Alan Epstein is quick to tell you he's a "jet engine guy" - just in case you haven’t guessed as much from the turbine engine parts strewn around his office or the museum on his lab’s ground floor, which includes a rare example of a 1944 German engine that helped kick off the jet age. For the director of MIT’s Gas Turbine Laboratory, who stands a slightly stooped five foot six, the fascination has to do with raw power. “The engines on a Boeing 747 shove air through at Mach 1 with 120,000 pounds of force,” says Epstein. “The engines on three 747s put out as much power as a nuclear power plant.”
Gas turbines powered much of 20th-century technology, from commercial and military aircraft to the large gas-fired plants that helped supply U.S. electricity. But these days it isn’t the hulking machines in the lab’s museum that capture Epstein’s enthusiasm. Instead it’s a jet engine shrunk to about the size of a coat button that sits on the corner of his desk. It’s a Lilliputian version of the multiton jet engines that changed air travel, and, he believes, it could be key to powering 21st-century technology.
Though the turbine’s blades span an area smaller than a dime, they spin at more than a million revolutions per minute and are designed to produce enough electricity to power handheld electronics. In the foreseeable future, Epstein expects, his tiny turbines will serve as a battery replacement, first for soldiers and then for consumers. But he has an even more ambitious vision: that small clusters of the engines could serve as home generating plants, freeing consumers from the power grid, with its occasional black- and brownouts. The technology could be especially useful in poor countries and remote areas that lack extensive and reliable grids for distributing electricity. A comparison to how the continuous shrinkage of the integrated circuit drove the microelectronic revolution is tempting. “Just as PCs pushed the computing infrastructure out to users, microengines could push the energy infrastructure of society out to users,” says Epstein.
Epstein’s immediate goal, however, is to use these miniature engines as a cheap and efficient alternative to batteries for cell phones, digital cameras, PDAs, laptop computers, and other portable electronic devices. The motivation is simple: batteries are heavy and expensive and require frequent recharging. And they don’t produce much electricity, for all their size and weight.
The consequences of these failings go beyond consumer inconvenience. Today’s soldiers are often forced to lug around brick-sized batteries to power their high-tech gear. And hamstrung by short-lived power supplies, designers of next-generation electronics are frequently forced to leave out energy-hungry improvements and features like bigger, brighter screens and more powerful processors. Take, for example, the “ultimate PDA” from Frog Design, a Sunnyvale, CA–based firm specializing in industrial design. The device combines multiple cell-phone and Wi-Fi radio protocols, GPS location, a projection screen, the functionality of a laptop, and the ability to browse through video libraries and play full-length movies. But it exists only as a mock-up; it would drain any reasonably sized battery in half an hour. With functions like GPS location and radio communications, “you’re just eating through batteries,” says Valerie Casey at Frog Design.
A micro gas turbine engine would change all that. It could run for ten or more hours on a container of diesel fuel slightly larger than a D battery; when the fuel cartridge ran out, a new one could be easily swapped in. Each disposable cartridge would pack as much energy as a few heavy handfuls of lithium-ion batteries. As a result, a small pack of the cheap and light cartridges could power a PDA or cell phone through several days of heavy usage, no wall-outlet recharging required—a highly attractive feature for soldiers in remote locations or travelers. What’s more, the miniature turbine takes up about a quarter of the volume of a typical cell-phone battery.
Not that a micro engine is without drawbacks. It would shoot a tiny stream of hot exhaust gas, for one thing, making it more suitable for devices clipped to belts or carried in briefcases than for those stuffed in pockets. The engine itself would get hot, though an exhaust suppressor would easily keep devices from getting much warmer than they do today. But for many energy-hungry applications, says Epstein, a tiny turbine’s remarkable power output would far outweigh any disadvantages. Suggests Epstein, “You don’t need a very good jet engine to do better than batteries.”Grounded
Epstein started thinking about building a jet engine on a chip nearly a decade ago. At the time, microelectromechanical systems (MEMS) were picking up speed. Techniques had emerged for carving new types of features into the surfaces of slabs of silicon, including sealed chambers and pipes and moving parts like spinning wheels—most of the parts needed for a gas turbine engine. Less clear at first was what one would do with a miniature fuel-burning engine. “We thought we’d be able to get the cost way down if we could figure out a reason for needing a lot of them,” says Epstein. “But the only thing we could see doing with tiny engines was flying tiny airplanes, and that seemed stupid. Of course, we hadn’t counted on the DoD.”
Sure enough, the U.S. military was suddenly gung ho over the idea of 15-centimeter-long planes that could carry small cameras for surveillance. The engineers at Epstein’s lab were somewhat less enthusiastic; they suspected that getting jet chips that were airworthy would take a couple of decades. Then Epstein latched onto a more immediate military need: freeing soldiers from the batteries that many of them have to lug around to power radios, GPS receivers, night-vision goggles, and other gadgets. And unlike a miniature aircraft engine, a battery-replacing jet chip would have enormous commercial potential.
Other materials scientists and engineers were already beginning to work on ways to shrink power-producing machines to supplement or replace batteries, creating a new field called “power MEMS.” The most popular approach involved shrinking fuel cells, which typically pass hydrogen through a membrane that pulls electrons out to create an electric current. But Epstein was convinced gas turbines were a better way to go, because of their unmatched ability to wring power out of hydrocarbon fuels. The technology becomes even more appealing where minimizing weight and volume is critical, as with portable devices. A jet chip would be at most half the size of a micro fuel cell of equal energy capacity. A gas turbine should also be relatively easy to fabricate, figured Epstein, because it could be built entirely out of silicon, using standard fabrication techniques.
Though Epstein envisioned his micro version working roughly the same way a conventional gas turbine does, much about micro jet engines was a mystery. Would silicon crumble under 1,300 °C temperatures? Could microscopic bearings handle a million-plus revolutions per minute? With funding from the U.S. military, Epstein tapped into the expertise of neighboring MIT labs in fluid mechanics, materials science, structural engineering, and microfabrication. The project team eventually swelled to dozens of researchers, including Mark Spearing, a materials engineer charged with finding ways to keep the silicon microstructures intact under furious heat and pressure. “Most MEMS chips involve etching small structures up to 10 microns tall,” says Spearing. “We were going to parts that are hundreds of microns tall.”In Hand
Earlier this year, Epstein and his coworkers finished making engines in which each of the individual parts functions: the combustion chamber burns fuel, and the turbine blades spin. The resulting device is sealed all around, with holes on the top and bottom for air intake, fuel intake and exhaust. One shortcoming: it doesn’t run continuously. The obstacle, says Epstein, is imperfections that imbalance the blades and cause them to wobble. “We think we know what to do to correct it,” he says. “The problem is that it takes three months to get new parts when you make an adjustment, so we’re just waiting for the new parts.” Epstein predicts the chip will be functioning within months—a little ahead of schedule. Spearing estimates a version capable of putting out enough power to run devices would take two to three years more, with another year or two beyond that to produce a marketable version.
That means conceding an early lead in the power MEMS race to fuel cells, which are already hitting the market. Albany, NY–based MTI Micro Fuel Cells is preparing to launch one the size of a deck of cards for use in handheld industrial devices such as radio-frequency ID-tag readers and has plans to roll out a slightly smaller version for cell phones, PDAs, and digital cameras. Medis Technologies of New York City intends to sell a $20 disposable micro fuel cell next year.
“Our competition is fuel cells, absolutely,” says Epstein. But he insists that turbine chips can make up any lost ground. “Up to now a few million dollars has been invested in microturbines, compared to the billions invested in fuel cells,” he points out. Epstein’s faith is fueled by the inherent advantages he sees in turbines. Even micro fuel cells are larger, and they’re much more finicky about fuel than a turbine engine. But in the end, it all comes down to power. Most micro fuel cells struggle to put out a watt or two, while Epstein’s prototypes could provide 15 to 20 watts, more than enough to keep a power-hungry handheld device going. Laptop computers can require 50 watts, but a few turbines working together could easily pump that much power out. Likewise, Epstein envisions that a cluster of tiny engines, each capable of producing up to a hundred watts, could supply a home with an efficient and reliable source of electricity.
That switchover will surely take time. But Epstein sees it as the natural extension of the remarkable progress jet engines have made throughout the second half of the 20th century, from the novel fighter planes that appeared in World War II to the behemoth engines that power today’s jumbo jets. And though Epstein predicts that, from an engineer’s point of view, his tiny chip-based turbines will initially perform more like the pioneering jets of the 1940s than like today’s superefficient gas turbines, he is fully confident in the technology’s vast potential to evolve. Indeed, the aging engines in his lab’s museum are an ever present reminder of the gas turbine’s awesome power.
20 October 2004