*2.1.2. Parallel system*

**Figure 1.** Conventional, hybrid and universally-electric propulsion system topology [5]

The topological options for the application of electric drives to the propulsion system of transport aircraft are detailed in the following sections. The discussion covers serial, parallel

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

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

**2.1. Topological options for electric drive application**

as well as integrated arrangements.

integration (see Section 2.2.3).

*2.1.1. Serial system*

118 New Applications of Electric Drives

A parallel system is characterized by mechanical nodes that connect the different systems. The most common parallel approach is the installation of an electric motor on the low-pressure shaft of a gas-turbine in order to support the operations of the gas-turbine or even drive by itself the propulsor device during segments of the mission [19–22]. Because of the benefit of utilizing battery on the overall propulsion system efficiency, the electric motor is commonly powered by batteries but the utilization of fuel cells is also conceivable. However, it was found that driving simultaneously the shaft of the gas-turbine by an electric motor influences dramatically the operation of the gas-turbine. The simultaneous operation of the electric motor forced notably the gas-turbine to operate into part-load impairing its efficiency. Moreover, due to the modification of the operating line of the gas-turbine components, the margin to surge might also become critical. Practical engineering solutions need to be envisioned for the integration of the electric motor in the environment of the gas-turbine. Parallel integration of an electric motor on the low-pressure shaft could consequently disrupt contemporary design axioms of gas-turbines.
