Information about Tidal Power

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Tidal energy, sometimes called tidal power, is a form of hydropower that exploits the movement of water caused by tidal currents or the rise and fall in sea levels due to the tides. Although not yet widely used, tidal power has potential for future electricity generation and is more predictable than wind energy and solar power. In Europe, tide mills have been used for over a thousand years, mainly for grinding grains.

Tidal power can be classified into two main types:

• Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines, in a similar way to wind mills use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact.
• Barrages make use of the potential energy from the difference in height (or head) between high and low tides. Barrages suffer from the problems of very high civil infrastructure costs,few viable sites globally and environmental issues.

Modern advances in turbine technology may eventually see large amounts of power generated from the ocean especially tidal currents using the tidal stream designs. Tidal stream turbines may be arrayed in high velocity areas where natural flows are concentrated such as the west coast of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in south east Asia and Australia. Such flows occur almost anywhere where there are entrances to bays and rivers, or between land masses where water currents are concentrated.

A factor in human settlement geography is water. Human settlements have often started around bays, rivers, and lakes. Future settlement may one day be concentrated around moving water, allowing communities to power themselves with non-polluting energy from moving water.

Tidal stream power

A relatively new technology tidal stream generators draw energy from currents in much the same way as wind turbines. The higher density of water, 832 times the density of air, means that a single generator can provide significant power.

Similar to wind power, selection of location is important for the tidal turbine. Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses. The following potential sites have been suggested:

• Pentland Firth in Scotland
• Channel Islands in the United Kingdom
• Cook Strait in New Zealand
• Strait of Gibraltar
• Bosporus in Turkey
• Bass Strait in Australia
• Torres Strait in Australia
• Strait of Malacca between Indonesia and Singapore
• Bay of Fundy in Canada.
• East River in New York City
• Vancouver Island Canada
• Strait of Magellan south of mainland Chile

Prototypes

Several commercial prototypes have shown promise. Trials in the Strait of Messina, Italy, started in 2001^[1] and Australian company Tidal Energy Pty Ltd^[2] undertook successful commercial trials of highly efficient shrouded turbines on the Gold Coast, Queensland in 2002. Tidal Energy Pty Ltd has commenced a rollout of shrouded turbines for remote communities in Canada, Vietnam and Torres Strait in Australia and following up with joint ventures in the EU.



During 2003 a 300 kW Periodflow marine current propeller type turbine was tested off the coast of Devon, England, and a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. Another British device, the Hydro Venturi, is to be tested in San Francisco Bay.^[3]

Although still a prototype, the world's first grid-connected turbine, generating 300 kW, started generation on November 13 2003, in the Kvalsund, south of Hammerfest, Norway, with plans to install a further 19 turbines.^[4][5]

SeaGen, a commercial prototype design will be installed by Marine Current Turbines Ltd in Strangford Lough in Northern Ireland at the end of 2007. The turbine could generate up to 1.2 MW and will be connected to the grid.^[6]

Verdant Power.^[7] is running a prototype project in the East River between Queens and Roosevelt Island in New York City

Shrouded Tidal turbines

An emerging tidal stream technology is the shrouded tidal turbine enclosed in a Venturi shaped shroud or duct producing a sub atmosphere of low pressure behind the turbine, allowing the turbine to operate at higher efficiency (than the Betz Limit ^[8] of 59.3%) and typically 3–4 times higher power output ^[9] than a turbine of the same size in free stream.

As shown in the CFD (Computational Fluid Dynamics) generated figure here it can be seen that a down stream low pressure (shown by the gradient lines) draws upstream flow into the inlet of the shroud from well outside the inlet of the shroud. This flow is drawn into the shroud and concentrated (as seen by the red coloured zone). This augmentation of flow velocity corresponds to a 3–4 times increase in energy available to the turbine.

Therefore a turbine located in the throat of the shroud is then able to achieve higher efficiency, and an output 3–4 times the energy the turbine would be capable of if it were in open or free stream. For this reason shrouded turbines are not subject to the properties of the Betz Limit.

Considerable commercial interest has been shown in recent times in shrouded tidal stream turbines as it allows a smaller turbine to be used at sites where large turbines are restricted. Arrayed across a seaway or in fast flowing rivers shrouded tidal stream turbines are easily cabled to a terrestrial base and connected to a grid or remote community. Alternatively the property of the shroud that produces an accelerated flow velocity across the turbine allows tidal flows formerly too slow for commercial use to be utilised for commercial energy production.

While the shroud may not be practical in wind, as the next generation of tidal stream turbine design it is gaining more popularity and commercial use. A shrouded tidal turbine is mono directional and constantly needs to face upstream in order to operate. It can be floated under a pontoon on a swing mooring, fixed to the seabed on a mono pile and yawed like a wind sock to continually face upstream. A shroud can also be built into a tidal fence or barrage increasing the performance of the turbines.

Cabled to the mainland they can be grid connected or can provide energy to remote communities where large civil infrastructures are not viable. Describe as eco benign the slow R.P.M. of tidal stream open turbines does not interfere with marine life or the environment and have little if any visual amenity impact. They are ideal for remote communities that are far from grid connected infrastructure such as islands and rivers.

SHROUDED TURBINE ADVANTAGES.

• A shroud of suitable geometry can increase the flow velocity across the turbine by 3–4 times the open or free stream velocity allowing the turbine to produce 3–4 times the power then the same turbine minus the shroud.
• More power generated means greater returns on investment for investors.
• The number of suitable sites is increased as sites formally to slow for commercial development become viable.
• Where large cumbersome turbines are not suitable smaller shrouded turbines can be sea bed mounted in shallow rivers and estuaries allowing safe navigation of the water ways.^[10]
• Hidden in a shroud a turbine is less likely to to be damaged by floating debris.
• Bio-fouling is also reduced as the turbine is shaded from natural light in shallow water also,
• The increased velocities through the turbine effectively water blast the shroud throat and turbine clean as bio-organisms are unable to attached at increased velocities. ^[11]

SHROUDED TURBINE DISADVANTAGES.

• All shrouded turbines are directional, fixed shrouds may not capture flow efficiently - in order for the shroud to produce maximum efficiency to use both flood and ebb tide they need to be yawled like a windmill on a pivot or turntable, or suspended under a pontoon on a marine swing mooring allowing the turbine to always face upstream like a wind sock.
• Shrouded turbines need to be below the mean low water level. This can be accomplished by marine mono piles to to the sea/riverbed or suspended under a pontoon where inclement surface events don't buffet the turbine.
• Shrouded turbine loads are 3–4 times those of the open or free stream turbine, so a robust mounting system is necessary. However this mounting system needs to be designed in such a way as to prevent turbulence being spilled onto the turbine or high pressure waves occurring near the turbine and detuning performance. Streamlining the mounts and or including structural mounts in the shroud geometry performs two functions, that of supporting the turbine and providing a net benefit of 3–4 times the power output.

Energy calculations

The energy available from these kinetic systems can be expressed as:

• P = Cp x 0.5 x ρ x A x V³

Where:
Cp is the turbine coefficient of performance
P = the power generated (in W)
ρ = the density of the water (seawater is 1025 kg/m³)
A = the sweep area of the turbine (in m²)
V³ = the velocity of the flow cubed (i.e. V x V x V)

• Relative to an open turbine in free stream. Shrouded turbines are capable of higher efficiencies as much as 3–4 times the power of the same turbine in open flow. ^[12]

Commercial Calculator

Prices paid for electricity varies around the globe. The kilowatt price can vary from 10–15 British Pence in the UK to 30–40 US cents to 50–60 US cents or more if the area is remote and electricity supplies come from diesel generators.

The following equation can be used to calculate the revenue from a tidal stream turbine. By substituting variables such as size of the turbine, flow velocity and price into the equation it is possible to accurately predict an annual return.

Keeping in mind this equation does not include the cost of civil infrastructure which would vary with manufacturer and from site to site.

In order to calculate the revenue that a tidal stream generator would return the following equation can be used as a guide. Assuming 1000 meters of cabling then the following would be a close approximation.

Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y (x 3 for shrouded turbines)

Where:
Cp = the turbine coefficient of performance (say 20% for free stream or 60% for shrouded)
ρ = the density of the water (seawater is 1025 kg/m³ or 998 kg/m³ for fresh water)
A = the sweep area of the turbine (in m²)
V³ = the velocity of the flow cubed (i.e. V x V x V)
Hr = the number of hours per day that the turbine would operate at maximum efficiency (12-22 hours for tidal and 24 for run of river)
LL* = x .95 line losses (multiply by .95 )assuming a 5% loss in a cable run of 1000 meters
Gearbox and Generator Losses* = x .95 (multiply by .95) assuming 5% for gearbox and generator losses
$ = insert the price per kilowatt that would be paid (prices vary with location)
Year = 350 days (allowing 15 days per year for servicing cleaning etc if necessary)
Shrouded turbine = 3 times (augementation factor of the shroud)

• May vary from manufacturers.

For example a tidal stream turbine with a sweep area of 1m² at a site with a 3 m/s flow velocity operating at peak/maximum for 12 hours earning 10 cents per kilowatt would be,

Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y(x 3 for shrouded turbines)

Annual Revenue = 0.20 x 0.5 x 1025 x V³ x 12 x 0.95 x 0.95 x 0.10 x 350 (x 3 for shrouded turbines)

Revenue Revenue = $10,490.20 ($31,470.62 for shrouded turbines)

Keeping in mind this is only a 1m² sized turbine in 3m/s flow velocity for only 12 hours per day. Many commercial turbines are 20–30 times or greater in size in faster flow velocity at 20 or more hours per day. Run of river turbine would operate for 24 hours per day.

From the above equation it can be demonstrated that the predictability of tidal power holds very great potential and interest for renewable investment dollars. Wind and solar are unpredictable by nature but tidal stream can be predicted years in advance allowing businesses to plan years in advance.

As the flow velocity doubles the the Velocity term in the equations causes the increase 8 times.

i.e. 2 x 2 x 2 = 8 and 4 x 4 x 4 = 64-12/2222222 super super.

So to the revenue from the turbine increases 8 fold when the flow velocity doubles. The same turbine given in the example above if installed in a 6 m/s velocity would return 8 times or $83,920 (or $251,760 for a shrouded turbine) from every square meter of sweep area of the turbine.

This becomes a compelling argument for the commercialisation of tidal stream technologies.

High Velocity Sites a Commodity

As mentioned above, "a factor in human settlement geography is water. Human settlements have often started around bays rivers and lakes. Future settlement may one day be concentrated around moving water, allowing communities to power themselves with non-polluting energy from moving water".

Sites with high tidal stream velocities are becoming a highly sought after commodity when selcetion of a tidal stream site is under consideration. In some instances Government Entities in North America [citation needed] have begun legislating to prevent a gold rush mentality with unscrupulous entrepreneurs with no access to technology speculating and claim jumping sites.

Variable Output and Value Adding of Energy

Tidal power schemes do not produce energy all day. A conventional design, in any mode of operation, would produce power for 6–10 hours per flood and ebb tide or 12–20 hours in every 24 hour period depending on the site.

As the tidal cycle is based on the rotation of the Earth with respect to the moon (24.8 hours), and and an 18.6 year epoch cycle, electricity demand would not always coincide with supply. However, the tides are relatively reliable and more predictable than other alternative energy sources, such as wind.

Tidal stream turbines deployed in run of rivers location are not subject to tidal cycles and can produce energy 24 hour a day. Deployed from the banks of rivers close to the end user/s they avoid many complex issues of a salt water marine environment.

Value adding the electricity produced is one method of using energy produced without the limitations. For example high quality potable water can be produced from using energy to power reverse osmosis plant. As water is easily stored it matters little if a the tide is flowing at 3 am when a load on a grid is less.

Water is a high value commodity that can exceed that of fossil fuels. In Australia the petroleum industry is worth AU$250 million annually where as the bottled water market is AU$350 and growing rapidly.

Hydrogen can also be produced from the electrolysis of water and electricity from tidal stream generators. Hydrogen has been used to supply the buses in German cities for 30 or more years and is predicted to become the replacement to fossil fuels as hydrogen is virtually pollution free.

Source of the Energy

Because the tidal forces are caused by interaction between the gravity of the Earth, Moon and Sun, tidal power is essentially inexhaustible and classified as a renewable energy source. In fact though, the ultimate energy source is the rotational energy of the Earth, which will not run out in the next 4 billion years, although the Earth's oceans may evaporate away in 2 billion years.

Barrage tidal power

With only three operating plants globally Rance River, Bay of Fundy and Kislaya Guba the barrage method of extracting tidal energy involves building a barrage as in the case of the Rance River in France. The barrage turbines generate as water flows in and out the esturary bay or river. These systems are similar to a hydro dam that produces Static Head or pressure head (a height of water pressure). When the water level outside of the basin or lagoon changes relative to the water level inside, the turbines are able to produce power. The largest such installation has been working on the Rance river, France, since 1966 with an installed (peak) power of 240 MW, and an annual production of 600 GWh (about 68 MW average power).

The basic elements of a barrage are caissons, embankments, sluices, turbines and ship locks. Sluices, turbines and ship locks are housed in caisson (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons.

The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate and rising sector.

Barrage systems are affected by problems of high civil infrastructure costs associated with what is in effect a dam being placed across estuarine systems, and the environmental problems associated with changing a large ecosystem.

Ebb generation

The basin is filled through the sluices until high tide. Then the sluice gates are closed. (At this stage there may be "Pumping" to raise the level further). The turbine gates are kept closed until the sea level falls to create sufficient head across the barrage, and then are opened so that the turbines generate until the head is again low. Then the sluices are opened, turbines disconnected and the basin is filled again. The cycle repeats itself. Ebb generation (also known as outflow generation) takes its name because generation occurs as the tide ebbs.

Flood generation

The basin is filled through the turbines, which generate at tide flood. This is generally much less efficient than ebb generation, because the volume contained in the upper half of the basin (which is where ebb generation operates) is greater than the volume of the lower half (and making the difference in levels between the basin side and the sea side of the barrage), (and therefore the available potential energy) less than it would otherwise be. This is not a problem with the "lagoon" model; the reason being that there is no current from a river to slow the flooding current from the sea.

Pumping

Turbines are able to be powered in reverse by excess energy in the grid to increase the water level in the basin at high tide (for ebb generation). This energy is more than returned during generation, because power output is strongly related to the head. If water is raised 2 ft (61 cm) by pumping on a high tide of 10 ft (3 m), this will have been raised by 12 ft (3.7 m) at low tide. The cost of a 2 ft rise is returned by the benefits of a 12 ft rise.

Two-basin schemes

Another form of energy barrage configuration is that of the dual basin type. With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are placed between the basins. Two-basin schemes offer advantages over normal schemes in that generation time can be adjusted with high flexibility and it is also possible to generate almost continuously. In normal estuarine situations, however, two-basin schemes are very expensive to construct due to the cost of the extra length of barrage. There are some favourable geographies, however, which are well suited to this type of scheme.

Environmental impact

The placement of a barrage into an estuary has a considerable effect on the water inside the basin and on the ecosystem. Many governments have been reluctant in recent times to grant approval for tidal barrages.

Turbidity

Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.

Salinity

As a result of less water exchange with the sea, the average salinity inside the basin decreases, also affecting the ecosystem. "Tidal Lagoons" do not suffer from this problem.

Sediment movements

Estuaries often have high volume of sediments moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.

Fish

Fish may move through sluices safely, but when these are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through. Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15% (from pressure drop, contact with blades, cavitation, etc.). Alternative passage technologies (fish ladders, fish lifts, etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing.

Energy calculations

The energy available from barrage is dependant on the volume of water. The potential energy contained in a volume of water is :

where:
h is the height of the tide
M is the mass of water = 1025 kg per cubic meter (seawater varies between 1021 and 1030 kg per cubic meter)
g is the acceleration due to gravity = 9.81 meters per second squared at the Earth's surface.

A barrage is therefore best placed in a location with very high-amplitude tides. Suitable locations are found in Russia, USA, Canada, Australia, Korea, the UK. Amplitudes of up to 17 m (56 ft) occur for example in the Bay of Fundy, where tidal resonance amplifies the tidal range.

• Simple Approximation: P=hrk, where P is power in watts, h is height in meters, r is rate in cubic meters per second, and k is 7,500 watts (assuming an efficiency factor of about 75 percent).

Economics

Tidal barrage power schemes have a high capital cost and a very low running cost. As a result, a tidal power scheme may not produce returns for many years, and investors may be reluctant to participate in such projects.

Governments may be able to finance tidal barrage power, but many are unwilling to do so also due to the lag time before investment return and the high irreversible commitment. For example the energy policy of the United Kingdom^[13] recognizes the role of tidal energy and expresses the need for local councils to understand the broader national goals of renewable energy in approving tidal projects. The UK government itself appreciates the technical viability and siting options available, but has failed to provide meaningful incentives to move these goals forward.

Mathematical modelling of tidal schemes

In mathematical modelling of a scheme design, the basin is broken into segments, each maintaining its own set of variables. Time is advanced in steps. Every step, neighbouring segments influence each other and variables are updated.

The simplest type of model is the flat estuary model, in which the whole basin is represented by one segment. The surface of the basin is assumed to be flat, hence the name. This model gives rough results and is used to compare many designs at the start of the design process.

In these models, the basin is broken into large segments (1D), squares (2D) or cubes (3D). The complexity and accuracy increases with dimension.

Mathematical modelling produces quantitative information for a range of parameters, including:

• Water levels (during operation, construction, extreme conditions, etc.)
• Currents
• Waves
• Power output
• Turbidity
• Salinity
• Sediment movements

Energy efficiency

Tidal energy has an efficiency of 80% in converting the potential energy of the water into electricity, which is efficient compared to other energy resources such as solar power or fossil fuel power plants.

Global environmental impact

A tidal power scheme is a long-term source of electricity. A proposal for the Severn Barrage, if built, has been projected to save 18 million tonnes of coal per year of operation. This decreases the output of greenhouse gases into the atmosphere.

If fossil fuel resource is likely to decline during the 21st century, as predicted by Hubbert peak theory, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy.

Operating tidal power schemes

• 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.^[14] It has 240 MW installed capacity.
• The first (and only) 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.^[15] It has 18 MW installed capacity.
• A small project was built by the Soviet Union at Kislaya Guba on the Barents Sea. It has 0.5 MW installed capacity.
• China has apparently developed several small tidal power projects and one large facility in Jiangxia.
• China is also developing a tidal lagoon near the mouth of the Yalu.^[16]
• Scotland has committed to having 18% of its power from green sources by 2010, including 10% from a tidal generator. The British government says this will replace one huge fossil fuelled power station.^[17]
• South African energy parastatal Eskom is investigating using the Mozambique Current to generate power off the coast of KwaZulu Natal. Because the continental shelf is near to land it may be possible to generate electricity by tapping into the fast flowing Mozambique current.^[18]

Tidal power schemes being considered

In the table, "-" indicates missing information, "?" indicates information which has not been decided

Country	Place	Mean tidal range (m)	Area of basin (km²)	Maximum capacity (MW)
Argentina	San Jose	5.9	-	6800
Australia	Secure Bay	10.9	-	?
Canada	Cobequid	12.4	240	5338
	Cumberland	10.9	90	1400
	Shepody	10.0	115	1800
	Passamaquoddy	5.5	-	?
India	Kutch	5.3	170	900
	Cambay	6.8	1970	7000
South Korea	Garolim	4.7	100	480
	Cheonsu	4.5	-	-
Mexico	Rio Colorado	6-7	-	?
	Tiburon	-	-	?
United Kingdom	Severn	7.8	450	8640
	Mersey	6.5	61	700
	Strangford Lough	-	-	-
	Conwy	5.2	5.5	33
United States	Passamaquoddy Bay, Maine	5.5	-	?
	Knik Arm, Alaska	7.5	-	2900
	Turnagain Arm, Alaska	7.5	-	6501
	Golden Gate, California[19]	?	-	?
Russia[20]	Mezen	9.1	2300	19200
	Tugur	-	-	8000
	Penzhinskaya Bay	6.0	-	87000
South Africa	Mozambique Channel	?	?	?

See also

• Run-of-the-river hydroelectricity
• Wave power
• World energy resources and consumption
• Aquanator

Patents

• U.S. Patent 6,982,498 , Tharp, January 3, 2006, Hydro-electric farms
• U.S. Patent 6,995,479 , Tharp, February 7, 2006, Hydro-electric farms
• U.S. Patent 6,998,730 , Tharp, February 14, 2006, Hydro-electric farms

References

Other sources

• 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. [26 July 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.

External links

• Energy Information
• Climate Change Chronicles -- Article about new tidal power technology
• Location of Potential Tidal Stream Power sites in the UK
• University of Strathclyde ESRU -- Summary of tidal and marine current generators
• University of Strathclyde ESRU-- Detailed analysis of marine energy resource, current energy capture technology appraisal and environmental impact outline
• Renewable Energy UK Tidal -- Articles about tidal power generation.
• The British Library - finding information on the renewable energy industry
• Independent Online - information about South African ventures into coastal current power
• State explores renewable energy powered by tides
• Sustainable Development Commission - Report looking at 'Tidal Power in the UK', including proposals for a Severn barrage

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