A Method for the Conceptual Design of Hybrid Electric Aircraft

As the aviation sector keeps expanding, a growing interest in technologies that can reduce the dependency from non-renewable energy sources, both for economic and environmental reasons, has led researchers to investigate the opportunities offered by the electrification of flight. It is theorized that the use of electrically powered machines as means of propulsion could reduce the emission impact of future aircraft through a combination of a higher energy conversion efficiency and synergies with the overall aircraft system. However, fully electric designs are not viable in the near and medium term as the performance characteristics of the electric devices are still not comparable with the achievements in terms of high specific weights of modern fuel-burning propulsive system. A proposed solution while the technology maturity level of the electric devices advances, is to electrify only a fraction of the aircraft system. The use of two energy sources in the propulsive system opens the design space and allows for the experimentation with novel aircraft configurations that could lead to interesting fuel and energy consumption reductions. However, established methods for aircraft design become obsolete as the complex configurations and control strategies cannot be modelled. Without fully taking advantage of the degrees of freedom introduced by the hybridization of the propulsive system, it is expected that little improvements will be seen as the resulting designs would not differ much from the modern aircraft. The objective of this thesis is to develop a conceptual design procedure that can be applied to size and analyse any hybrid electric architecture using electro-chemical batteries as the electric energy source. A conceptual design method needs to be simple and adaptable so that a wide range of designs with morphologically different propulsive system architectures can be quickly compared and improved. The proposed approach distils the main components required in a hybrid electric architecture into a base building block that can be used to model an arbitrarily complex system using few control parameters. The control strategy determines the power flows inside the propulsive system contemporaneously defining the type of architecture and allowing the estimation of power and energy requirements. The electric and conventional devices are sized with these requirements using low order methods that can be applied to a wide range of power ratings. The impact of the hybridization is considered both in the energy consumption estimation and in the evaluation of point performance constraints. The proposed method is validated in two steps. First, the performance estimations derived from the main components models are compared to published data or higher order method results. Discrepancies and limitations are discussed and solutions to improve the accuracy are proposed. Then, a conventional twin-engine turboprop aircraft is designed and the results are compared to assess if the integrated approach that puts the constraint analysis in communication with the point mass estimations and sizing procedures leads to valid results. The method is then used for a design space exploration exercise where three different hybrid electric powertrain architectures are studied for varying technology maturity levels and mission control strategies. The most promising propulsive system operational strategy is then selected for each design and studied in more depth. The sensitivity of each aircraft to the main design parameters is then assessed in the final study. The results indicate that the electrification of the propulsive system can lead to fuel and energy consumption reductions if the battery specific energy improves beyond 500 Wh/kg, a value already double of what is possible with today's technologies. The improvement of the other devices such as electric motors and electric power distribution sub-system is less critical but still necessary if large fuel load reductions are to be accomplished. Finally, the full potential of the hybrid electric powertrain can be achieved only by taking advantage of synergies with the other aircraft disciplines and by carefully selecting an appropriate control strategy.