Energy and Human Ambitions on a Finite Planet, 2021a

14 Biological <strong>Energy</strong> 233
in 2 years, therefore taking two years to produce as much as went into
its manufacture—paying for itself, energetically.
In a self-supporting sense 23 the net energy is x − 1 for an EROEI of x:1.
In other words, an EROEI of 1.25:1 only “really” produces 0.25 units of
exportable energy for every one unit invested, if that one invested unit
comes from the 1.25 units extracted in a closed system. In this case, for
every one unit netted, 24 4 units went in <strong>and</strong> 5 came out–only 1 of the 5 24: . . . multiplying the 0.25 net by 4
free <strong>and</strong> clear.
Example 14.3.3 A self-contained operation to produce ethanol manages
to only use its own ethanol to run the entire operation of growing,
harvesting, <strong>and</strong> processing the crops to produce ethanol. In one year,
the operation produced a total of 250,000 L of ethanol at an EROEI
of 1.25:1. How much ethanol were they able to export/sell from the
operation?
The 1.25 number is associated with total production, which is 250,000 L
in this case. Multiplying both sides of the 1.25:1 ratio by 200,000 results
in an output:input ratio of 250,000:200,000 meaning that the operation
required 200,000 L of input. Thus the operation was able to deliver
50,000 L to market.
23: . . . if the energy extracted is then used
as the input to extract more
Low EROEI cuts into the effective available resource, dem<strong>and</strong>ing investment
of precious energy. As conventional resources are exhausted,
forcing us to lower-EROEI deposits, even if we keep up with energy
dem<strong>and</strong> in absolute terms, 25 the net energy available will decline as a
greater fraction of the harvest must go back into extraction.
Example 14.3.4 What would have happened to an early agricultural
society if the EROEI of growing food 26 slipped below 1:1, if all of the
energy used to harvest the food came from workers <strong>and</strong> animals fed
by the same food?
25: . . . e.g., same number of barrels of oil
produced each year
26: ...aformofenergy
At 1:1, every unit of energy extracted requires one unit of investment.
Then 100% of the energy is spent acquiring energy, leaving no energy
for other functions of the society (shelter, defense, etc.). Such a marginal
existence could not be maintained, so some minimum exists below
which the society becomes non-viable.
Note that many of the entries in Table 14.1 have low numbers, translating
to a tough life in which a substantial fraction of the total energy resource
is dedicated to continued energy procurement. Biological forms of
energy are not superstars in this regard.
Box 14.4: Eating Our Fossil Fuels
Relatedly, <strong>and</strong> in a familiar context, the food industry in the U.S. today
expends about 10 kcal of mostly fossil-fuel energy for every 1 kcal of
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.