Energy and Human Ambitions on a Finite Planet, 2021a

14 Biological <strong>Energy</strong> 229
efficiency is calculated as the ratio of output to input: about 2% in
this case.
14.2 Biomass
Biological mass, or biomass, has long been utilized to supplement our
energy needs, via controlled use of fire starting hundreds of thous<strong>and</strong>s
of years ago. Burning wood or other plant matter, <strong>and</strong> in some places
dried animal feces 6 counts as utilization of biomass. Wood provides
about 4 kcal of energy per gram when burned, or about 16 MJ/kg—much
like proteins or carbohydrates 7 in our diet. Burning of biomass is most
typically used for heating <strong>and</strong> cooking within individual homes.
Example 14.2.1 A 10 kg bundle of dry firewood is used to heat a home
that requires 4,000 W to stay warm. How long will the wood last? 8
Every gram of wood has 4 kcal or about 16 kJ of energy. We get 4,000 W
by burning one gram every 4 seconds: 16 kJ/4 s is 4 kW. Each kilogram
will therefore take about 4,000 s (a little over an hour) <strong>and</strong> the whole
bundle will be gone after 11 hours.
6: . . . such as cow dung
7: Fats <strong>and</strong> hydrocarbons like fossil fuels
are 2–3 times more energy dense.
8: . . . assuming a wood stove or other efficient
device to prevent most heat from just
escaping through the chimney
In the U.S. in 2018, 2.36 qBtu of the 101.25 qBtu total came from burning
wood, <strong>and</strong> an additional 0.5 qBtu came from incinerating waste products
. Thus about 2.8% (0.1 TW) of U.S. energy comes from biomass. Out
of the 11.41 qBtu of all renewables in 2018, biomass accounted for 25% of
the U.S. renewable budget. 9
[34] [34]: U.S. <strong>Energy</strong> Inform. Administration
(2011), Annual <strong>Energy</strong> Review
Globally, biomass use is estimated to be more important, at 6%, 10
constituting more than a third of global renewable resources (Fig. 7.8;
p. 109). The high use of biomass in the wider world is a reflection on the
difference between developed countries like the U.S. <strong>and</strong> developing
countries that are more likely to rely on more primitive forms of energy
like firewood <strong>and</strong> animal dung. Since most biomass around the world is
burned for individual use, emissions controls are essentially non-existent,
resulting in high levels of pollution—smoke <strong>and</strong> harmful chemicals that
would be scrubbed out of a power plant’s exhaust. 11
Box 14.3: Life is Thin <strong>and</strong> Precious
The total mass of living organisms on Earth is estimated to be about 2
trillion tons [94]. Having a density similar to that of water, 2 × 10 15 kg,
if spread uniformly around the planet, would stack to 4 mm high! Put
another way, a r<strong>and</strong>om line projecting upward from the surface of
the earth would go through about 4 mm of living matter, on average.
That’s a pretty thin shell of life!
If we tried to substitute our 18 TW global power dem<strong>and</strong> by burning
9: . . . while 23% was hydro; 22% was wind;
20% were biofuels, 8% was solar, <strong>and</strong> 2%
was geothermal; see Table 10.3 (p. 170)
10: ...down considerably from ∼25% in
1950
11: Note that CO 2 is common to both, <strong>and</strong>
is not scrubbed out of power plant exhaust,
comprising the bulk of the emissions.
[94]: Elhacham et al. (2020), “Global humanmade
mass exceeds all living biomass”
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.