New turbogenerators emerge for hybrid-electric power sector

Tests of the high-power, high-voltage system included two sets of electric motors-generators, power converters, power transmission and power control systems, which successfully demonstrated performance and operation of the components in a replicated flight environment. The trials “proved that it can work reliably at altitude,” Hegeman says.

Operational modes evaluated in the facility included power transfer from side to side, power assist to a simulated engine and aircraft power generation. The implementation of energy storage was also simulated. “The energy flows in different directions,” Hegeman says. “You can use it to run the motor for the electric assist, or you can drive the motor to generate that electricity to flow it into the battery. So it’s back and forth, and there are transients there as well. So all those situations have been tested.”

“We’ll then do ground testing, where the actual CT7, the electric motor and the power electronics are sitting in the same nacelle on a test stand at Peebles,” Hegeman says, referring to the company’s outdoor test facility in Ohio. “Then it goes on the aircraft.”

For this phase, GE has partnered with Boeing and its subsidiary Aurora Flight Sciences, which is providing the aircraft, modification work, aircraft integration and flight testing. This work includes nacelle manufacturing, flight deck interface design and software, aircraft-level performance analysis and systems integration. BAE Systems will provide energy management components, including the battery and cabling. Flight tests are expected to begin in the mid-2020s.

As part of its rapidly expanding electric-propulsion portfolio, Rolls-Royce is also developing an all-new turbogenerator for use in advanced air mobility applications, including primary propulsion for eSTOL aircraft and as a range extender for eVTOL vehicles. The new turbogenerator could significantly augment the performance of the P-Volt all-electric version of the eight-passenger P2012 Traveller, which Rolls is developing with Italian general aviation company Tecnam.

With two 320-kW electric motors and an 1,100-kg (2,430-lb.) battery pack replacing a pair of 275-kW piston engines, the P-Volt is expected to fly 85 nm when it enters service in the second half of this decade, increasing to 145 nm by 2030 as battery technology improves. With the addition of a turbogenerator, however, the aircraft’s range could be extended to around 300 nm, says Matheu Parr, customer business director at Rolls-Royce Electrical.

The scalable system is therefore intended to complement the company’s electric-propulsion offerings—enabling extended range on sustainable aviation fuels (SAF), and later as it comes available, through hydrogen combustion. The new, small gas turbine at the heart of the turbogenerator is under development at Rolls-Royce’s Dahlewitz facility in Germany, where it is expected to begin running by the end of the year, Parr says.

Although Rolls originally looked at using the company’s Allison-designed M250 turbine as the basis for the turbogenerator, it later revised the plan to go with a clean-sheet configuration incorporating a two-stage axial compressor and advanced materials. “When we look at the turbogenerator market at the moment, it  [consists of] repurposed auxiliary power units and helicopter gas turbines,” Parr says. “Given what the market demands in terms of fuel efficiency and power density, we see a strong opportunity for a new turbogenerator. It’s very much a clean sheet, because when you have the opportunity to design a turbogenerator for the first time. How do you make that as efficiently as possible?”

Instead of Rolls’ usual focus on lower specific fuel consumption, the chief design drivers are low overall maintenance and operating costs while simultaneously achieving a high power density. “We’re demonstrating a core at 780 kW, with a target power density at that rating of around 4 kW/kg,” Parr says. “The gas turbine is about 1 m (3.3 ft.) long, plus an electrical machine of about 0.5 m on the end of that, which is turning that mechanical power into electrical power. We are demonstrating it close to 800 kW, because that gives us all the kind of technology challenges we need. And we are working with customers to understand what is the right value that you need for a product.”

Rolls says the turbogenerator system will be scaled to serve a power range between 500 kW and two megawatts. The company says this will allow it to provide power systems for aircraft on “new longer routes that our electric battery-powered aircraft can also support.” The turbogenerator is targeted for a service entry date around 2027.