France's Turbotech plans to flight test its under-development turbogenerator later this year. It is offering the technology to developers of new hybrid-electric aircraft.

While some hybrid-electric aircraft systems under development use an engine connected to a generator through a gearbox of some sort, Turbotech has developed a compact all-in-one unit it describes as a “game-changer.” This uses a microturbine, a generator, and a heat exchanger, all on the same shaft that spins at 9,000 rpm providing it with a high power-to-weight ratio.

According to sales director Marie-Claire Camin, the shortcomings of today’s fully-electric vehicles, such as short-range and low payload, weigh against the benefits of having zero carbon dioxide emissions and reduced noise. While hybrid propulsion systems can increase the range to that of conventionally powered aircraft there are tradeoffs.

“Hybrid-electric is the best available solution right now,” Camin told last week's Vertical Flight Society Electric VTOL Symposium, noting that, “the hybrid-electric architecture has to be optimized in order not to waste the benefits which are gained by using it.

According to Camin, the turbogenerator, which has been bench tested for the past two years, can be used in flight as either supplemental propulsion to extend range, or more importantly to charge the batteries in flight. It will be available in either a 55 kW or 90 kW output version that is approximately 39 inches long (just under one meter), 16 inches in diameter (40 cm), and weighs between 121 and 141 pounds (55 to 64 kg, depending on the model).

Camin equated the weight of the 55 kW unit along with another 110 pounds (50 kg) of fuel as having comparable energy to one ton of batteries. While the turbogenerator is not intended to replace aircraft batteries it can ensure they never fall below a certain level of charge and can prevent full discharge cycles which can limit a battery’s lifespan. “It just means it’s a powerful enabler for electrical propulsion,” she said. “It's a powerful system to add to a battery pack.”

The unit can burn a variety of fuels including conventional Jet A, sustainable fuels, and natural gas. Turbotech is working to modify it to burn hydrogen which would result in zero carbon emissions.

The company claims the turbogenerator’s design, which incorporates a regenerative cycle, has improved fuel efficiency. Hot exhaust gasses pass through the heat exchanger which preheats the air before it enters the combustion chamber, reducing fuel usage. Once the exhaust gasses are finally expelled, the temperature has been reduced by more than half, from 700 degrees C to 300 degrees. In addition, the heat exchanger also acts as a muffler, reducing the noise of the operating unit.

Camin declined to comment further on the flight test schedule citing confidentiality agreements with partners but stated that a turboprop-based system will also fly this year.

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