Energy and Human Ambitions on a Finite Planet, 2021a

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14 Biological <strong>Energy</strong>
The renewable energy options discussed thus far have been rather
different from the chemically-stored thermal energy provided by fossil
fuels. These sources—hydroelectricity, wind, <strong>and</strong> solar—are good at
making electricity, but are intermittent to various degrees <strong>and</strong> are not
directly suitable for transportation, except via bulky batteries.
Biologically-based energy is more similar to fossil fuels in that it is a
form of chemical energy burned to create thermal energy. We will focus
on two major forms: solid biomass <strong>and</strong> liquid biofuels. The latter is
well-suited to transportation: one of the few renewable energies that can
make this claim. In some cases, the same plant can produce either food
or bio-energy—depending on whether it is eaten by another biological
form or by a machine.
14.1 Photosynthesis ........ 227
14.2 Biomass ............ 229
14.3 Biofuels ............ 230
EROEI ............. 231
EROEI of Biofuels ...... 234
14.4 Upshot: Nature’s Burn ... 235
14.5 Problems ............ 236
Ultimately, biologically-based energy is a form of solar energy, creating
chemical storage by means of photosynthesis. Fossil fuels are also an
ancient form of biofuel, deriving from photosynthetic energy captured
millions of years ago. So sunlight is the actual energy source, <strong>and</strong>
photosynthesis is the mechanism by which the energy is stored in
chemical form.
14.1 Photosynthesis
This textbook will not focus on the complex mechanisms responsible
for photosynthesis, but rather will describe the net result <strong>and</strong> efficiency.
Photosynthesis involves the absorption of individual solar photons
that ultimately facilitate the movement of electrons in order to change
bonding structures, forming sugars, cellulose, <strong>and</strong> other materials used
to construct a plant. The fundamental chemical reaction is depicted in
Figure 14.1: Cartoon version of photosynthesis,
providing a graphical representation
of Eq. 14.1. Water, CO 2 , <strong>and</strong> sunlight are inputs.
The leaf “exhales” oxygen <strong>and</strong> keeps
sugar (only part of the final sugar molecule
is pictured here).
Photo Credit: Rudy <strong>and</strong> Peter Skitterians.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.