Energy and Human Ambitions on a Finite Planet, 2021a

14 Biological <strong>Energy</strong> 237
4. How many logs of firewood per day (whose parameters are specified
in Problem 3) would you need to burn to provide 5,000 W of
heating to a house?
5. Replicate the conclusion of Box 14.3 by assuming one-quarter of
the 2 trillion tons (5 × 10 1 4 kg) of mass is combustible at 4kcal/g.
How long—in years—would this amount of energy last if burning
at 18 TW?
6. Given the energy densities of ethanol vs. octane (gasoline), how
many liters of ethanol does it take to replace one liter of gasoline,
if the densities are 789 g/L for ethanol <strong>and</strong> 703 g/L for octane?
7. If 50 kJ of energy are spent to extract 1 MJ of energy content in the
form of coal, what is the EROEI?
8. Re-express an EROEI of 1.5:1 in terms of how many total units of
energy must be produced in order to extract one unit of net energy
in a self-supporting operation.
9. Imagine that the extraction of a low-EROEI biofuel is performed
using energy derived from that biofuel alone—in other words, a
self-contained operation not using any other (external) form of
energy. We can think of the situation thusly: for each hectare of
l<strong>and</strong> producing fuel for external use, some additional l<strong>and</strong> must
be dedicated to raising the energy used to perform the extraction
operation—like an overhead. If the EROEI is 1.5:1, 34 how much
total l<strong>and</strong> area would need to be devoted to the endeavor for
every hectare (or any area unit you wish) that contributes to net
production? See Problem 8 for a related scenario.
34: You can always multiply both sides by
the same factor to make both sides integers,
if this is easier to underst<strong>and</strong>.
10. It takes a certain energy investment to fabricate a solar panel.
Referring to Table 14.1, figure out how many years of the panel’s
output energy it takes to “pay off” the investment if the EROEI
estimate assumed operation for 30 years.
11. Our modern food industry has an EROEI of 0.1:1. In pre-industrial
settings, when energy investment for food production was in the
form of muscle power (animal <strong>and</strong> human), why would a 0.1:1
EROEI for food have been untenable? Next, describe the conditions
for an exact break-even food EROEI of 1:1. What would this mean
in terms of where effort/energy goes? What would this leave for
building shelters, cathedrals, or esthetic pursuits?
12. Parallel the development in the text for the l<strong>and</strong> required for corn
ethanol if it is self-sufficient 35 in the case for EROEI of 1.5:1—near
the optimistic end of the range. How much larger is this than the
area now devoted to corn, <strong>and</strong> how does it compare to total arable
l<strong>and</strong> in the U.S.?
35: . . . i.e., relying on its own energy to
re-invest in its extraction
13. Using the setup for Problem 12, how large would the required corn
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.