CONFÉRENCIERS > Conférence 6

Towards greener aircrafts: Multidisciplinary Design Optimization of hybrid electric propulsion system

Xavier ROBOAM, LAPLACE, University of Toulouse, France


The aviation sector currently accounts for around 2-3% of global CO2 emissions and a “flight shame” feeling is asserting itself in a portion of the population, especially among young people. Thus, Electrification is a key driver in this direction especially with hybrid and full electric architectures. Three main directions are on the road of hybrid electric aircrafts: 1. the thermal engine optimization thanks to the electric boost capabilities offered in hybrid architectures; 2. The aerodynamic optimization thanks to electric distributed propulsion capabilities; 3. The energy management optimization that will be the main focus of this keynote. But electrical technologies and electrified powertrains are heavier than thermal ones, while snowball effects are typical in aeronautic domain: ”more embedded weight in devices and systems means an increased wing surface also involving more fuel burn”. In the case of a regional flight (400 nm range), 1 additional ton would account for 6% more fuel burn. In that context, the ”hunt for kilos” is opened and huge level of power integration and energy efficiency is required to make electric solutions competitive. But if a high level of integration is required at technological device level, the ”system integration’’ optimizing main couplings at powertrain or if possible at aircraft level is a second major driver to lower the whole embedded weight. For that purpose, multidisciplinary design optimization (MDO) involving integrated devices through surrogate models is appropriate. In order to emphasize these issues, this keynote will describe a MDO process of the powertrain system for a series hybrid electric architecture applied to a regional flight mission. This powertrain associates gas turbines and electric (typically batteries or fuel cells) sources, a ”ultra HVDC” (beyond the kV) electric distribution with its power electronics (multilevel inverters at MW range) supplying a surface-mounted PMSM in order to maximize the specific power and efficiency of each device. These latter devices must be cooled by very efficient and compact thermal devices. This actuation system drives propellers sized for a typical regional aircraft beyond 5MW at the whole. This study has been achieved in the EU Cleansky II program, in the ‘’HASTECS’’ project (Hybrid Aircraft: reSearch on Thermal and Electric Components and Systems). 3 Research labs were associated to face these challenges: the Laplace and Cirimat in University of Toulouse and the P’ institute in ENSMA Poitiers. Very high levels of integration have been reached in HASTECS with specific powers higher than 30kW/kg for power electronics including high performance 2 phase cooling system and more than 10kW/kg for electric machine with its cooling and also fulfilling partial discharge tolerance. The main technological choices will be described for power electronics and actuation devices emphasizing the huge influence of the “thermal challenge”. A tradeoff between high density batteries and PEM fuel cell with hydrogen storage has been also assessed which the main results will be summarized. But system couplings between technological devices involve multi-physical aspects (electric, magnetic, mechanical, thermal, partial discharges and insulation, energy management and flight mission). Optimizing the whole powertrain by taking account of the snowball effects in this hybrid aircraft is a complex problem. A MDO process will be described aiming at reducing the objective functions with regard to a wide set of constraints. Some of these constraints are verified during a flight mission typical of a regional flight. During that virtual flight, sequence after sequence, an energy management system (EMS) strategy must also be proposed to share powers between both power sources (gas turbines and fuel cells). Several results will be presented showing, among other, the crucial sensitivity of the electro-thermal coupling. The DC bus distribution level constitutes a key coupling inside this power chain: the MDO formulation leads to an optimized tradeoff on this key parameter involving power sources, power electronics, cables, electric motors with partial discharges tolerant windings. Furthermore, ”unforeseen” systemic couplings are analyzed. To conclude, key figures in terms of both embedded weight and fuel burn are displayed for optimal solutions of the series hybrid electric power chain with reference to an optimized full thermal chain. Some prospects towards a ”zero emission aircraft” will close the presentation


Xavier Roboam received the Ph.D. Degree of ‘Université de Toulouse’, France in 1991. He is full-time CNRS researcher (Directeur de Recherches) in the Laboratory of Plasma and Conversion of electrical Energy (LAPLACE) of Université de Toulouse since 1992. He is currently the deputy Director of LAPLACE. He has been consultant expert for Airbus in the framework of “new architectures and integrated design of electrical networks and systems” during 10 years until 2016. He has published more than 300 references with 12 edited books. His current research interests include integrated optimal design of multi-field systems for transport and embedded networks (more electrical aircraft) or smart microgrids.

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