Energy and Human Ambitions on a Finite Planet, 2021a

17 Comparison of Alternatives 297
232
233U
from Th), the proliferation aspect is severely diminished for
232
thorium due to a highly radioactive U by-product 20 <strong>and</strong> virtually no 20: . . . making it deadly to rogue actors
easily separable plutonium.
Geothermal Heating allowing Depletion (Sec. 16.1.2; p. 277): A vast
store of thermal energy sits in Earth’s crust, permeating the rock <strong>and</strong>
moving slowly outward. Ignoring sustainable aims, boreholes could be
drilled a few kilometers down to extract thermal energy out of the rock
faster than the geophysical replacement rate, effectively mining heat as
a one-time resource. In the absence of water flow to distribute heat, dry
rock will deplete its heat within 5–10 meters of the borehole in a matter
of a few years, requiring another hole 10 m away from the previous, in
repeated fashion. The recurrent large-scale drilling operation across the
l<strong>and</strong> qualifies this technique as moderately difficult.
The temperatures are marginal for running heat engines to make electricity
with any respectable efficiency, 21 but at least the thermal resource
will not suffer intermittency problems during the time that a given
hole is still useful. Kilometer-scale drilling hurdles have prevented this
technique from being demonstrated at geologically normal (inactive)
sites. Public acceptance may be less than lukewarm given the scale
of drilling involved, dealing with tailings <strong>and</strong> possibly groundwater
contamination issues on a sizable scale. While a backyard might accommodate
a borehole, it would be far more practical to use the heat
for clusters of buildings rather than for just one—given the effort <strong>and</strong>
lifetime associated with each hole.
Geothermal Heating, Steady State (Sec. 16.1.1; p. 276): Sustainable extraction
of geothermal heat—replenished by radioactive decay within
the Earth—offers far less total potential, coming to about 13 TW of flow
if summed across all l<strong>and</strong>. And to get to temperatures hot enough to
be useful for heating purposes, boreholes at least 1 km deep would be
required. It is tremendously challenging to cover any significant fraction
of l<strong>and</strong> area with thermal collectors 1 km deep. As a result, a yellow score
for the abundance factor may be generous. To gather enough steady-flow
heat to provide for a normal U.S. home’s heating dem<strong>and</strong>, the collection
network would have to span a square 200 m on a side at depth, which is
likely unachievable.
Biofuels from Crops (Sec. 14.3; p. 230): While corn ethanol may not even
be net energy-positive, sugar cane <strong>and</strong> vegetable oils as sources of biofuel
fare better. But these sources compete with food production <strong>and</strong> arable
l<strong>and</strong> availability. So biofuels from crops can only graduate from “niche”
to moderate scale in the context of plant waste or cellulosic conversion.
The abundance <strong>and</strong> demonstration fields are thus split: food crop energy
is demonstrated but severely constrained in scale. Cellulosic matter
becomes a potentially larger-scale source but is undemonstrated. 22
Growing <strong>and</strong> harvesting annual crops on a relevant scale constitutes
a massive, perpetual task <strong>and</strong> thus scores yellow in difficulty—also
driving down EROEI.
21: . . . especially given that many easier options
are available for producing electricity
Note that technologies known as “geothermal”
heat pumps are not accessing an energy
resource; they are simply using a large
thermal mass against which to regulate temperature.
22: ...tothepoint that perhaps this should
even be red
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.