Hydrocarbons are well known for releasing pollutants when burned. However, as it now seems, it may not always be necessary to burn them when generating energy. A promising approach, emerging from the research stage – into the commercialization stage – is solid-oxide fuel cell (SOFC) technology. The potential is made clear through a partnership between Mohawk Innovative Technology and Velo3D.
The US 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,” said Jose Luis Cordova, Ph.D., VP of Engineering at Mohawk Innovative Technology Inc. (MITI). Working on several 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.
“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,” said Jose Luis 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.”
Although more than 40,000 units of 100-kilowatt fuel cells (each able to power 50 homes) were shipped worldwide in 2019, the widespread adoption of the technology has been limited due to many of the SOFC components being expensive to manufacture, and these components wearing out quickly thanks to the exposure to the very gases that make their operation so efficient.
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 off-gas 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. The rest of the approximately 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,” said Jose Luis 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.”
“SOFC balance-of-plant designers were thinking that this blower would be an off-the-shelf unit,” said Jose Luis 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 and an installation-cost reduction from an average of $12,000/kWe (kilowatt of electrical energy) to $900/kWe.
“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,” said Jose Luis Cordova.
AM 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. The latest versions demonstrated no significant degradation in parts or output and the 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 the longest life, this part must be made from expensive, high-strength, nickel-based, corrosion-resistant superalloy materials like Inconel 718 or Haynes 282, which are difficult to machine or cast. Achieving optimal aerodynamic efficiency in an impeller requires complex three-dimensional geometries that are a challenge to manufacture. In addition to this, due to the incipient nature of the current SOFC market, impellers are produced in relatively small batches, and economies of scale are difficult to realize.
As you can imagine, additive manufacturing provided a compelling answer for bringing the costs of production down. 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,” said Jose Luis 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 LPBF provider Velo3D.”
Collaborating on capabilities
“With its goal of reducing costs and improving the performance of SOFCs, the DOE is enthusiastic about innovative manufacturing methods such as AM,” said Jose Luis 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.”
“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,” said Matt Karesh, Velo3D’s Mohawk-project leader.
The cost efficiency of AM
“Our traditional, subtractively-manufactured impeller wheels were running up to $15,000 to $19,000 a piece,” said Jose Luis 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,” said Jose Luis Cordova.
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 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 Hannah Lea, a mechanical engineer at Mohawk.
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,” said Hannah Lea. “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,” said Jose Luis Cordova. “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 (60mm 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,” said Velo3D’s Matt 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.”
According to Mohawk engineer, Rochelle Wooding, surface finish was another focus. “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.”
Future testing, forward outlook
The next steps are to retrofit the AORBs with the new impellers and test 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,” said Jose Luis Cordova. 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. 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,” concluded Jose Luis Cordova.