Why there are no heat engine powered laptops, tablets, mobile phones etc? I mean batteries will probably never reach power density of a fuel, so why not use it instead?
I don’t believe it is impossible to build engine small enough, but maybe there is some limit not allowing small engines to be powerful enough?
Maybe there is a problem with dissipating excessive heat?
Is this idea really that unfeasible?
Space, weight, noise, exhaust, fuel supply. Just for starters.
It’s definitely feasible. (This device burns wood, but a petrol-fueled version would obviously work.)
Just how practical this would be is open to question.
Here is a butane-powered fuel cell charger for laptops and phones. I think fuel cells are more efficient than internal combustion engines and produce less waste heat.
heat would be the big one; even a small fuel burning engine is going to get incredibly hot. then you have to have a fuel store for it; and the smaller an engine gets the less efficient it is. Then there’s the exhaust; they would pump out noxious fumes meaning you couldn’t use your device in enclosed spaces. Noise would be a problem. If you wanted to take care of the last two you’d need a catalyst to clean up pollutants (which would get hotter than hell) and a muffler.
plus, an engine that small would pretty much have to be a two-stroke, so there’s another efficiency hit and noise penalty.
A tiny engine to power a cell phone will be extremely inefficient. It is always advantageous for any given fuel to convert the chemical potential energy into useful electrical energy on a big scale. For example, utility power plants reach the 55-60% efficiency level with various energy recapture cycles (like gigawatt sized hydrocarbon burning power plants), whereas car-sized engines have an efficiency of 20%. Who knows what efficiency to expect from a 2 watt engine, but I bet it’s terrible (sorry I can’t be bothered to do the analysis, maybe someone else will want to jump in here). So from a first order analysis it can be shown to be economically unfeasible, not to mention all the other problems already mentioned.
Tempted to see if I could make a small sterling engine to run a generator to charge my phone.
Energy inefficient I am sure.
As pointed out above fuel cells have been tried for this and look borderline promising - and have advantages over batteries like instant refilling , but that converts chemical into electrical w/o going through heat and mechanical to get there.
Apart from all the practical problems listed by others, there’s a more fundamental problem: yes, hydrocarbons are more energy dense than lithium batteries, but you also need an engine and a generator.
So basically your volume and size would scale n for a battery but n/10+100 (to use some nonsense numbers) for the fuel/engine/generator combo. That’s going to work well if you need to power a phone for a year from a single charge, but it’s useless if you can just recharge a battery every night.
For cars hydrocarbon power works out much better because they need much more power and you need motion and not electricity, so battery-powered ones still need a motor and hydrocarbon-powered ones don’t need a generator. And yet, we’re moving towards battery-powered cars.
I wouldn’t mind a miniature gas turbine to charge my phone on camping trips, though.
Isn’t there a problem with fuel cells concerning generating sufficient power? I mean there were talks about them in media for couple years (maybe even a decade) already but I haven’t seen any breakthroughs or practical implementations…
as with most things, there can be quite a long time between “working proof of concept” and “ready to be a shipping product.”
How small of a nuclear reactor would be possible? Three Micrometer Island?
Apart from any of the engineering difficulties, we habitually keep these devices in a powered state in our pockets. How convenient is that going to be, with an internal combustion engine running in there?
You changed units in the wrong direction, it’s centimeters. Wikipedia:
“Of all the common nuclear fuels, Pu-239 has the smallest critical mass. A spherical untampered critical mass is about 11 kg (24.2 lbs),[1] 10.2 cm (4”) in diameter."
Of course you may want to reserve some extra space for a boiler, steam turbine, generator and, last but not least, shielding.
If you want to go small and don’t mind some radiation, antimatter would probably be the way to go.
There are commercial available hydrogen fuel cells for electric forklifts that directly replace the conventional lead-acid battery. However the cost, weight, complexity, and availability of fuel means there will probably never be a AAA fuel cell.
As to IC engines I see two problems. The power density is TOO high and I see problems of scale in making a tiny engine/generator - say the size of a marble. Most technologies have a application sweet spot and the support systems for such an engine might be larger than the engine - the main shortcoming of the automotive Wankel engine.
I wonder though if there are any possibilities for building an internal combustion engine that’s optimized for this kind of use. Maybe burn a small amount of fuel in a chamber slowly, and then use the expanding gasses to put pressure on piezo crystals to generate electricity?
Hmm…that’s creative…I like it. It’s certainly better that miniaturization or some type of solid state energy conversion.
all depends on how you look at it.
any high grade fuel will waste energy in a heat engine as mentioned.
a Stirling engine can use low grade heat that other heat engines can’t.
If you’re willing to include such devices as RTGs, almost arbitrarily small, but those are all less than 10% efficient; most are in the range of 3-7% efficiency. I don’t know how big of an RTG you’d need to make a real difference when it comes to powering (or charging) a cell phone, for example, and I’m pretty sure fuel costs would be prohibitive.
Can’t believe no one has posted this one yet:
Note: It is a commercial, but relevant to the subject.