peter green wrote:

>> I'm not a chem engineer, but a friend that /is/ tells me fossil fuels
>> require four times the yield of energy to recover it from the earth and
>> process it.
> 
> then ask him where the hell that energy comes from. 
> 
> I'm not saying that all fossil fuel recovery that happens is positive in
> terms of usefull energy output to usefull energy input but on average
> they must be otherwise fossil fuels could not be the dominant
> contributor to our supply of usefull energy.

The calculation goes like this I think:

- There's 100 TJ worth of chemical energy in the form of crude oil in a
well.
- To extract and refine it, you need 80 TJ. 
- Leaves you with 20 TJ of useful chemical energy from fossil fuels.

The energy comes from the well. It costs four times the resulting useable
energy to extract and refine it. There is still a positive outcome, because
the loss in available energy stored in the well doesn't go into the
calculation. So the last (and usually skipped) calculation step is:

- After using 20 TJ of fossil energy, we (globally speaking) have 100 TJ of
fossil energy less than we had before.

All this using Tsu's numbers. I don't know if they are correct. But they
could be -- there's no general principle that says they can't be. 

The reason why, despite such a claimed inefficiency, these fuels can be the
dominant contributor is that the owners sell their fossil energy stock
relatively cheaply. (Which probably will change as these stocks get near
their end. Which then probably will also change the energy supply
structure.)

Gerhard

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