*2.1.3. Distributed parallel system*

In view of these challenges and motivated by the search for more synergistic integration, an innovative approach to a parallel hybrid-electric propulsion system was proposed by Pornet and Isikveren [23] taking advantage of distributed propulsion technology (see Section 2.2.3). Instead of coupling the electric motor to the shaft of the gas-turbine, the electric motor is coupled directly to the shaft of the propulsor and the combination electric motor and propulsive device (called electric-fan) is integrated on the aircraft as an additional bill-of-material item to the conventional combustion based engines. Concrete aircraft concepts would be a tri-fan aircraft with two fans conventionally powered by gas-turbines 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 aero-propulsive-structural integration.

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 schematic representation of this topology is given in Figure 2.

## *2.1.4. Integrated system*

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 momentum.
