Casting Complexity in Conformable Tank
Michigan’s REL Inc. aims to improve natural gas storage with an innovative aluminum casting produced via a modified low pressure permanent mold process.
Nicholas Leider, Associate Editor
(Click here to see the story as it appears in the February issue of Modern Casting.)
Metalcasters are well aware of the premium placed on energy efficiency across all industrial sectors. Whether the motive is commercial, environmental or regulatory, metalcasters know sacred cows cannot get in the way of progress. Casting buyers and end users want the highest quality performance at minimal cost, so suppliers continually seek ways to deliver improved mechanical and physical properties.
One challenge faced by REL Inc., Calumet, Michigan, a material process development company with casting capabilities, was repackaging fuel storage. Compressed natural gas (CNG) is commonly used to supplement traditional gasoline-powered engines. But improving the storage of CNG—most commonly in long cylindrical tubes—may make the cheaper, cleaner-burning fuel a commercial competitor to standard gasoline.
REL Inc., through a grant from a U.S. Department of Energy program, cast a technically viable conformable tank in an aluminum 206 alloy. The so-called conformable tank, cast in aluminum via a low pressure permanent mold process, better uses allowable packaging space to store as much as 35% more CNG. The shape of the tank also improves flexibility in the placement of the fuel, allowing it to be more commercially viable than the torpedo-shaped tanks currently in use. REL is now partnering with a major CNG supplier and a local university in a three-year, $2.1 million project to prove the technology is commercially viable.
Public Investment Kick-Starts Development
REL Inc., with much of its experience in squeeze casting, was capable of infiltrating metal into ceramic preform structures, with gaps as small as 10 microns. Adam Loukus, vice president, REL, saw potential in casting a tank with a thin internal structure to minimize the amount of tank material while still having the ability to hold high-pressure gas.
“Conventionally, you use cylindrical geometry when storing gas at high pressure,” said Josh Loukus, president, REL Inc. “That’s nature’s shape. Put those cylindrical tanks in the trunk, and as soon as you do that, you can’t put your groceries in there. With a conformable design, with a box, you can pack fuel more efficiently inside a vehicle.”
With the idea of a conformable CNG tank, REL applied for funding under the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) program. In July 2012, REL Inc. was awarded a $3 million grant to pursue its research.
The final goal being a design improvement from the cylindrical tanks, REL Inc.’s design team started with a squared structure. The corners were rounded to minimize high stress points. Through numerous iterations—including designs with pillar and bubble shaped internal structures—the REL Inc. team eventually pursued a path that already existed in nature.
The so-called Schwarz P surface features two intertwined cubic lattices. This design, which consists of two fully independent and intertwined cavities, features a high surface-to-volume ratio. Such a design is found in the skeletal plate of sea urchins, a fact that reinforced the REL Inc. team’s belief it was a naturally efficient design. The constant curvature, though, meant the Schwarz P design was especially difficult in simulation.
“Because of the constant curvature of the internal structure, it is extremely difficult to model efficiently. The structure is such that we need to develop simplifying techniques to model and analyze the final tanks. It almost cripples the CAD and FE programs we use,” Adam Loukas said. “While the structure is efficient, it’s computationally taxing.”
The final casting would be a single aluminum component, but it would require a number of sand cores, which could be stacked atop one another to produce the complex cavity. REL Inc. produced the majority of these cores in house, though some were 3-D printed for prototype models.
Castings to Meet Demanding Parameters
The tank needed to operate at 3,600 psi, though it had to withstand a burst pressure of 8,100 psi. This demand placed a premium on uniformity in casting. REL Inc. had some experience with the low pressure permanent mold process, which was appealing because it could reduce turbulence in filling the mold, but the company partnered with Eck Industries Inc., Manitowoc, Wisconsin, to produce the first version in 7075 aluminum, also produced in a low pressure permanent mold.
Those first articles proved promising enough for REL Inc. to invest in a low pressure permanent molding line at its facility in Calumet. Engineers then opted to cast the tanks in A206, one of the highest strength structural alloys.
“It wouldn’t do us much good to have great properties at the top of the tank if we have trouble near the gates,” Adam Loukus said. “It required a lot of process improvements and changes to the low pressure process. We needed to optimize the gating. We needed to introduce the metal in the most uniform manner possible.”
Once REL Inc. was able to produce acceptable castings, processing after solidification was relatively minor. The complex cores needed to be burned out, the gates removed and the tanks heat treated.
Proving the technology works is one thing; establishing a path to commercial feasibility is another. REL Inc., with the success of its ARPA-E project, sees the first half of the equation solved; the cast A206 aluminum tank can improve CNG storage. Now, the goal is to produce the tanks in a commercially viable way.
“It’s important to find resources to make that transition,” Adam Loukus said. “Who’s going to help you go from proving out the technology to getting prototypes out on the road?”
Now that REL Inc. is approaching the end of its participation in ARPA-E, the company has reached a partnership with Houston-based Southwestern Energy (SWN), the third largest producer of natural gas in the continental U.S. Improving CNG storage and use in automobiles and trucks holds obvious commercial appeal for SWN, which led to the company’s $2.1-million, three-year investment in December 2014.
“We want to get the tank under the bed,” said John Gargani, vice president, SWN. “Tanks today sit in the bed and take up a lot of the space, which is a drawback to the consumer.”
Michigan Technology Univ., Houghton, Mich., also will participate in the project by helping to optimize material used for the tank. MTU will also conduct testing of the tanks after they are cast by REL Inc.
“There will be mechanical engineering and materials science students and faculty working together at Tech,” said Greg Odegard, professor of computational mechanics, MTU. “Once the tank is fabricated, REL will give it back to us and we’ll test it—put it on a pickup truck, mount it, drive it around. REL and Michigan Tech are positioned ideally to develop this tank.”
With its own fleet of CNG vehicles, SWN has the potential to be REL Inc.’s first customer. A more environmentally friendly fuel than gasoline, CNG is also more affordable, costing drivers between $1-$1.50 per gallon. While recent decreases in gas prices limit cost savings in the near future, the CNG’s cost savings compared to gasoline are expected to be a major advantage of CNG-fueled vehicles in the long-term.
“We have proven the technology is viable,” Adam Loukus said. “The next step is a matter of making it competitive. I think this project has shown a lot of potential. Now it’s about improving our processes and materials so these castings are the best we can produce.”