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]
The 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.
I 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?
Ethanol, 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.
Well, 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.
Lighter 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.)
> OTOH, they're so lightweight that they cannot sustain high compressions (hence anti-knock formulations, most notoriously with leaded fuels).
Anti-knock capability of a fuel has very little to do with how "lightweight" they are. Methane, the lightest hydrocarbon and gaseous at any kind of condition you'll find in an engine, has an octane rating of 120. And diesel fuel, substantially heavier than gasoline, as a much lower octane rating than gasoline.
What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Anti-knock additives (initially ethanol or methanol, later tetraethyl lead, now ... other stuff, including again alcohol) brought up compression ratios and engine efficiency / power. This information I'm remembering from Yergin's The Prize, FWIW.
Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
> What I was aware of was that early automobiles typically ran on what we'd now call "distillate", which were lighter fractions of petroleum, some just barely liquid (I don't know specific components), with a result that air-fuel mixes ignited readily at low compression ratios (say, 6:1, as opposed to current petrol engines which are in the range generally of 8:1 to 12:1, with some high-performance engines going as hihg as 16:1).
Early gasoline was more or less output straight from the refinery distillation tower, yes. Octane rating varied a lot depending on the quality of the crude oil, but usually something in the range of 50-70. Thus necessitating the low compression ratios on those early gasoline engines. But the volatility of that gasoline was approximately similar to modern day gasoline.
What was then developed were various further processing steps to improve the octane rating of gasoline (and as the demand for gasoline increased, to increase the fraction of gasoline that you could get from a given amount of crude oil), like dehydrogenation, catalytic cracking, alkylation etc. First these were used for producing high octane aviation gasoline, but after WWII these processes were also put into use to produce automotive gasoline, enabling higher compression ratios in cars. Anti-knock additives helped a bit as well.
> This information I'm remembering from Yergin's The Prize, FWIW.
A pretty good book, I hear. I should read it.
> Diesel operates at generally higher compression ratios, 14:1 to 23:1 per Wikipedia, which I thought translated to higher octane equivalent, but whatever's impeding ignition point isn't that. I know some (most?) diesel engines are fuel-injected, which permits timing of fuel introduction at maximum compression, but not all as I understand.
Diesels inject ALL of the fuel during the combustion stroke. During the compression stroke, they only compress air. Which is why they can have so high compression ratios, there's no fuel vapor mixed with the air that may ignite and cause knock or detonation. Due to the high temperature and pressure in the air caused by the compression, the fuel ignites by itself more or less immediately as it's injected. No spark plug needed.
If you think about it, diesels want something which is sort-of the opposite of an anti-knock (octane) rating. You want the fuel to ignite by itself as soon as it's injected, not resist ignition. For diesel fuel this scale is called the 'cetane' rating, FWIW.
> I'm doing some online sleuthing about this as I'm curious. Volatility itself may play a role, where petrol vapourises whilst diesel aerosolises. The latter is still a fuel-air suspension but with much lower equivalent surface area (and hence, ignition rate) than a vapour would be.
I believe you're sort-of right here. Diesel fuel is injected under high pressure, modern common-rail injection systems reach injection pressures of up to 2000 bar FWIW, which causes the fuel to be atomized into small droplets. The actual burn process AFAIU is sort-of a liquid burn process where fuel vaporizes from the droplets and immediately ignites.
On The Prize, it's really phenomenal, and that's from someone who disagrees pretty strongly with Yergin on his general cozyness to the petroleum industry and enthusiasm for its future prospects. As a history the book is a brilliant work, there's an accompanying PBS/BBC miniseries, and the wealth of information contained (and number of head-turning new-to-me revelations) can't be briefly described. If you're into that sort of thing, I'd also recommend as much of Vaclav Smil as you can stand, though would suggest starting with Energy and Civilization, a look at human history through the lens of energy.
The octane ratings you give are about what I recall from Yergin's description (if that's where I first heard it, again, somewhat vague decade-plus recollection).
My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Otherwise, appreciate the additional knowledge, it fits pretty well with my own weaker understanding. Interesting especially about cetane. Looking that up, the name comes from Hexadecade, a/k/a C16H34, or a sixteen-chain hydrocarbon (double the carbon-atom count of octane, a/k/a C8H18).
> My understanding of diesel ignition is somewhat informed by WWII-era triple-expansion steamships, which burned bunker fuel, that requiring a lot of heating (utilising spent steam) just to get it flowing toward the boiler, then again getting toasted immediately before going into the burners. External combustion, obviously, but the challenge of getting a very nonvolatile fuel to burn left an impression. That engine room visit left an impression as well....
Yes, bunker fuel is very much non-volatile stuff. As an aside, they did eventually figure out that you could run slow-running big diesel engines on that stuff too. Perhaps you've seen pictures of such massive engines big as houses, if not e.g. https://www.youtube.com/watch?v=K30_jf-aA_U
These big diesels have some things in common with the old triple-expansion steam engines, e.g. they are directly connected to the propshaft (and thus they turn slowly, about 100 rpm max or thereabouts), and the engines themselves are reversible, so there's no need for any reduction gearing or gearboxes.
Similarly to steam ships, they need steam lines in the fuel tanks to heat the fuel so it can be pumped, and then further heated to 130C or thereabouts in order to be injected. So they need a small auxiliary steam boiler just for producing the steam to heat the fuel; modern ships often have an 'exhaust heat recovery boiler', which as the name implies utilizes the hot exhaust from the main engine to produce the steam, so that the auxiliary boiler isn't needed when the main engine is running.
Marine powerplants are indeed cool, and yes, I've seen a few prior videos, both in operation and maintenance with workers literally crawling through the engines.
Many 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.
Now 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
I 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?
Hard 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.
Here'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]
Suggesting 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.
There's millions of radial turbines in cars around the world today. They use an internal-combustion engine for their combustor, and they're called turbo-chargers.
While true, they are not sustaining combustion within the turbo. I believe this is what makes the problem more difficult, and it sounded like what the OC was suggesting.
Turbos float on a layer of motor oil, and have a crude design compared to combustion-sustaining turbines.
What 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).
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.
> Food is a stupid argument invented by the oil companies.
It's not that stupid, we still have many millions of people in utter food insecurity, and not just in the "third world" but also across our Western nation.
IMHO, people should come before cars when it comes to distribution of food, and we should electrify automotive to get rid of the entire issue anyway. What few renewable fuels we have, we will sooner or later need to power air flight and ocean-crossing ships, as we do not have any alternative to some sort of combustible fuel for either purpose.
We have a food distribution problem. The world makes more than enough food to feed everyone.
As an aside, corn ethanol is not made with the type of corn that could be used for human consumption. We could use those same fields to produce human food, but refer back to the first paragraph of this post for why we don't do that.
The GP was talking about microgrids in neihgborhoods (presumably fixed, permanent installations), so there isn't really a reason to worry too much about weight.
Reliability, efficiency/cost of operation, and noise is probably the priorities that come way ahead of weight.
Small and micro gas turbines are already used in the sort of Combined Heat and Power plants used to power commercial / retail buildings, refrigerated warehouses, light industrial units and the like. There's no reason why they shouldn't be used for residential neighbourhoods, especially in colder areas where district heating would be a major selling point.
Compared to reciprocating engine-powered CHP, they tend to produce a slightly higher proportion of their energy output in the form of heat than electricity, and the plant is about about 60% of the volume and half the weight - so they make most sense in constrained spaces or for rooftop installations.
Maybe 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.
A 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.
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.
> 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
[1] https://en.wikipedia.org/wiki/Capstone_Green_Energy