People Love Hybrid Cars. Get Ready for Hybrid Planes

Worldwide aviation accounts for about 2.5% of global carbon emissions, and while that might sound minuscule, the industry is expected to expand steadily in coming decades just as the world is scrambling to slash greenhouse gases to head off the ravages of climate change. 

That makes reining in aircraft emissions an increasing priority—but how? Modern gas turbines used in jet aircraft are already highly efficient, with little room for further optimization, so solving the problem mandates new approaches.

Now, a team led by University of Maryland professors Christopher Cadou in aerospace engineering and Eric Wachsman in materials science and engineering has its sights set on a breakthrough: a hybrid gas turbine/fuel cell system that can be used to power large aircraft such as the Boeing 737 that generate more than 85% of aviation emissions globally.

Their approach, supported by funding from the Advanced Research Projects Agency-Energy, Raytheon and other industry partners, is to marry a traditional gas turbine with a new, ultra-high performance fuel cell to create a system that performs better than either component by itself, Cadou said.

“It’s a practical way to realize the improvements in overall efficiency (and thus reduced emissions) offered by electric propulsion without waiting for the needed improvements in hydrogen or battery technology,” he said. “Just as importantly, it’s a fuel-flexible solution that can be used with today’s fossil fuels as well as tomorrow’s carbon-neutral fuels.”

Gas turbines work by burning fuel with air to heat up pressurized gases, which expand through a nozzle to produce thrust. Relatively few components are required, and thus, power-to-weight and power-to-size ratios are very large. Fuel cells also oxidize fuel, but do it electrochemically, directly producing the electricity that drives electric motors to produce thrust. The fuel cell system requires more components to work, but because of the direct electrochemical conversion of fuel to electricity, it’s more efficient than a gas turbine.

“We have developed record-high power density fuel cells that put them closer to turbines, but with greater fuel efficiency for electric power production. This is a great opportunity to integrate the two technologies to achieve a hybrid solution that further advances the electrification of flight,” said Wachsman.

The combination makes sense from an operational perspective, the researchers said: Pilots sometimes need to adjust power quickly, for example during a go-around (or aborted landing). But fuel cells don’t easily respond to sudden changes, so a pilot confronted with a rapid-response situation might not be able to adjust the power in time.

“When you’re climbing and need a lot of power, the turbine will be making more power and the fuel cell will be making less,” Cadou said. “If you need to floor it or pull back quickly, the turbine can do it. When you’re in cruise, the turbine will be making less power and the fuel cell will be making more power. You’re able to better manage energy.”

The system incorporates new fuel cell technology from Wachsman’s group that achieves unprecedented levels of power-to-size and durability. It also incorporates regenerative use of electrical energy, with energy given up by the aircraft as it descends being stored for use later. The principle is familiar to anyone who has driven a Prius or a Tesla; when braking, kinetic energy is fed back into the electric motor, conserving energy.

During the initial two years of the project, the team will design the system, model its performance characteristics, and show that it will meet the target performance specifications. In the final two years, a prototype will be built and tested in a simulated flight cycle, taking advantage of the state-of-the-art facilities and resources available at UMD. 

Cadou is focusing on systems modelling, pressurized fuel cell testing and, later, manufacturing the full system; Wachsman, who directs the Maryland Energy Innovation Institute, will develop the fuel cell, working together with Associate Research Professor Yi-Lin Huang and the students in the Wachsman group. In addition, a fuel cell research group at the Colorado School of Mines will develop fuel cell reformer materials and architectures that will maximize the system’s power-to-weight ratio and overall reliability.

In keeping with ARPA-E’s push for industry-transforming research, the team’s goal is to develop a commercially viable technology that reach the market within the decade—and one day be recognized as a milestone. 

“My metric for success is to be able to walk into the Smithsonian Air and Space Museum in 25 or 30 years and see a hybrid turbine/fuel cell on display as an example of state-of-the-art aircraft engine technology,” Cadou said.