Pratt & Whitney is hopeful that a revolutionary hydrogen combustion system that uses water vapour recovered from the exhaust stream to increase engine efficiency could be ready for service entry by 2035.

The engine manufacturer on 21 February revealed that it had been awarded $3.8 million by the US Department of Energy through the OPEN21 scheme run by its Advanced Research Projects Agency-Energy (ARPA-E) for early stage research work on a project P&W calls the Hydrogen Steam Injected, Inter‐Cooled Turbine Engine (HySIITE).


Source: Pratt & Whitney

P&W believes geared turbofan architecture could be coupled with steam-injection hydrogen combustor

P&W claims the technology will cut carbon dioxide emissions to zero, slash nitrous oxide emissions by 80%, and reduce fuel consumption for next-generation single-aisles by 35% compared with the current kerosene-fuelled PW1100G.

Vince Sidwell, director of advanced concepts and technology at the East Hartford, Connecticut-headquartered firm, says the 24-month initiative is designed to develop critical elements of the system.

These include the heat exchanger, which is used in the exhaust stream to turn water vapour into steam, and the combustor.

At the end of the two-year effort – which is likely to kick-off later in 2022 – the system will be at technology readiness level 2 or 3, says Sidwell, and could then transition to a second phase of development.

“Completing the HySIITE project will put us in a position to propose a ground demonstration,” he says.

P&W has not conducted any “detailed planning” around a follow-on project, but he sees a roadmap under which a commercial powerplant could arrive by the middle of next decade.

“I can say that we envision this to be 2035-plus entry into service potentially,” says Sidwell. But he cautions that date will depend on the development of an “airframe that’s compatible”, alongside hydrogen delivery infrastructure at airports.

The HySIITE system takes advantage of the incredibly low temperature of cryogenic liquid hydrogen state – around -252°C (-421°F) – to support the water recovery, steam injection process.

P&W believes this factor will allow it to reduce the size, and therefore weight, of the heat exchanger required to enable the system.

“We think there’s possibility there and part of that is driven by the unique properties of hydrogen; the fact that you have a large source of cold – cryogenic hydrogen,” says Sidwell.

“Is it possible to create a system that has flight weight and flight volume? That’s what the HySIITE contract will let us understand.”

HySIITE will inject steam into the combustor to increase its power, and water vapour into the compressor to act as an intercooler, allowing more efficient compression, and into the turbine as a coolant. These processes work together to increase the overall system efficiency of the engine core.

Sidwell says aside from the increased power in the combustor, steam injection also lowers the temperature, reducing nitrous oxide emissions.

Although a proportion of the water vapour produced through hydrogen combustion will be recycled back into the engine through the system’s semi-closed-loop architecture, the net combustion moisture ejected as part of the exhaust stream remains the same. Hydrogen fuel combustion will produce fewer particulate emissions, potentially decreasing contrail formation. Sidwell says further research will be needed “to understand that impact”.

Initial activity under the project will consist of a “system evaluation to understand the requirements and the suitability of key components”, says Brent Staubach, associate director of advanced concepts and innovation at P&W, a process he hopes that can be completed by year-end.

A detailed design phase will follow, including of sub-elements, ultimately generating laboratory-based “feasibility testing towards the end of the [ARPA-E] contract”, says Staubach.

Alterations to the combustor will focus on both enabling the steam-injection process and changing certain materials to avoid any weakening of metallic parts through a reaction with hydrogen – an effect known as hydrogen embrittlement.

In addition, hydrogen burns in a different way to kerosene, with different flame speeds and temperatures, affecting the combustor dynamics, says Staubach. “Hydrogen as a fuel has different combustion properties, so that’s another development area.”

While HySIITE deals with the efficiency of the engine core, Sidwell says the technology could be easily married with the next generation of P&W’s geared turbofan architecture to also improve propulsive efficiency.

One potential barrier to the adoption of hydrogen as a future fuel for aviation is its likely cost versus standard jet fuel.

Although Sidwell declines to speculate on the future price of hydrogen, he adds: “I will say that the efficiency of HySIITE will offset some of the penalties associated with putting a large hydrogen tank in an airframe from a performance standpoint; we think the HySIITE configuration is an enabler for hydrogen as fuel because it offsets some of those losses.”

Work on the project is also being carried out at the Raytheon Technologies Research Center – a facility run by P&W’s parent company, but located on the engine maker’s East Hartford campus.

The ARPA-E initiative is designed to fund high-risk, high reward programmes. “So they intentionally fund things that are lower confidence but have a high pay-off,” says Sidwell.