Aircraft Engine Manufacturers Are Still Sceptical About Hydrogen.
Start-up companies are providing a lot of the energy behind the rising momentum for hydrogen propulsion in aviation. As they struggle to secure the investment they need to fuel their ambitious business plans, many of them have recruited small teams of well-educated, mostly young engineers. As their own cheerleaders, these businesses are prone to highlighting the benefits of hydrogen while presenting the possible drawbacks as problems waiting to be solved.
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So, where does this leave the much larger and more developed current aircraft engine manufacturers in terms of providing a carbon-neutral course for aviation? Qatar Airways CEO Akbar Al Baker challenged GE Aviation in April to speed up the production of a brand new engine that would help the industry meet its 2050 goal of zero carbon emissions.
GE's strategy calls for a step-by-step approach focused on factors like increased use of renewable aviation fuel (SAF). By the mid-2020s, the company hopes to be able to demonstrate a new hybrid-electric propulsion system on a regional airliner.
Rolls-Royce is taking a similar path, having acquired Siemens' eAircraft division and making a major investment in electric propulsion. Simultaneously, it has continued to work on making its current turbofans use less jet-A fuel while also encouraging the use of SAF.
Since the late 1950s, when it was working on Project Suntan with Lockheed Martin to create an alternative to the Blackbird SR71 military surveillance aircraft, which had to fly at altitudes of 100,000 feet, Pratt & Whitney has been considering the case for hydrogen. Despite being a very powerful propellant, this experience made Michael Winter, the US group's senior fellow advanced technology, all too aware of the challenges the fuel presented.
Winter told AIN that hydrogen has three times the energy density of kerosene but needs three to four times the volume. “When you store it, it's under a lot of pressure and at minus 253 degrees Celsius. There is less room for travellers on commercial aeroplanes. It takes 15% of the energy contained in the fuel to transform it into a liquid state [for easier use], so you want to recover that energy.” This could entail using a heat exchanger, as was done for Project Suntan, as well as hundreds of miles of pipes.
Winter said, “Pratt & Whitney sees promise in hydrogen, but there are numerous technological challenges and limitations.” “It's only one direction among many to make aviation more energy-efficient and environmentally friendly, and [we] will be prepared to help all of them with [our] technology.”
Other more immediate solutions include improving the performance of existing gas turbine engines; Pratt & Whitney claims to have reduced fuel burn by 16 percent with its geared turbofan (GTF) technology, which is largely focused on propulsive efficiency. It claims there will be more advancements in the future, as it recently tested engine designs with up to an 18:1 bypass ratio in partnership with NASA (compared with the 12 to 13 bypass ratio of current GTF engines).
Apart from that, Pratt & Whitney is focusing on ground-based hydrogen technologies, such as electricity generation. The company has been developing dual-fuel approaches involving natural gas and hydrogen in Asia, for example. It's also working on a collaboration with the US Department of Energy's ARPA-E programme to see whether ammonia can be used to store hydrogen.
The process for producing hydrogen is a key factor in determining the fuel's overall environmental viability. Most hydrogen generated today is classified as "brown" because it needs electricity to generate, and therefore producing so-called "green" hydrogen necessitates reliable sources of "green" electricity, such as solar, tidal, or wind energy.
Nonetheless, Pratt & Whitney believes that with certain modifications to the fuel handling system, combustors, and injector nozzles, existing engines may be converted to directly burn hydrogen. According to the business, this should be regarded as a longer-term transformation for aviation, as current propulsion technology is squeezed for even more efficiencies by methods that may involve the more widespread use of SAFs.
MTU Aero Engines, based in Germany, wants to play a leading role in promoting hydrogen adoption. It announced a collaboration with the DLR Aerospace Research Center last year to convert a Dornier 228 regional airliner to operate on liquid hydrogen direct combustion. As they plan for the first flight of a technology demonstrator in 2026, the partners hope to be able to begin ground testing subsystems for the project by the end of June 2021.
One of the 19-seat aircraft's two Honeywell TPE331 turboprop engines will be replaced by a 500 kW electric motor driven by hydrogen fuel cells and driving a propeller. MTU is in charge of propulsion, while DLR is in charge of system integration and certification.