Energy and Human Ambitions on a Finite Planet, 2021a

10 Renewable Overview 166
The sun will continue to shine no matter how many solar panels we
set out. Wind is replenished daily by the sun heating the l<strong>and</strong> <strong>and</strong>
driving air currents. Solar energy drives the hydrological cycle, refilling
the reservoirs behind hydroelectric dams. Plants grow back to replace
harvested ones—again thanks to the sun. Ocean currents <strong>and</strong> waves are
also driven by the sun, via wind.
Nothing, of course, lasts forever, but the sun will continue to operate
in its current mode for billions of years more, <strong>and</strong> this is long enough
to count as indefinite, for our purposes. Table 10.1 classifies various
sources as to whether they are alternatives or renewables, along with
justifications. The items with asterisks are technically not renewable, but
last long enough that we can treat them as such in a practical sense (see
Box 10.1).
Table 10.1: <strong>Energy</strong> classification. Asterisks indicate non-replenished, but long-lasting sources.
Resource Chapter Alternative? Renewable? Reason
Petroleum 8 No No finite supply in ground
Natural Gas 8 No No finite supply in ground
Coal 8 No No finite supply in ground
Hydroelectric 11 Yes Yes Sun generates rain <strong>and</strong> refills reservoirs
Wind 12 Yes Yes Sun generates daily by heating Earth surface
Solar 13 Yes Yes Sun will last billions of years
Biomass (wood) 14 Yes Yes Sun grows more
Nuclear Fission 15 Yes No finite supply of fissile material in ground
Nuclear Fusion 15 Yes Yes* billions of years of deuterium; not tritium/lithium
Geothermal 16 Yes Yes* finite, but large; rate-limited
Tidal Capture 16 Yes Yes* can drive Moon away eventually
Ocean Currents 16 Yes Yes Sun/wind-driven
Waves 16 Yes Yes Sun/wind-driven
Just because a resource is renewable does not mean it is limitless. 5 We
only have so much l<strong>and</strong>, nutrients, <strong>and</strong> fresh water to grow biomass,
for example. Cutting trees down faster than they grow back would
result in depleting the resource—possibly permanently if the l<strong>and</strong> is
altered severely enough that trees do not grow back. Installing turbines
throughout the ocean to capture ocean currents, 6 would eventually
create enough impediment to the flow that it could stop altogether.
Box 10.1: About Those Asterisks. . .
The items sporting asterisks in Table 10.1 deserve additional explanation
as to why they are not technically renewable, even if the
depletion timescales are extremely long.
Capturing all available tidal energy would end up accelerating the
moon’s egress from Earth, 7 eventually causing loss of the resource. 8
About half of the geothermal energy store represents a one-time
5: Ultimately, only so much sunlight strikes
the earth.
6: This would be a hugely expensive <strong>and</strong>
impractical undertaking, but helps illustrate
the point that renewable does not mean
unlimited.
7: ...nowat3.8cm/year
8: It would take hundreds of millions of
years to “accomplish” this (see Sec. D.4;
p. 402).
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.