Biomedical Engineering Reference
In-Depth Information
materials. The net increase in the CO
2
in atmosphere is due to a combination of the short
average life span of the harvest biomass before converted to CO
2
and the decrease in the
amount of standing biomass. Burning of biomass for electricity and/or heat emits CO
2
shortly after harvesting. If one focuses on some special cases of applications, for example
long-lasting materials, the net effect can be the opposite: a net capture of CO
2
from
atmosphere.
For the same token, converting grassland to forest can dramatically increase the CO
2
capture from atmosphere to be stored as standing biomass. The net storage of biomass by
grass and agricultural crops is negligible as compared with forest or trees. For example,
the CO
2
storage due to forest growth in China and the developed world is substantial
when compared with their pre-industrialized past due to the shifting of low-productive agri-
culture crop land to forest.
15.6. SOLAR ENERGY
Energy source sustainability and environmental impacts are two key factors in evaluating
the benefits of energy source exploitation.
Fig. 15.14
shows a schematic of the major energy
flow to meet the needs of human society today. One can imagine that only energy originated
from the sun may be considered as renewable on the planet. While nuclear energy could be
lost if not harvested for human use, its deposit is limited and will run out long before the
sun's ray. Nevertheless, nuclear energy use is a positive step toward energy sustainability
transitioning out of fossil energy use. Geothermal energy refers to the energy available based
on the temperature difference between atmosphere and that deep underneath the earth or
deep in water. Exploiting the high temperature deep under the earth surface is the worst
choice for energy production as promoting the temperature change inside the planet can
lead to irreversible damage to the whole ecological system. Like nuclear energy, geothermal
energy use does not contribute directly to CO
2
emission. However, removing thermal energy
from the earth core is an act that cannot be reversed. The earth core is cooling slowly. Main-
taining slow decaying in the temperature gradient along the radius of the earth is the ulti-
mate sustainability goal toward long-time survival of the ecological systems we can
envision today. Petroleum, gas, and coal, even if they were formed from biomass, are limited
in their deposits and their recycle is at best questionable.
Primary
Source
Human
Needs
Intermediates
Primary
Secondary
Animals
Biomass
Food
Sun's
ray
Wind
Hydro
Energy
Liquid fuel
Transport
Heat
Lighting
Geothermal
Batteries
Electricity
Nuclear
Petroleum, coal, gas
Minerals
Hydrogen
FIGURE 15.14
A schematic of major pathways of energy flow to meet human needs.
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