IMOTHEP assessed the benefit of hybridization on four different configurations of hybrid aircraft covering regional and short medium range (SMR) missions and representing different levels of disruption in aircraft design. This assessment was performed in close connection with the investigation and preliminary design of the electric components and architecture of the hybrid power train. From a first conceptual aircraft design, target specifications were defined for the architecture and components of the hybrid propulsion chain, triggering the investigation of technological solutions with a twenty-year timeframe perspective. The performance resulting from these analyses of the electric components and power chain were synthesized through two subsequent aircraft design loops, with an increasing level of fidelity, to assess the potential fuel burn reduction of the selected aircraft configurations, compared to conventional technologies extrapolated to 2035.
Designs were proposed for all the components of the propulsion chain, with performances that over-perform current state of the art and are generally close to the projections for 2035, using medium aggressive assumptions. Safe an operable electrical architecture could be designed for all the studied configurations. Solutions for power management scheme were also identified, covering various flight situations. Failure case analyses were performed and evidenced the need for increasing the reliability of electric components to avoid frequent failure occurrences in case of distributed propulsion. Thermal management of electrical machines also emerged as a key design issue for HEP power trains. Finally, two classes of systems emerge based on the involved power and the required voltage for power distribution. Regional aircraft involve electric machines with power up to one MW and seem feasible with 800 V electric distribution. Associated technologies are believed to reach TRL 6 by 2030. On the contrary, SMR aircraft require electric machine in the range from one to 10 MW and multi-kilovolt power distribution. Corresponding technologies are only believed to reach TRL 6 after 2035, pushing any SMR application to a longer term. Especially, electric wiring interconnection system (EWIS) appears as a critical challenge involving a disruption with current systems (540 V) and severe physical issues such as partial discharges or arcing.
Finally, when integrating all the power train's characteristics in the aircraft overall design and optimisation, none of the studied SMR configurations exhibited a benefit of hybridization compared to conventional technologies projected to 2025. Moreover, results from sensitivity analyses did not suggest that improving the performances of electric components beyond those reached by the design performed within IMOTHEP were likely to change significantly the conclusions. For regional aircraft, it was found that a purely turboelectric propulsion chain (i.e. using electricity produced on board the aircraft by turbomachinery) was not promising, while parallel hybrid, using electricity stored in batteries to assist thermal propulsion, could bring a fuel burn reduction in the order of 10%, but on short range only, typically 200 nm. The most promising configuration identified within IMOTHEP is a hybrid "plug-in" configuration, an aircraft flying in full electric mode over 200 nm and using a thermal range extender for 600 nm missions. Efficiency gain in block energy could reach 55% on 200 nm, while a 20% fuel burn reduction was possible on 600 nm.
