8 ms·

Iron has a specific energy of 4.9 MJ/kg (for comparison: wood 18, coal 26-33, natural gas 53.6, jet fuel 43, diesel 45.6, gasoline 46.4). So having ships, plan

by nuccy 6y ago

Iron has a specific energy of 4.9 MJ/kg (for comparison: wood 18, coal 26-33, natural gas 53.6, jet fuel 43, diesel 45.6, gasoline 46.4). So having ships, planes or even cars powered by burning iron seems to be unfeasible (at least from energy production standpoint). Albeit iron has much higher density, so if weight is not a problem then it may work. Iron has energy density of about 40 MJ/liter which is comparable with coal, diesel, petrol, which have ~34-40 MJ/liter.

Karliss 6y ago
That's still 5-10 times higher than lithium based batteries (0.3-1MJ/kg) and those are being used in cars and almost good enough for some airplane use cases. Heat engine efficiency isn't as good as electric motors, but it isn't bellow 20%. Downside is that recharging by converting back to iron probably can't be done within device. While pouring in metal powder in for fast refilling might be doable getting it out is probably messier.
- BlueTemplar 6y ago
  You would need a steam engine to convert heat into work, which is clearly out of the question for planes, and might make cars and trucks impractical. Maybe rail and boats though ?
- nuccy 6y ago
  Yes, but even best combustion engines have at very best 50% efficiency using direct transformation of thermal energy through gas expansion into mechanical energy. This involves numerous technologies, which better mix fuel and air, inject fuel directly, adjust timing of injection and exhaust following the RPMs, etc. Cars don't generate steam or use any other indirect method of transformation of thermal energy into mechanical energy, due to losses and impracticality (otherwise we would see other types of combustion engines). Burning of powder iron would not work with normal ICEs since powders are not liquids, they are abrasive and burn residuals are not gaseous (rust dust, which is even more abrasive). At very least this is non trivial engineering challenge. So 5-10 times lower specific energy of lithium batteries wins because of simplicity (and lower weight) of whole power-train (inverters, engines) and their high overall efficiency (90-95%). But indeed, on big ships (e.g. tankers), which can have a steam turbines, iron might be a feasible fuel (especially if using onboard solar power while cruising it is possible to recover part of it as a fuel again).
  - ClumsyPilot 6y ago
    I this is is largely on point - this should work for larger ships that have steam turbines. Also, said turbines have better efficiency than an ICE engine.
    - xyzzyz 6y ago
      They don't, actually. Steam turbines are less, not more efficient than marine diesel engines. In modern marine propulsion, turbines are used pretty much only on gas and coal carriers.
  - ncmncm 6y ago
    Ships get well north of 70% efficiency on the same cycle: technically, the Diesel cycle. (It is named after the person, not the fuel.) Most cars run on the Otto cycle, less efficiently. (It is named after the person, not the vehicle.:-)
trhway 6y ago
>So having ships, planes or even cars powered by burning iron seems to be unfeasible aluminum of course. It is cheap too and has higher specific energy than iron. Using aluminum-air fuel cell instead of burning makes the efficiency about 2 times higher. The actual cars powered by aluminum have range up to 2000km per refueling.
ClumsyPilot 6y ago
It us perfectly feasible on ships, they are not restricted on mass very much and their fuel is dirty and not very efficient. Although we could have really fast nuclear powered container ships and the whole problem goes away

by nuccy 6y ago

Karliss 6y ago

That's still 5-10 times higher than lithium based batteries (0.3-1MJ/kg) and those are being used in cars and almost good enough for some airplane use cases. Heat engine efficiency isn't as good as electric motors, but it isn't bellow 20%. Downside is that recharging by converting back to iron probably can't be done within device. While pouring in metal powder in for fast refilling might be doable getting it out is probably messier.

BlueTemplar 6y ago
You would need a steam engine to convert heat into work, which is clearly out of the question for planes, and might make cars and trucks impractical. Maybe rail and boats though ?
nuccy 6y ago
Yes, but even best combustion engines have at very best 50% efficiency using direct transformation of thermal energy through gas expansion into mechanical energy. This involves numerous technologies, which better mix fuel and air, inject fuel directly, adjust timing of injection and exhaust following the RPMs, etc. Cars don't generate steam or use any other indirect method of transformation of thermal energy into mechanical energy, due to losses and impracticality (otherwise we would see other types of combustion engines). Burning of powder iron would not work with normal ICEs since powders are not liquids, they are abrasive and burn residuals are not gaseous (rust dust, which is even more abrasive). At very least this is non trivial engineering challenge. So 5-10 times lower specific energy of lithium batteries wins because of simplicity (and lower weight) of whole power-train (inverters, engines) and their high overall efficiency (90-95%). But indeed, on big ships (e.g. tankers), which can have a steam turbines, iron might be a feasible fuel (especially if using onboard solar power while cruising it is possible to recover part of it as a fuel again).
- ClumsyPilot 6y ago
  I this is is largely on point - this should work for larger ships that have steam turbines. Also, said turbines have better efficiency than an ICE engine.
  - xyzzyz 6y ago
    They don't, actually. Steam turbines are less, not more efficient than marine diesel engines. In modern marine propulsion, turbines are used pretty much only on gas and coal carriers.
- ncmncm 6y ago
  Ships get well north of 70% efficiency on the same cycle: technically, the Diesel cycle. (It is named after the person, not the fuel.) Most cars run on the Otto cycle, less efficiently. (It is named after the person, not the vehicle.:-)

trhway 6y ago

>So having ships, planes or even cars powered by burning iron seems to be unfeasible aluminum of course. It is cheap too and has higher specific energy than iron. Using aluminum-air fuel cell instead of burning makes the efficiency about 2 times higher. The actual cars powered by aluminum have range up to 2000km per refueling.

ClumsyPilot 6y ago

It us perfectly feasible on ships, they are not restricted on mass very much and their fuel is dirty and not very efficient. Although we could have really fast nuclear powered container ships and the whole problem goes away