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ADDITIVE MANUFACTURING KEY TO CLEAN ENERGY OF THE FUTURE

Mohawk Innovative Technology has teamed with Velo3D to boost efficiency and
durability, and to take 60% off the price of offgas recycle blowers used in fuel
cells that produce cleaner energy at a lower cost. The switch to AM has been an
eye-opener.

Keeping the electricity grid up and running through summer heat waves and winter
deep freezes is an ongoing balancing act. Power lines that stretch for miles are
vulnerable to wind and fire. Surges in demand for heating and cooling strain
capacity, which can lead to blackouts. Air pollution is an ongoing issue.
Although alternative-energy solutions such as solar- and wind-power are rising
up the supply curve, meeting today's energy needs still requires the use of
traditional fuel sources to balance the mix.

Views of oil-free anode offgas recycle blowers (AORBs) made by Mohawk Innovative
Technology Inc. (MITI)

 

 

Of course, hydrocarbons can release pollutants when burned. But what if you
never ignited them?

A promising approach, now emerging from the research stage into
commercialization, is solid-oxide fuel cell (SOFC) technology. The U.S.
Department of Energy (DOE) has invested in SOFCs for years ($750 million since
1995, according to their website) as part of the ongoing effort to decarbonize
energy production. The DOE describes an SOFC as an electrochemical device that
produces electricity directly from the oxidation of a hydrocarbon fuel (usually
natural gas), while eliminating the actual combustion step.

Basically, an SOFC acts like an infinite-life battery that is constantly being
recharged -- without burning the gas that recharges it.

Small Package, Big Energy Output

"Solid oxide fuel cells are very attractive because they produce a lot of energy
in very small packages," says Jose Luis Cordova, Ph.D., VP of Engineering at
Mohawk Innovative Technology Inc. (MITI). Working on a number of DOE-funded
programs, Mohawk is a 28-year-old, Albany, New York-based company specializing
in "CleanTech" -- the design of high-efficiency, cost-effective, environmentally
low-impact, oil-free turbomachinery products including renewable energy
turbogenerators, oil-free turbocompressors/blowers, and electric motors.

Mohawk's Hannah Lea, Jose Luis Cordova and Rochelle Wooding

 

 

"SOFCs are compact and can be built at a factory, then transported to the
specific site where they're needed to support distributed-energy production,"
says Cordova. "Contrast that with the usual centralized, multimegawatt power
plant that takes billions of dollars and many years to set up. SOFCs are also
very efficient. Unlike a regular battery, they don't lose power over time
because as long as you supply the reagents you can continue the electrochemical
reactions pretty much indefinitely."

Sounds ideal, and more than 40,000 units of 100-kilowatt fuel cells (each able
to power 50 homes) were shipped worldwide in 2019. But there have been bumps in
the road slowing more widespread adoption of the technology: Many SOFC
components are expensive to manufacture and, due to exposure to the very gases
that make their operation so efficient, they wear out frustratingly quickly.

Facing Cost and Durability Issues

To help overcome such challenges, Mohawk has designed some of those critical
parts for longer lives and greater efficiency. One example is the anode offgas
recycle blower (AORB), an essential component of the "balance of plant" -- the
machinery that supports the SOFC's fuel stack.

During operation, each fuel cell only uses about 70% of the gas it's fed; some
30% passes right through the system along with water (a product of the
electrochemical reaction). "You don't want to throw away the leftover gas or
water, you want to send them back to the beginning of the process," says
Cordova. "And that's where the AORB comes in; it's essentially a low-pressure
compressor or fan that recycles the exhaust and returns it to the front of the
fuel cell."

Cross section of a Mohawk AORB highlights the main internal components.

 

 

"SOFC balance-of-plant designers were thinking that this blower would be an
off-the-shelf unit," says Cordova [a typical 250 kW SOFC plant would employ two
of them]. "But due to the process gases in the system, traditional blowers tend
to corrode and degrade; the hydrogen in the mixture attacks the alloys the
blowers are made of and also damages the magnets and electrical components of
the motors that power the blowers. Most blowers also contain lubricants, like
oil, that degrade as well. So, you end up with very low-reliability blowers --
representing a significant portion of the balance-of-plant cost -- and your SOFC
plant needs an overhaul every two- to four-thousand hours."

This statistic falls far short of the DOE's goal of an operating lifetime of
40,000 hours for a typical SOFC -- as well as an installation-cost reduction
from an average of $12,000/kWe (kilowatt of electrical energy) to $900/kWe.


VIDEO: How Mohawk is Printing the Future of Energy

Time to rethink those blowers! "So, we realized that Mohawk's proprietary,
oil-free, compliant foil bearing (CFB) technology, specialized coatings, and
decades of turbomachinery expertise were a good fit for this challenge," says
Cordova.

Additive Manufacturing Offers Answers

DOE funding provided the means for Mohawk to design and test AORB prototypes in
a demonstrator SOFC power plant run by FuelCell Energy. Rigorous testing under
realistic operating conditions measured durability and performance, with the
latest versions demonstrating no significant degradation in parts or output and
complete elimination of any performance or reliability issues.

Yet the cost of an AORB remained prohibitively high -- in large part due to its
high-speed centrifugal impeller, which operates continuously under extreme
mechanical and thermal stress. For longest life, this part must be made from
expensive, high-strength, nickel-base, corrosion-resistant superalloy materials
like Inconel 718 or Haynes 282 that are difficult to machine or cast.

First trial group of AORB impellers -- a) solid CAD model, b) in Velo3D printer
during manufacture, c) complete build plate removed from AM system

 

 

In addition, achieving optimal aerodynamic efficiency in an impeller requires
complex three-dimensional geometries that are a challenge to manufacture. And
because of the incipient nature of the current SOFC market, impellers are
produced in relatively small batches, and economies of scale are difficult to
realize.

How to bring that cost down? Additive Manufacturing (AM, aka 3D printing)
provided a compelling answer.

While the original project with FuelCell Energy was evolving, Mohawk was also
getting calls from R&D groups looking for help with their own fuel- cell
component designs. "Because many of these manufacturers and integrators were
still at the research stage, each one had a different operating condition in
mind," says Cordova. "Using traditional manufacturing, to make just the handful
of the custom impeller wheels or volutes they wanted, would have been extremely
expensive. So that's where we started looking at AM; we did our own research
into AM system makers and connected with laser-powder-bed fusion (LPBF) provider
Velo3D."

Collaborating on Capabilities

With its goal of reducing costs and improving performance of SOFCs, the DOE is
enthusiastic about innovative manufacturing methods such as AM, says Cordova.
"Their funding [through The Small Business Industrial Research Project] supports
our current partnership with Velo3D as well as our previous one with FuelCell
Energy. An additional benefit is that this work is helping advance 3D printing
technology in general as we learn more and more about its capabilities and
potential."

A 3D-printing trial build with multiple impeller designs for an AORB.

 

 

Velo3D's Mohawk-project leader Matt Karesh agrees. "Working hand in hand with
companies like Mohawk, who are willing to collaborate with us and give us
feedback, drives progress on our internal process parameters and capabilities,
and helps direct us as to how to make our print methodologies better," he says.

A Nice Price Surprise

The switch to AM was an eye-opener: "Our traditional, subtractively manufactured
impeller wheels were running up to $15,000 to $19,000 apiece," says Cordova.
"When we 3D printed them, in small batches of around eight units rather than one
at a time, this dropped to $500 to $600 -- a very significant cost reduction.

"As well as cutting manufacturing costs, LPBF is the one technology that could
provide us with the design flexibility we were looking for. AM is indifferent to
the number of impeller blades, their angles, or spacing -- all of which have a
direct impact on aerodynamic efficiency. We now have the geometric precision
needed to achieve both higher-performance rotating turbomachinery designs and
reduce associated manufacturing costs."

Picking the Perfect Alloy

For 3D printing impellers on a Velo3D Sapphire system (at Duncan Machine, a
contract manufacturer in Velo3D's global network), the choice was made to use
Inconel 718 -- one of the nickel-based alloys with a strong temperature
tolerance that withstand the stress of rotation best.

An SOFC power system with AORB highlighted. Photo courtesy of FCE

 

 

"Inconel was very attractive to us because it's chemically inert enough and
retains its mechanical properties at pretty high temperatures that definitely
surpass aluminum or titanium," says Mohawk mechanical engineer Hannah Lea.

Although Velo3D had already certified Inconel 718 for their machines, Mohawk did
additional material studies to add to the body of knowledge about the 3D-printed
version of the superalloy. "Our tests demonstrated that LPBF 3D-printed Inconel
718 had mechanical properties, like yield stress and creep tolerance, that were
higher than those of cast material," Lea says. "This was more than adequate for
high-stress centrifugal blower and compressor applications within the
operational temperature range."

Iteration Made Easy

As their impeller work progressed, Mohawk's engineers collaborated with Velo3D
experts on design iterations, modifications and printing strategies. "It was
really interesting because we didn't have to make any major design changes to
the original impeller we were working with -- with Velo3D's Sapphire system we
could just print what we wanted," says Cordova. "We did do some process
adjustments and tweaking in terms of support-structure considerations and
surface-finish modifications."

Mohawk engineer Rochelle Wooding at work on an AORB test rig.

 

 

Of course, tweaking is just another day in the office for design engineers. As
the impeller project progressed, AM provided much faster turnaround times than
casting or milling would have allowed, since parts could be printed, evaluated,
iterated and printed again quickly. In subsequent 3D printing runs, multiple
examples of old and new impeller designs could be simultaneously made on the
same build plate to compare results.

The relatively small size of the impellers (60 millimeters in diameter)
necessitated the team's development of a "sacrificial shroud" -- a temporary
printed enclosure that held the blades true during manufacturing.

Sacrificial Shrouds and Smoother Surfaces

"What was really interesting about this approach is that shrouded impellers are,
for most current additive technologies, basically untouchable because of all the
traditional support structures they require," says Velo3D's Karesh. "We used a,
not support-free, but reduced-support approach. Mohawk was saying, 'we don't
need the shroud in the end, but the shroud makes our part better, so we'll
attach this thing that's typically extremely hard to print -- and just cut it
off after.' Using Velo3D's technology, they were able to build that disposable
shroud onto their impeller, get the airfoil and flow-path shapes they wanted,
and then it was a very simple machining operation to remove the shroud."

Cost comparison of conventional manufacturing versus LPBF-3D printing shows the
significant material savings possible.

 

 

Surface finish was another focus. Says Mohawk engineer Rochelle Wooding, "The
surface was a bit rough in our early iterations. What was interesting about the
sacrificial shroud was that it gave us a flow path through the blades that we
could use to correct for roughness using extrusion honing; it took some further
iteration to determine how much material to add to the blades to achieve the
required blade thickness that we wanted. The final surface finish we achieved is
comparable to that of a cast part and suits our purposes aerodynamically."
What's more, all critical design dimensions enabling proper impeller operation
were within tolerances.

Future Testing, Forward Outlook

Next steps are retrofitting AORBs with the new impellers and testing them in
field conditions.

"We expect that successful execution of these two tasks will fully demonstrate
that 3D-printed Inconel parts delivered by LPBF technology are a viable and
reliable alternative for manufacturing turbomachinery components," says Cordova.
Work is already underway using AM for other blower parts like housings and
volutes.

Cordova is particularly proud of the professional credentials and work ethic of
the two young women engineers who've been engaged in these recent in-house
projects. "I never really feel like I have an easy day here," Wooding says. "But
I really enjoy that!"

She's also keenly aware of the greater value of what they are doing: "Through
these DOE-funded projects we've been able to develop a library of common parts.
Based on the original idea, we now have at least three completely different
platforms that can serve different power capabilities to support progress for
the clean energy of the future."

Want more information? Click below.

Velo3D

Mohawk




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