Pushing the Envelope
Pushing the envelope
Brayton cycle for hydrogen-powered engines
Robert Coppinger
Hydrogen has been a popular option in the greener aviation and net zero carbon dioxide emissions debate. All-electric power and hybrid gas turbine battery designs have been proposed for air taxi, regional airliners and short-haul flights but the debate for long-haul seems to have found its climate change solution in hydrogen.
In 2020, Boeing Australia and that country’s Commonwealth Scientific and Industrial Research Organisation jointly published a report on aviation and hydrogen. That year, Airbus unveiled its 200 passenger, 3,700km range hydrogen aircraft concept with a service entry goal of 2035. In February this year, Airbus announced its ZEROe hydrogen demonstrator, a modified Airbus A380. The same month, Pratt & Whitney announced it had won a US government Department of Energy (DoE) contract to develop hydrogen-fuelled propulsion technology.
The hard work of turning concepts into highly reliable powerplants, using the turbine and its long-proven Brayton thermodynamic cycle, has begun. The adoption of hydrogen, as a fuel, also has the double challenge of the supply infrastructure that must replace the existing kerosene facilities for storage and transportation.
The hydrogen replacing the ubiquitous drop-in fuel is expected to be in the form of a cryogenic liquid, existing at minus 258°Celsius, whether it is on the ground or on the aircraft. How hard this work with cryogenic hydrogen is going to be is highlighted by the two-year DoE propulsion project’s limited goals. It will only develop components of a hydrogen-burning turbine to a technology readiness level (TRL) of three to four. A TRL of zero is an idea, a TRL of nine is an operational system, an engine, rocket or aircraft. A TRL of three to four is a laboratory bench or a test rig demonstration.
As Pratt & Whitney, and any engine maker, will readily admit, the engines are the biggest obstacle in developing a new aircraft. The DoE project is called hydrogen steam-injected, inter-cooled turbine engine or HySIITE. There is no such thing as hydrogen steam. That is a misnomer. Instead, water is injected into the combustor, becoming steam in the process. The hydrogen is not only combusted though. It is also used to capture what would otherwise be waste heat. The liquid hydrogen will be routed from the fuel tanks to a heat exchanger at the engine exhaust to absorb some of the heat.
Pratt & Whitney said that the heat exchanger used to collect this otherwise wasted energy is the most difficult of the HySIITE technologies being developed. The engine maker is creating the exchanger, which needs to be of a certain size and weight, with the help of Raytheon. The exchanger’s physics is understood but the engineering needs to be conquered. Pratt & Whitney admits the additional subsystems to make the hydrogen engine work will add weight and it will be heavier than a standard kerosene fed turbine, but the increased efficiency will make up for the mass penalty. This is not a new trade-off for jet engines. Their mass increased with the adoption of high bypass ratio designs and the resulting efficiency gains overcame the drawbacks of that added bulk.
While the exchanger preheats the hydrogen before combustion, the water injection is being used to cool, not heat, the combustor. Water is a by-product of hydrogen combustion because combustion is an oxidisation reaction. Water is made of oxygen and hydrogen and this by-product is to be collected and fed back into the engine cycle. Injecting water into the combustor to reduce the temperature may seem odd but it is a constant pressure that is needed – a feature of the Brayton cycle. If the temperature can be kept as low as is effective for combustion, it reduces the output of foliage damaging, ozone-producing nitrogen oxides, an important emission reduction.
While HySIITE is planned to last 24 months, follow-on projects for ground test demonstrations could take place. The 24-month project is one piece of a much broader strategy to improve engines for Pratt & Whitney. The firm sees this hydrogen technology being compatible with its geared turbofan engine. Pratt & Whitney is hoping to achieve more than a TRL of three, something between three and four with HySIITE, yet it is cautious on the timing of hydrogen’s eventual introduction, which for some of its engineers may not come until after 2035. Those TRL levels of four, five, six and up to nine, are going to be a long haul for the engine makers.