The Oil Drum: Net Energy | EROI Update: Preliminary Results using Toe-to-Heel Air Injection

78 comments on EROI Update: Preliminary Results using Toe-to-Heel Air Injection

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Pitt the Elder on March 20, 2009 - 10:08am

In that case the oft-cited EROI of 8:1+ is over-stated, because the bagasse is not counted as an energy input.

That's because it's not an energy input for anyone outside the ethanol plant's walls.

Society at large gives the ethanol producer 1 GJ.
The ethanol producer gives society at large 8 GJ.

How is this not EROEI 8:1 for society?

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The problem here is that you're erroneously conflating two things:

The EROEI of farming sugar cane.
The efficiency of converting sugar cane into ethanol.

#1 has an EROEI of about 13:1, while #2 has an efficiency of about 65%, and combining these two gives us the 8:1 EROEI of cane ethanol.

Think of it this way: suppose farming cane was EROEI of 100:1, but the conversion was only 8% efficient. This still returns 8 units of energy to society for every 1 unit it invests in the process, but your calculation would say it took 1 unit of farming energy + 92 units of bagasse energy to generate 8 units of ethanol energy, for a massive energy loss of 8:93. This scenario shows the error in your calculation more clearly: the calculation predicts a huge net energy loss (93 units -> 8 units = -85 units), when in fact there is a net energy gain (1 unit -> 8 units = +7 units).

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Nate Hagens on March 24, 2009 - 10:06pm

Pitt - I disagree. That lignin could be used for something else. The above graphics show the same process (cellulosic or sugar cane ethanol) measured in two ways. The bottom diagram shows traditional EROEI = energy out / energy in. The top graphic indicates that the intermediate step biomass K2 (the bagasse) has an energy opportunity cost as it could be used for other energy uses. This 'loss' of availability has to be considered, which translates to EROEI = (Eout +(E lost-Ein)) / E lost.

Here are numbers from our pending publication:

What does this imply for the EROI of cellulosic ethanol? Drawing on a review conducted by Hammerschlag [11] of four net energy studies, we averaged the energy inputs and outputs to produce estimates for system energy flows for cellulosic production. Flows follow Figure 3a with the exception that the available energy from Biomass A (the lignin) is higher than the required input to the ethanol processing system, thus yielding an additional energy output. Estimates are as follows on a per liter of ethanol basis:
Energy In = 5.3 MJ.
Energy from Biomass A = 32.5 MJ.
Energy into the biomass processing system = 29.0 MJ.
The surplus energy from Biomass A that is outputted = 3.5 MJ.
Ethanol production (Biomass B) = 23.6 MJ.
Using the intuitive definition, the EROI measure would be Eout ( = 23.6 + 3.5 = 27.1 MJ) divided by Ein ( = 5.3 MJ) for an EROI of 5.5, significantly higher than soy biodiesel or starch ethanol. However, using equation (1), we have:

EROI = 27.1 + (34.3-5.3) / 34.3 = 1.7

where E lost = 5.3 + 29.0 = 34.3 MJ. This value is only marginally better than reported EROI measures for starch-based ethanol [7]. A similar exercise shows that the high EROI numbers for Brazilian sugar-cane based ethanol, which uses the bagasse as an intermediate input, are also overestimations.

In a different economy the bagasse might be used in many other energy technologies; heat, biogas, electricity, etc. that would have higher social efficiencies. So the bagasse has to be considered an energy opportunity cost, and if not counted as an input it will overstate the EROEI. This is relevant if we would change how society used energy. There would be less need for high energy surplus liquid fuels if things were done more locally, or with more electricity, etc. Again, this gets at energy quality and is not something I would debate too an extreme, but I think it is correct.

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