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I've always been fascinated by the power density potential of the gas turbine. Especially the micro turbine class. > The MT power-to-weight ratio is better th
by bob1029 2y ago
I've always been fascinated by the power density potential of the gas turbine. Especially the micro turbine class.
> The MT power-to-weight ratio is better than a heavy gas turbine because the reduction of turbine diameters causes an increase in shaft rotational speed. [0]
> A similar microturbine built by the Belgian Katholieke Universiteit Leuven has a rotor diameter of 20 mm and is expected to produce about 1,000 W (1.3 hp). [0]
Efficiency is not fantastic at these scales. But, imagine trying to get that amount of power from a different kind of thermodynamic engine with the same mass-volume budget. For certain scenarios, this tradeoff would be amazing. EV charging is something that comes to mind. If the generator is only 50lbs and fits within a lunch box, you could keep it in your car just like a spare tire. I think the efficiency can be compensated for when considering the benefits of distributed generation, cost & form factor.
One of the other advantages of the smaller engines is that you can use techniques that are wildly infeasible in larger engines. For example, Capstone uses a zero-friction air bearing in their solutions:
> Key to the Capstone design is its use of air bearings, which provides maintenance and fluid-free operation for the lifetime of the turbine and reduces the system to a single moving part. This also eliminates the need for any cooling or other secondary systems. [1]
[0] https://en.wikipedia.org/wiki/Microturbine https://en.wikipedia.org/wiki/Microturbine
[1] https://en.wikipedia.org/wiki/Capstone_Green_Energy https://en.wikipedia.org/wiki/Capstone_Green_Energy
- nick3443 2y agoTiny nitro RC engines can produce 1+ horsepower in engines that weight 1/2 lb.
- esperent 2y agoI guess nobody cares about efficiency in their model car engine, so it doesn't matter if you need to refuel every 5-10 minutes. But that would be a problem for pretty much any other use case. Does anyone know how the efficiency per liter of engine volume compares to these small turbine engines?
- nick3443 2y agoHard to compare as it depends on application, the shaft speed differential is huge or you're comparing jet thrust to propeller thrust. I don't think small turbines (like RC jet turbine size) are usually very efficient as they are working against the surface area ratio of heat loss through the chamber walls and Reynolds number effects on the turbine blades.
- bob1029 2y agoHow long can these engines be ran at rated power before you have to overhaul or replace?
- jcgrillo 2y ago30-50hr before they need to be re-sleeved and given a new piston
- mppm 2y agoThe reason why microturbines are not taking off is, as you mentioned, low efficiency. "Not fantastic" is a bit of an understatement. Especially if you want the turbine to be reasonably cheap (no superalloys, etc) and if it runs below maximum capacity, you'd probably be happy to get 15-20% out of it, not even half of what is achievable with ICEs of the same size. There are not many applications where power-to-weight-ratio is important enough to overcome that limitation.
- ahartmetz 2y agoI just calculated it for 100 ml of methanol. 4.4 kWh/l / 10 * 0.15 = 66 Wh. Enough to charge a laptop once. Yeah, I expected more from chemical fuel somehow. Gasoline and diesel have twice the energy density, but do you really want to carry that smelly, messy stuff with you?
- kragen 2y agoEthanol, canola oil, or baby oil might be reasonable things to carry with you if you want to lighten your backpack or just reduce your risk of blindness.
- ahartmetz 2y agoWell, obviously you are not supposed to drink it! For reasons that I don't know, methanol is more commonly used as fuel than ethanol. A nice thing about methanol and ethanol is that they evaporate without a trace if there is a minor spill. That is not true for most any distilled petroleum product or any vegetable oils.
- dredmorbius 2y agoLighter weights of petroleum oils (from petrol through natural gas) are highly volatile and will typically evaporate with minimal (though probably nonzero) residue. That's what makes them attractive as fuels generally as they require little persuasion to vapourise. OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels). Vegetable oils are nonvolatile, but also generally nontoxic and hence mostly environmentally benign. (You can choke a river or foul ground-dwelling creatures given sufficient quantities, but a few 100 ml won't cause major problems.)
- grapesodaaaaa 2y agoSuggesting a turbine could go in a gas car on size/weight alone isn’t a great idea. I’m saying this as someone in the aviation industry. Turbines are amazing pieces of machinery and incredibly reliable, BUT incredibly expensive to operate. They require all kinds of specialized maintenance and what I would call “exotic” oils that won’t break down in the harsh environment. It’d make a really great generator for a vehicle, but I don’t think the economics will work out for a family car anytime soon.
- pm90 2y agoWhat about for “microgrids”? If it was possible for a household (or neighborhood) to install one and run completely on corn based ethanol… that might be something better than the IC generators we have today (I understand that corn ethanol isn’t completely green).
- chiph 2y agoMaybe for a remote cabin? One thing I think might be a problem when grid-connecting them is their lower rotational inertia might make it harder to match/keep frequency. Unless it has very good speed regulation.
- salynchnew 2y agoWhy wouldn't this remote cabin be better off with wind or solar?
- chiph 2y agoThose would be my first choice too but there might be trees that block the sunlight, or are taller than a wind turbine tower. They'd certainly be quieter than a microturbine and not need fuel brought in.
- SoftTalker 2y ago> corn based ethanol This is an idea that needs to go away. We should not burn food for fuel, and there are a lot of externalities in growing corn and then turning it into ethanol that people are not considering. Corn-based ethanol is just a very inefficient form of solar energy. Use solar panels instead and skip the middleman.
- pfdietz 2y agoNow look at the power density (and power/$) of rocket engines. A Falcon 9's Merlin 1D engine is reported to cost $400K. Its jet kinetic power in vacuum is 1.5 GW, in an engine with a mass of ~500 kg. $0.27/kW is insanely cheap for a heat engine.
- generj 2y agoA Merlin’s lifetime run-time, even with 25+ reuse launches, is just a hair over two hours (162 first stage time times 25 times two for the static burn). That’s assuming the high reuse stages keep all the engines even. There are likely some compromises engineers can make when the engine is only running for that amount of time with refurbishment in between each 6 minute runtime.
- deleted 2y ago[deleted]
- RachelF 2y agoMany years ago, I worked for what would now be called a startup building small gas turbines. The turbine was impressive, 400hp in something the size of 2 shoe boxes. However, it spun at 120,000rpm, which meant either a very heavy gearbox or electrical generator had to be connected to it. High rpms, noise and the difficulty in adjusting the power output quickly, killed the project.
- faceloss 2y ago[dead]
- LtdJorge 2y agoNow I'm thinking of the Koenigsegg Dark Matter, "an 800 hp, 1250 Nm patent-pending Raxial Flux e-motor". What if it was used as a generator? It's 39Kg, although 6 phase. Well, I'm dumb, it says max motor RPM 8500, so I don't think you'd get close to what's needed as a generator :D
- mschuster91 2y ago> Well, I'm dumb, it says max motor RPM 8500, so I don't think you'd get close to what's needed as a generator :D 20:1 reduction gear, off you go.
- hinkley 2y agoI had no idea Capstone was still around. Their idea was cogeneration, but I’m not sure if the math works out if you have a low efficiency turbine. We just usually don’t need that many BTUs to run a water heater and furnace versus electricity to run everything else. And with heat pumps becoming more of a thing that’s just becoming more apparent.
- jabl 2y agoWell, per wikipedia: "On September 28, 2023, Capstone Green Energy declared Chapter 11 bankruptcy"
- araes 2y agoHere's the PowerMEMS Project page from the link you're referring to. Unfortunately, seems like the last update was from 2010. Haven't heard much since. [1][2][3] [1] Turbine Overview: https://www.powermems.be/gasturbine.html https://www.powermems.be/gasturbine.html [2] Turboshaft Setup: https://www.powermems.be/Turboshaft.html https://www.powermems.be/Turboshaft.html [3] 1,200,000 RPM on Aerodynamic Bearings Test Runs: https://www.powermems.be/Pen_setup.html https://www.powermems.be/Pen_setup.html There's a little bit further from the author (Tobias Waumans) afterward, yet not much publication [4] [4] https://scholar.google.com/scholar?hl=en&as_sdt=0%2C13&q=T.+Waumans&btnG= https://scholar.google.com/scholar?hl=en&as_sdt=0%2C13&q=T.+... Mostly a summary pub on the work on the aerodynamic bearing setup in Journal of Micromechanics and Microengineering [5] [5] Aerodynamic Bearing (pdf): https://lirias.kuleuven.be/retrieve/160403 https://lirias.kuleuven.be/retrieve/160403
- djmips 2y agoCapstone filed for chapter 11 in 23. I wonder what's the fate of their tech.