**2.2. Enabling technologies for electric drive application to transport aircraft**

Technologies considered as key enablers for the deployment of electric drives to the propulsion system of transport aircraft are discussed in this section. The application of the field of superconductivity to electric propulsion system is first reviewed followed by a discussion about distributed propulsion technology.

### *2.2.1. Energy storage*

Technology improvement of the electrical components discussed in Section 2 in terms of gravimetric specific power and efficiency are of particular importance for the application

**Figure 2.** Distributed parallel hybrid-electric topology for distributed fans arrangement

while the remaining fan is driven by an electric motor (or vice versa with two electric fans and one turbofan aft-fuselage mounted), or, as a quad-fan aircraft equipped with two turbofans and two electric fans. The integrated prospects of this latest concept are the subject of the investigation presented in Section 4.4. This approach offers numerous advantages compared to mounting the electric motor on the low-pressure shaft of the gas-turbine. As the conventional system is decoupled from the electrical system, design and operation of the conventional and electrical system are independent. As a result, contemporary design and off-design heuristics of the gas-turbine are not perturbed by the introduction of the electrical system. Moreover, it reduces the system complexity and clears out the integration challenges of the electric motor in the environment of a gas-turbine. By thoughtful sizing and operational strategy of the gas-turbine and the electric motor, the efficiency of the hybrid propulsion system can be optimized by running the conventional and electrical system close to their peak efficiencies. This innovative parallel hybrid arrangement marries up perfectly with distributed propulsion technology opening potentials for tightly

While in the architecture investigated in [23], the electric motors are powered solely by batteries, a further topological evolution of the distributed propulsion system can be conceived with the introduction of a turboelectric approach. By equipping the gas-turbine with a generator, additional electric power can be transmitted to the electric motors. This system approach reduces the technological level requirement imposed to the battery in terms of gravimetric specific energy while enabling significant increase in system efficiency when using the highly efficient battery system for propulsion. This topology is also interesting as it enables the possible combination of charge sustaining and charge depleting strategies of the batteries for optimum energy management [24, 25]. Charge sustaining strategy, recharging the battery with the generator utilizing excess power of the gas-turbine during segments of the mission, would reduce the integrated battery pack mass and volume requirement. A

While not illustrated in Figure 1, another recent approach for hybrid-electric system to be considered is the so-called integrated system [26, 27] which consists of electrifying part of the core cycle of a gas-turbine. A possible configuration for hybrid-electric integrated system was proposed by Schmitz and Hornung [28] investigating the electrification of the high-pressure compressor stages of a gas-turbine. Still in a pioneering phase, few publications are currently available on this topic but it is definitely an application to follow closely as it gathers

**2.2. Enabling technologies for electric drive application to transport aircraft**

Technologies considered as key enablers for the deployment of electric drives to the propulsion system of transport aircraft are discussed in this section. The application of the field of superconductivity to electric propulsion system is first reviewed followed by a

Technology improvement of the electrical components discussed in Section 2 in terms of gravimetric specific power and efficiency are of particular importance for the application

aero-propulsive-structural integration.

*2.1.4. Integrated system*

120 New Applications of Electric Drives

momentum.

*2.2.1. Energy storage*

schematic representation of this topology is given in Figure 2.

discussion about distributed propulsion technology.

of electric drive to transport aircraft. But most importantly, the evolution of energy storage capability, in particular, the progress made in battery technology is of great relevance for establishing the competitiveness of hybrid-electric and universally-electric aircraft. Gravimetric specific power (the amount of power delivered per unit of mass) and gravimetric specific energy (amount of energy contained per unit of mass) are the key metrics for evaluating the battery application. These parameters are not independent characteristics and they are determined during battery design [29]. As a result, the combined enhancement of the specific energy and the specific power turns out to be a greater technological challenge than the single criterion optimization. Additional aspects such as gravimetric specific energy density, efficiency, design service goal, operating temperature, discharge behavior, safety, reliability and environmental sustainability need also to be considered when selecting the battery technology [30].

Lithium battery technology is regarded as a promising option for application in aviation. State-of-the-art lithium batteries reach specific energy of circa 200 Wh/kg at cell-level [31]. Extensive research focusing on improving the cathode and anode materials as well as on enhancing electrolyte properties [30, 32] aim to scale up the power and energy capability of batteries for aircraft application. Specific energy of around 400 Wh/kg is predicted to be achieved at cell-level through future progress in lithium battery technology [33].

Breakthrough in battery technology could be achieved through the development of innovative battery concept as indicated by open battery systems like zinc-air, aluminium-air and lithium-air [32, 34]. Lithium-air batteries are considered with attention for aircraft application with estimated theoretical gravimetric specific energy from 1000 Wh/kg [33, 35] up to 2000 Wh/kg [36] at cell-level. Not currently commercially available, market readiness of lithium-air is expected for a time-line horizon of 2030 [37].
