out extensive testing of its 2.5MW
generator at a specialist facility
in Trondheim, Norway as well as
completing testing of a full-scale, flightrepresentative
thermal management
rig, and developing a control system for
controlling the engine and electrical
equipment together.
Reflecting on some of the key
lessons learned throughout this process
Armesmith said that one of the biggest
challenges was the development of the
electric distribution network that forms
the connection between the 3000V
DC power generation system, the high
voltage battery and the electric motor.
“At 2MW of power we’ve got to push up
the voltage otherwise the current would
be absolutely enormous and the cable to
carry that current would be enormous
and the forces due to that current would
be unmanageable,” she explained.
With the highest voltage system
flying today 270V DC, developing a
3000V system presented a huge step
up and raised a number of significant
challenges. One of these relates to
the effects of a phenomenon called
Paschen’s Law, which observes that
the voltage required to break through
insulation reduces with altitude,
therefore making phenomena like
arcing and corona more likely.
This has led to a lot of work on
insulation systems. “Protection,
insulation and separation is our strategy
on this,” said Armesmith, “we’re testing
a lot of components at the moment to
deal with this.”
Another key challenge has been
producing and integrating the wide
variety of bespoke components required
for such a novel system, many of
which - Armesmith explained - simply
aren’t available off the shelf. “We’ve
had to build everything from scratch.
We’re pushing the laws of physics on
developing these components and
integrating them into the system.
We’re trying to replace a gas turbine in
a nacelle with an electric motor firing
a fan. How do you package all of that?
How do you support that motor with
the oil you need to lubricate it? How to
do you take the heat out of the motor?
Where do you put that heat? How do you
integrate the heat exchanger? How do
you insulate your control systems? How
do you route your cables? Where does all
of your instrumentation go? It has been
really challenging to package all of that
neatly into the shape of gas turbine.”
Unsurprisingly scaling all of the
components down to aerospace
specification in a system like this
presents some pretty acute thermal
management challenges, and the group
has also made some key advances here.
“Any time you touch anything electrical
you generate loss which generates heat,”
said Armesmith. “If you convert AC
to DC that’s lossy, if you transmit the
power that’s lossy. We’re having to make
sure the cables aren’t too hot, and we’re
having to take heat out of the motors,
the batteries, and the power electronics.
When you’re at 2MW even with
equipment that’s 98 per cent efficient,
that’s a lot of heat to take out.”
June 2020 / www.theengineer.co.uk 28
still get that from a motor on an aircraft, but you’ve got
a big inertia on the fan. It then reacts against that in an
electrically driven fan, so if you’re thinking you’re going
to get quick acceleration then maybe you won’t because
of the aerodynamics of the fan. Fan engineers are really
interested in all of this because it’s making them go back to
first principles.”
Clearly, there’s a big difference between testing all of
this in a laboratory on the ground and taking it into an
aircraft, and there are undoubtedly lessons that won’t
now be learned.
But Armesmith is currently focused on the upcoming
ground testing program (set to take place at a new testbed
facility in Bristol later this year) which will see the
team test a system that integrates a 2.5MW generator
powered by an AE2100 gas turbine engine, a 3,000V
distribution system, plus new power control and thermal
management systems.
And the legacy of this work will - she said - be hugely
important. “We will have a complete integrated power and
thermal management system that is fully integrated and
The E-Fan X generator
Aerospace
Gxxxxxx
ground tested. That means we will be in a great position
whenever anyone is ready to create a hybrid-electric
demonstrator vehicle – we will be ready to go. By fully
testing the power generation system on the ground we
will have enough knowledge to be very well advanced to
integrate that further into a future aircraft.”
It’s hard to know exactly what further lessons would
have been learned, or indeed whether the project would
have reached its original planned conclusion, if Covid-19
hadn’t struck.
And whilst the initiative’s cancellation will certainly
be seen by some as a major setback on the path to low
carbon flight, Armesmith believes that, in the longer
term, the case for green aviation will be stronger than
ever. “I believe in our post-COVID 19 world that people
will still want to fly and see other people and make new
discoveries, and they will possibly be even more aware
of our environment. And when the market is ready, the
drive to sustainable aviation will resume. Hybrid-electric
will be part of that movement and we will be ideally
positioned to be a pioneer.”
She added that dissipating heat from
electrical equipment is particularly
challenging because of the relatively
low temperatures involved, and the
low temperature difference (Delta T)
between the system and the ambient
temperature. “The difference here
is that a gas turbine is usually really
hot and you’re taking heat out from
very high temperatures to ambient
air. The electrical equipment doesn’t
like to be very hot – you’re looking
at temperatures of 100 – 200oC and
dissipating that heat is really hard.”
More generally, the project has
also helped Rolls-Royce engineers
develop their understanding of the
exceptionally complicated relationship
between electricity and thrust on an
aircraft. “If you look at electric cars the
torque is massive,” she said. “And you
/www.theengineer.co.uk