Environmental Engineering Reference
In-Depth Information
Arelated way to heat or cool a building is geother-
mal exchange or geoexchange. Buried pipes filled with a
fluid move heat in or out of the ground or from nearby
bodies of water, depending on the season and the heat-
ing or cooling requirements. In winter, for example,
heat is removed from the fluid in the buried pipes and
blown through house ducts. In summer, this process is
reversed. According to the EPA, geothermal exchange
is the most energy-efficient, cost-effective, and envi-
ronmentally clean way to heat or cool a building.
We have also learned to tap into deeper, more con-
centrated underground reservoirs of geothermal en-
ergy. One type of reservoir contains dry steam with wa-
ter vapor but no water droplets. Another consists of
wet steam, a mixture of steam and water droplets. A
third is hot water trapped in fractured or porous rock at
various places in the earth's crust.
If such geothermal sites are close to the surface,
wells can be drilled to extract the dry steam, wet
steam, or hot water (Figure 13-2). It can then be used to
heat homes and buildings or to spin turbines and pro-
duce electricity.
Three other nearly nondepletable sources of geo-
thermal energy exist. One is molten rock (magma). An-
other is hot dry-rock zones, where molten rock that has
penetrated the earth's crust heats subsurface rock to
high temperatures. A third source is low- to moderate-
temperature warm-rock reservoir deposits. Heat from
such deposits could be used to preheat water and run
heat pumps for space heating and air conditioning.
Hot dry-rock zones can be found almost anywhere
8-10 kilometers (5-6 miles) below the earth's surface.
Research is being carried out in several countries to see
whether these zones can provide affordable geother-
mal energy.
Currently, about 22 countries (most of them in the
developing world) extract enough energy from geo-
thermal sites to produce about 1% of the world's elec-
tricity. Geothermal energy is used to heat 85% of Ice-
land's buildings, produce electricity, and grow most of
that country's fruits and vegetables in greenhouses
heated by geothermal energy.
Geothermal electricity meets the electricity needs
of 6 million Americans and supplies 6% of California's
electricity. The world's largest operating geothermal
system, called The Geysers, extracts energy from a dry-
steam reservoir north of San Francisco, California.
Currently, heat is being withdrawn from this geother-
mal site about 80 times faster than it is being replen-
ished, converting this renewable resource to a non-
renewable source of energy. In 1999, Santa Monica,
California, became the first city to get all its electricity
from geothermal energy. Figure 13-44 (p. 322) lists the
advantages and disadvantages of using geothermal
energy.
T rade-Offs
Methanol Fuel
Advantages
Disadvantages
High octane
Large fuel tank
needed
Some reduction in
CO 2 emissions
Half the driving
range
Lower total air
pollution (30-40%)
Corrodes metal,
rubber, plastic
Can be made
from natural gas,
agricultural
wastes, sewage
sludge, garbage,
and CO 2
High CO 2 emissions
if made from coal
Expensive to
produce
Can be used to
produce H 2 for
fuel cells
Hard to start in
cold weather
Figure 13-43 Trade-offs: general advantages and disadvan-
tages of using methanol as a vehicle fuel compared to gasoline.
Critical thinking: pick the single advantage and the single dis-
advantage that you think are the most important.
used to produce useful chemicals like the petrochemi-
cals made from petroleum and natural gas.
x
H OW W OULD Y OU V OTE ? Should we greatly increase our
dependence on ethanol and methanol as vehicle fuels? Cast
your vote online at http://biology.brookscole.com/miller11.
13-6
GEOTHERMAL ENERGY
Science: Tapping the Earth's
Internal Heat
We can use geothermal energy stored in the earth's
mantle to heat and cool buildings and to produce
electricity.
Geothermal energy consists of heat stored in soil, un-
derground rocks, and fluids in the earth's mantle. We
can tap into this stored energy to heat and cool build-
ings and to produce electricity.
Geothermal heat pumps can exploit the difference
between underground and surface temperatures in
most places and use a system of pipes and ducts to
heat or cool a building. These devices extract heat from
the earth in winter. In summer, they can store heat re-
moved from a house in the earth. They are a very effi-
cient and cost-effective way to heat or cool a space.
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