Energy and Human Ambitions on a Finite Planet, 2021a

14 Biological <strong>Energy</strong> 236
We conclude by listing some pros <strong>and</strong> cons for biologically-derived
energy, beginning with the advantageous aspects:
◮ Biofuels offer a possible liquid fuel substitute to support transportation
needs;
◮ Biological energy relies on dependable solar input, replenished as
harvested stocks grow back;
◮ Biofuels represent a form of storage of solar energy, mitigating
intermittency;
◮ Methods for growing <strong>and</strong> harvesting crops are well established;
◮ Burning biomass is low-tech <strong>and</strong> likely to remain part of our
energy portfolio.
And the less savory aspects:
◮ It is difficult to scale biological energy to meaningful multi-terawatt
levels;
◮ Heavy reliance on biological energy co-opts earth’s biology <strong>and</strong>
displaces natural habitat;
◮ Cultivating biofuels competes with food production for water <strong>and</strong>
l<strong>and</strong> resources;
◮ Low EROEI for biofuels reduces net energy available;
◮ Smoke <strong>and</strong> other pollutants from burning biomass can be problematic.
14.5 Problems
1. A large tree might have a trunk 0.5 m in diameter <strong>and</strong> be 40 m tall.
Even though it branches out many times, pretend all the wood
fits into a cylinder maintaining this 0.5 m diameter for the full 33: i Water has a density of 1,000 kg/m 3 .
height of the tree. Wood floats, 33 so let’s say it has a density around
800 kg/m 3 . How many kilograms of CO 2 did this tree pull out of Hint: carbon dioxide is 44/12 times the mass
the atmosphere to get its carbon, if we treat the tree’s mass as 50% of plain carbon.
carbon?
2. Using the geometry <strong>and</strong> density of the tree in Problem 1, ifthe
resulting wood has an energy density similar to carbohydrates
(4 kcal/g), <strong>and</strong> the tree spent 50 years accumulating this bulk while
receiving an average of 250 W/m 2 of solar input over 5 months
each year in a leafy area averaging 200 m 2 to receive sunlight, what
is the net photosynthetic efficiency of the tree?
3. The U.S. gets 2.4 qBtu per year of energy from burning biomass
(mostly firewood). At an energy density of 4 kcal per gram, <strong>and</strong> a
population of 330 million, how many 5 kg logs per year does this
translate to per person?
Now if you could just think of a way to put
the answer on a log scale, ha ha.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.