Energy and Human Ambitions on a Finite Planet, 2021a

14 Biological <strong>Energy</strong> 232
using oil as the energy source for the original exploration, building
the equipment, running the drill, <strong>and</strong> collecting/storing the product.
An EROEI of 100:1 means that for every barrel of oil that goes into the
process, 100 barrels come out. That’s a good deal. A high EROEI means
nearly “free” energy: low effort for high reward.
As we progress to more challenging oil resources, 21 the EROEI drops—
now around 10–15:1 for conventional oil <strong>and</strong> as low as 3:1 for tar s<strong>and</strong>s
. Table 14.1 provides one set of EROEI estimates for various sources.
Note that estimates vary due to difficulties in proper accounting of all
energy inputs, so don’t take these numbers literally—just as approximate
guides.
21: . . . deep water, fracking, tar s<strong>and</strong>s
[95] [95]: Hall et al. (2014), “EROI of different
fuels <strong>and</strong> the implications for society”
Source EROEI Est. Source EROEI Est.
Hydroelectric 40+ Solar PV 6
Wind 20 Soy Biodiesel 5.5
Coal 18 Nuclear Fission 5
Oil 16 Tar S<strong>and</strong>s 3–5
Sugar Cane Ethanol 9 Heavy Oil (Can., Ven.) 4
Natural Gas 7 Corn Ethanol 1.4
Table 14.1: EROEI estimates for various
sources [96]. For example, Wind has an
estimated EROEI of 20:1. See Table 7.1
(p. 106) for a refresher on how much energy
we get from various sources. Canada <strong>and</strong>
Venezuela tend to have heavy oil deposits.
If life were a video game, we would look at Table 14.1, decide that
hydroelectric <strong>and</strong> wind are “the best,” cursor over to them <strong>and</strong> “plus”
those two up until we’re getting all our energy from these low-energyinvestment
sources. But of course the world is constrained, placing real
limits to what is possible. We saw in Chapter 11 <strong>and</strong> Chapter 12 that
hydroelectricity <strong>and</strong> wind cannot be expected to provide more than a
few terawatts, leaving a large shortfall. Meanwhile, solar has the largest
raw potential. In other words, it is useful to appreciate the EROEI of
various resources, but EROEI is not the sole determining factor of what
is practical. A low EROEI can be tolerable if abundance makes up for
it.
For resources whose energy investment is mostly up-front, before production
begins, the resulting EROEI depends critically on how long the
resource will provide energy. After all, the energy return gets larger
the longer the facility can operate, while the investment part may be
essentially done <strong>and</strong> unchanging. It can be difficult to predict how long
a resource will last, which is part of why EROEI estimates are just that:
approximate guidelines.
Example 14.3.2 Let’s say a particular wind turbine achieves a 20:1
EROEI after operating for a 40 year lifetime. How many years-worth
of its energy output went into constructing <strong>and</strong> installing it?
Each year the turbine produces some amount of energy, which we can
call E (in Joules, for instance). In this case, it will produce 40E Joules in
its lifetime. Since EROEI is 20:1, it must have taken 40E/20 2E Joules
of input energy to create. At a rate of E per year, it will produce 2E
22 22: . . . because the delivered energy is 20
times the input energy
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.