Jet Engine on a Chip

Roland Piquepaille writes "Today, our handheld devices are powered by batteries, which are heavy and inconvenient. Fuel cells are just arriving on the market as a replacement. But there is a new contender: micro gas turbine engines under development at the MIT. Engineers there shrunk jet engines to the size of a coat button. And their blades which span an area smaller than a dime can spin a million times per minute and produce enough electricity to power your PDA or your cell phone. While there are still a few hurdles to overcome, these micro turbine engines should be operational in two or three years, with commercial products available four years from now. These micro jet engines also have the potential to free soldiers or travelers from carrying heavy batteries. The engineers even think their engines on a chip could be used in poor countries to bring electricity there. This summary gives you the essential details about a technology which promises to free us to carry extra fuel instead of batteries."

Comments

Re:Cool, but misleading title

[kaszeta]

Gas Turbines are some of the most efficient fuel -> energy converters known to man.

Actually, in terms of the overall thermodynamic efficiency, they aren't all that great. 40% efficiency is *very* good for a Brayton cycle (i.e. turbine engine) system, but is fairly easily done with a large-scale steam system. Microturbines tend to run around 25%, which means that (a) you need a fairly big recuperator to run efficiently (which doesn't seem to be part of the MIT design), and (b) you need to be able to reject a lot of waste heat (so running your laptop on one of these means you'll be blowed 200+ watts out the back).

Not that gas turbines are without their advantages. Their specific power (weight per kW) is very good, so for the same amount of power the engine is very light compared to most other engine types (which is why they use them in aircraft). They also start and stop quickly compared to steam turbine systems. And they can be nicely combined with other systems like a steam system to make a combined cycle, the whole system can be fairly efficient.

But, by themselves, they aren't all that efficient.

Can we PLEASE STOP linking to this guy's blog???

[sczimme]

In all seriousness, why does /. continue to link to his ramblings instead of to articles that contain real, useful, technical content?

Yes, this is probably off-topic (as in "not about tiny turbines") but it is still relevant. At least give us the option to ignore him.

Storage

[Andy Dodd]

Long-term storage of hydrogen is still a bit of a problem. Hydrogen has a tendency to penetrate ANYTHING you try to store it in, resulting in hydrogen embrittlement. In short, anything you store hydrogen in (esp. pressurized hydrogen) will eventually become weakened by the hydrogen permeating it.

Re:Cool, but misleading title

[AKAImBatman]

Why do you say that gas turbines are some of the most efficient fuel to energy converters known to man? Every link I can find in a google search says otherwise.

That used to be true, but the current breed of Gas Turbines are amazingly efficient. From wikipedia:

They can be particularly efficient -- up to 60 percent -- when waste heat from the gas turbine is recovered by a conventional steam turbine in a combined cycle.

The primary issue in obtaining high efficiencies is in (as you stated) efficiently recycling the waste heat. I can only assume that the inventors would be attempting to shrink the secondary cycle along with the gas turbine. The physics really aren't all that different, so it should just be a matter of materials.

Also from the wikipedia link above:

Typical micro turbine efficiencies are 20 to 35 percent. When in a combined heat and power system, overall efficiencies of greater than 90 percent may be achieved.

See same story from 1997

[Animats]

Here's a very similar article from 1997, from the same guy at MIT, making about the same claims. They sounded closer to success back then.

They've been working on this since 1993, and in 1997 they said they'd have it working in three years. In 2004, they say they'll have it working in three years.

It doesn't work yet. They can fabricate the individual parts, but it doesn't really generate power.

It's not an unreasonable idea, but if this was going to work, there should already be little gas turbine powerplants a few inches long, machined out of metal by standard techniques. The smallest turbines available weight around 1.5Kg, and are used for model aircraft, and they don't have to run for very long. There's a "microturbine" industry, but they mean 10KW units taller than a man.

Little turbines are hard. Automotive turbines and light-plane turbines have been attempted many times, but have never been cost-effective.

Two lessons for you:

[Engineer-Poet]

1. Wikipedia is not trustworthy.
2. The microturbines the authors appear to be talking about are in the neighborhood of 30 kilowatts and bigger, not 15 watts. A non-micro turbine would have an output of megawatts; some are capable of hundreds of megawatts.