Jet Engine on a Chip 463
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."
Re:exaust (Score:4, Informative)
Well, your cell phone only needs about a watt, a PDA about 2-10 watts, and your laptop about 20-100 watts. If you consider that cars produce kilowatts of constant power output, you should realize that the amount of exhaust shouldn't be anywhere close to what your car puts out.
In addition, these turbines will probably use something a smidge cleaner than gasoline. Even kerosine is better, but ethanol would probably rank the cleanest.
Speaking of kerosine, these turbines shouldn't even be as back as burning a kerosine lamp.
Re:Cool, but misleading title (Score:5, Informative)
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??? (Score:5, Informative)
In all seriousness, why does
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 (Score:5, Informative)
Re:Cool, but misleading title (Score:5, Informative)
That used to be true, but the current breed of Gas Turbines are amazingly efficient. From wikipedia [wikipedia.org]:
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.
Re:Brilliant (Score:1, Informative)
Of course, all that goes straight out the window if you get energy from nuclear, wind, hydro, or solar.
See same story from 1997 (Score:5, Informative)
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 [bairdtech.com] 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: (Score:5, Informative)
Re:exaust (Score:2, Informative)
The potential problem with these things... (Score:4, Informative)
That's why you don't see very many working concepts of small aircraft (the kind that fit in the palm of your hand) with what most people recognize as wings. They're usually equipped with small flat-plate type wings, or a ribbon-like system like on a cuttlefish.
And the reason that many folks that do happen to understand the physics don't try and do things at such small scales is that the problem is difficult. Not impossible, but difficult.
As a person with a background in fluid mechanics, I don't see how the approach in the article will ever work well or efficiently. It might work, but it's not using the kind of principles that you need. (The whole point of my post is that you can't scale a device down without adjusting or remaking how it does what it does. The physics change.)
Re:exaust (Score:3, Informative)
The article mentions diesel, which makes the whole thing sound like it will be messy and smelly. Changing cartridges you'd probably get some in the air, and it doesn't smell good. Using the device, then (because you're in a hurry, say) quickly sticking it in your shirt pocket while the engine is still winding down, you'd be smelling like diesel exhaust the rest of the day. Yuck...
Re:Somebody want to geek out for me? (Score:4, Informative)
The sun is a mass of incandescent gas... ^H^H oops sorry... wrong comment.
Here's the lowdown on fart gas content, for those interested in such things:
(source: Facts on Farts [heptune.com])
What is fart gas made of?
The composition of fart gas is highly variable.
Most of the air we swallow, especially the oxygen component, is absorbed by the body before the gas gets into the intestines. By the time the air reaches the large intestine, most of what is left is nitrogen. Chemical reactions between stomach acid and intestinal fluids may produce carbon dioxide, which is also a component of air and a product of bacterial action. Bacteria also produce hydrogen and methane.
But the relative proportions of these gases that emerge from our anal opening depend on several factors: what we ate, how much air we swallowed, what kinds of bacteria we have in our intestines, and how long we hold in the fart.
The longer a fart is held in, the larger the proportion of inert nitrogen it contains, because the other gases tend to be absorbed into the bloodstream through the walls of the intestine.
A nervous person who swallows a lot of air and who moves stuff through his digestive system rapidly may have a lot of oxygen in his farts, because his body didn't have time to absorb the oxygen.
According to Dr. James L. A. Roth, the author of Gastrointestinal Gas (Ch. 17 in Gastroenterology, v. 4, 1976) most people (2/3 of adults) pass farts that contain no methane. If both parents are methane producers, their children have a 95% chance of being producers as well. The reason for this is apparently unknown. Some researchers suspect a genetic influence, whereas others think the ability is due to environmental factors. However, all methane in any farts comes from bacterial action and not from human cells.
Re:See same story from 1997 (Score:3, Informative)
And, if I recall correctly, they use fuel very inefficiently; the fuel consumption is like nothing else you've ever seen. The friction losses on this thing would probably be far worse, so unless there's breakthroughs in the design elsewhere I'd be very surprised if you could carry enough fuel to keep the things powered up.
Re:exaust (Score:2, Informative)
And, interestingly enough, jet fuel IS kerosene. Jet fuel is sold as "JET-A", and it's essentially kerosene like you would put in a kerosene heater. Any traveler who has walked out onto an airport tarmac to board a flight has smelled the exhaust from the jets, and it smells just like a kerosene heater.
Re:1 million rpm? (Score:2, Informative)
Lets assume r=0.25 cm (the centre of gravity of a blade with the 5 mm length).
The speed of the centre of gravity= 1.000.000rpm
Lets assume the blades have a total weight of 0.1g (very assumable, the thing is about 1cm*1cm*2mm, the fill factor should be less than 0.3, lets assume 10g/cm^3.
That means the energy stored in the blades is 0.5 m v^2= 0.5*10^-4kg*67600m^2/s^2=3.38 joule.
Less than a airgun.
Inside a metal enclosure.
No danger.
Re:What about noise polution? (Score:0, Informative)
Does anyone remembers the old ultrasonic remote-controls of the 80s? I can tell you one thing: they worked on cats too, as well as on TVs!
Re:1 million rpm? (Score:4, Informative)
Assuming the (very rough) idea of the blades as a solid disc, a 10 mm turbine blade (which is what is suggested for a 20 W turbine, running at 100krpm, from other experimental papers I've seen) comes out as follows:
I =
Density of silicon nitride, a commonly mentioned blade material, is 3.28 g/cc.
Volume of a solid disc 10 mm blade, assuming it is 1 mm in thickness (a value pulled from some of the experimental papers), would be pi*r^2*h, giving us pi*25*10^-6*1*10^-3, or 2.5*10^-8 m^3.
2.5*10^-8 m^3 is 2.5*10^-2 cm^3, yielding a mass of 3.28*2.5*10^-2, or 8.2*10^-2 grams, which is 8.2*10^-5 kg.
Thus, moment of inertia is
The correct equation for energy is
So, at 100krpm (2*pi*100000/60 rads/s), the turbine I'm thinking of is carrying:
Even if its spinning at 1 million RPM, we get:
TNT explodes with an energy of 2.175*10^6 J/kg, meaning that the turbine disintegrating at 1 million would yield something roughly equivalent to 0.025 grams of TNT. Not exactly a big explosion.
Yes, it is spinning very, very fast, but it is also very very small and very very light. These counterbalance the speed.