**4.3. Hybrid-electric clean sheet design**

A hybrid-electric clean sheet design was investigated on a twin engine narrow body transport aircraft by Pornet et al. [22]. The topological approach of the hybrid-electric propulsion system is identical to the one presented in Section 4.2. Driving an electric motor on the low-pressure shaft has strong influences on the gas-turbine operational characteristics as highlighted in Section 2.1.2. These aspects become more predominant with increasing electric motor power. In order to not modify the contemporary design heuristics of the gas-turbine and to avoid the negative operational influence of the electric motor, the operational strategy selected in this design was to switch-off the gas-turbine during cruise while the electric motor drives by itself the shaft of the propulsor. By equipping only one gas-turbine with an electric motor, the useful degree-of-hybridization Hp*use* achieved during cruise is 50%. The other segments of the mission are performed conventionally using the two gas-turbines. By comparing the integrating performance of the hybrid-electric transport aircraft sized for interval design ranges between 500 nm (926 km) and 2100 nm (3889 km) against a suitably projected conventional twin-engine narrow-body aircraft, the prospects in terms of block fuel reduction, change in aircraft size and in vehicular efficiency were investigated for different range applications. In addition, the effect of the battery technology level was assessed for gravimetric specific energy at cell-level varying between 750 Wh/kg and 1500 Wh/kg.

The analysis of the relative change in block fuel versus the relative change in block ESAR is illustrated in Figure 5. When assuming a battery gravimetric specific energy of 1500 Wh/kg, the highest potential block fuel burn reduction of 20% was achieved at a design range of 1100 nm (2037 km). Block fuel reduction is achieved due to the utilization of the electrical energy and because of the increase in overall propulsion system efficiency resulting from the use of the highly efficient electrical system. An improvement in overall propulsion system efficiency of 30% during cruise was evaluated. At 1100 nm (2037 km), around a neutral change in vehicular efficiency was reached compared to an advanced gas-turbine only reference aircraft. It was observed that for increasing design ranges the potential inblock fuel reduction decreases, however, the vehicular efï ˇn ˛Aciency is signiï ˇn ˛Acantly diminished. In other words, the hybrid-electric transport aircraft requires more energy than the reference aircraft for the same transport task. This trend is explained by the increasing electrical energy demand during cruise, which results in increasing the total battery mass required. The resulting sizing cascade effects, leading to a large increase in MTOW as indicated in Figure 6, are the main cause of the block ESAR reduction. At 1100 nm (2037 km), the MTOW of the hybrid-electric aircraft is increased by 25% compared to an advanced gas-turbine only aircraft. While remaining energy neutral, the block fuel potential reduces for lower design ranges. It is explained by the fact that less electrical energy is used during the reduced portion of the cruise and consequently less block fuel reduction can be achieved. Because of the less pronounced sizing effects and the improvement in overall propulsive efficiency, the total energy consumption remains about neutral. When evaluating hybrid-electric aircraft, it is important to focus not only on fuel burn reduction but to consider also the overall energy consumption. The consumption of electrical energy will affect the operating cost of the hybrid aircraft with respect to the fluctuation of the electric energy price and moreover as the electrical energy might certainly not be produced only through renewable energy sources, its production will impact any carbon life cycle assessments that are undertaken.

Identical trends were observed for lower battery technology levels. However, a lower battery gravimetric specific energy results, for the same amount of energy required, in higher battery mass requirement, which considerably amplifies the sizing cascading effects. It leads consequently in a diminution of the potential fuel burn reduction and in a stronger degradation of the vehicular efficiency. Moreover, the design range at which the largest fuel consumption reduction occurs is reduced to 750 nm (1389 km) and to 900 nm (1667 km) at a specific energy of 750 Wh/kg and 1000 Wh/kg respectively. At these points the block fuel reduction is 9% and 14% and the change in block ESAR is -7% and -4% respectively. In these regards, the potential segment application for this concept turns out to be the regional market. In the context of this investigated concept, a battery technology level of at least 1000 Wh/kg should be reached to enable significant emissions reduction.

The integration of annexed technologies including for instance aerodynamic tailoring technology and flexible, adaptive structures [3] and the consideration of novel aircraft morphologies could lead to improvement in vehicular efficiency. However, the purpose of

**Figure 5.** Relative change in block fuel and in block ESAR for a Hp*use* of 50% in cruise [22]

The other segments of the mission are performed conventionally using the two gas-turbines. By comparing the integrating performance of the hybrid-electric transport aircraft sized for interval design ranges between 500 nm (926 km) and 2100 nm (3889 km) against a suitably projected conventional twin-engine narrow-body aircraft, the prospects in terms of block fuel reduction, change in aircraft size and in vehicular efficiency were investigated for different range applications. In addition, the effect of the battery technology level was assessed for gravimetric specific energy at cell-level varying between 750 Wh/kg and 1500 Wh/kg.

128 New Applications of Electric Drives

The analysis of the relative change in block fuel versus the relative change in block ESAR is illustrated in Figure 5. When assuming a battery gravimetric specific energy of 1500 Wh/kg, the highest potential block fuel burn reduction of 20% was achieved at a design range of 1100 nm (2037 km). Block fuel reduction is achieved due to the utilization of the electrical energy and because of the increase in overall propulsion system efficiency resulting from the use of the highly efficient electrical system. An improvement in overall propulsion system efficiency of 30% during cruise was evaluated. At 1100 nm (2037 km), around a neutral change in vehicular efficiency was reached compared to an advanced gas-turbine only reference aircraft. It was observed that for increasing design ranges the potential inblock fuel reduction decreases, however, the vehicular efï ˇn ˛Aciency is signiï ˇn ˛Acantly diminished. In other words, the hybrid-electric transport aircraft requires more energy than the reference aircraft for the same transport task. This trend is explained by the increasing electrical energy demand during cruise, which results in increasing the total battery mass required. The resulting sizing cascade effects, leading to a large increase in MTOW as indicated in Figure 6, are the main cause of the block ESAR reduction. At 1100 nm (2037 km), the MTOW of the hybrid-electric aircraft is increased by 25% compared to an advanced gas-turbine only aircraft. While remaining energy neutral, the block fuel potential reduces for lower design ranges. It is explained by the fact that less electrical energy is used during the reduced portion of the cruise and consequently less block fuel reduction can be achieved. Because of the less pronounced sizing effects and the improvement in overall propulsive efficiency, the total energy consumption remains about neutral. When evaluating hybrid-electric aircraft, it is important to focus not only on fuel burn reduction but to consider also the overall energy consumption. The consumption of electrical energy will affect the operating cost of the hybrid aircraft with respect to the fluctuation of the electric energy price and moreover as the electrical energy might certainly not be produced only through renewable energy sources,

its production will impact any carbon life cycle assessments that are undertaken.

Wh/kg should be reached to enable significant emissions reduction.

Identical trends were observed for lower battery technology levels. However, a lower battery gravimetric specific energy results, for the same amount of energy required, in higher battery mass requirement, which considerably amplifies the sizing cascading effects. It leads consequently in a diminution of the potential fuel burn reduction and in a stronger degradation of the vehicular efficiency. Moreover, the design range at which the largest fuel consumption reduction occurs is reduced to 750 nm (1389 km) and to 900 nm (1667 km) at a specific energy of 750 Wh/kg and 1000 Wh/kg respectively. At these points the block fuel reduction is 9% and 14% and the change in block ESAR is -7% and -4% respectively. In these regards, the potential segment application for this concept turns out to be the regional market. In the context of this investigated concept, a battery technology level of at least 1000

The integration of annexed technologies including for instance aerodynamic tailoring technology and flexible, adaptive structures [3] and the consideration of novel aircraft morphologies could lead to improvement in vehicular efficiency. However, the purpose of

**Figure 6.** Relative change in MTOW for a Hp*use* of 50% in cruise [22]

this study was to capture a true potential resulting from the integration of hybrid-electric propulsion system compared to gas-turbine only aircraft.

Another aspect investigated in this study is the volumetric constraint for the housing of the battery within the cargo of the fuselage. According to an assumed density of 1000 kg/m3 including the volume of the battery, of the thermal management and of the housing, the provision of a standard cargo volume per PAX for regional aircraft of 0.14 m<sup>3</sup> is indicated by a triangle in the Figures 5 and 6. For instance, assuming a battery specific energy of 1000 Wh/kg, the concept is volumetrically constrained for design ranges above 1100 nm (2037 km). Possible evolution of the fuselage geometry, with minor-to-modest aerodynamic and mass penalties, towards double-bubble cross-section could be conceived to free up the design space from this volumetric limitation.

The analysis of this clean sheet design reveals shifts in aircraft design paradigm due to the nature of the hybrid-electric propulsion system as fuel burn reduction can be achieved while the MTOW of the aircraft is increased and moreover fuel burn reduction does not mean automatically an improvement in vehicular efficiency.

While this investigation was aimed to get first insights into the sizing impact of hybrid-electric propulsion system at aircraft level, the full benefit of hybrid-electric technology will be reached through holistic integration of the propulsion system at aircraft level. As defended by Moore and Fredericks [54], the full potential of hybrid-electric aircraft will be demonstrated only once the synergistic benefits from the integration of the hybrid-electric propulsion system at aircraft level are fully understood. In this regard, an innovative hybrid-electric approach is proposed in the next section with the assessment of a distributed hybrid-electric clean sheet design.
