Energy and Human Ambitions on a Finite Planet, 2021a

10 Renewable Overview 167
deposit of heat left over from the collapse/formation of the earth, 9
the other half coming from radioactive decays of elements ultimately
tracing to ancient astrophysical cataclysms. 10 For both the formation
<strong>and</strong> radioactive contributions, the supply is not replenished after its
use, although the timescale for the radioactive decays to fade away is
billions of years.
Nuclear fusion needs deuterium <strong>and</strong> tritium. 11 Roughly one out of
every 10,000 hydrogen atoms is deuterium, so ocean water (H 2 O) will
have enough deuterium to last billions of years. Tritium, however, is
not found naturally <strong>and</strong> must be synthesized from lithium, in finite
supply. Details will follow in Chapter 15.
9: ...aconversion of gravitational potential
energy
10: . . . primarily supernova explosions <strong>and</strong>
neutron star mergers
11: Eventually it is hoped that only deuterium
could be used.
10.3 Renewable <strong>Energy</strong> Budget
Notice that all of the unqualified 12 “Yes” entries in Table 10.1 originate 12: . . . i.e., no asterisk
from the sun. For that matter, fossil fuels represent captured ancient
solar energy, stored for all these years. The sun sends energy toward the
earth at a rate of 1,360 W/m 2 . Multiplying this by the projected area 13 of 13: See Example 10.3.1.
the earth (πR 2 ⊕ ≈ 1.28 × 1014 m 3 ) results in 174,000 TW of solar power
intercepting the earth. This number absolutely dwarfs the 18 TW societal
energy budget of all humans on Earth. Figure 10.1 shows graphically
what happens to this energy input.
174,000 TW (100%)
solar input
atmospheric
reflection
22.6%
29.3%
reflected
39,500 TW (22.6%)
atmospheric
absorption
900 TW (0.5%) wind
44 TW geothermal
100 TW (0.06%)
photosynthesis
83,000 TW (48%)
absorbed at
surface
6.7% reflection
from surface
5 TW (0.003%) ocean currents
44,200 TW
(25.4%)
evaporation
3 TW
tidal
Figure 10.1: <strong>Energy</strong> inputs to the earth, ignoring
the radiation piece (since that is an
output channel). About 70% of Incoming
solar energy is absorbed by the atmosphere
<strong>and</strong> l<strong>and</strong>, while about 30% is immediately
reflected back to space (mostly by clouds).
About half of the energy absorbed at the
surface goes into evaporating water, while
smaller portions drive winds, photosynthesis
(l<strong>and</strong> <strong>and</strong> sea), <strong>and</strong> ocean currents.
Additional non-solar inputs are geothermal
<strong>and</strong> tidal in origin [63–65].
Example 10.3.1 Solar Input: Because we will encounter solar power
flux many times in this textbook, this is a good opportunity to spell
out some key numbers <strong>and</strong> concepts.
First, sunlight arriving at the top of Earth’s atmosphere delivers energy
at a rate of 1,360 Joules per second per square meter (1,360 W/m 2 ),
which is known as the solar constant [4].
[4]: Kopp et al. (2011), “A new, lower value
of total solar irradiance: Evidence <strong>and</strong> climate
significance”
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.