Big steps toward high-performance hybrid propulsion technologies in next-generation aircraft
Merging expertise in electrical, chemical and thermal engineering, the EU-funded HASTECS project assessed and optimised architectures for hybrid propulsion – an evolving technology that could yield many reductions in CO2 emissions in future aircraft. As in hybrid cars, the technology supplements traditional fuel-burning internal combustion engines with electric motors that use energy from auxiliary power sources such as batteries or fuel cells. Despite their promising potential, electrical systems require advances in power electronics to handle the ever-increasing loads and need to dissipate the heat created by losses within the electrical power chain. Furthermore, the additional weight and fuel consumption introduced by the extra electric components need to be offset by very high-energy performance systems. “Focusing on regional aircraft designed to fly up to 70 passengers on short-haul routes, we developed models and designed tools to optimise hybrid propulsion. Our activities ranged across the whole spectrum of hybrid electric power chain: electric machines, cooling, power electronics and thermal management,” notes Xavier Roboam, research director at the LAPLACE laboratory at the University of Toulouse in France and HASTECS coordinator.
High-performance electric motors
LAPLACE researchers unveiled electric motors with high specific powers that exceeded 11 kW/kg (including the cooling device). The high electrical efficiencies reported (over 97 %) are attributed to the high-performance permanent magnet synchronous motor equipped with a Halbach array. Special windings and ultra-thin magnetic sheets that limit high-frequency losses were also crucial for obtaining such excellent efficiencies. High electromechanical conversion efficiencies need to go hand in hand with equally powerful cooling systems. To this end, researchers from the Prime Institute used a glycol–water mixture as a coolant for the rotor and the stator and added cooling pipes within the stator slots to achieve even higher specific powers. “Achieving specific powers of over 10 kW/kg in aircraft motors is a real breakthrough. Electric vehicle performances pale in comparison. The motor of Tesla electric vehicles, together with its cooling system, displays a specific power of less than 4 kW/kg,” explains Roboam.
Optimising power electronics, cooling and high-voltage distribution
Impressive work was also conducted by the LAPLACE researchers in the control and conversion of electric power. The joint use of a high-voltage bus (2 kV) with optimised mechanical structure, the best electronic components 7th generation insulated gate bipolar transistors and modulation strategies optimised for various multilevel converters proved to be particularly effective in designing compact and efficient power electronics systems. The Prime Institute, together with LAPLACE Lab researchers, also proposed a high-performance pumped two-phase cooling system. “Inside the power electronic system, 1 kg of the cooling device can remove 4.5 kW of heat losses, allowing the specific power of the power conversion system to exceed 30 kW/kg, well beyond the ambitious targets set by the aeronautical industry,” notes Roboam. “In the effort to raise the capacity of electric power sources by researching unprecedented voltage levels, a particular challenge has to do with partial discharges,” explains Roboam. This phenomenon, caused by the fatigue in insulating materials exposed to high voltage levels, degrades equipment and may also cause power loss. To counter this, the LAPLACE researchers proposed the use of more resistant (partial discharge tolerant) insulating materials compatible with voltage buses whose voltage ranges between 1.5 and 2 kV. The optimisation of the overall powertrain has completed the studies conducted by HASTECS, highlighting unexpected interactions. HASTECS has been a real technological and scientific success, demonstrating on-board power component designs with high specific powers. “Our groundbreaking work paves the way for the first practical powertrains in future hybrid electric aircraft that are more respectful of the environment,” concludes Roboam.
Keywords
HASTECS, cooling, specific power, hybrid propulsion, power electronics, electric motor, partial discharges, LAPLACE laboratory, University of Toulouse, high-voltage distribution
