Tidal power

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

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, cross flow 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.

Historically, tide mills have been used both in Europe and on the Atlantic coast of North America. The incoming water was contained in large storage ponds, and as the tide went out, it turned waterwheels that used the mechanical power it produced to mill grain.[1] The earliest occurrences date from the Middle Ages, or even from Roman times.[2][3] It was only in the 19th century that the process of using falling water and spinning turbines to create electricity was introduced in the U.S. and Europe.[4]

The world's first large-scale tidal power plant is the Rance Tidal Power Station in France, which became operational in 1966. It was the largest tidal power station in terms of power output, before Sihwa Lake Tidal Power Station surpassed it. Total harvestable energy from tidal areas close to the coast is estimated to be around 1 terawatt worldwide.[5]

Generation of tidal energy

Variation of tides over a day
Main articles: Tide and Tidal acceleration

Tidal power is taken from the Earth's oceanic tides; tidal forces are periodic variations in gravitational attraction exerted by celestial bodies. These forces create corresponding motions or currents in the world's oceans. Due to the strong attraction to the oceans, a bulge in the water level is created, causing a temporary increase in sea level. When the sea level is raised, water from the middle of the ocean is forced to move toward the shorelines, creating a tide. This occurrence takes place in an unfailing manner, due to the consistent pattern of the moon’s orbit around the earth.[6] The magnitude and character of this motion reflects the changing positions of the Moon and Sun relative to the Earth, the effects of Earth's rotation, and local geography of the sea floor and coastlines.

Tidal power is the only technology that draws on energy inherent in the orbital characteristics of the Earth–Moon system, and to a lesser extent in the Earth–Sun system. Other natural energies exploited by human technology originate directly or indirectly with the Sun, including fossil fuel, conventional hydroelectric, wind, biofuel, wave and solar energy. Nuclear energy makes use of Earth's mineral deposits of fissionable elements, while geothermal power taps the Earth's internal heat, which comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[7]

A tidal generator converts the energy of tidal flows into electricity. Greater tidal variation and higher tidal current velocities can dramatically increase the potential of a site for tidal electricity generation.

Because the Earth's tides are ultimately 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 resource. Movement of tides causes a loss of mechanical energy in the Earth–Moon system: this is a result of pumping of water through natural restrictions around coastlines and consequent 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 its formation. During the last 620 million years the period of rotation of the earth (length of a day) has increased from 21.9 hours to 24 hours;[8] in this period the Earth has lost 17% of its rotational energy. While tidal power will take additional energy from the system, the effect is negligible and would only be noticed over millions of years.[9]

Generating methods

The world's first commercial-scale and grid-connected tidal stream generator – SeaGen – in Strangford Lough.[10] The strong wake shows the power in the tidal current.

Tidal power can be classified into four generating methods:

Tidal stream generator

Tidal stream generators (or TSGs) make use of the kinetic energy of moving water to power turbines, in a similar way to wind turbines that use wind to power turbines. Some tidal generators can be built into the structures of existing bridges, involving virtually no aesthetic problems. Land constrictions such as straits or inlets can create high velocities at specific sites, which can be captured with the use of turbines. These turbines can be horizontal, vertical, open, or ducted and are typically placed near the bottom of the water column.[11]

Tidal barrage

Main article: Tidal barrage

Tidal barrages make use of the potential energy in the difference in height (or hydraulic head) between high and low tides. When using tidal barrages to generate power, the potential energy from a tide is seized through strategic placement of specialized dams. When the sea level rises and the tide begins to come in, the temporary increase in tidal power is channeled into a large basin behind the dam, holding a large amount of potential energy. With the receding tide, this energy is then converted into mechanical energy as the water is released through large turbines that create electrical power through the use of generators.[12] Barrages are essentially dams across the full width of a tidal estuary.

Dynamic tidal power

Main article: Dynamic tidal power
Top-down view of a DTP dam. Blue and dark red colors indicate low and high tides, respectively.

Dynamic tidal power (or DTP) is an untried but promising technology that would exploit an interaction between potential and kinetic energies in tidal flows. It proposes that very long dams (for example: 30–50 km length) be built from coasts straight out into the sea or ocean, without enclosing an area. Tidal phase differences are introduced across the dam, leading to a significant water-level differential in shallow coastal seas – featuring strong coast-parallel oscillating tidal currents such as found in the UK, China, and Korea.

Tidal lagoon

A newer tidal energy design option is to construct circular retaining walls embedded with turbines that can capture the potential energy of tides. The created reservoirs are similar to those of tidal barrages, except that the location is artificial and does not contain a preexisting ecosystem.[11] The lagoons can also be in double (or triple) format without pumping[13] or with pumping[14] that will flatten out the power output. The pumping power could be provided by excess to grid demand renewable energy from for example wind turbines or solar photovoltaic arrays. Excess renewable energy rather than being curtailed could be used and stored for a later period of time. Geographically dispersed tidal lagoons with a time delay between peak production would also flatten out peak production providing near base load production though at a higher cost than some other alternatives such as district heating renewable energy storage. The proposed Tidal Lagoon Swansea Bay in Wales, United Kingdom would be the first tidal power station of this type once built.[15]

US and Canadian studies in the twentieth century

The first study of large scale tidal power plants was by the US Federal Power Commission in 1924 which if built would have been located in the northern border area of the US state of Maine and the south eastern border area of the Canadian province of New Brunswick, with various dams, powerhouses, and ship locks enclosing the Bay of Fundy and Passamaquoddy Bay (note: see map in reference). Nothing came of the study and it is unknown whether Canada had been approached about the study by the US Federal Power Commission.[16]

In 1956, utility Nova Scotia Light and Power of Halifax commissioned a pair of studies into the feasibility of commercial tidal power development on the Nova Scotia side of the Bay of Fundy. The two studies, by Stone & Webster of Boston and by Montreal Engineering Company of Montreal independently concluded that millions of horsepower could be harnessed from Fundy but that development costs would be commercially prohibitive at that time.[17]

There was also a report on the international commission in April 1961 entitled "Investigation of the International Passamaquoddy Tidal Power Project" produced by both the US and Canadian Federal Governments. According to benefit to costs ratios, the project was beneficial to the US but not to Canada. A highway system along the top of the dams was envisioned as well.

A study was commissioned by the Canadian, Nova Scotian and New Brunswick governments (Reassessment of Fundy Tidal Power) to determine the potential for tidal barrages at Chignecto Bay and Minas Basin – at the end of the Fundy Bay estuary. There were three sites determined to be financially feasible: Shepody Bay (1550 MW), Cumberline Basin (1085 MW), and Cobequid Bay (3800 MW). These were never built despite their apparent feasibility in 1977.[18]

Tidal power development in the UK

The world's first marine energy test facility was established in 2003 to start the development of the wave and tidal energy industry in the UK. Based in Orkney, Scotland, the European Marine Energy Centre (EMEC) has supported the deployment of more wave and tidal energy devices than at any other single site in the world. EMEC provides a variety of test sites in real sea conditions. Its grid connected tidal test site is located at the Fall of Warness, off the island of Eday, in a narrow channel which concentrates the tide as it flows between the Atlantic Ocean and North Sea. This area has a very strong tidal current, which can travel up to 4 m/s (8 knots) in spring tides. Tidal energy developers currently testing at the site include: Alstom (formerly Tidal Generation Ltd); ANDRITZ HYDRO Hammerfest; OpenHydro; Scotrenewables Tidal Power; Voith.[19] The resource could be 4 TJ per year.[20]

Current and future tidal power schemes

Tidal power issues

Environmental concerns

Tidal power can have effects on marine life. The turbines can accidentally kill swimming sea life with the rotating blades, although projects such as the one is Strangford feature a safety mechanism that turns off the turbine when marine animals approach.[39] Some fish may no longer utilize the area if threatened with a constant rotating or noise-making object. Marine life is a huge factor when placing tidal power energy generators in the water and precautions are made to ensure that as many marine animals as possible will not be affected by it. The Tethys database provides access to scientific literature and general information on the potential environmental effects of tidal energy.[40]

Tidal turbines

The main environmental concern with tidal energy is associated with blade strike and entanglement of marine organisms as high speed water increases the risk of organisms being pushed near or through these devices. As with all offshore renewable energies, there is also a concern about how the creation of EMF and acoustic outputs may affect marine organisms. It should be noted that because these devices are in the water, the acoustic output can be greater than those created with offshore wind energy. Depending on the frequency and amplitude of sound generated by the tidal energy devices, this acoustic output can have varying effects on marine mammals (particularly those who echolocate to communicate and navigate in the marine environment such as dolphins and whales). Tidal energy removal can also cause environmental concerns such as degrading farfield water quality and disrupting sediment processes. Depending on the size of the project, these effects can range from small traces of sediment build up near the tidal device to severely affecting nearshore ecosystems and processes.[41]

Tidal barrage

Installing a barrage may change the shoreline within the bay or estuary, affecting a large ecosystem that depends on tidal flats. Inhibiting the flow of water in and out of the bay, there may also be less flushing of the bay or estuary, causing additional turbidity (suspended solids) and less saltwater, which may result in the death of fish that act as a vital food source to birds and mammals. Migrating fish may also be unable to access breeding streams, and may attempt to pass through the turbines. The same acoustic concerns apply to tidal barrages. Decreasing shipping accessibility can become a socio-economic issue, though locks can be added to allow slow passage. However, the barrage may improve the local economy by increasing land access as a bridge. Calmer waters may also allow better recreation in the bay or estuary.[41] Nova Scotia Canada - August 2004, a Humpback whale swam through the open sluice gate at slack tide, ending up trapped for several days before eventually finding its way out to the Annapolis Basin.[42]

Tidal lagoon

Environmentally, the main concerns are blade strike on fish attempting to enter the lagoon, acoustic output from turbines, and changes in sedimentation processes. However, all these effects are localized and do not affect the entire estuary or bay.[41]

Corrosion

Salt water causes corrosion in metal parts. It can be difficult to maintain tidal stream generators due to their size and depth in the water. The use of corrosion-resistant materials such as stainless steels, high-nickel alloys, copper-nickel alloys, nickel-copper alloys and titanium can greatly reduce, or eliminate, corrosion damage.

Mechanical fluids, such as lubricants, can leak out, which may be harmful to the marine life nearby. Proper maintenance can minimize the amount of harmful chemicals that may enter the environment.

Fouling

The biological events that happen when placing any structure in an area of high tidal currents and high biological productivity in the ocean will ensure that the structure becomes an ideal substrate for the growth of marine organisms. In the references of the Tidal Current Project at Race Rocks in British Columbia this is documented. Also see this page and Several structural materials and coatings were tested by the Lester Pearson College divers to assist Clean Current in reducing fouling on the turbine and other underwater infrastructure.

Structural Health Monitoring

The high load factors resulting from the fact that water is 800 times denser than air and the predictable and reliable nature of tides compared with the wind makes tidal energy particularly attractive for Electric power generation. Condition monitoring is the key for exploiting it cost-efficiently.[43]

See also

Notes

  • Baker, A. C. 1991, Tidal power, Peter Peregrinus Ltd., London.
  • Baker, G. C., Wilson E. M., Miller, H., Gibson, R. A. & Ball, M., 1980. "The Annapolis tidal power pilot project", in Waterpower '79 Proceedings, ed. Anon, U.S. Government Printing Office, Washington, pp 550–559.
  • Hammons, T. J. 1993, "Tidal power", Proceedings of the IEEE, [Online], v81, n3, pp 419–433. Available from: IEEE/IEEE Xplore. [July 26, 2004].
  • Lecomber, R. 1979, "The evaluation of tidal power projects", in Tidal Power and Estuary Management, eds. Severn, R. T., Dineley, D. L. & Hawker, L. E., Henry Ling Ltd., Dorchester, pp 31–39.

References

  1. ↑ Ocean Energy Council (2011). "Tidal Energy: Pros for Wave and Tidal Power".
  2. ↑ "Microsoft Word - RS01j.doc" (PDF). Retrieved 2011-04-05.
  3. ↑ Minchinton, W. E. (October 1979). "Early Tide Mills: Some Problems". Technology and Culture (Society for the History of Technology) 20 (4): 777–786. doi:10.2307/3103639. JSTOR 3103639.
  4. ↑ Dorf, Richard (1981). The Energy Factbook. New York: McGraw-Hill.
  5. ↑ Schweitzer, Sophia. "Will Tidal and Wave Energy Ever Live Up to Their Potential?". Yale Environment 360. Retrieved 16 October 2015.
  6. ↑ DiCerto, JJ (1976). The Electric Wishing Well: The Solution to the Energy Crisis. New York: Macmillan.
  7. ↑ Turcotte, D. L.; Schubert, G. (2002). "Chapter 4". Geodynamics (2nd ed.). Cambridge, England, UK: Cambridge University Press. pp. 136–137. ISBN 978-0-521-66624-4.
  8. ↑ George E. Williams (2000). "Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit". Reviews of Geophysics 38 (1): 37–60. Bibcode:2000RvGeo..38...37W. doi:10.1029/1999RG900016.
  9. ↑ "Tidal Energy". stanford.edu.
  10. ↑ Douglas, C. A.; Harrison, G. P.; Chick, J. P. (2008). "Life cycle assessment of the Seagen marine current turbine". Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment 222 (1): 1–12. doi:10.1243/14750902JEME94.
  11. 1 2 "Tidal - Capturing tidal fluctuations with turbines, tidal barrages, or tidal lagoons". Tidal / Tethys. Pacific Northwest National Laboratory (PNNL). Retrieved 2 February 2016.
  12. ↑ Evans, Robert (2007). Fueling Our Future: An Introduction to Sustainable Energy. New York: Cambridge University Press.
  13. ↑ "Hydrological Changing Double Current-typed Tidal Power Generation" (video). Retrieved 2015-04-15.
  14. ↑ "Enhancing Electrical Supply by Pumped Storage in Tidal Lagoons" (PDF). Retrieved 2014-03-13.
  15. ↑ Elsevier Ltd, The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, United Kingdom. "Green light for world's first tidal lagoon". renewableenergyfocus.com. Retrieved 26 July 2015.
  16. ↑ "Niagara's Power From The Tides" May 1924 Popular Science Monthly
  17. ↑ Nova Scotia Light and Power Company, Limited, Annual Report, 1956
  18. ↑ Chang, Jen (2008), "6.1", Hydrodynamic Modeling and Feasibility Study of Harnessing Tidal Power at the Bay of Fundy (PDF) (PhD thesis), Los Angeles: University of Southern California, retrieved 2011-09-27
  19. ↑ "EMEC: European Marine Energy Centre". emec.org.uk.
  20. ↑ M. Lewis, S.P. Neill, P.E. Robins, M.R. Hashemi, Resource assessment for future generations of tidal-stream energy arrays, Energy, Volume 83, 1 April 2015, Pages 403-415, ISSN 0360-5442, http://dx.doi.org/10.1016/j.energy.2015.02.038. (Creative Commons)
  21. ↑ L'Usine marémotrice de la Rance Archived April 8, 2005, at the Wayback Machine.
  22. 1 2 "Hunt for African Projects". Newsworld.co.kr. Retrieved 2011-04-05.
  23. ↑ "Tidal power plant nears completion". yonhapnews.co.kr.
  24. ↑ "Nova Scotia Power - Environment - Green Power- Tidal". Nspower.ca. Retrieved 2011-04-05.
  25. ↑ "China Endorses 300 MW Ocean Energy Project". Renewableenergyworld.com. Retrieved 2011-04-05.
  26. ↑ "Race Rocks Demonstration Project". Cleancurrent.com. Retrieved 2011-04-05.
  27. ↑ "Tidal Energy, Ocean Energy". Racerocks.com. Retrieved 2011-04-05.
  28. ↑ "Information for media inquiries". Cleancurrent.com. 2009-11-13. Retrieved 2011-04-05.
  29. ↑ Korea's first tidal power plant built in Uldolmok, Jindo Archived May 23, 2012, at the Wayback Machine.
  30. ↑ "Tidal energy system on full power". BBC News. December 18, 2008. Retrieved March 26, 2010.
  31. ↑ $ 3-B tidal power plant proposed near Korean islands Archived January 7, 2010, at the Wayback Machine.
  32. ↑ "Islay to get major tidal power scheme". BBC. March 17, 2011. Retrieved 2011-03-19.
  33. ↑ "India plans Asian tidal power first". BBC News. January 18, 2011.
  34. ↑ "1st tidal power delivered to US grid off Maine", CBS MoneyWatch, September 14, 2012
  35. ↑ "Turbines Off NYC East River Will Create Enough Energy to Power 9,500 Homes". U.S. Department of Energy. Retrieved 13 February 2012.
  36. ↑ Oliver, Antony (9 June 2015). "Swansea Tidal Lagoon power plant wins planning permission". http://www.infrastructure-intelligence.com/. Retrieved 6 July 2015. External link in |work= (help)
  37. ↑ Macalister, Terry. "Tidal power firm signs deal to sell electricity to EDF Energy" The Guardian, 25 September 2014.
  38. ↑ , Ellie. "First full-scale tidal generator in Wales unveiled: Deltastream array to power 10,000 homes using ebb and flow of the ocean" Daily Mail, 7 August 2014.
  39. ↑ "Tidal Energy Technology Brief" (PDF). International Renewable Energy Agency. Retrieved 16 October 2015.
  40. ↑ "Tethys".
  41. 1 2 3 "Tethys".
  42. ↑ "Whale still drawing crowds at N.S. river". The Globe and Mail.
  43. ↑ "Structural Health Monitoring in Composite Tidal energy converters".

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