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Lightweight and efficient power conversion (including inverters and rectifiers) is an evolving research area. A detailed discussion of the operating principles of power converters is beyond the scope of this review, but the major design choices being ex- plored include semiconducting materials and cooling systems. Jansen et al. provides an up-to-date summary of NASA-funded power conversion research [27]. Armstrong et al. [92] present a table summarizing estimated specific power and technology readi- ness of many kinds of electrical components. For megawatt-scale converters, NASA envisions pi/r ratings between 19–26 kW/kg and efficiencies above 99% [27]. The NAE committee projects pi/r of about 9 kW/kg by the N+3 generation [10]. At high power levels, large amounts of waste heat are generated with even highly- efficient conventional electronics. Freeman et al. pointed out the memorable fact that a half-megawatt motor operating at a state-of-the-art 95% ηm produces as much waste heat as a barbecue grill [119]; therefore, superconducting motor/generators, power electronics, and conductors have been proposed as a way to raise efficiency and greatly reduce the thermal management problems introduced by megawatt-scale electric propulsion systems. Superconducting materials exhibit zero resistance at low operating temperatures, greatly reducing or eliminating Joule heating. For example, superconducting stator windings in motors and generators may improve efficiency sub- stantially but incur penalties for other reasons (such as eddy current losses and fault currents) [90, 95]. Conceptual trade studies so far have favored superconducting ar- chitectures for very high-power applications (such as the 300-passenger NASA N3-X) and conventional conductors for megawatt-class requirements and below (such as the 150-passenger NASA STARC-ABL) [56, 27]. We discuss thermal management in more detail in Section 6. 5.2 Batteries An electrochemical cell “converts stored chemical energy into electrical energy via the energy difference between the reactions occurring at the two electrodes” [135]. A battery is a practical electrical energy storage device consisting of one or more cells connected in series and/or parallel in order to provide desired output voltage, capacity, and power. An important parameter in the design and operation of battery-powered devices is the C-rate [136]: ζ(t)= I(t), Cnom where I(t) is the charge or discharge current (in A) and Cnom is the nominal charge capacity of the battery (in Ah). At a 1C discharge rate, the battery will be fully dis- charged from nominal capacity in one hour. The rated maximum power of a battery is directly proportional to the maximum C-rate. A high C-rate is desirable from an operational perspective, as it enables rapid recharging (and less downtime). Two re- lated metrics are the state of charge (SOC), which measures the percentage of charge capacity remaining, and the depth of discharge (DOD) which is simply 1 − SOC [136]. Each battery design has a characteristic voltage profile as a function of DOD. For lithium-ion batteries, voltage decreases slowly until a precipitous drop at about 90% DOD. Discharging past a threshold DOD (specific to the battery design) can cause 28PDF Image | Electric, hybrid, and turboelectric fixed-wing aircraft
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