44974649-Tidal-Energy by dandanhuanghuang


									                                          TIDAL ENERGY
Tidal Energy
Coastal areas with huge and flowing tidal waters carry vast potential energy.
11th Century England was the first to harness this energy, using water wheels to
produce mechanical power. These days the rise and fall of tides have become the
basis to produce electrical power similar to the principles of hydroelectric power

The daily rise and fall in the level of
ocean water relative to the coastline is
referred to as tide. Tides originate from
the motions of the earth, moon and sun.
The gravitational pull of the Moon and
Sun along with the revolution of the
Earth result in tides. (The magnitude of
the gravitational attraction of an object
is dependant upon the mass of an object and its distance.) The moon exerts a larger gravitational
force on the earth, though it is much smaller in mass, because it is a lot closer than the sun. This
force of attraction causes the oceans, which make up 71 percent of the earth's surface, to bulge along
an axis pointing towards the moon. Tides are produced by the rotation of the earth beneath this bulge
in its watery coating, resulting in the rhythmic rise and fall of coastal ocean levels.

The gravitational attraction of the sun also affects the tides similarly, but to a lesser degree. As well
as bulging towards the moon, the oceans also bulge slightly towards the sun. When the earth, moon
and sun are positioned in a straight line i.e on the occasion of a full or new moon, the gravitational
attractions are combined, resulting in very large spring tides. At half moon, the sun and moon are
positioned at right angles, resulting in lower neap tides. Coastal areas experience two high and two
low tides over a period of 24 hours and slightly above.

The presence of geographical features such as bays and inlets result in higher tides. To produce
enough amounts of power (electricity) that can be put to practical use, a difference of at least five
meters between high and low tides is a must. There are about 40 suitable sites around the world with
this kind of tidal range. The higher the tides, the greater is the amount of electricity that can be
generated from a given site. It is inversely proportional to the cost of electricity produced, making
such sites also more economical. Approximately 3000 GW (1 Giga Watt = 1 GW = 1 billion watts) of
energy are available from the tides, worldwide. However considering the limitations as mentioned
above, only about 2% (= 60 GW) can potentially be exploited for electricity generation.

Generating tidal energy
The technology required to convert tidal energy into electricity is comparable to technology used in
traditional hydroelectric power plants. The first requirement is a dam across a tidal bay or estuary.
However building a dam is expensive and the best sites are those where a bay has a narrow opening,
thus reducing the length of dam required. Gates and turbines are installed. When there is adequate
difference in the levels of the water on the different sides of the dam, the gates are opened. This
causes water to flow through the turbines, turning the generator to produce electricity.

Electricity is generated by water flowing both inwards and out of a bay. There are periods of
maximum generation every twelve hours, with no electricity generation at the six-hour mark in
between. The turbines may also be used as pumps to pump extra water into the basin behind the dam
at times when the demand on electricity is low. This water can later be released when the demand on
the system is very high, thus allowing the tidal plant to function like a "pumped storage" hydroelectric

Uses and economy
The friction of the bulging oceans acting on the spinning earth results in a very gradual slowing down
of the earth's rotation but this is not expected to impact us for billions of years. Therefore, for
practical purposes, tidal energy can be considered a sustainable and renewable source of energy. It
can prove to be a valuable source of renewable energy to an electrical system. The demand of
electricity from a grid varies with the time of the day. Tidal power, although variable, is reliable and
predictable and can make a valuable contribution to an electrical system, which has a variety of
sources. Tidal electricity provides a good alternative to conventional methods of generating electricity,
which would otherwise be generated by fossil fuel (coal, oil, natural gas) etc, thus reducing emissions
      of greenhouse and acid gases.

                                                     Dam across La Rance
                                                 Although the technology is well in place, tidal power is an
                                                 expensive affair. Operating and maintenance costs of
                                                 tidal power plants are very low because the fuel, being
                                                 seawater, is free. However the overall cost of electricity
                                                 generated is still very high. There is only one major tidal
                                                 generating station in operation - the 240-MW tidal plant
                                                 (1 megawatt = 1 MW = 1 million watts) at the mouth of
                                                 the La Rance river estuary on the northern French coast.
                                                 In operation since 1966, this plant has been a very
                                                 reliable source of electricity for France. Researchers are
                                                 examining the potential of several other tidal power sites
                                                 and some of the prospects include the Severn River Dam
      in western England, the Bay of Fundy in Canada, Cook Inlet in Alaska, and the White Sea in Russia.

      Environmental studies so far indicate that tidal energy does not result in the emission of gases
      responsible for global warming or acid rain associated with fossil fuel generated electricity. Use of tidal
      energy could also decrease the need for nuclear power, with its associated radiation risks. One
      concern is that the tidal flows caused by damming a bay or estuary could, result in negative impacts
      on the immediate environment. This is still unclear as very little is understood about how altering the
      tides can affect incredibly complex aquatic and shoreline ecosystems. However each specific site is
      different and the impacts depend greatly upon local geography. Local tides changed only slightly due
      to the La Rance dam, and the environmental impact has been negligible. It has been estimated that in
      the Bay of Fundy, tidal power plants could decrease local tides by 15 cm.

      The negative environmental impacts of tidal barrages are probably much smaller than those of other
      sources of electricity. Tidal energy has the power to generate significant amounts of electricity at
      suitable sites around the world. But the potential is yet to be fully explored.

Electricity generation
From Wikipedia, the free encyclopedia

Electricity generation is the process of creating electricity from other forms of energy.

The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the
British scientist Michael Faraday. His basic method is still used today: electricity is generated by the movement
of a loop of wire, or disc of copper between the poles of a magnet.[1]

For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes,
electricity transmission,distribution, and electrical power storage and recovery using pumped storage methods
are normally carried out by the electrical power industry.

Electricity is most often generated at a power station by electromechanical generators, primarily driven by heat
engines fueled by chemicalcombustion or nuclear fission but also by other means such as the kinetic energy of
flowing water and wind. There are many other technologies that can be and are used to generate electricity
such as solar photovoltaics and geothermal power.
Sources of electricity in the U.S. in 2009[2] fossil fuel generation (mainly coal) was the largest source.


Sources of electricity in France in 2006;[3] nuclear power was the main source.

Centralised power generation became possible when it was recognised that alternating currentpower lines can
transport electricity at very low costs across great distances by taking advantage of the ability to raise and
lower the voltage using power transformers.

Electricity has been generated at central stations since 1881. The first power plants were run on water
power[4] or coal,[5] and today we rely mainly on coal, nuclear, natural gas, hydroelectric, andpetroleum with a
small amount from solar energy, tidal harnesses, wind generators, andgeothermal sources.

[edit]Methods         of generating electricity

There are seven fundamental methods of directly transforming other forms of energy into electrical energy:

    Static electricity, from the physical separation and transport of charge (examples: triboelectric
     effect and lightning)

    Electromagnetic induction, where an electrical generator, dynamo or alternator transforms kinetic
     energy (energy of motion) into electricity

    Electrochemistry, the direct transformation of chemical energy into electricity, as in a battery, fuel
     cell or nerve impulse
   Photoelectric effect, the transformation of light into electrical energy, as in solar cells

   Thermoelectric effect, direct conversion of temperature differences to electricity, as
    in thermocouples, thermopiles, and Thermionic converters.

   Piezoelectric effect, from the mechanical strain of electrically anisotropic molecules or crystals

   Nuclear transformation, the creation and acceleration of charged particles (examples: betavoltaics or alpha
    particle emission)

Static electricity was the first form discovered and investigated, and the electrostatic generator is still used even
in modern devices such as the Van de Graaff generator and MHD generators. Electrons are mechanically
separated and transported to increase their electric potential.

Almost all commercial electrical generation is done using electromagnetic induction, in which mechanical
energy forces an electrical generator to rotate. There are many different methods of developing the mechanical
energy, including heat engines, hydro, wind and tidal power.

The direct conversion of nuclear energy to electricity by beta decay is used only on a small scale. In a full-size
nuclear power plant, the heat of a nuclear reaction is used to run a heat engine. This drives a generator, which
converts mechanical energy into electricity by magnetic induction.

Most electric generation is driven by heat engines. The combustion of fossil fuels supplies most of the heat to
these engines, with a significant fraction from nuclear fission and some from renewable sources. The
modern steam turbine invented by Sir Charles Parsons in 1884 - today generates about 80 percent of
the electric power in the world using a variety of heat sources.


Large dams such as Three Gorges Damin China can provide large amounts ofhydroelectric power; it will have a
22.5 GWcapability.
Susquehanna Steam Electric Station, anuclear power plant.

A combined cycle natural gas power plant near Orem, Utah.

All turbines are driven by a fluid acting as an intermediate energy carrier. Many of the heat engines just
mentioned are turbines. Other types of turbines can be driven by wind or falling water.

Sources include:

   Steam - Water is boiled by:

        Nuclear fission,

        The burning of fossil fuels (coal, natural gas, or petroleum). In hot gas (gas turbine), turbines are
         driven directly by gases produced by the combustion of natural gas or oil. Combined cycle gas turbine
         plants are driven by both steam and natural gas. They generate power by burning natural gas in a gas
         turbine and use residual heat to generate additional electricity from steam. These plants offer
         efficiencies of up to 60%.

        Renewables. The steam generated by:

            Biomass

            The sun as the heat source: solar parabolic troughs and solar power towers concentrate sunlight
             to heat a heat transfer fluid, which is then used to produce steam.
              Geothermal power. Either steam under pressure emerges from the ground and drives a turbine or
               hot water evaporates a low boiling liquid to create vapour to drive a turbine.

    Other renewable sources:

         Water (hydroelectric) - Turbine blades are acted upon by flowing water, produced byhydroelectric
          dams or tidal forces.

         Wind - Most wind turbines generate electricity from naturally occurring wind. Solar updraft towers use
          wind that is artificially produced inside the chimney by heating it with sunlight, and are more properly
          seen as forms of solar thermal energy.
[edit]Reciprocating            engines
Small electricity generators are often powered by reciprocating engines burning diesel, biogas or natural
gas. Diesel engines are often used for back up generation, usually at low voltages. However most large power
grids also use diesel generators, originally provided as emergency back up for a specific facility such as a
hospital, to feed power into the grid during certain circumstances. Biogas is often combusted where it is
produced, such as a landfill or wastewater treatment plant, with a reciprocating engine or a microturbine, which
is a small gas turbine.

A coal-fired power plant in Laughlin, Nevada U.S.A. Owners of this plant ceased operations after declining to invest in
pollution control equipment to comply with pollution regulations. [6]

[edit]Photovoltaic           panels
Unlike the solar heat concentrators mentioned above, photovoltaic panels convert sunlight directly to electricity.
Although sunlight is free and abundant, solar electricity is still usually more expensive to produce than large-
scale mechanically generated power due to the cost of the panels. Low-efficiency silicon solar cells have been
decreasing in cost and multijunction cells with close to 30% conversion efficiency are now commercially
available. Over 40% efficiency has been demonstrated in experimental systems.[7] Until recently, photovoltaics
were most commonly used in remote sites where there is no access to a commercial power grid, or as a
supplemental electricity source for individual homes and businesses. Recent advances in manufacturing
efficiency and photovoltaic technology, combined with subsidies driven by environmental concerns, have
dramatically accelerated the deployment of solar panels. Installed capacity is growing by 40% per year led by
increases in Germany, Japan, California and New Jersey.

[edit]Other    generation methods

Wind-powered turbines usually provide electrical generation in conjunction with other methods of producing power.

Various other technologies have been studied and developed for power generation. Solid-state generation
(without moving parts) is of particular interest in portable applications. This area is largely dominated
by thermoelectric (TE) devices, though thermionic (TI) and thermophotovoltaic(TPV) systems have been
developed as well. Typically, TE devices are used at lower temperatures than TI and TPV
systems. Piezoelectric devices are used for power generation from mechanical strain, particularly in power
harvesting. Betavoltaics are another type of solid-state power generator which produces electricity from
radioactive decay. Fluid-based magnetohydrodynamic (MHD) power generation has been studied as a method
for extracting electrical power from nuclear reactors and also from more conventional fuel combustion
systems. Osmotic power finally is another possibility at places where salt and sweet water merges (e.g. deltas,

Electrochemical electricity generation is also important in portable and mobile applications. Currently, most
electrochemical power comes from closed electrochemical cells ("batteries") [8], which are arguably utilized
more as storage systems than generation systems, but open electrochemical systems, known as fuel cells,
have been undergoing a great deal of research and development in the last few years. Fuel cells can be used
to extract power either from natural fuels or from synthesized fuels (mainly electrolytic hydrogen) and so can be
viewed as either generation systems or storage systems depending on their use.
Tidal energy use harnesses the water flow created primarily by the moon orbiting the Earth.
As water is pulled toward the gravity of the moon, currents are created that can turn
generator turbines.

Volumes have been written about tides and their effects on our planet. ThisWikipedia
Tides article is a good primer on the subject. It is noteworthy that all tidal energy does not
come from the moon. About a third of it comes from the gravitational influence of our sun.

The interplay of gravitational fields of the moon and the sun combined with the rotation of
Earth, creates a twice a day ebb and flow of the tides of our world that varies in height and

Those variations in height and strength are completely predictable. As we’ll see later, that
predictability is an important aspect of tidal energy use.

Though renewable, practical tidal energy use will be limited. Tidal flows are global, but the
key to using them economically is finding either natural high tidal flow areas, or large tidal
basins that can be easily dammed to channel water through turbines.


Tidal energy use involving dams creates many of the same environmental concerns as
damming rivers. Tidal dams restrict fish migration and cause silt build up which affects tidal
basin ecosystems in negative ways.

Systems that take advantage of natural narrow channels with high tidal flow rates have less
negative environmental impact than dammed systems. But they are not without
environmental problems.

Both systems use turbines that can cause fish kills. But these are being replaced by new,
more fish friendly turbines. The art and science of environmentally friendly hydro
engineering is well advanced and will certainly be applied to any tidal energy project.

But even with dams, the environmental impact of tidal energy projects may prove to be
smaller than our use of any other energy resource. Economics will severely limit the number
of tidal energy projects.


Tidal energy projects involving tidal dams are more expensive per KW of installed power
than similar size systems that use river dams. Tidal flow is intermittent. Twice a day tidal
flows go through a flood stage, slow down, stop, reverse into an ebb tide, slow down, stop,
and repeat the cycle.
This constant start and stop cycle creates intermittency problems similar to wind turbines
and wave generators. Though a tidal dam might be identical to a river dam in every way
including cost; the tidal dam will produce less than half the amount of electricity.

A typical average plant load factor for tidal energy generators is about 27%. Load factor
defines the amount of actual power output expected from a given capacity. Installed
generating capacity of 100 MW with a load factor of 27% would produce only 27 MW per
hour when averaged over a given time, usually a year.

That makes tidal energy expensive.

This U.S. Department of Energy Tidal Energy Report concludes that tidal power costs are
not competitive with fossil fuel plants.

But a private company, Blue Energy of Canada , believes that they can generate tidal
electricity at rates that are highly competitive with existing conventional power generators
at rates of less than $.05 per KWh.



The key to reliable, economic power from tidal energy involves properly engineered,
economical turbine generators placed at well researched sites with high tidal flow rates.

Because of intermittency and variable flow problems of tidal energy, it is a very limited
resource. The DOE Tidal Energy link, above, states that there are only about 40 really good
sites on Earth with high enough flows to be considered economically practical.

Few studies of tidal energy resources have been done, so information is sketchy at best.

The World Energy Council Ocean Current Report states that total electrical power available
from tidal energy use is about 450 GW of installed capacity. The report is a bit confusing to
read and appears to be mixing tidal information with other ocean current information. Still,
it’s worth reading.

That 450 GW figure seems to be compatible with the data on the WEC Tidal Energy page.

If we apply the .27 average load factor for tidal energy use, we can expect it to deliver
about 450GW x 24 hrs x 365 days x .27 LF = 1064 TWh (Terrawatt hours) annually, or a
little over 6% of global electrical demand.



Tidal energy projects involving dams would involve about the same aesthetic concerns as
other dams. But many of the systems that use natural tidal currents will be largely hidden
from view.
Natural current driven tidal generators can be built into the structure of existing bridges.
These generators will involve virtually no aesthetic problems.

And, the fact that tidal energy use will be extremely limited means that any aesthetic
concerns will also be limited.

Tidal energy use may not be a big player in our energy future, but it can make a

Tidal intermittency is completely predictable. Power output from tidal generators is also
completely predictable. That predictability makes tidal energy reliable and easy to integrate
with the existing electrical power grid. All of that makes it valuable.

Though tidal energy use will provide only a small portion of electrical grid demand, it can be
a reliable and important energy resource.

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