Microturbines headed back to the future

Nobody’s talking about generating 1.21 gigawatts of power as was required in the “Back to the Future” movie, but microturbine research is headed back to where it was in early 2007. Dr. Alan Epstein, one of the prime movers behind the micro-engine project at the Massachusetts Institute of Technology, expected to have a micro-engine producing power by the late spring of that year, and a fully integrated device ready for commercialization within 5 years.

In the interim, however, the research hit a wall at MIT and has moved to the University of Maryland in College Park, Md. Dr. Reza Ghodssi, a member of the MIT team from 1997 to 2000 and now the Herbert Rabin Distinguished Professor and Director of the Institute for Systems Research with the A. James Clark School of Engineering at the University of Maryland, continued to work on such a device—though with one major difference: Ghodssi is using stainless steel ball bearings to support the rotor as opposed to air bearings.

In 1995, while pondering how a large turbine could power an entire city, such as the one above, produced by Siemens AG, Dr. Alan Epstein first thought about creating a microturbine to meet one person’s needs. A cross section of the microturbine (left below) created at the University of Maryland reveals the location of the micro ball bearings, which are contained in a notch in the edge of the turbine below the rotors (right below). Main photo courtesy of Siemens AG; photos below courtesy Reza Ghodssi, Matthew McCarthy, C. Mike Waits, Mustafa Beyaz, Brendan Hanrahan, University of Maryland.

And Ghodssi seems confident a fully integrated microturbine will be ready to demonstrate within 4 to 5 years.

To truly appreciate this feat, we need to go back to 1995. That’s when it occurred to Epstein, now retired, that if a large turbine could power a city, then a tiny turbine should be able to supply enough electrical power for one person’s needs. It was, if you will, his “flux capacitor” moment.

With a team of researchers, the professor in MIT’s Department of Aeronautics and Astronautics set out to develop a tiny turbine, which Epstein theorized could have a thrust-to-weight ratio of 100:1. Perhaps with his own nod to the “Back to the Future” movie franchise, he playfully observed in a May 23, 1997, issue of Science that it would be theoretically possible to use some 1,400 microturbines to levitate a skateboard.

By early 2007, there were no levitating skateboards, but the MIT team had used silicon wafer technology to construct a prototype microturbine one layer at a time.

“We showed that we could run this [device] at high temperatures,” said Stuart Jacobson, the former deputy director of the MIT micro-engine project. “We burned inside the combustion chamber, [and] the device sped up as you would expect when you’re putting a high-temperature gas through the turbine. And that basically showed the next step—that we could integrate the turbine machinery, the bearings and the combustion chamber in a single device. And that’s where things sort of closed out.” Funding from the U.S. Army Research Laboratory came to an end.

While the team didn’t run into any “show stoppers,” noted Jacobson, the vital question that remained unanswered was whether the device could be manufactured at yield levels that would allow it to be sold at a reasonable price.

Ghodssi credits the MIT project for sparking the creation of the annual Power MEMS Conference and generating a body of knowledge, technology and basic science that benefits similar research elsewhere—including his own.

“When I went to Maryland,” Ghodssi recalled, “I realized that from my past work [at MIT and during his master’s research at the University of Wisconsin-Madison] that a lot of the difficulties of developing the microturbine really lie in the complexity of the fabrication because the scales are so rigid. To meet the specs, you need a perfect fabrication process that by itself is very complex because you have multilayer films coming together.

“So I decided to take an in-between approach,” Ghodssi continued. “I thought about using stainless steel ball bearings on a small scale.” Though doing so produces a slower device, the simplicity appears to have paid off. In the University of Maryland MEMS Sensors and Actuators Lab, Ghodssi said the microturbine has reached speeds up to 95,000 rpm.

“If you can reach 150,000 rpm,” he added, “you can start generating electricity.” And Ghodssi said he expects to do that very soon.

Using 285µm-dia. stainless steel ball bearings eases fabrication because the spinning plates sit on ball bearings. “You don’t have to fit them in place and hold them with pressurized air,” as with the MIT device, he noted.

A critical breakthrough for the use of ball bearings, said Ghodssi, came from Dr. C. Mike Waits (currently at the Army Research Lab in Adelphi, Md.), who introduced the idea of supporting the microturbine with a planar-contact encapsulated ball bearing.

With this encapsulated approach, the University of Maryland team no longer had to worry about handling the ball bearings one by one using tweezers. Ghodssi said encapsulating them made the fabrication process much more robust from a manufacturing standpoint.

Ghodssi said he hopes to have a microgenerator ready to add to the microturbine within the next year. The goal in the next 2 to 3 years is to integrate the microgenerator with an off-the-shelf micro-engine.

“So once the device is working,” observed Ghodssi, “it will be robust and reproducible.”

And once that moment arrives, the microturbine could power anything from unmanned aerial vehicles to any number of digital devices used by U.S. troops, who could then do away with about 20 lbs. of lithium-ion and other batteries that they now must carry into battle.

Asked whether the current microturbine device, which measures 23mm × 23mm × 1.5mm, could get smaller, Ghodssi noted that there are 150µm-dia. ball bearings available. But first he wants to prove the existing device can work.

Now, if only we could hop in the DeLorean, we could find out what happens! µ