SMA-based 'energy-scavenging device' could convert exhaust heat into useful power

  • 02-Mar-2010 06:52 EST
SMA heat exchange.jpg
General Motors is working on a device that may be able to extract energy from the waste heat in engine exhaust, perhaps enough to raise fuel economy by as much as 10%.

Getting something for nothing is almost always desirable. Witness the never-ending fascination with so-called perpetual motion machines. But given that thermodynamics proscribes such fanciful systems, probably the next best thing is a device that squeezes benefits out of what is normally wasted.

That’s the promise of an effort by General Motors to develop a simple mechanism that could recover a portion of the energy that is regularly lost in engine exhaust. About 40% of the energy content of the fuels burned by even the most efficient internal-combustion engines exits in the form of hot exhaust fumes.

Late last year, GM received $2.7 million in grant money from the U.S. Department of Energy’s Advanced Research Projects Agency - Energy (ARPA-e) to develop a prototype waste energy-recovery device that is based on shape-memory alloys (SMAs)—a kind of smart material that changes its physical dimensions when exposed to heat.

“When you heat an SMA wire, it gets stiffer and shrinks to its original, prestretched length,” explained Jan Aase, Director of GM’s Vehicle Development Research Laboratory in Warren, MI. “When you cool it, you can stretch it out with an opposing force—often, another SMA wire or return spring.”

The basic concept behind the proposed energy-scavenging device, he continued, is that “you wrap the wire around a pair of pulleys—one hot, one cold. As long as you maintain the temperature difference, the SMA wire will turn the pulleys.”

Aase noted that this principle underlies a popular science-education toy that features two pulleys, a length of helical SMA wire, and two reservoirs—one filled with hot water, the other with cold. “Just looking at it run, you’d think that it was some kind of perpetual motion machine, but it’s not,” he said. “It’s a functional engine.”

The same fundamental principles that make the toy work underlie the proposed energy-recovery device now under development, Aase continued. In one design concept, a radial array of cylindrical pulleys that looks a bit like a bundle of cigars fits around an auto exhaust pipe. Each inner tubular pulley is tied, via an SMA wire coil, to an outer pulley pipe. During operation, hot exhaust fumes would be directed through the inside pulleys while cooler, ambient air collected from outside would flow through the exterior ones. As the SMA wires are alternately cooled and heated by contact with the pulleys, they would contract and expand, causing the pulleys to move, and thus rotating the entire assembly. The spinning ensemble could drive an electric generator or a mechanical takeoff.

“This is a high-risk, high-impact project,” he warned—one eminently suited for governmental funding efforts aimed at promoting transformational technological change. “We don’t know as yet what the system efficiency might be or just how much energy we might be able to convert to useful power, but we’re hoping to get equivalent fuel-efficiency gains of up to 8 to 10% at highway speeds,” Aase stated. “If it were able to run the electrical accessories—the radio, A/C, power steering, etc.—it would constitute a significant improvement.”

And should the novel technology be successful in cars, the same basic device would probably be able to grab energy from the waste heat produced from stationary sources such as factory smokestacks, home furnaces, hot water heaters, and the like. “In fact, if this thing works, it would be a real game-changer,” he concluded.

The key to the project is an improved SMA that was developed in recent years by Dynalloy, GM’s SMA supplier and consultant in Tustin, CA. The SMA of interest is a titanium-nickel alloy that was originally produced at the U.S. Naval Ordnance Laboratory (now the Naval Surface Warfare Center). With the application of heat, the metal transforms from a martensitic to austenitic crystal structure, which alters its overall dimensions.

The material, known generally as Nitinol (N for nickel, Ti for titanium, and NOL for Naval Ordnance Lab), exhibits substantial resistance to corrosion and recovers well from relatively large deformations (typical strains of 3 to 5%). In the past, such smart metals could perform their expand/contract trick only so many times before microstructural fatigue set in, but specialists at Dynalloy have figured out how to reliably get tens—even hundreds—of millions of hot/cold actuation cycles out of the alloys.

“I have a toy butterfly that flaps its wings under Nitinol power when heated by sunlight,” Aase shared. “It just keeps going, so much so that the wings have lost their pigment from sun exposure.” He estimates that the flapping gadget has already undergone 1.5 million actuation cycles. “By the time I retire, it’ll have hit 10 million cycles.”

According to Wayne Brown, President and founder of Dynalloy, “the very high repeatability and stability of our Flexinol material is the result of our proprietary processing—that is, the way we draw the wire through the die and the way we anneal it afterward.” He cautioned, however, that “using this material requires a thorough understanding of its idiosyncrasies. You must recognize that how the material behaves depends on how it’s used.”

Brown advised that potential users need to study any new application in detail to match the material to the proposed use.

“Nitinol is a difficult material to handle,” noted Aase. “In applications, you have to keep your materials strain fairly conservative: 10 N of force is OK, 100 N is not.” But if successfully managed, “an SMA solution can be cheaper, lighter, and more reliable than small electric motors and solenoids.” SMA devices can be packaged in smaller volumes—“in places where motors don’t go.” They also offer quiet operation and do not emit electromagnetic noise.

GM has developed an extensive patent portfolio in SMA technology in recent years, having obtained 60 patents, and has come up with several hundred applications, some of which are targeted for production during the next few years. Devices are typically activated using electrical resistance heating, but ambient temperatures can be employed in certain cases.

The list of potential automotive uses is long, including seat components, mirrors, latches, handles, hatches, door locks, louvers, and valves. Other automakers such as Daimler and Porsche have successfully placed various SMA-based actuators into high-volume production during the last decade.

To have the best chance of getting things right, GM has assembled a team of specialists to produce the novel heat-recovery device. For material testing and characterization, the company is working with materials scientists at HRL Laboratories, a research facility that is jointly owned by GM and Boeing.

Predictive modeling of the system’s materials and mechanical behavior will be conducted at GM’s Technical Center in Bangalore, India. And the detailed design duties will be handled by researchers at the GM Collaborative Research Laboratory in Advanced Vehicle Manufacturing at the University of Michigan in Ann Arbor.

GM plans to have a 100-mW/g lab prototype ready for evaluation by ARPA-e by the end of 2010 and a bench demo model (1 W/g) completed by the end of the next year.

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