IEEE Electrification - June 2022 - 36

cooling arteries with its supply branches. Following the
blue arrows in the figure, this simplified flowchart shows
the liquid fuel distribution across the aircraft, from the
storage tank to the propulsors. Most of the liquid is used
for cooling the fully superconducting motor, HTS lines,
busbars, and current leads.
The superconducting engine (motor) with its field
winding and armature needs to be kept superconducting
at an optimum temperature. This is achieved by making
use of the high-heat-sinking capability of hydrogen by
using forced flow boiling through the helical motor windings
to fully exploit the latent heat of vaporization.
Superconducting power transmission lines must be
maintained below their critical temperature as well. Liquid
hydrogen also resides in cryogenic transfer lines and is
directed to flow over cold plates for heat-sinking cryogenic
power electronics and for cooling batteries.
From the motor, we drive the liquid flow to the cryogenic
inverter and many other, different cryogenic
power electronic components. Those are efficiently
heatsinked to run at their nominal optimal operating
temperature before entering a further heat exchanger.
The current design requires a liquid-cooling flow
through the superconducting motor of only approximately
0.1 kg/s, which would account for an approximate
30% drain of the liquid hydrogen tank. Liquid tank
withdrawal (not shown) is achieved with the use of specific
liquid hydrogen pumps that can provide the pumping
flow at light pump weight and durability for decades
and can be maintained and replaced, if required. For
LH2 Fuel
20 K
HTS Lines and Busbars
4.02 kW
SC Motor
30 K
Inverter
100 kW
4.32 kW
(per Motor)
20 K
Heater
H2
H2O + Air
Air
Coolant
216 K
Heat
Exchanger
216 K
Figure 1. The cryogenic flow circuit of an all-electric aircraft. SC: superconducting.
36
IEEE Electrification Magazine / JUNE 2022
443 K
50 K
GH2 Supply
(433 K)
Fuel Cell
aircraft applications, those pumps do not yet exist but
are being developed.
One major problem with pumps is the susceptibility to
cavitation the pump experiences and the immediate
destruction of piston rings. Furthermore, one pump stack
is designed for supporting four tanks. Attached to the
pump outlet are transfer lines, called cable cryostats, that
can be either flexible or rigid. Fortunately, those cable
cryostats have been available since the 1960s and are now
at a very high technical readiness level (TRL), making
them ready to be integrated in the airframe structure. The
same applies to cable cryostats with integrated superconducting
power lines. The design of those lines has recently
been revisited by researchers at the Karlsruhe Institute of
Technology, resulting in very-low-heat-leaking lines.
A bypass line also leads the liquid hydrogen flow
directly to a heat exchanger to generate additional gaseous
flow for maintaining the fuel cells. As the fuel cell
technology advances, the hydrogen gas flow that accounts
for 70% of the liquid hydrogen tank drain will be reduced.
Besides, there may be components in an aircraft that can
benefit from operating at a cryogenic temperature that we
are not aware of yet. Many components tend to work and
respond much faster due to the low electrical resistance of
their metallic components.
Depending on the aircraft range (long-range), the fuel
cell requirement determines the liquid hydrogen tank volume
and size. As shown in Figure 1, the cryogenic mass
flow through motor and HTS lines is also returned to the
fuel cell after component cooling has been accomplished.
Current Lead
Ambient Air
(216 K)
30 K
100 K
Air at
2.5 Bar
Chiller
Air Supply
(433 K)
Compressor
Air/H2O
Products
(443 K)
Turbine
IEEE Electrification - June 2022

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