The Oil Drum: Net Energy | A Net Energy Parable: Why is ERoEI Important?

156 comments on A Net Energy Parable: Why is ERoEI Important?

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Oilrig medic on August 4, 2006 - 9:10am

Some inputs are "free", light from the sun, coal in the ground etc. you have to count exploration/manufacturing costs all the way to energy consumer. It can never be infinite and if it is less than 1 it is not worth doing. Arguably if it is near 1 its not worth doing.

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slaphappy on August 7, 2006 - 12:44am

So what you're really saying is this...

We have a process for converting these "free" input to useful output.
We consider the utility of this process based on only the amount of non-free inputs it consumes to produce outputs, not on how efficiently it converts the free inputs to useful outputs.

Is that really what you're saying, because that is what EROEI is. And it can be infinite, look at my example above. What exactly is wrong with it?

If you have an ethanol operation that runs everything off of ethanol, then energy input is zero (0), whereas "energy returned" is (presumably) something not zero. If energy returned is zero, then it isn't a source of energy, case closed. If it is not zero, then the EROEI is infinite, by definition. How is this a useful number again?

Seems like nomenclature to befuddle the foolish if you ask me. Use efficiency, it has some basis in science, and makes perfect sense. Such as "with what efficiency does this ethanol process turn sunlight into ethanol?". Answer that question (it isn't terribly hard), and you know, for instance, how many acres of land would be needed to produce X units of ethanol. Simple, useful.

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Robert Rapier on August 7, 2006 - 6:15pm

I think you have gotten confused about what EROEI means. It is simply the energy ouput divided by the direct energy inputs. In your example, let's say we have a process that runs off of ethanol. We run the process on ethanol, and we capture a bit of solar energy from growing the corn. If we input 1 BTU of ethanol in producing the corn and turning it into ethanol, and we end up with 1.1 BTUs of ethanol, the EROEI is 1.1, not infinity. Just because you ran the process off of ethanol and produced ethanol doesn't mean you didn't have net energy inputs.

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slaphappy on August 8, 2006 - 11:48pm

That's so irrelevant though. Ok, if you count it that way, then a process that didn't use any fuel (like having peasants harvest it and not using irrigation...) would have a vastly higher EROEI, even though it would be wildly less efficient than just using the ethanol from the output to run the machinery.

How can this be a meaningful number if it's boosted dramatically by disposing of the machines, even as that causes output to plummet.

What exactly are you trying to measure? The efficiency of converting sunlight into fuel? EROEI doesn't even touch on that problem, so what good is it?

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step back on August 9, 2006 - 3:17pm

Slaphappy,

IMHO you are correct to recognize that eROI is an accounting game just like $ROI is.

In the case of eROI, if we stoped allowing ourselves to not-count energy inputs that are "free" (don't cost money) then eROI will always be less than unity because of the entropy laws of thermodynamics --all real world transformation processes are lossy and energy is conserved (assuming no E=mc^2 stuff allowed).

However, if we choose to exclude the money-wise "free" parts of the energy inputs and to exclude the one-time energy costs for manufacturing the contraptions (e.g. ethanol plant) used in transformation (e.g. corn to ethanol), then we get some sort of partly-economic, partly-physics measure of the long term payback we get for our efforts.

We can use this eROI number for comparing one type of apples against another, for example, switch grass ethanol versus corn ethanol --which has higher eROI assuming conversion plants for each are equal?

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slaphappy on August 13, 2006 - 2:14pm

I'll grant you most of that (and the E=mc2 stuff doesn't really pose a problem, matter is nothing more than "frozen" energy anway), but the comparison between switchgrass and corn, I don't agree with.

If switchgrass had an EROEI of 1.5, but produced (net, after all fuels and such are accounted for) 2 units of fuel per unit of land, and corn had an EROEI of 1.2, but produced (net, again) 25 units of fuel per unit of land, which is really better? Seems to me that in this (contrived, I know) scenario the corn would be better. Neither of us would have any trouble coming up with complete numbers that would make the above work out (switchggrass uses y units of fuel, and produces z units of mass per acre, etc...)

What is the point of EROEI again? It doesn't seem to be actually measuring anything significant. As near as I can tell, it's measuring the boundaries between the various corporations and occupations that run the system. Seriously, why bother?

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