IEEE Electrification Magazine - June 2020 - 24

nonlinear; for example, the largest proportional increase
in aircraft range is produced with the most immediate
improvements in battery specific energy.
Regarding energy storage, battery systems are not the only
medium that can be utilized to provide electrical power. As
mentioned previously, turboelectric configurations have been
proposed to take advantage of the dense chemical energy
found in hydrocarbon fuels, which can be converted to electrical power through a turboshaft-driven electric generator.
Other gaseous or liquid fuels can also be used, due to their
relatively high specific energy, and converted into electrical
power through other means. For these
cases, fuel cells are often used due to
their high electrochemical conversion
efficiency, which varies as a function of
output power (current density).
For example, the peak conversion
efficiency of modern proton exchange
membrane fuel cells operating on
hydrogen have reached values of
approximately 60-70%. While these
systems are canonically envisioned for
use with hydrogen, some fuel cell systems are also compatible with other
energy carriers, such as methanol,
ammonia, ethanol, and even some
heavier hydrocarbons. Some of the
disadvantages of fuel cell systems, though, are the high
weight these power conversion devices have and the significant cost of certain materials (e.g., platinum) often used in
their production. Rapid improvements have been made in
both of these areas in recent history, but further development is still necessary before the integration benefit of fuel
cells becomes clear.
Certain types of fuel cells may also have long startup
times and generally are unable to respond to rapid changes in power demand. For this reason, many fuel cell architectures are designed with a supplementary battery and/
or ultracapacitor system, both to handle dynamic changes
in power and shave peak power demands during takeoff
and climb, permitting reductions in the required weight/
sizing of fuel cell stacks.
The aforementioned limited availability of high-power,
flight-weight motors and generators has been met with a
recognized need for improved rated power and specific
power of electric machines. The Boeing 787 defines the
current technology in aviation-rated electrical machines
with four 250-kW electric generators, each with a specific
power of 2.2 kW/kg. The electrical power produced by
these generators is not intended for propulsion but, rather,
the auxiliary systems and hotel loads on the aircraft.
These capabilities are quite far from the multiple megawatt-class electric machines that are required to meet the
demands for aircraft propulsion with suitable specific
power above roughly 6.5 kW/kg. To address this need,
researchers are turning to novel machine architectures

and topologies. For particularly high-power applications,
superconducting machine systems are being considered
to meet aggressive weight and size constraints.
In a similar fashion, high-power, lightweight, high-efficiency power electronic converters are also required for the
motor drives and energy management system. Systems of
varying levels of power electronics integration have been
explored. In one extreme, the frequency is controlled at the
generator, and power is distributed at the same frequency to
multiple electric propulsors. At the other extreme, rectified
power from generators or energy storage, or a combination of
both, is processed with an energy management system and distributed in dc
form to multiple inverters that independently drive individual propulsors.
Intermediate solutions are also
possible with a few inverters driving a
collection of motors. In vehicles with
ac distribution, doubly fed induction
machines could be employed to re--
duce the rating of the power converter,
especially if only a limited range of fan
speed is needed. With the rapid ad--
vance in power electronics technology,
especially with wide-bandgap devices,
such as silicon carbide and gallium
nitride, the use of full power conversion combined with compact, highly efficient synchronous
machines tends to be the most attractive. In addition to the
power density and efficiency improvements being sought,
such converters are also expected to incorporate fault tolerance and protection functions.
New approaches for the distribution of high electrical
power in the flight environment have been recently studied. The overall weight and reliability of power distribution
systems have been shown to vary substantially depending
on the distribution architecture, protection scheme, component redundancy, operating voltage, and management
approach. Since commercial aircraft operate at high altitudes where the atmospheric pressure decreases, partial
discharge can occur across transmission lines at lower
voltages than would be observed in ground-based systems. If lower voltages need to be utilized, the conductors
must then be sized to carry a larger current to meet power
requirements and, as a result, become heavier.
Furthermore, the protection system must be sized to
interrupt larger currents. The inclusion of new insulation
materials and terminating schemes may assist with preventing partial discharge at altitude, and novel conductor
designs could reduce the associated weights of these systems. The importance of effective thermal management
has also become apparent as the field of electrified aircraft
propulsion has progressed. In terms of the propulsive
power requirements at scales in the 10s and 100s of megawatts, efficiency losses of 10-15% across the entire electrical system can result in enormous thermal sources that

Continued research
into electrical
components is
necessary to produce
systems that are
appropriate for
aircraft propulsion.

I E E E E l e c t r i f i cati o n M agaz ine / J UN E 2020

IEEE Electrification Magazine - June 2020

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