Much of the energy behind the growing momentum for hydrogen propulsion in aviation is now coming from start-up companies. Many of these have assembled small teams of well-educated, generally young engineers as they scramble to secure the investment they need to fuel their ambitious business plans. As their own cheerleaders, these companies generally are pre-disposed to emphasizing the positives around hydrogen, while portraying the potential negatives as solutions waiting to happen.
So where does this leave the much larger and more established existing aircraft engine manufacturers, when it comes to offering aviation a path to carbon neutrality? In April, Qatar Airways CEO Akbar Al Baker challenged GE Aviation to accelerate the development of a completely new engine that would support industry targets of achieving net-zero carbon emissions by 2050.
GE has indicated its plan calls for an incremental approach based on factors such as increased use of sustainable aviation fuel (SAF). The company also expects to be ready to demonstrate a new hybrid-electric propulsion system on a regional airliner by the mid-2020s.
Rolls-Royce is taking a similar approach, having made a significant investment in electric propulsion with the acquisition of Siemens’s eAircraft division. At the same time, it has continued efforts to make its existing turbofans burn less jet-A fuel, while also supporting the expansion of SAF usage.
Pratt & Whitney has been considering the case for hydrogen since the late 1950s when it was involved in Project Suntan with Lockheed Martin in trying to develop an alternative to the Blackbird SR71 military surveillance aircraft that needed to operate at altitudes of 100,000 feet. Michael Winter, the U.S. group’s senior fellow advanced technology, said that this experience made it all too conscious of the challenges the fuel posed, despite being a very effective propellant.
“Hydrogen is three times the energy density of kerosene but takes between three and four times the volume,” Winter explained to AIN. “When you store it, there is great pressure and also minus 253 centigrade temperatures. For commercial airliners, there is less room for passengers. To put it in a liquid state [for more convenient use] takes 15 percent of the energy stored in the fuel, so you want to recover that energy.” That could involve the use of a heat exchanger, which is what was done for Project Suntan, along with hundreds of miles of pipes.
“Pratt & Whitney sees potential in hydrogen, but there are plenty of technical challenges and limitations,” Winter said. “It’s just one path in an array of solutions to make aviation more energy-efficient and environmentally sustainable and [we] will be ready to support any of these with [our] technology.”
Other more immediate solutions include making existing gas turbine engines more efficient, and Pratt & Whitney claims to have achieved a 16 percent fuel burn reduction with its geared turbofan (GTF) technology, largely based on propulsive efficiency. It believes there are more improvements to come as, in collaboration with NASA, it has recently tested engine designs with up to an 18:1 bypass ratio (compared with the 12 to 13 bypass ratio of current GTF engines).
That apart, Pratt & Whitney is also working on ground-based applications for hydrogen, such as electricity generation. In Asia for example, the company has been developing dual-fuel approaches involving natural gas and hydrogen. It also is involved in a project with the U.S. Department of Energy’s ARPA-E program to explore ways to store hydrogen with ammonia.
The method for hydrogen production is a critical element in assessing the overall environmental sustainability of the fuel. Today, most hydrogen produced is designated as "brown" because it takes electricity to produce and so to produce so-called "green" hydrogen requires assured sources of ‘green’ electricity, produced through methods such as solar, tidal, or wind energy.
Nonetheless, Pratt & Whitney can see a path to directly burning hydrogen in existing engines with some changes to the fuel handling system, combustors, and injector nozzles. In the company’s view, this should be viewed as a longer-term transition for aviation, while further efficiencies are squeezed from existing propulsion technology through methods that could also include the more widespread use of SAFs.
For its part, Germany’s MTU Aero Engines wants to play a leading role in supporting the adoption of hydrogen. Last year it launched a partnership with the DLR Aerospace Research Center to convert a Dornier 228 regional airliner using fuel cells that they are jointly developing and validating. The partners, who have jointly deployed a team of around 80 experts, expect to be ready to start ground testing subsystems for the project before the end of June 2021 as they prepare for the first flight of a technology demonstrator in 2026.
The 19-seat aircraft will have one of its two Honeywell TPE331 turboprop engines replaced by a 500 kW electric motor, powered by electricity produced by hydrogen fuel cells and driving a propeller. DLR is managing the flight project, which involves providing and operating the research aircraft. It is also responsible for the integration and certification of the powertrain, which MTU is developing along with the hydrogen fuel cell that is powering the unit. The research institute is also providing its expertise in the fields of flight testing and aircraft dynamics and aeroelasticity.