shape of aircraft
Another key challenge is that although hydrogen provides three times the energy density of kerosene per unit of mass, it requires four times the volume of kerosene to achieve the same result. So, whether the aircraft employs hydrogen turbines or hydrogen fuel cells to drive electric motors, there are major implications for the frame of the aircraft.
Either the cargo capacity, the number of passengers or both must be reduced to accommodate a hydrogen fuel source, or the entire aircraft configuration must be re-imagined.
One exciting possibility is a blended wing body (BWB) aircraft like the Airbus ZEROe BWN concept aircraft,10 in which the wings and fuselage are blended into a single entity (figure 2). Also known as a ‘flying wing,’ the entire aircraft provides the lift required for flight.
A major advantage of a flying wing configuration is the increased fuselage space can be used for carrying payloads such as cargo, passengers, batteries, hydrogen and fuel cells.
Figure 4. The Simcenter Amesim model enables engineers to evaluate the thermodynamic cycle of the hydrogen-powered turbofan.
Digitalization and the digital twin
As the design of the BWB suggests, finding solutions to enable hydrogen-powered aircraft involves far more than re-engineering the propulsion system alone. It requires a convergence of design domains and a coordinated effort between all the engineering disciplines involved in aircraft development.
Engineering data from all these interrelated domains – propulsion, fluids, thermal, mechanical, dynamics, acoustics, and more – must be shared between teams in an efficient manner so designers can continue operating efficiently within their native development environments.
The complexity, combined with cost, time and resource limitations, means that evolving a series of physical prototypes is not a viable design strategy. Instead, the way forward lies in digitalization.
Engineers are using Simcenter simulation and testing solutions, part of the Siemens Xcelerator portfolio, to bring together the various disciplines required to design and build hydrogen-powered aircraft.
The Simcenter provides an integrated design suite that can fully support multi-disciplinary aerospace engineering teams, helping them model, analyze and test the impact of alternative energy sources and propulsion systems – in short, enabling the creation of a physics-based digital twin.
Multi-disciplinary design capability
Within the Simcenter environment, the simulation modeling capabilities enable the evaluation of engine architectures, gas turbines, fuel storage, fuel cells, batteries and other components, as well as their weight (figure 4).
In parallel, engineers can then factor in thermal and mechanical 3D simulations as well as computer-aided design (CAD) capabilities to design each of these subsystems.
From the sloshing of cryogenic fuels and measurement of turbine inlet temperatures to dynamic system response and component durability, advanced physics are delivered in robust and validated Simcenter models (figure 5).
Automated workflows enable extended analysis of the design space to identify any conflicts from the various disciplines. Components such as the burner and vanes, the engine, the various subsystems and eventually the entire aircraft can be designed using a similar approach to meet precise design specifications.
Simcenter models are generated and executed with real-world fidelity to allow aerospace companies to design and deliver real-world systems (figure 6). In addition, the outputs can be combined with the Siemens Xcelerator portfolio to evaluate component and system manufacturability.
Supporting the journey to sustainability
These activities are just the beginning of a decades-long effort to re-imagine aircraft configurations and address materials supply chains, energy production, distribution and logistics networks, airport fuel delivery systems, and more.
The capabilities of the Siemens Xcelerator portfolio and the Simcenter tool suite are focused on supporting the digitalization efforts required to scale the aviation industry into this sustainable future.
Figure 5. This multi-domain design exploration rendering of a hybrid cryogenic H2 burn propulsion system was generated using Simcenter 3D, Simcenter STAR-CCM+, Simcenter Ames and HEEDS software tools to accurately represent the design's aeroelasticity.
Figure 6. This multi-physics design exploration of an H2 micro mix burner leverages NX CAD, Simcenter STAR-CCM+ and Simcenter 3D driven by the HEEDS automated optimization tool. (source: B&B AGEMA, RWTH Aachen and Kawasaki)
References
2. https://www.spikeaerospace.com/how-many-passengers-are-flying-right-now/
3. https://www.bbc.com/future/article/20210401-the-worlds-first-commercial-hydrogen-plane
4. https://afdc.energy.gov/fuels/hydrogen_production.html
5. https://www.energy.gov/eere/fuelcells/hydrogen-storage
6. https://www.plm.automation.siemens.com/global/en/our-story/customers/siemens-energy/93022/
7. https://www.plm.automation.siemens.com/global/en/our-story/customers/b-b-agema/98716/
8. https://webinars.sw.siemens.com/en-US/simulation-for-digital-testing-with-bb-agema/
9. https://webinars.sw.siemens.com/en-US/aerospace-defense-aircraft-propulsion-system-simulation
10. https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe