IEEE Electrification Magazine - June 2020 - 22

Naturally, a fully electric architecture consists of one or
more motor-driven propulsors, which are powered by one
or more electrical energy (e.g., battery) or electrochemical
power conversion (e.g., fuel cell) systems. Given the aforementioned limitation in specific energy of available electrical energy storage systems, many recent efforts have
focused on hybrid electric configurations, in which gas
turbines are utilized in conjunction with energy storage
systems and electric machines. The inclusion of conventional fuels provides a stepping stone to leveraging the
beneficial aspects of electrification, such as reduced operating costs and reduced greenhouse gas emissions, while
making fewer compromises in aircraft performance, such
as range, weight, and turnaround time.
A relevant example includes the series hybrid configuration, where the gas turbine power is converted to electrical power through a generator, which
is combined with stored electrical
energy to power one or more electrically driven propulsors. This series
hybrid configuration offers a means
to enabling distributed electric propulsion, for example, with the re--
duced impact on range that naturally
comes from the lower specific energy
of most electrical energy storage systems. In contrast, a parallel hybrid
configuration utilizes stored electrical energy to drive an electric
machine, which is coupled to the
mechanical power of a turbine-driven propulsor. Such an architecture
allows electric boost motors to augment the peak power demands during the takeoff and climb flight
phases and permits the use of smaller engine cores that are better tailored for cruise requirements. These
systems would be very familiar to
those working on hybrid electric automotive powertrains.
Concepts have also been developed for turboelectric
drivetrains, for which conventional kerosene-based fuels
are used to power a gas turbine and nearly all (fully turboelectric) or part (partial turboelectric) of the associated
shaft power is extracted by a generator. This power is then
used to operate one or more electrically driven propulsors.
Turboelectric configurations permit the power generation
process to be centralized into a smaller quantity of highpower, efficient turbine cores. The thrust contribution of
electrically driven propulsors can then be spread across a
larger region of the vehicle and allow avenues for larger
effective fan areas.
In the pursuit of improved propulsive efficiency, many
ultrahigh bypass turbofans also feature increasing fan areas
but are limited by a required minimum clearance distance
between the ground and the turbofan nacelle. By providing

flexibility in the placement of individual propulsors, series
hybrid and turboelectric architectures alleviate some of
these challenges. However, a disadvantage of turboelectric
configurations is the associated reliance on conventional jet
fuels with no immediate path for carbon-neutral energy
storage. Such a system would be familiar to those working
on diesel-electric locomotives or ships.

Electrified Aircraft Design
Since the challenges of electrical energy storage are exacerbated by an increase in the scale of the aircraft platform,
a number of researchers are turning toward battery hybrid
electric architectures for regional-class aircraft that are
typically designed to carry between 48 and 86 passengers
across short-haul flights of 600 nmi or less. These efforts
offer a near-term approach to reduce fuel burn and, for
some missions, operational costs.
The global average passenger flight
has a range just above this 600-nmi
capability, suggesting that hybrid
electric aircraft designed with this
range in mind would capture a
great portion of air travel demand.
Example range profiles from notable airport hubs are shown in Figure 7 for reference.
This approach would require that
air carriers make modifications to
their typical concept of operations
and means for allocating aircraft to
associated routes, though the prospect of reduced operating costs
offers an appealing reason to deviate
from current business practices. This
class of regional aircraft also features
lower-power propulsion systems
than single- and twin-aisle aircraft,
which decreases the aggressive im--
provements in rated power required
from electrical components, permitting nearer-term viability. It should be noted that this class of aircraft currently
does not contribute significant portions of aviation-related
fuel burn, potentially limiting the impact of electrified propulsion solutions at this scale. This trend, however, could
change if new operations models are adopted with regional hybrid electric aircraft.
With increasing vehicle scale, passenger capacity, and
range, trends in propulsion electrification are observed to
shift toward lightly hybridized configurations and kerosene-fueled turboelectric or alternative energy-driven
fully electric systems. While single-aisle and twin-aisle
aircraft concepts better target the classes associated with
the greatest fuel burn and emissions, they also rely on
technologies and developmental improvements that give
them an entry into service timetable further into the
future. In essentially all circumstances, key technological

The decoupling of
power production
and expenditure
enables ultrahigh
effective bypass
ratios to be
achieved, since
many geometric
constraints of
the fan design
are relieved.

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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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