Hydrogen Facts
I thought it might be a good idea to produce a Hydrogen Facts thread, which may be particularly useful for anyone interested in its use as a fuel.
This idea of this thread then is to provide a handy source of reference and somewhere that information gathered from numerous sources can all be found in one place.
I’ve noticed over the years that there appears to be much confusion surrounding the energy released from combusting hydrogen, so thought that clearing this up would be a good starting point.
Most of the confusion seems to arise because comparisons of energies released by various fuels is not always compared fairly, or indeed accurately. For example, comparing fuels by volumes can be very misleading, especially in the case of hydrogen due to its extremely low density.
The following table is known to be accurate, and gives a figure for the energy released when a mass of 1kg of the fuel is completely burned in oxygen.
Carbon, C (as in coal) C(solid) + O2(gas) = CO2(gas)...................................................................................33,000kJ
Methane, CH4 (as in natural gas) CH4(g) + 2O2(g) = CO2(g) + 2H2O(g).......................................................50,000kJ
Octane, C8H18 (as in petrol) 2C8H18(liquid) + 25O2(g) = 16CO2(g) + 18H2O(g............................................45,000kJ
Methanol, CH3OH 2CH3OH(liquid) + 3O2(g) = 2CO2(g) + 4H2O(g)...............................................................20,000kJ
Carbohydrates (sugars and starches)…..products are CO2(g) and H2O(liquid).........................................average 17,000kJ
Animal Fats……….products are CO2(g) and H2O(liquid)......................................................variable, but typically 40,000kJ
Hydrogen, H2 2H2(g) + O2(g) = 2H2O(g)............................................................................................120,000kJ
So, if we are comparing energies released by mass, then hydrogen clearly possesses the greatest energy by far.
The problem is that hydrogen liquid has a low density, so as a fuel, larger volumes of it are required when compared to say petrol.
For example liquid hydrogen has a density of just 72 kilograms per cubic metre, petrol is around 737kg/m3 and water 1000kg/m3.
So while mass for mass hydrogen releases the greatest amount of energy upon combustion in oxygen, its low density means that volume for volume, around 4 times more liquid hydrogen would be needed to equal that of a given volume of petrol. Hence hydrogen, even in liquid state would require substantially greater volumes to be carried on board a vehicle in order to get it to travel for the same distance as a vehicle fueled with petrol.
Of course, when we are talking about powering a vehicle with liquid hydrogen as fuel, there is also the issue of not only producing the hydrogen initially, but also in compressing it into liquid state.
Here’s an interesting piece I found:
Here's what Pimentel (1996, p. 211-212) has to say.
In terms of energy contained, 9.5 kg of hydrogen is equivalent to 25kg of gasoline ( Peschka 1987). Storing 25 kg of gasoline requires a tank with a mass of 17 kg, whereas the storage of 9.5 kg of hydrogen requires 55kg, (Peschka 1987). Part of the reason for this difference is that the volume of hydrogen fuel is about 4 times greater for the same energy content of gasoline. Although the hydrogen storage vessel is large, hydrogen burns 1.33 times more efficiently than gasoline in automobiles ( Bockris and Wass 1988). In tests a BMW 745h liquid-hydrogen test vehicle with a 75 kg tank and the energy equivalent of 40 liters of gasoline had a cruising range in traffic of 400 km, or a fuel efficiency of 10 km per liter ( Winter 1986).
Carrying liquid hydrogen around as a fuel has never really appealed to me due to the problems associated with it in liquid state. So whether it be used to drive and ICE or a hydrogen fuel cell to power an electric motor, the problem of carrying, storing and indeed refuelling are obstacles I feel too great to make it worth the effort. This of course is to say nothing of having to produce the hydrogen in the first place.
Now here’s the thing, and the beauty of water. Any given volume of water - even though it contains the oxygen atom as well as hydrogen - actually contains more hydrogen than pure liquid hydrogen of an equal volume. This for me is the real appeal of on-demand production of hydroxy from water.
Of course we are extremely hampered by the conservation of energy laws, but that is not to say that some laws cannot be rewritten if new information comes to light or new discoveries are made that perhaps question whether or not these laws truly take into account every factor. By this I mean that some laws generally work great and calculations are accurate within most useable parameters, but the same law can fall apart under certain specific conditions whereby the physics we know seems to go out of the window. I believe Ohm’s Law is one such case. So there are always possibilities – always hope!
Interesting you-tube video detailing the amount of hydrogen (gas) required to run a vehicle:
http://www.youtube.com/watch?v=7qzrI20VPCwNote: If anyone finds that I’ve provided incorrect figures anywhere, either through a typo or otherwise, please speak up and I’ll correct any mistakes.
** rather than adding further new posts of information that may inevitably be lost between discussion posts, I will always update this first post in order to keep all the relevant information in one place and hence easy to find.
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Topic: Hydrogen Facts (Read 5669 times)
