**5. Results and Discussion**

From the Monte Carlo simulation, the decomposed distribution of the possible flying range based upon the specific energy of batteries and the specific power of an electric motor is illustrated in Figure 1. In addition, the figure also provides numeric details from each scenario. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 5 of 5

**Figure 1.** Decomposed distribution of flying range for an eight-seat turboprop with varying technological improvements of the powertrain. **Figure 1.** Decomposed distribution of flying range for an eight-seat turboprop with varying technological improvements of the powertrain.

The three different color groupings denote the three levels of specific energy of batteries. The gradations of the colors within each group are indicative of the distinct levels of motor specific power. The numerical descriptives in the legend can be directly derived from the probability distribution figure. For example, the minimum value for sc1, 53, is The three different color groupings denote the three levels of specific energy of batteries. The gradations of the colors within each group are indicative of the distinct levels of motor specific power. The numerical descriptives in the legend can be directly derived

The first takeaway that one can be derived from the decomposition is that the specific energy of batteries is, indeed, a critical factor. Namely, any improvement in the specific energy produces a substantial gain in the number of extra kilometers flown. With only minor overlap, the three groups (of colors) appear distinctively partitioned on the graph,

Secondly, the effect of the increasing motor specific power is also substantial, but nonlinear. With the existing levels of battery specific energy, improving the motor provides only incremental benefits. However, with a higher specific energy of batteries, the effect of improving the motor becomes more and more pronounced. For example, one can readily observe the difference between the right edge of sc1 and sc2 (178 − 164 = 13) and sc7 and sc8 (571 − 531 = 40)—a veritable two-fold increased improvement in flight range. This occurs because the weight saved from using a motor with higher specific power provides more of a benefit if batteries also have higher specific energy. The consequences of this observation lead to an important implication. Although with the existing levels of battery specific energy, investing in electric-motor R&D might not look particularly beneficial, it will lead to significant, tangible, positive differences when conducted in simul-

thereby suggesting a strong influence from the underlying factor.

taneous conjunction with the development of battery technology.

from the probability distribution figure. For example, the minimum value for sc1, 53, is the leftmost edge of the whole distribution.

The first takeaway that one can be derived from the decomposition is that the specific energy of batteries is, indeed, a critical factor. Namely, any improvement in the specific energy produces a substantial gain in the number of extra kilometers flown. With only minor overlap, the three groups (of colors) appear distinctively partitioned on the graph, thereby suggesting a strong influence from the underlying factor.

Secondly, the effect of the increasing motor specific power is also substantial, but nonlinear. With the existing levels of battery specific energy, improving the motor provides only incremental benefits. However, with a higher specific energy of batteries, the effect of improving the motor becomes more and more pronounced. For example, one can readily observe the difference between the right edge of sc1 and sc2 (178 − 164 = 13) and sc7 and sc8 (571 − 531 = 40)—a veritable two-fold increased improvement in flight range. This occurs because the weight saved from using a motor with higher specific power provides more of a benefit if batteries also have higher specific energy. The consequences of this observation lead to an important implication. Although with the existing levels of battery specific energy, investing in electric-motor R&D might not look particularly beneficial, it will lead to significant, tangible, positive differences when conducted in simultaneous conjunction with the development of battery technology.

However, the impact from improving the electric motor, alone, is also nonlinear. Specifically, within the same level of battery specific energy (e.g., the on the horizon), the difference between the right edges of existing and under development motor specific power, sc7 and sc8 (571 − 531 = 40) is higher than the difference between under development and futuristic, sc8 and sc9 (594 − 571 = 24). Indeed, increasing the electric motor specific power from 5 to 10 kW/kg generates a 100% yield in specific power, and thus, the output flight range, while from 15 to 20 kW/kg, there is only a 33% increase. Although this linear function of relative specific power yield might be obvious for engineers, financial decisionmakers involved in investment planning may consider absolute values (km, €, etc.)—thus, the relation becomes nonlinear. Irrespective of the starting point, a five kW/kg increase in specific power may well imply similar levels of R&D investment costs. However, the benefit—and thus the pay-off of such an investment—will differ dramatically, depending on the specific power of the motor that has already been achieved. Consequently, the value of every "next step" in electric motor development should be weighed against its cost.

The three main conclusions from the SimDec decomposition can be summarized as:


Although our analysis focuses on the flying range, the derived distances directly translate into economic and environmental benefits. Any extended flying ranges directly correspond to an increased number of possible routes or pairs of towns that can be connected. In Europe, fewer than a hundred airports connect towns within a distance of 50 km between each other. However, if the inter-town distance is expanded to 500 km, nearly a thousand airports could be connected. Apart from the increased flying distance, the improved powertrain technology enables electrifying larger planes that can transport more passengers or larger loads per flight. Furthermore, fully electrified aircraft fleets would entirely eliminate the emissions from fuel combustion. The corresponding environmental impact would, therefore, be dependent upon which electricity is used to charge the batteries or, more specifically, from the specific combination of each respective country's actual power mix. Even so, there are still other life cycle emissions aspects that are associated with specific aircraft design and operations. More detailed life-cycle estimates of the environmental impacts of such air-traffic electrification are presented in [17].
