Tony Smith wrote:

> 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


PS For the sake of global understanding... I'd like to spread the word
about the apostrophe as thousands separator. Is "10,000 kg" ten kg (with a
precision of 1 g) or ten thousand kg? Depends on your position on the globe
and your understanding of the context. OTOH, "10'000 kg" is usually
unambiguous. I don't remember where I first read about using the apostrophe
as thousands separator, but among the available options (including the ISO
suggested thin non-breaking space) it seems to be the easiest to use that
avoids the ambiguity that both dot and comma necessarily have. (And before
you think "this is an English mailing list, and in English-speaking
countries it has always been the decimal point and the thousands separator
comma, think again... :)

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