November 2004

Power on a Chip

Batteries are heavy and inconvenient. Their successors could be tiny jet engines that provide more than enough power for cell phones and PDAs.

By David H. Freedman

Alan Epstein is quick to tell you he's a "jet engine guy" - just in case you havent guessed as much from the turbine engine parts strewn around his office or the museum on his labs ground floor, which includes a rare example of a 1944 German engine that helped kick off the jet age. For the director of MITs 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 isnt the hulking machines in the labs museum that capture Epsteins enthusiasm. Instead its a jet engine shrunk to about the size of a coat button that sits on the corner of his desk. Its 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 turbines 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.

Epsteins 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 dont produce much electricity, for all their size and weight.

The consequences of these failings go beyond consumer inconvenience. Todays 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, CAbased 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, youre 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 requireda highly attractive feature for soldiers in remote locations or travelers. Whats 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 turbines remarkable power output would far outweigh any disadvantages. Suggests Epstein, You dont need a very good jet engine to do better than batteries.