Tidal Power

Define:

Tidal power  is  the only  form of energy which derives directly  from  the  relative motions of  the  Earth–Moon  system, and  to a  lesser extent  from  the Earth–Sun  system. The  tidal  forces produced by  the Moon and Sun,  in  combination with Earth’s  rotation,  are responsible  for  the  generation  of  the  tides. Tidal power  is  the only  form of energy which derives directly  from  the relative motions of  the  Earth–Moon  system, and  to a  lesser extent  from  the Earth–Sun  system. The  tidal  forces produced by  the Moon and Sun,  in  combination with Earth’s  rotation,  are  responsible  for  the  generation  of  the  tides.

Tidal power, also called tidal energy, is a form of hydropower that converts the energy of tides into electricity or other useful forms of power.

Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power. Among sources of renewable energy, tidal power has traditionally suffered from relatively high cost and limited availability of sites with sufficiently high tidal ranges or flow velocities, thus constricting its total availability. However, many recent technological developments and improvements, both in design (e.g. dynamic tidal power, tidal lagoons) and turbine technology (e.g. new axial turbines, crossflow turbines), indicate that the total availability of tidal power may be much higher than previously assumed, and that economic and environmental costs may be brought down to competitive levels.

Because  the Earth’s  tides are caused by  the  tidal  forces due  to gravitational  interaction with  the Moon and Sun, and the Earth’s rotation,  tidal power is practically inexhaustible and classified as a  renewable energy source.

Global Scenario:

  • The first tidal power station was the Rance tidal power plant built over a period of 6 years from 1960 to 1966 at La Rance, France. It has 240 MW installed capacity.
  • The first tidal power site in North America is the Annapolis Royal Generating Station, Annapolis Royal, Nova Scotia, which opened in 1984 on an inlet of the Bay of Fundy.  It has 20 MW installed capacity.
  • The Jiangxia Tidal Power Station, south of Hangzhou in China has been operational since 1985, with current installed capacity of 3.2 MW. More tidal power is planned near the mouth of the Yalu River.
  • The first in-stream tidal current generator in North America (Race Rocks Tidal Power Demonstration Project) was installed at Race Rocks on southern Vancouver Island in September 2006.  The next phase in the development of this tidal current generator will be in Nova Scotia.
  • A small project was built by the Soviet Union at Kislaya Guba on the Barents Sea. It has 0.4 MW installed capacity. In 2006 it was upgraded with a 1.2MW experimental advanced orthogonal turbine.
  • Jindo Uldolmok Tidal Power Plant in South Korea is a tidal stream generation scheme planned to be expanded progressively to 90 MW of capacity by 2013. The first 1 MW was installed in May 2009.
  • A 1.2 MW SeaGen system became operational in late 2008 on Strangford Lough in Northern Ireland.
  • 254 MW Sihwa Lake Tidal Power Plant in South Korea is under construction and planned to be completed by the end of 2010.
  • The contract for an 812 MW tidal barrage near Ganghwa Island north-west of Incheon has been signed by Daewoo. Completion is planned for 2015.
  • A 1,320 MW barrage built around islands west of Incheon is proposed by the Korean government, with projected construction start in 2017.
  • Other South Korean projects include barrages planned for Garorim Bay, Ansanman, and Swaseongho, and tidal generation associated with the Saemangeum reclamation project. The barrages are all in the multiple-hundred megawatts range.
  • Estimates for new tidal barrages in England give the potential generation at 5.6GW mean power.
Station Capacity (MW) Country Comm
Rance Tidal Power Station 240 France 1966
Annapolis Royal Generating Station 20 Canada 1984
Jiangxia Tidal Power Station 3.2 China 1980
Kislaya Guba Tidal Power Station 1.7 Russia 1968
Strangford Lough SeaGen 1.2 United Kingdom 2008
Uldolmok Tidal Power Station 1.0 South Korea 2009

Indian Scenario:

  • A British tidal energy company, Atlantis Resources, is expected to set up a tidal power plant with the capacity to generate over 250 MW in the Gulf of Kutch or Khambhat.
  • India’s first attempt to harness tidal power for generating electricity would be in the form of a three MW plant proposed at the Durgaduani creek in Sundarbans delta of West Bengal.
  • The Gulf of Kutch and Gulf of Cambay in Gujarat and Ganga delta in the Sunderbans, the world’s largest mangrove, are the three sites identified as potential areas for tidal power generation.

How Tide Generates:

Tidal  energy  is  generated  by  the  relative  motion  of  the  water  which  interact  via  gravity.  Periodic changes  of water  levels,  and  associated  tidal  currents,  are  due  to  the  gravitational  attraction  by  the  Sun  and Moon. The  magnitude  of  the  tide  at  a  location  is  the  result  of  the  changing  positions  of  the Moon  and  Sun relative to the Earth, the effects of Earth rotation, and the local shape of the sea floor and coastlines. Because  the Earth’s  tides are caused by  the  tidal  forces due  to gravitational  interaction with  the Moon and Sun, and the Earth’s rotation,  tidal power is practically inexhaustible and classified as a  renewable energy source. A  tidal  generator  uses  this  phenomenon  to  generate  electricity. The  stronger  the  tide,  either  in water level height or tidal current velocities, the greater the potential for tidal electricity generation. Tidal  movement  causes  a  continual  loss  of  mechanical  energy  in  the  Earth–Moon  system  due  to pumping of water through the natural restrictions around coastlines, and due to viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth  to slow in the 4.5 billion years since formation.  During  the  last  620 million  years  the  period  of  rotation  has  increased  from  21.9 hours  to  the 24 hours we  see now;  in  this period  the Earth  has  lost 17% of  its  rotational energy. While  tidal power may take additional energy  from  the  system,  increasing  the  rate of  slowdown,  the effect would be noticeable over  millions of years only, thus being negligible.  Dynamically speaking, the earth and  the Moon are  two masses  that display centrifugal  forces on one another.  First, we must consider a particle of mass m which is located on the earth‘s surface. Given Newton‘s  law of gravitational state we introduce the equation:

F = G m1m2 R2

Where  F  is  the  force created between mass1 and mass2, G  is  the  universal gravitational constant whose value depends only on the chosen units of mass, length, and  force

(typically  6.67 x 10-11 N m2 kg-2). If we then take  the difference between  the  force  towards  the moon and  the  force  necessary  for earth‘s  rotation we generate the tidal producing force.

Tidal Force = 2Gmm1a (1.2) R3

Where m is the mass of  the earth, a is the mean radius of the earth and R is the distance between earth and  the  lunar  surface. The  net effect of  this equation  is  to displace particle m1 away  from  the center  of  the earth. Thus, we can conclude  that diurnal  tides are  generated because  the maxima and minima  in each daily rotation are unequal in amplitude. (Pugh 64) This is ultimately, in its simplest form, the process behind the half-day cycle which  results  in a period of 12 hours 25 minutes between  successive high waters. Figure 1.3 demonstrates Tidal Force and its  tendency to create bulging at  the water‘s surface;  thus making  for the differential sloshing effect.

Spring-neap tides are a second significant tidal pattern  type. The  fortnightly modulation in semidiurnal   tidal amplitudes is due to the various combinations of lunar and solar semidiurnal tides. The minimum ranges occur  at  the  first  quarter  and  last  quarter. This  is  because  at  times  of  spring  tides  the  lunar  and  solar  forces combine  together, but at  neap  tides  the  lunar and  solar  forces are out of phase and  tend  to cancel.  (Pugh 82) Figure 1.4 illustrates the difference between Neap and Spring ellipses; notice during the Spring Tide, the ellipse is drawn outward toward the Sun, allowing for increases tidal activity in terms of range. During  the Neap Tide, one  gets  a  significant  decrease  in  tidal  activity  due  to  the  gravitational  strain  at  the  poles  instead  o f  at  the Equator.  Unfortunately,  although  predictable,  this  tidal  pattern  makes  for  increased  variation  in  terms  of expected power output; if tidal power produced 25% of a large city‘s power peak load, the city would be forced to  find  another  source  of  power  during  times  of Neap Tide. This has  always  been  a  significant  factor when considering tidal energy schemes as a significant portion of a population‘s energy requirement.

How it Works:

In order to create enough electricity to be economically feasible, the size and configuration of the structure has to be increased tremendously. Tidal Energy consists of generating kinetic energy from potential energy. If falling water is forced through ducts with rotators attached to them, the rotors will turn driving electric generators (Mc Gown 182). Generating electricity from tides is very similar to hydroelectric generation, except the tides flow in two directions rather than one. For tidal power, the most common generating system is the ebb generating system. In the scheme, a dam, or barrage is constructed across an estuary. The tidal basin is allowed to fill when the sluice gates are opened and high tide is in. The gates are then closed when the tide turns trapping the water behind the gates. Once low tide is reached, the gates are opened the water flows through the turbines located underneath the water generating electricity. The basic concept for this type of scheme is extremely similar to that used at the Eling Mill. The schematic below shows the basic concept used in an ebb generation scheme.

In some cases, double effect turbines are used, which are able to generate electricity when then basin is filling. In this scheme, sluice gates located on either side of the turbine are opened, when the tidal basin is low, and the sea is at high tide level. Water will rush into the tidal basin, turning the turbines and generating electricity. This occurs until the water level on either side of the barrage is equal. At this point, the sluice gates are closed until the sea is at its low tide height. When this occurs, the gates are opened and water flows from the basin to the sea, generating electricity a second time.

Construction

In terms of construction, caissons, which are large units of concrete or steel that, are manufactured at shore-based construction yards are delivered to water sites by barges and then positioned by cranes to allow for the structures to correctly settle on the marine floor. Overall, this is an extremely expensive process. Another method calls for constructing diaphragm walls of reinforced concrete within a temporary sand island. But the approach offers no significant cost advantages over caissons and studies for the proposed Mersey Barrage in the United Kingdom indicate that the use of diaphragm walling could prolong construction time by about two years. (Johansson 519)

Location

Historically, tidal mills were usually built on inlets branching off tidal estuaries. An estuary is a wide part of a river where it meets the sea. It creates a unique environment because both freshwater and saltwater are present. Tidal estuaries are characterized by narrow, shallow channels with a relatively constant width and depth. Tides are greatly amplified in these areas of smaller volume, which causes the tide to travel up the river. Tidal ranges vary greatly from once place to another because of the geography of the land, but the most suitable tidal ranges are between five and ten meters.

Tidal Barrage

The tidal barrage is similar to a dam, which creates a tidal basin used for electricity generation. The structure is extremely large, spanning the entire width and height of the estuary. The bottom of the barrage is located on the sea floor and the top is above the highest level that the water can get at high tide.

Evolution of Turbine Types

Waterwheel Turbines

Waterwheels were used from the invention of the tidal mill until the

Undershot Wheel                                                                                Overshot Wheel

Breast Wheel

The first turbine used was the basic undershot waterwheel. This is probably the oldest type of waterwheel dating back over two thousand years. It is mounted vertically on a horizontal axle and it has flat boards located radially around a rim. It is turned by water flowing under the wheel and striking the boards.

The second type of turbine used was an overshot waterwheel. The overshot wheel is much more efficient than the undershot wheel. Again, this turbine is mounted vertically on a horizontal axle, but the overshot wheel has buckets mounted around the rim. Water from above flows into the buckets causing one side of the wheel to be heavier. Gravity then acts on the heavier side causing the wheel to turn.

The third type of turbine used was a breast-shot waterwheel. This type of wheel was developed in the late middle ages and combines the previous two waterwheels. It has buckets on a rim that face the opposite direction of the buckets on the overshot wheel. Water then fills the buckets at the middle of the wheel. Again, gravity acting upon the water in the buckets causes the wheel to turn.

Recent Turbine Developments

Bulb turbines incorporated the generator-motor unit in the flow passage of the water. These turbines are used at the La Rance power station in France. The main drawback is that water flows around the turbine, making maintenance difficult.

Rim turbines allow the generator to be mounted in the barrage, at right angles to the turbine blades. It is difficult to regulate the performance of these turbines and it is unsuitable for use in pumping.

Once the development of more tidal schemes occurs, additional types of turbines will be tested and implemented.

HOW IT WORKS

During high tide, water will flow from sea to tidal basin through turbine, thus producing electricity. During low tide, water will flow from tidal basin to sea through turbine producing electricity.

Advantages

  • Once you’ve built it, tidal power is free.
  • It produces no greenhouse gases or other waste.
  • It needs no fuel.
  • It produces electricity reliably.
  • Not expensive to maintain.
  • Tides are totally predictable.
  • Offshore turbines and vertical-axis turbines are not ruinously expensive to build and do not have a large environmental impact.

Disadvantages

  • A barrage across an estuary is very expensive to build, and affects a very wide area – the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. Fish can’t migrate, unless “fish ladders” are installed.
  • Only provides power for around 10 hours each day, when the tide is actually moving in or out.
  • There are few suitable sites for tidal barrages.

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