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 black-outs. 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.
Hydrocarbons can release pollutants when burned, but what if you never ignited them? A promising approach, 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 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,” said Jose Luis Cordova, Ph.D., vice president of engineering at Mohawk Innovative Technology Inc.
Working on a number of DOE-funded programs, Mohawk is an Albany, N.Y. based company specializing in the design of high-efficiency, cost-effective, environmentally low-impact, oil-free turbomachinery products.
“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,” Cordova said. “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.”
More than 40,000 units of 100-kW 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,” Cordova said. “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.
“SOFC balance-of-plant designers were thinking that this blower would be an off-the-shelf unit,” Cordova continued. “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, and your SOFC plant needs an overhaul every 2,000-4,000 hours.”
This statistic falls far short of the DOE’s goal of an operating lifetime of 40,000 hours for a typical SOFC.
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 (Fig. 1) 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. 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) 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,” Cordova said. “Using traditional manufacturing to make just the handful of the custom impeller wheels or volutes they wanted would have been extremely expensive. 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.”
Fig. 1. Oil-free anode offgas recycle blowers (AORBs) made by Mohawk Innovative Tech-nology Inc.
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.
“Their funding [through The Small Business Industrial Research Project] supports our current partnership with Velo3D as well as our previous one with FuelCell Energy,” Cordova said. “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 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,” Cordova said. “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.”
Fig. 2. Cross section of a Mohawk AORB highlights the main internal components
Fig. 3. First trial group of AORB impellers: a) solid CAD model; b) in Velo3D metal 3D-printer during manufacture; c) complete build plate removed from AM system
Picking the Perfect Alloy
For 3D-printing impellers on a Velo3D Sapphire metal 3D-printing system (Fig. 3) at Duncan Machine, a contract manufacturer in Velo3D’s global network, the choice was made to use Inconel 718 (Table 1) – one of the nickel-based alloys with a strong temperature tolerance that can withstand the stress of rotation best.
“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,” said 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 said. “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. Velo3D’s Sapphire system could just print what we wanted,” Cordova said. “We did do some process adjustments and tweaking in terms of support-structure considerations and surface-finish modifications.”
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 mm in diameter) necessitated the team’s development of a “sacrificial shroud” – a temporary printed enclosure that held the blades true during manufacturing.
Fig. 4. An SOFC power system with AORB highlighted (photo courtesy of FuelCell Energy) // Courtesy of FuelCell Energy
Sacrificial Shrouds and Smoother Surfaces
“What was really interesting about this approach is that shrouded impellers are, for most current additive technology, basically untouchable because of all the traditional support structures they require,” said Matt Karesh, Velo3D’s Mohawk project leader. “We used a 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.”
Surface finish was another focus.
“The surface was a bit rough in our early iterations,” said Mohawk engineer Rochelle Wooding. “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.
Fig. 5. Mohawk’s Hannah Lea, Jose Luis Cordova and Rochelle Wooding (left to right)
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,” Cordova said.
Work is already underway using AM for other blower parts like housings and volutes.
“Through these DOE-funded projects, we’ve been able to develop a library of common parts,” Wood-ing said. “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.”
For more information: Lynn Manning is a science and technology writer based in Providence, R.I. She is also president of Parker Group, a business-to-business public relations firm with expertise in promoting industrial and high-technology products. She can be reached at email@example.com.
All graphics supplied by the author, except where noted.
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