> > Let's compare energy densities, MJ per litre (so volume, 
> not weight):
> > 
> > Helium fusion       - 8,570,000,000,000,000,000,000,000
> > U-235 fission       - 1,700,000,000
> > Petrol              - 34.6
> > Liquid hydrogen     - 8
> > Compressed hydrogen - 4.7
> > Li-ion battery      - 1.5
> > Lead-acid battery   - 0.15
> > Hydrogen gas        - 0.01079
> > Water               - 0
> 
> > One source claimed it takes 10kWh (36MJ) to crack a litre 
> of water.  
> > So you're turned 36MJ into about 11MJ... and you haven't 
> started the 
> > engine yet.
> > 
> > To match a litre of petrol, you need around 3,000 litres of 
> hydrogen 
> > gas. Ha, you say, 3 cubic metres is nothing!  Just look at 
> all those 
> > little bubbles...
> 
> I'm not advocating the use of water, but your calculation 
> seems pretty lopsided to me. 
> 
> When you take into consideration the 36MJ that are needed to 
> transform the (vastly available) water into something that 
> can be used to propel a car (hydrogen), shouldn't you 
> consider also the energy that is needed to transform the (not 
> so vastly available, but that's a different issue) raw crude 
> into petrol? How does the calculation look then?
> 
> For the reason I already mentioned (no bookkeeping of public 
> resources), for most of the processes listed we don't have 
> this data, and where we have it, it is unlikely that it goes 
> deeper than one stage (where there are usually many stages). 
> This is extremely unlucky, because discussions like this one 
> are bound to end up in beliefs for the lack of knowledge 
> about what we're doing.
> 
> Gerhard


It is a bit lopsided, I was being generous to the hydrogen.  The
water/hydrogen calc also leaves out "where does the electricity to crack the
water come from?".  

The free energy crowd seem to think electricity + water --> hydrogen -->
fuel cell --> moving car + water + electricity & repeat.  That would work if
hydrogen had about 10 times the energy content.  That would make it a fuel,
not waste.

I'm sure there's a number out there some about how efficient petrol is, that
is the total cost from finding, extracting, refining etc, but I figure it's
a fair way on the positive side.  I'd guess the energy needed to 'create' a
litre of petrol is less than 34.6MJ.  Its main advantage is it's a very
compact energy store, and easy to handle.  Electricity is similar, but
suffers by being hard to store.  A decent battery would help.

Oil is like me giving you a dollar, and getting 5 back.  Water/hydrogen is
handing over the same dollar, and getting 33 cents back (on a very good
day).

There's nothing inherently wrong with running a car on hydrogen, except for
the fact hydrogen is a bit hard to come by.  Electrolysis of water isn't the
solution.

Tony

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