Energy and Human Ambitions on a Finite Planet, 2021a

7 The <strong>Energy</strong> L<strong>and</strong>scape 112
<strong>and</strong> their total qBtu amounts. Treat “< 1%” as 0.5%. Do these add
to 13 qBtu, as they should, within rounding error? 23
5. Figure 7.2 hides contributions of sources to end sectors behind
the “electric black box.” 24 Following similar logic to that in the
margin, <strong>and</strong> using results from Problem 4, figure out “corrected”
values for what percentage of coal provides energy to each of the
four end-sectors (re-distributing the 91% going to electricity into
end-sectors). 25
6. Figure 7.2 makes it look as if residential dem<strong>and</strong> is satisfied
without coal or nuclear, but 42% of residential dem<strong>and</strong> comes
from electricity, which does depend in part on coal <strong>and</strong> nuclear.
Using numbers derived in Problem 3, <strong>and</strong> following a logic similar
to that in Problem 5 <strong>and</strong> Example 7.1.1, redistribute this 42%
residential contribution from electricity into its primary sources
to ascertain what fraction of residential dem<strong>and</strong> comes from each
of the five source categories. For instance, petroleum would be the
direct 8% plus 42% times the fraction (or percentage) of electricity
coming from petroleum. 26
7. While no energy source is free of environmental harm, arguably
the last four entries in Table 7.1 are the cleanest, requiring no
burning <strong>and</strong> no evidently problematic “waste.” What percentage
of the total U.S. energy is in this “clean” form, at present?
23: This is a great way to check the correctness
of your answers.
24: For example, the figure indicates that
17% of natural gas goes directly to residential
end-users. But a substantial fraction of
natural gas (35%) also goes to electricity,
<strong>and</strong> 38.5% of electrical output goes toward
residential use—a result of Problem 4. So
the fraction of natural gas ending up satisfying
residential dem<strong>and</strong>s is the direct 17%
plus 38.5% of 35%, adding to 30.5%.
25: The four numbers you get should add
to 100%, within rounding error.
26: Make sure your five numbers add to
100%, within rounding error.
8. Let’s say that in the course of one year a county in Texas produces
5 million kWh of electrical output from wind, <strong>and</strong> also pumps
100,000 barrels of oil from the ground containing a (thermal)
energy content of about 6 GJ per barrel. What percentage of total
energy production came from wind, if scaling wind in terms of
thermal equivalent, as explained in Box 7.2?
9. Referring to Figure 7.4, what is the fastest-growing energy source
in the U.S., <strong>and</strong> is it one of the fossil fuels?
10. If the approximately linear trends for recent increases in solar <strong>and</strong>
wind seen in Figure 7.5 were to continue at the current (linear)
pace, approximately how long would it take for the pair of them
to cover our current ∼ 100 qBtu per year dem<strong>and</strong>? 27
11. If the downward trend in U.S. coal use continues at its current
pace, approximately what year would we hit zero?
i Keep it simple, as there is no single
correct way to extrapolate this far into the
future; just explain your approach.
27: We can hope to see faster-than-linear
expansion in renewables, but this question
asks what would happen without dramatic
changes to the recent trends.
12. Globally, do any of the resources appear to be phasing out, as coal
is in the U.S. (as in Problem 11)? If so, how long before we would
expect to reach zero usage, globally, based on simple extrapolation?
13. Globally, would you say that renewable energy sources are climbing
faster than the combined fossil fuels, or more slowly? Can we
therefore confidently project a time when renewables will overtake
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.