Nuclear waste and its disposal Category: Pollution and waste disposal The disposal of radioactive waste is a significant prob-lem for the nuclear power industry and society as a whole..
Trang 1the organization to ensure public health and safety but
provides no specific guidance on how far the mandate
must be pursued
NRC safety decisions have been criticized both by
the nuclear industry and by environmental interests
The industry contends that NRC regulations have
sometimes been unnecessary, counterproductive, and
overly prescriptive in techniques for achieving safety
Environmental interests have asserted that the NRC
has compromised safety to ensure the economic
via-bility of nuclear projects
Safety concerns reached a peak when a reactor
ac-cident occurred at the Three Mile Island nuclear
plant in Pennsylvania in 1979 In response to
investi-gations of the accident, the NRC reformed its
licens-ing and regulatory processes However, no new plants
were begun, and a number of nuclear projects then in
progress were canceled
William C Wood
Web Site
U.S Nuclear Regulatory Commission
http://www.nrc.gov/
See also: Atomic Energy Acts; Atomic Energy
Com-mission; Energy economics; Nuclear energy; Nuclear
waste and its disposal; Three Mile Island nuclear
acci-dent
Nuclear waste and its disposal
Category: Pollution and waste disposal
The disposal of radioactive waste is a significant
prob-lem for the nuclear power industry and society as a
whole Various methods of burying and destroying the
material have been proposed.
Background
Unwanted radioactive materials are classified as
low-level, transuranic, or high-level waste or spent nuclear
fuel, depending on the concentration of radioactivity
and the half-life of the radioactive material In some
cases radioactive tailings from uranium mines are also
of concern Low-level waste (LLW), such as syringes
contaminated by radioactive pharmaceuticals, is much
less dangerous than the high-level waste (HLW) or
spent fuel rods generated by nuclear reactors
Trans-uranic waste (TRU) is radioactive waste with a level of radioactivity greater than 100 nanocuries per gram, a half-life greater than twenty years, and a composition
of elements with atomic numbers higher than 92 (ura-nium) In the United States, the 1980 Low-Level Ra-dioactive Waste Policy Act makes individual states responsible for the development of low-level waste disposal sites in conformity with licensing rules estab-lished by the U.S Nuclear Regulatory Commission Disposal of transuranic waste, high-level waste, or spent fuel rods, however, must be accomplished on the national level, as specified by the Nuclear Waste Policy Act of 1982 Some countries, such as France and Japan, do not consider spent fuel rods as waste be-cause they can be reprocessed into fuel (this process does generate TRU, however) Across the globe, trans-uranic waste and disposal of high-level nuclear waste and spent fuel rods continues to pose problems Be-cause nuclear power is seen as a potential means for alleviating global warming, the disposal of nuclear waste has become of increasing concern
Spent Fuel Rods and High-Level Waste Most electricity continues to be produced through the burning of fossil fuels, predominantly coal Some countries, such as France, rely heavily on nuclear en-ergy for their electric power; even the United States, a major consumer of fossil fuels, obtains 20 percent of its electric power from nuclear energy Worldwide,
440 nuclear power reactors are in operation in thirty countries Their operation is associated with the accu-mulation of large amounts of high-level waste and transuranic waste as well as with the production of highly radioactive spent fuel rods
The fuel for a typical reactor is a rare isotope of ura-nium, uranium 235, which is mixed with common uranium (uranium 238); the enrichment ratio is 1 to
30 The fuel is consumed through a nuclear reaction, called fission, a process in which atoms of uranium are broken into radioactive fragments The energy re-leased in fission becomes heat, part of which is con-verted into electricity Once or twice a year a reactor must be shut down for refueling The removed waste
is usually stored in a local isolated area for several years, because it is highly radioactive and its presence
in the biosphere would be a great danger to all living organisms However, this type of procedure is not sufficient: Tens of thousands of years are necessary to reduce the radioactivity in spent fuel to a nonthreat-ening level
Trang 2High-level nuclear waste includes liquids derived
from fuel reprocessing, solidified liquids and spent
nuclear fuel that have not been reprocessed, and
by-products from the nuclear weapons industry,
includ-ing residues from the reprocessinclud-ing of out-of-date
nu-clear weapons Like spent fuel rods, HLW poses severe
environmental problems if not disposed of properly
Nuclear waste generated by the nuclear weapons
industry poses a special set of problems Some of this
waste is either the same sort of high-level or low-level
waste as that generated by other nuclear industries or
a substantial amount of transuranic material Nuclear
warheads contain highly radioactive material that
poses potential danger if not treated properly Some
weapons are reprocessed into fuel, although this
pro-cess is not without hazard and produces substantial
HLW and TRU wastes Some people advocate
repro-cessing the weapons into new weapons, and others
call for destruction Some “spent” uranium is
repro-cessed into artillery shells In addition, the U.S
De-partment of Energy’s Nuclear Weapons Complex has
maintained sixteen major weapons sites throughout
the country, generating a variety of nuclear wastes
Two of the oldest—at Oak Ridge, Tennessee, and
Hanford, Washington—began operation during
World War II, generating large quantities of nuclear
waste that has been stored on-site Some of the liquid
storage tanks at Hanford are corroding; in others,
chemical reactions are under way; and in some cases,
knowledge of what is in the storage tanks no longer
exists The Russian military site at Mayak—which has
been the source of several accidents and deaths
in-volving substantial radioactive contamination of the
land, water, and atmosphere—poses even more of a
problem than U.S weapons sites because of the poor
safety and waste-disposal practices followed in the
past Some of the same problems as those
experi-enced in the United States are also present in nuclear
weapons sites in countries such as the United
King-dom and France
Although there have been few accidents involving
nuclear reactors, the 1986 accident at Chernobyl in
the Ukraine (then part of the Soviet Union)
pro-duced a large amount of nuclear waste The reactor
site and several hundred square kilometers of
sur-rounding countryside became contaminated In this
case the contamination became so extensive that the
Russian and Ukrainian governments sealed off the
area, in essence leaving the region contaminated
be-cause no other solution existed The reactors were
entombed in concrete, although some doubt exists as
to how successful this process has been
Burying Nuclear Waste The nuclear industry faces the critical challenge of isolating radioactive waste from the human habitat In the United States, for example, there are 131 sites in thirty-nine states storing HLW One possible method
of disposal is to bury spent fuel and HLW deep in geo-logically stable formations, a solution that the U.S Na-tional Research Council advocates A site has been se-lected for that purpose: Yucca Mountain, which is 145 kilometers northwest of Las Vegas, Nevada Construc-tion is under way, and the U.S government has spent millions of dollars developing the site, but it is not likely to be ready until 2015 or later The Yucca Moun-tain site is geologically stable, with low precipitation The water table lies, on average, 300 meters below the repository tunnels According to the plan, high-level waste would be stored in corrosion-resisting titanium
The Hanford nuclear reservation in Richland, Washington, pre-pares for the acquisition of 2,000 metric tons of radioactive waste to
be inserted into the holes in the floor pictured above (AP/Wide
World Photos)
Trang 3alloy containers with a drip shield, to prevent
ground-water from damaging the containers, and monitored
for fifty years The depository would be sealed to
pre-vent human interference However, the project is
con-troversial Some scientists think that an unacceptable
number of radioactive atoms would leak into the
bio-sphere with slowly percolating water or steam
gener-ated by radioactive heat Others fear the possibility of
future volcanic activities and earthquakes
Several other countries are also engaged in the
de-velopment of underground storage sites for nuclear
waste, and this has become the likely method for
dis-posal worldwide Most European countries have not
selected sites for the disposal of HLW The United
Kingdom continues to store nuclear waste on-site and
has selected no permanent repository site, nor has
France Germany is considering a sites at an
aban-doned iron-ore mine and a salt dome at Gorleben
Finland, on the other hand, is developing an
under-ground site at Olkiluoto for HLW Belgium is
consid-ering a site for deep deposition in a clay formation In
the first decade of the twenty-first century, no country
had a site for HLW in operation, and many, such as
Canada, did not appear to have any possible sites
Some countries with small volumes of nuclear waste
or no suitable geological formations ship their
nu-clear waste to countries such as the United States
Although environmental questions have been
raised concerning Yucca Mountain, the site is
in-tended to provide a stable and safe repository for
nu-clear waste In the past, the former Soviet Union used
a process of underground injection at three sites for a
good deal of its LLW such as strontium 90 and cesium
137 Radioactive atoms have migrated from these sites
into nearby water supplies and soil, creating
environ-mental problems
The Yucca Mountain tunnels are large enough to
hold the radioactive waste already accumulated in the
United States New sites, however, will be needed to
store the HLW that will be produced in the twenty-first
century and beyond Proliferation of such sites is not
desirable
TRU waste in the United States is handled at the
Waste Isolation Pilot Project (WIPP) site near
Carls-bad, New Mexico, which began operation in 1999
WIPP and the Yucca Mountain site are under control
of the U.S Department of Energy The WIPP disposes
of TRU waste in salt-dome formations located 655
me-ters below the surface Questions remain about WIPP,
most notably pertaining to the possibility of migration
of radioactive material to the surface through cracks
in the salt Most nations seem to be following a path of underground disposition for TRU
Low-level nuclear waste presents its own set of problems, the most formidable of which is the volume
of material Although not highly radioactive, LLWs— such as those generated by hospitals or contaminated clothing or building materials—may be solids, liq-uids, or even gases In many cases, efforts are made to turn these materials into solids so that they can be dis-posed of more readily They still require special han-dling because of their radioactivity and the possibility
of contamination with other hazardous substances LLWs are treated in a variety of ways In many cases, countries are engaging in burial of LLWs, often in shallow trenches Because LLWs have a high volume, their disposal requires a large site LLWs have in-creased in volume as, for example, nuclear medicine facilities have expanded In the United States, the Low Level Waste Policy Act provided that states would enter into compacts and develop LLW sites Because the disposal of nuclear waste is a controversial politi-cal issue, state governments have done little to de-velop such disposal sites By the early twenty-first cen-tury, the only two sites in operation were at Barnwell, South Carolina, and Richland, Washington, with Barnwell handling much of the LLW
One means of disposal that has been considered and rejected is dumping radioactive material in deep trenches in the ocean or even in lakes or rivers The Soviet Union has engaged in such practices in the past, leading to substantial contamination
Two other means of disposal of HLW and TRU wastes have also been considered and rejected One is
to store this waste beneath polar ice caps Another is
to load the waste on rockets and send it into space Both alternative pose enormous technological prob-lems More important, they also pose substantial risks
to human health and safety
Transforming Radioactive Waste Not everyone agrees that spent fuel should be buried without preliminary processing In France, for exam-ple, where 80 percent of electricity is nuclear, the pol-icy is to process spent fuel so that some can be reused before the remainder is buried Chemical processing
is already used to extract valuable by-products, such as plutonium, from spent fuel One isotope of pluto-nium that is as fissionable as urapluto-nium 235 has already been used to manufacture new fuel One by-product
Trang 4of this process, however, is weapons-grade plutonium.
France and the United Kingdom are developing a
process to recycle spent control rods without
produc-ing weapons-grade plutonium The Russian
Federa-tion stores most high-level nuclear waste on-site but
has a small reprocessing facility at Chelyabinsk-65 and
is developing a larger reprocessing facility at
Krasno-yarsk, scheduled to be operational in 2015
Proposals have been developed to “incinerate”
spent fuel Nuclear incineration refers not to
chemi-cal burning but to nuclear reactions by which
long-lived radioactive atoms are transmuted into
short-lived or nonradioactive atoms Pyrometallurgical
processes are intended to produce a mix of TRU
ele-ments instead of plutonium, which is produced in
conventional reprocessing processes Because
pluto-nium can be used to make nuclear weapons, the
United States banned this sort of reprocessing in
1977, although it is done in some countries, most
no-tably France Pyrometallurgical recycling produces
fission products and transuranics that are unsuitable
for weapons and also current reactors However, they
are suitable for what are called fast neutron reactors
Because fast neutron reactors neither use nor
gener-ate pure plutonium at any stage, they pose less of a
threat for weapons production China and India have
considered fast reactors, but none are currently
un-der consiun-deration in the United States
Other means of destroying high-level wastes have
been considered One involves the use of a nuclear
in-cinerator that would subject the waste material to a
flux of neutrons of an intensity that is several orders of
magnitude higher than occurs in an ordinary reactor
Research groups in the United States, Switzerland,
and Japan have engaged in research involving
incin-eration, but there are no commercial applications yet
The prevailing means for the disposition of
nu-clear waste remains underground disposition in
sta-ble geologic formations Such a solution has some
long-term problems, such as site maintenance and
intergenerational equity questions
Challenges for the Future
Many of the technological issues facing the disposal of
nuclear waste are being resolved Underground
dis-posal still has some potential problems, but it is clearly
superior to leaving spent control rods, HLW, and TRU
waste in temporary storage facilities on-site at power
reactors or government research facilities Long-term
on-site disposition raises numerous questions, such as
safety from terrorists, environmental contamination, and cost Some remaining concerns regarding under-ground disposition include the potential for migra-tion of radioactive material through the surrounding material to the surface and some difficulties concern-ing transportation of radioactive material to disposal sites The major obstacle to underground disposal re-mains societal Many people are not confident in the technology for disposal or are concerned about long-term consequences of underground burial, such as migration of radioactive material into water tables
or human access Other people distrust governmen-tal bodies, especially when they have not been trans-parent in decision making in the past However, above-ground storage on site is not a feasible long-term solution for HLW, LLW, spent nuclear fuel, and TRU waste Failure to resolve questions of waste disposal are increasing potential costs and have hampered the development of the nuclear power industry, which has the potential for helping to resolve energy needs and provide an alternative fuel not linked to global warming
Ludwik Kowalski, updated by John M Theilmann
Further Reading
Gerrard, Michael B Whose Backyard, Whose Risk: Fear and Fairness in Toxic and Nuclear Waste Siting
Cam-bridge, Mass.: MIT Press, 1994
Hambin, Jacob Darwin Poison in the Well: Radioactive Waste in Oceans at the Dawn of the Nuclear Age New
Brunswick, N.J.: Rutgers University Press, 2008 Hannum, William H., Gerald E Marsh, and George S
Stanford “Smarter Use of Nuclear Waste.” In Oil and the Future of Energy Guilford, Conn.: Lyons
Press, 2007
Johnson, Genevieve Fuji Deliberative Democracy for the Future: The Case of Nuclear Waste Management in Can-ada Toronto: University of Toronto Press, 2008 Lochbaum, David A Nuclear Waste Disposal Crisis Tulsa,
Okla.: PennWell Books, 1996
Long, Michael E “America’s Nuclear Waste: The Search for Permanent Solutions Heats up as Tons
of Highly Radioactive Sludge, Spent Fuel, and
Con-taminated Soil Pile up Around the Nation.” Na-tional Geographic 202, no 1 (2002): 2.
Macfarlane, Allison M., and Rodney C Ewing, eds
Uncertainty Underground: Yucca Mountain and the Na-tion’s High-Level Nuclear Waste Cambridge, Mass.:
MIT Press, 2006
Murray, Raymond L Understanding Radioactive Waste.
Trang 55th ed Edited by Kristin L Manke Columbus,
Ohio: Battelle Press, 2003
National Research Council Disposition of High-Level
Waste and Spent Nuclear Fuel: The Continuing Societal
and Technical Challenges Washington, D.C.:
Na-tional Academy Press, 2001
Risoluti, Piero Nuclear Waste: A Technological and
Politi-cal Challenge Berlin: Springer Verlag, 2004.
Rogers, Kenneth A., and Marvin G Kingsley
Calcu-lated Risks: Highly Radioactive Waste and Homeland
Se-curity Burlington, Vt.: Ashgate, 2007.
Saling, James H., Audeen W Fentiman, and Y S
Tang, eds Radioactive Waste Management 2d ed.
Philadelphia: Taylor & Francis, 2002
Savage, David, ed The Scientific and Regulatory Basis for
the Geological Disposal of Radioactive Waste New York:
John Wiley, 1995
Vandenbosch, Robert, and Susanne E Vandenbosch
Nuclear Waste Stalemate: Political and Scientific
Contro-versies Salt Lake City: University of Utah Press,
2007
Web Sites Nuclear Energy Institute Nuclear Waste Disposal http://www.nei.org/keyissues/nuclearwastedisposal U.S Nuclear Regulatory Commission
Radioactive Waste http://www.nrc.gov/waste.html See also: Air pollution and air pollution control; Bio-sphere; Department of Energy, U.S.; Electrical power; Energy economics; Food chain; International Atomic Energy Agency; Nuclear energy; Plutonium; Resources
as a source of international conflict; Three Mile Is-land nuclear accident; Uranium; Water pollution and water pollution control
Trang 6Ocean current energy
Category: Energy resources
The use of ocean currents as an energy source carries
great potential, but development has proceeded slowly
because the cost is not competitive with that of other
en-ergy sources.
Background
Just as winds flow through the Earth’s atmosphere,
currents flow throughout the world’s oceans These
currents are a potential power source as great as wind,
although winds harnessed for power have greater
speed than the currents The energy available in a
fluid flow varies both with velocity (by the square) and
with density:
Kinetic Energy = (Density) × (Velocity)2
Because water is nearly eight hundred times denser
than air (1,000 and 1.27 kilograms per cubic meter,
re-spectively), a current of 1.6 kilometers per hour has as
much energy as a wind of 45 kilometers per hour,
which is considered an excellent average speed for
wind energy Furthermore, currents are more
de-pendable than winds and flow in a constant direction
Ocean Temperature and Salinity
Ocean currents are caused by differences in
tempera-ture and salinity For example, as water near the
poles is cooled, its density increases, and much of
this cooler water sinks toward the ocean floor From
there it flows toward the equator, displacing warmer
water as it goes Meanwhile, water near the equator is
warmed, becoming less dense It tends to flow along
the surface toward the higher latitudes to replace the
sinking denser water
The Gulf Stream is such a current It starts from an
area of warm water in the equatorial Atlantic and in
the Gulf of Mexico This warm water flows generally
northward parallel to the coast of North America and
bends gradually to the right due to the rotation of the
Earth This tendency to curve (right in the Northern
Hemisphere, left in the Southern Hemisphere) is
called the Coriolis effect, and it bends the flow
north-east as the West Wind Drift, bringing warm, moist air
to Western Europe It continues south as the Canaries Current (carrying cooler water) past western North Africa Finally, the bending turns back west toward North America as the North Equatorial Current Similar circular patterns, or gyres, occur in all the world’s oceans, with many locations having great po-tential for electrical power generation For instance, the Gulf Stream has more energy than all the world’s rivers combined The area off Florida might yield 10,000 megawatts (10 billion watts) without observ-able change in the heat flow to Europe
Salinity differences also cause major flows The most easily tapped salinity currents are those between
a sea with high evaporation and the open ocean High-salinity water flows along the bottom from the Mediterranean Sea, for instance, while less saline At-lantic water flows in to replace it (German subma-rines used these currents during World War II for drifting silently past the major British base at Gibral-tar.) Two lesser potential sources of current power are tidal currents and the currents at the mouths of rivers
Methods for Harnessing Ocean Currents Electrical power generation from currents requires three things: mooring power stations to the ocean floor, generating power, and transmitting power to customers on shore
Mooring and transmitting power are related eco-nomic constraints on ocean current power Although
an underwater cable from a mid-Atlantic power sta-tion could technically supply power, deeper mooring lines and longer cables eventually cost more than the power delivered Thus, ocean current stations, if built, will tend to be near shore on the continental shelf and slope before investors attempt to moor a plant to the depths of the ocean floor
Using currents in deeper and more distant waters will require some means of energy storage This issue has been considered in design studies for ocean ther-mal energy conversion (OTEC) power stations, which would harness the temperature difference between warm tropical waters and the colder deep waters
Trang 7Elec-tricity could be used for some energy-intensive
pro-cess (such as refining aluminum) or for electrolyzing
hydrogen from water Hydrogen could be used to
syn-thesize chemical products, such as ammonia or
meth-anol Once the potential of current power is proven,
investors may consider the second set of risks
inher-ent in such mid-ocean vinher-entures
Among various proposals, two methods have been
studied in detail: turbines and sets of parachutes
on cables Turbines were first proposed by William
Mouton, who was part of a study team led by Peter
Lissaman of Aerovironment, Inc Their design is
called Coriolis In the study design, one 83-megawatt
Coriolis station has two huge counter-rotating fan
blades (so it does not pull to one side), roughly 100
meters in diameter The blades move slowly enough
for fish to swim through them
With blades so large, neither rigid blades nor the
central hub could be made strong enough without
be-ing too heavy and expensive However, a catenary
(free-hanging, like the cables of the Golden Gate
Bridge), flexible blade can be held in the proper
shape by the current while the generators are in a rim
around the blades The rim also acts as a funnel to
in-crease current speed past the blades and as an air
res-ervoir for raising the station when necessary
Another concept is parachutes on cables, which
was proposed by Gary Steelman His water low-velocity
energy converter (WLVEC) design is an endless loop
cable between two pulleys, much like a ski-lift cable
Parachutes along the cable are opened by the current
when going downstream and closed when coming
back upstream The WLVEC is cheaper than Coriolis,
but there is a question of how well any fabric could
withstand sustained underwater use
Ocean currents are sufficiently powerful and
pre-dictable to supply electricity effectively However, costs
of competing fossil fuels must rise significantly before
investors will overcome their timidity about
construct-ing offshore power plants However, test projects in
the United States, China, Japan, and the European
Union, in particular Britain, Ireland, and Portugal,
continue with high expectations
Roger V Carlson
Further Reading
Charlier, Roger Henri, and John R Justus “Ocean
Current Energy Conversion.” In Ocean Energies:
En-vironmental, Economic, and Technological Aspects of
Al-ternative Power Sources New York: Elsevier, 1993.
Congressional Research Service Energy from the Ocean.
Honolulu: University Press of the Pacific, 2002
Goldin, Augusta Oceans of Energy: Reservoir of Power for the Future New York: Harcourt Brace Jovanovich,
1980
Lissaman, P B S “The Coriolis Program.” Oceanus 22,
no 4 (Winter, 1979/1980): 23
_ “Tapping the Oceans’ Vast Energy with
Un-dersea Turbines.” Popular Science 221, no 3
(Sep-tember, 1980)
Noyes, Robert, ed Offshore and Underground Power Plants Park Ridge, N.J.: Noyes Data, 1977.
Web Site Minerals Management Service, U.S
Department of the Interior Ocean Current Energy
http://ocsenergy.anl.gov/guide/current/index.cfm See also: Energy storage; Ocean thermal energy con-version; Ocean wave energy; Oceans; Tidal energy
Ocean thermal energy conversion
Category: Energy resources
In some tropical regions of the Earth, there is virtually limitless energy in the ocean for possible conversion to electric power The efficiency of the conversion is very low, however, and the engineering problems are chal-lenging Development of ocean thermal energy conver-sion (OTEC) has been slow.
Background
In tropical oceans, the temperatures of warm and cold layers of water may differ significantly even though the layers are less than 1,000 meters apart This phenome-non results from global circulation currents caused by the Sun Solar energy warms water near the surface, and colder, more dense water moves to lower depths
At the same time, the rotation of the Earth causes the cold water to flow from the poles toward the tropics
As it is warmed, this cool water then rises toward the surface as its density decreases, causing the warm surface water to flow toward the polar regions, where
it is cooled
Differences of 20° to 25° Celsius over a distance of
500 to 1,000 meters are found in the Caribbean Sea
Trang 8and the Pacific Ocean near the
Hawai-ian Islands In accordance with the
second law of thermodynamics,
ther-mal energy from the warm layer can be
used as a “fuel” for a heat engine that
exhausts energy to the cool layer
Typi-cally, the warm layer has a
tempera-ture between 27° and 29° Celsius, and
the cool layer is between 4° and 7°
The second law of thermodynamics
indicates that the maximum efficiency
of the conversion from thermal
en-ergy to mechanical enen-ergy will be very
low For example, if the warm layer is
at 25° Celsius and the cold layer is at
5°, the maximum efficiency will be less
than 7 percent; even this figure is
be-tween two and three times the actual
efficiency that can be achieved in an
energy conversion plant
History
The concept of OTEC was first
sug-gested in 1882 by the French physicist
Jacques Arsène d’Arsonval, but it was
not until 1926 that the French scientist Georges Claude
made an attempt to implement the idea at Matanzas
Bay, Cuba The facility in Cuba was a small, land-based
plant which was so inefficient that it required more
power to operate than it produced It ran for only a
few weeks Beginning in the 1960’s, improvements in
design and materials led to considerable research
Feasibility as a practical method of power generation
was first demonstrated in the 1980’s
Advances in OTEC have depended on
governmen-tal support In the mid-1970’s, only the U.S and
Japa-nese governments were supporting research and
de-velopment The French government later became
interested, and sponsorship followed in the
Nether-lands, the United Kingdom, and Sweden
Basic Designs
Broadly speaking, designs are either open cycle (OC)
or closed cycle (CC) In the OC method, the
incom-ing warm seawater is continuously sent into an
evapo-rator operating at low pressure, where a small portion
of the water “flashes” into steam The steam in turn
passes through a turbine connected to an electric
power generator The low-pressure steam leaving the
turbine is then cooled and condensed in a heat
ex-changer by the cold seawater stream The condensed water is fresh water, the salt of the ocean having been left behind in the evaporator Hence, this water can be used for drinking and other household uses
In the CC process, heat from the warm stream is transferred in a heat exchanger to a “working fluid” such as propane or ammonia This fluid is vaporized and passed through a turbine generator in the same fashion as in the OC process The vapor leaving the turbine is then condensed in a second heat exchanger The condensate is recycled to the first exchanger, where it is again vaporized Thus, the working fluid is never in direct contact with the seawater Some hybrid plants have been designed which are combinations of
OC and CC technology
Though the first plant was a land-based unit, some plant designs involve plants located offshore, possibly floating or submerged One of the key elements in the process is the water pipe which carries the cold water
to the plant This pipe is typically between 1 and 2 kilo-meters long Originally, Claude used a corrugated steel pipe, 1.6 meters in diameter, which was fragile and not corrosion-resistant Steel has been replaced
by fiberglass-reinforced plastic or high-density poly-ethylene Diameters larger than this have been
con-The ocean thermal energy conversion plant in Keahole Point, Hawaii, opened in 1974 and became one of the top facilities of its kind in the world (United States
Depart-ment of Energy)
Trang 9sidered in some studies but are not feasible owing to a
lack of flexibility
Engineering Problems
Designs for OTEC plants with power capacities on the
order of 10 megawatts or more have been made, but
actual plants have been much smaller, with outputs on
the order of tens of kilowatts In spite of these
rela-tively small outputs, the equipment and the
engineer-ing problems are challengengineer-ing Both cold and warm
water flow rates are large because the efficiency of the
conversion process is so low The seawater carries
con-siderable dissolved gases, notably nitrogen and
oxy-gen, and these gases must be vented if flash
evapora-tion is used The presence of noncondensable gases
poses difficult problems both in the evaporator which
precedes the power turbine and in the condenser
which follows it These gases not only increase the
sizes of the units but also, because they are below
at-mospheric pressure, must be pumped out to maintain
the vacuum levels in the process
The CC method can avoid some of these problems
The operating pressures in the cycle using propane
are relatively high, so a turbine of reasonable size can
be used Moreover, because the pressures are greater
than atmospheric, vacuum and deaeration problems
are eliminated The CC process introduces additional
problems, however, owing to the heat-transfer steps
between the working medium and the hot and cold
water
Advantages
In view of the very low efficiency of OTEC, it may seem
hard to imagine how the process can be profitable
However, the “fuel” is free and virtually unlimited In
addition, the OC process can produce sizable
quanti-ties of fresh water, which is often valuable in places
where OTEC plants are located Some OC plants may
even be profitable on the basis of their freshwater
pro-duction alone Nevertheless, OTEC, even in the best
of circumstances, poses both engineering and
eco-nomic challenges that will continue to hamper its
de-velopment for many years
Thomas W Weber
Further Reading
Avery, William H., and Chih Wu Renewable Energy from
the Ocean: A Guide to OTEC New York: Oxford
Uni-versity Press, 1994
Charlier, Roger Henri, and John R Justus “Current
Assessment of Ocean Thermal Energy Potential.”
In Ocean Energies: Environmental, Economic, and Tech-nological Aspects of Alternative Power Sources New
York: Elsevier, 1993
Congressional Research Service Energy from the Ocean.
Honolulu, Hawaii: University Press of the Pacific, 2002
Goldin, Augusta Oceans of Energy: Reservoir of Power for the Future New York: Harcourt Brace Jovanovich,
1980
Krock, Hans-Jurgen, ed Ocean Energy Recovery: Proceed-ings of the First International Conference, ICOER ’89.
New York: American Society of Civil Engineers, 1990
Sorensen, Harry A Energy Conversion Systems New
York: J Wiley, 1983
Takahashi, Patrick, and Andrew Trenka Ocean Ther-mal Energy Conversion New York: John Wiley, 1996.
Tanner, Dylan “Ocean Thermal Energy Conversion:
Current Overview and Future Outlook.” Renewable Resources 6, no 3 (1995): 367-373.
Web Sites State of Hawaii, Department of Business, Economic Development, and Tourism Ocean Thermal Energy
http://hawaii.gov/dbedt/info/energy/renewable/ otec
U.S Department of Energy Ocean Thermal Energy Conversion http://www.energysavers.gov/renewable_energy/ ocean/index.cfm/mytopic=50010
See also: Electrical power; Ocean current energy; Ocean wave energy; Oceans; Tidal energy
Ocean wave energy
Category: Energy resources
A number of designs for harnessing wave energy have been proposed, and some are in use on various scales, but the vast potential of this power source has not been tapped because of the uncertainties and expense in-volved.
Trang 10Waves crashing against a beach are a vast, almost
mys-tical, display of mechanical power For centuries
peo-ple have sought ways of tapping it In 1799, a father
and son named Girard applied to the French
govern-ment for a patent on a wave-power device They noted
that waves easily lifted even mighty ships Hence, a
le-ver from a ship to shore could power all manner of
mills (There are records of Girard mills on rivers, but
the wave machine was probably never built.)
Hun-dreds of patents later, wave power is still largely a
dream, although a dream that is approaching reality
The Nature of Waves
Waves are the product of wind blowing on the ocean
surface The energy available comes from the wind
speed and the distance (or “fetch”) that the wind
blows: A breeze blowing on a small bay produces
rip-ples, whereas a hurricane blowing across several
hun-dred meters builds hill-sized waves Waves hitting a
beach can be the result of a storm on the opposite side
of an ocean From that standpoint, waves are a
collect-ing and concentratcollect-ing mechanism for wind power
However, there is some loss of wave energy over great
distances, so the best places to take advantage of wave
potential are along high-wind coasts of the temperate
and subpolar latitudes Specific regions of the world
with strong wave actions include the western coasts of
Scotland, northern Canada, southern Africa,
Austra-lia, and the northwestern coasts of the United States
Water waves mostly consist of a circular motion of
the water molecules as the wave energy continues
until it meets a barrier, such as a shoreline Then the
energy hurls water and pieces of the shore until
grav-ity pulls them back Ultimately, the energy is
trans-formed into heat, hardly noticed in the water Along
the way, the energy is vast The North Pacific is
esti-mated to have a flux of 5 to 50 megawatts of
mechani-cal energy per kilometer
One limitation of wave energy is that timing and
power are variable (although not as much as with
winds) The crests and troughs of one storm may be
out of phase with another, in which case they largely
cancel each other out Winds may be low, or they may
be directly against waves approaching the power plant
Any of these factors can limit power production at
unpredictable times Conversely, waves from two or
more storms may be in phase and stack, creating
mon-ster waves that have been observed as high as 34
me-ters in the open ocean Extraordinary waves have
been the destruction of countless ships and of more than one wave power station They are probably the greatest obstacle to widespread use of wave energy
Methods for Harnessing Ocean Waves Electrical power generation from waves requires three things: mooring the power stations to the ocean floor
or building along the coast, generating power, and transmitting the power to customers inland As with wind energy, a useful fourth item would be storage to deal with low-wave days
Building and power transmission are straightfor-ward operations, because most wave-harvesting de-signs are on or near shore Even though these installa-tions must be reinforced against especially strong waves, they do not have the cost and complexity of deepwater structures
Proposed energy-harvesting techniques have great variation because many researchers have been at-tempting to harness wave potential The researchers face three major problems First, generators face the previously mentioned fluctuations in awesome power Second, wave power is large but moves at a slow pace, and the machinery to obtain high speed (needed for
an electric generator) is expensive Third, complex hinges, pistons, and other moving parts need fre-quent replacement in the salty ocean environment The simplest approach is a ramp and dam facility that traps water splashing above sea level Draining water goes down a pipe (penstock) to turbines, just as
in a hydroelectric dam The “Russel rectifier” is sort of
a dam with chambers and flaps so that both rising and falling waves cause water in a turbine to flow continu-ously in the same direction The various dam schemes are familiar and can be built on land The disadvan-tage is that power dams must be large and capable of surviving the surf; thus they are expensive
The “dam atoll” is an open-ocean variant of the ramp A half-submerged dome in the ocean bends waves around it so that waves come in from all sides, just as with coral atolls The water sloshes to a central drain at the top and drains back through a penstock The central collection increases efficiency, and being
a floating structure allows submerging below the waves during major storms However, increased dis-tance from shore increases power transmission costs Air pressure can translate slow wave motion into a fast spin In many schemes, waves rise and fall either in
a series of open rooms at the bottom of a floating structure or in cylinders at the end of funnel-shaped