*2.1.1. Serial system*

A general characterization of a serial system is given by the electrical nature of the node connecting the different systems composing the propulsion power-train. The most common serial arrangement is known as turboelectric [9–12]. It denotes a serial system where electric power is produced by a generator driven by a combustion engine typically a gas-turbine. Efficiency improvement in the propulsion system results notably from the advantage that the gas-turbine operation is now decoupled from the operational constraints of the propulsor [9, 13]. The system efficiency and mass can consequently be optimized by operating the gas-turbine and the propulsor close to their peak efficiency. However, because of the greater bill-of-material due to the additional electrical components, in the propulsion chain, the weight of the system is expected to increase compared to a conventional propulsion system [9]. In order to make this approach viable at aircraft level, this penalizing aspect needs to be overcome by any improvements in system efficiency and/or structural-aero-propulsive integration (see Section 2.2.3).

A serial system using fuel-cells as means of electrical energy generation can be entertained. The efficiency of advanced fuel cells including the balance-of-plant is forecast to reach efficiency levels comparable to that of advanced gas-turbines. However, the power density of fuel cell stack is expected to remain much lower than that of a gas-turbine [14]. The weight penalty might consequently outweigh potential benefits. The unique utilization of fuel cells for providing the power requirement for the propulsion of transport aircraft remains consequently challenging [15].

A serial system whose electrical energy providers are batteries solely was dubbed as universally-electric architecture [4]. The clear advantage of utilizing battery in a propulsion system is the efficiency. With values remaining above 90% during a complete mission profile [16] significant improvement in overall propulsion system efficiency can be achieved compared to a conventional system. However, the gravimetric specific energy (the amount of energy content per unit mass) of advanced batteries is expected to remain relatively low, with a factor of about 8 compared to fossil fuel based on a complete system exergy analysis [17]. Consequently, the weight of the battery and its detrimental sizing cascading impacts on the overall aircraft penalizes the efficiency benefit.

As a result of all these considerations, serial arrangement combining for instance a turboelectric system with battery and/or fuel cell system are considered [14] to draw the advantages of each of the system and to create system synergies in order to achieve greater system performance to the detriment of a higher system complexity. This is notably the approach undertaken by Airbus with the E-Thrust concept [18], which combines a turboelectric system with batteries. Innovative, synergistic integrated serial systems need to be further investigated at aircraft level in order to assess the full potential of hybrid-electric serial propulsion system.
