450 CHAPTER 18 Renewable Energy Resources
Jim Richardson/NG Image CollectionNG MapsGlobal
Locator- Define biomass and outline its use as a source
of energy. - Compare the potential of wind energy and
hydropower.
S
ome renewable energy sources indirectly use
the sun’s energy. Combustion of biomass
(organic matter) is an example of indirect so-
lar energy because plants use solar energy for
photosynthesis and store the energy in biomass. Windmills,
or wind turbines, use wind energy to generate electricity.
The damming of rivers and streams to generate electricity
is a type of hydropower—the energy of flowing water.
Biomass Energy
Biomass, one of the oldest fuels known to humans, con-
sists of materials such as wood, fast-growing plant and
algal crops, crop wastes, sawdust and wood chips, and
animal wastes. Biomass contains
chemical energy that comes from
the sun’s radiant energy, which
photosynthetic organisms use to
form organic molecules. Biomass is a renewable form of
energy if managed properly.
Biomass fuel, which may be a solid, liquid, or gas, is
burned to release its energy. Solid biomass fuels such as
wood, charcoal (wood turned into coal by partial burn-
ing), animal dung, and peat (partly decayed plant matter
found in bogs and swamps) supply a substantial portion
of the world’s energy. At least half of the human popula-
tion relies on biomass as their main source of energy. In
many developing countries, wood is the primary fuel for
cooking and heat (Figure 18.7).
It is possible to convert biomass, particularly animal
wastes, into biogas. Biogas, which is usually composed
of a mixture of gases (mostly methane), is like natural
gas. It is a clean fuel—its combustion produces fewer
pollutants than either coal or solid biomass. In India
and China, several million family-sized biogas digesters
use microbial decomposition of household and agricul-
tural wastes to produce biogas for cooking and lighting
(Figure 18.8). When biogas conversion is complete,
the solid remains are removed from the digester and
used as fertilizer.
Biogas has the potential to power fuel cells to gener-
ate electricity. A pilot program at Boston’s main sewage
treatment plant began producing electricity from biogas
in 1997. Sewage sludge in large biogas digesters produces
methane, which is then burned in a methane fuel cell to
produce enough electricity for 150 homes. Like the hy-
drogen fuel cells discussed earlier in the chapter, meth-
ane fuel cells produce relatively few pollutants.
Biomass can also be converted into liquid fuels,
especially methanol (methyl alcohol) and ethanol (ethyl
alcohol), which can replace gasoline in internal com-
bustion engines. In many parts of the world, automotive
fuels must contain 10 percent or more ethanol. Bio diesel,
made from plant or animal oils, is becoming popular
as an alternative fuel for diesel engines in trucks, farm
equipment, and boats. The oil is often refined from
waste oil produced at restaurants (such as the oil used to
make french fries); biodiesel burns much cleaner than
diesel fuel.Indirect Solar Energy
LEARNING OBJECTIVES
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Firewood is the major energy source
for most of the developing world.
Photographed in Garadawa, Niger.How much firewood is used for household
heating where you live? for cooking? Is it
biomass Plant and a necessity?
animal material used
as fuel.GLOBALLOCAL