IEEE Electrification - March 2021 - 99

Boiler

Hydrogen

LH2 Tank

Cryogenic Cooling Circuit
dc
dc

Fuel Cell

Boiler

Converter

+
-
Battery

Capacitors

Inverter

Motor

Fan/
Hydrogen Turbine Propeller

Figure 8. A parallel hybrid fuel cell-based drive system for a single propeller with cryogenically cooled power conversion.

One of the main drawbacks of the fuel cell all-electric
propulsion system is the lack of significant power density. In this regard, the fuel cell hybrid electric propulsion
system (FC-HEPS) can take advantage of the higher
power densities of the hydrogen turbines with the climate neutrality of fuel cells. Figure 8 illustrates how this
type of system could be arranged. In its configuration,
the fuel cell is the major power source in cruise mode,
where low emissions are critical. In addition, the hydrogen turbine is strategically sized to deliver the required
thrust for takeoff and climbing. As a result, the major
parts of the flight do not emit NOX, and it could also lead
to fewer contrails.
The system is feasible and interesting for the shortrange aircraft segment (165 PAX, 2,000 km). For this particular application, the fuel cell system will have a power
rating greater than 10 MW, and thus, it will be crucial
that the cryogenic cooling opportunities from the LH2 are
sufficiently utilized. Another important issue is the fact
that the parallel hybrid configuration adds complexity to
the certification process. Seamless and optimal interactions with the electric propulsor and the hydrogen combustor are important challenges.

Hydrogen Combustion Propulsion-With
an Auxiliary Electric System
In a hydrogen combustion propulsion (HCP) system
with an auxiliary electric system, the aircraft propulsion can originate completely from the direct combustion of hydrogen fuel, just like kerosene, to create
thrust. Figure 9 depicts how the system would look
with a single fan with few components. This is a system that is employed in cases where the weight of the
FCS will be too high for propulsion. However, the FCS
could be utilized to produce auxiliary electrical power
in this kind of aircraft.
The system is technically feasible for medium-range
aircraft (250 PAX, 7,000 km), but it will have significantly
higher costs than conventional aviation. As displayed in

Figure 8, the system needs heat to boil the LH2 before it is
burned. However, there is less of a need to cryogenically
chill the power components, so a synergy is less likely due
to lower levels of electrification.

Hydrogen Turbo Electric Propulsion System
As an alternative to the direct burning of hydrogen, a
turbo electric propulsion system (TEPS) can be utilized. It
allows turbo electric distributed propulsion, which has the
potential to be the next disruptive technological breakthrough. It has a drawback of slightly lower energy conversion efficiency during cruise mode than direct burning,
but the propulsive efficiency can be improved, and it has
more flexibility in terms of new ultra-efficient aerodynamic designs and can optimize efficiency during takeoff and
climbing. The excellence of the propulsive efficiency is
determined by the bypass ratio of the fan/propeller, which
can be carefully controlled electrically.
However, there are added weight penalties from electrical power conversion components. It would, therefore,
require superconducting power conversion to save the
added weight and minimize the electrical losses. Superconducting solutions could make the conversion efficiency nearly as high during cruise mode and further improve
the propulsive efficiency. The turbo electric solution can
reduce the overall power consumption by letting the
hydrogen turbine operate at its optimum point during the
whole flight (improved gas turbine cycle). Moreover, it can

Hydrogen

Fuel Cell Hybrid Electric Propulsion System

Heat
LH2
Tank
Boiler

Hydrogen Turbine

Fan/
Propeller

Figure 9. Hydrogen direct-burning propulsion for a single propeller.

IEEE Electrific ation Magazine / MARCH 2 0 2 1

IEEE Electrification - March 2021

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