Air transport is facing an important challenge when it comes to the reduction of its environmental footprint. Hybrid electric aircraft offer a potential solution to reduce in-flight emissions. The essence of hybrid electric aircraft is to combine multiple energy sources and energy conversion systems, in order to improve the overall energy efficiency and synergies, which could result in reduced emissions. Conventional design methods for aircraft are based on a single energy source and gas turbines, at which power production and propulsion are combined. However, hybrid electric aircraft allow for multiple energy sources and a decoupling of the propulsive device from the power producing device. These two fundamental differences open a new design space for future aircraft. As conventional design methods are not able to cope with the extended design space of hybrid electric aircraft, the necessity arises for novel conceptual design methods. In the context of hybrid electric aircraft, the energy sources are considered to be a combination of electrochemical batteries and conventional
jet fuel in this thesis. One of the effects of increased electrification is the generation of heat, due to inefficiencies. As hybrid electric aircraft further increase the electric power demand, even more heat will be generated. It is expected that the emitted heat will affect the weight, drag, and performance of hybrid electric aircraft, due to additional systems to control and dissipate excessive heat. As high-power systems require cooling to prevent performance degradation and battery thermal runaways, an adequate thermal management system is needed to control the thermal behaviour of hybrid electric powertrain components. For this reason, a coupled design approach between the hybrid electric powertrain and the thermal management system should be applied. Moreover, the sizing of thermal management system components is based on the thermal behaviour of these powertrain components. Therefore, modelling of the thermal effects is needed to acquire improved insights in the performance characteristics of hybrid electric aircraft during the conceptual design phase. The objective of this thesis is to identify the impact of including a thermal management system on the power, weight, and volume usage for a parallel-hybrid electric ATR72-600 regional aircraft reference design case, that excluded the sizing of a cooling system. A sizing methodology was developed for this case study. The sizing includes only the electric part of the powertrain with a coupled thermal management system. The latter is a combination of a closed-loop liquid cooling system and an air cycle system to pre-cool ambient air. The combination of all components that are sized are described by the term "system". Furthermore, the same battery is used to power both the powertrain and the thermal management system. The validation of the defined system is performed at individual system component level, as no similar adequate reference data could be found in literature for this thermally-controlled electric propulsion system. The validation of the battery, electric motor, heat exchanger, and air cycle system is emphasised by comparing their performance behaviour to reference data. The system is used to assess its impact with near-term and mid-term component technology levels, at which it is sized for hot day conditions. Flight missions characteristics are taken from the reference design case and
used as input to capture the main sizing characteristics. The results indicate a low feasibility potential if near-term technology levels are used. A higher feasibility is obtained with mid-term technology levels. The battery is identified to be the most critical component in terms of power, weight, and volume usage, which are driven by three interacting mechanisms. These are the electric propulsion power, the required cooling power, and the thermal capacity as a function of the battery weight. Concluding, the significant impact of a thermal management system can not be neglected in conceptual design studies for hybrid electric regional aircraft.