Tidal Power

Tidal power, sometimes 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 power and solar power. Historically, tide mills have been used, both in Europe and on the Atlantic coast of the US. The earliest occurrences date from the Middle Ages, or even from Roman times.

Generation of Tidal Energy

Variation of tides over a day

Tidal power is the only form of energy that 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. Other sources of energy originate directly or indirectly from the sun, including fossil fuels, conventional hydroelectric, wind, biofuels, wave power, and solar. Nuclear energy is derived using radioactive material from the earth, and geothermal power uses the heat of magma below the earth's crust.

Tidal energy is generated by the relative motion of the earth, sun, and moon, which interact via gravitational forces. 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 the earth's rotation, and the local shape of the sea floor and coastlines.

A tidal energy generator uses this phenomenon to generate energy. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal energy 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 its 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. Tidal power may take additional energy from the system, increasing the rate of slowing over the next millions of years.

Categories of Tidal Power

Tidal power can be classified into two main types:

• Tidal stream systems make use of the kinetic energy of moving water to power turbines, in a similar way that windmills use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact as compared to barrages.

• Barrages make use of the potential energy in the difference in height (or head) between high and low tides. Barrages suffer from very high civil infrastructure costs, a worldwide shortage of viable sites, and environmental issues.

Modern advances in turbine technology may eventually see large amounts of power generated from the ocean, especially from tidal currents using the tidal stream designs. Tidal stream turbines may be arrayed in high-velocity areas where natural tidal current flows are concentrated, such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in Southeast 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.

Tidal Stream Generators

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 at low tidal flow velocities (compared with wind speed).

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, near headlands, or between islands or other land masses. The following potential sites have been suggested:

• Cook Inlet in Alaska
• Pentland Firth in Scotland
• Dee estuary in Wales
• Pembrokeshire in Wales
• River Severn between Wales and England 
• Solway estuary (Morecambe Bay) in England
• Humber estuary in England
• Mersey river in England
• Channel Islands in the English Channel, off the French coast
• 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 in Canada
• Strait of Magellan south of mainland Chile
• Golden Gate in the San Francisco Bay
• Piscataqua River in New Hampshire

Prototypes

Several prototypes have shown promise, with many companies making bold claims, some of which are yet to be independently verified, or operated commercially for extended periods to establish performances and rates of return on investments.

Trials in the Strait of Messina, Italy, started in 2001, and the Australian company Tidal Energy Pty. Ltd. 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 their efficient shrouded turbine (the turbine resembles a jet turbine engine and is capable of converting 60% of the kinetic energy in the flow) for a remote Australian community in northern Australia where there exist some of the fastest flows ever recorded (11 m/s, 21 knots); two small turbines will provide 3.5 MW. Another, larger, 5 m diameter turbine, capable of 800 kW in 4 m/s of flow, is planned for deployment as a tidal powered desalination showcase near Brisbane Australia in October 2008.

The SeaGen rotors in Harland and Wolff, Belfast, before installation in Strangford Lough

The world's first commercial tidal stream generator—SeaGen—in Strangford Lough. The strong wake shows the power in the tidal current.

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. Oscillating stream power extraction had, in fact, already been proven with the omni- or bi-directional Wing'd Pump windmill. Another device, the Hydro Venturi, is to be tested in San Francisco Bay.

Although still a prototype, a turbine, generating 300 kW, started generation on November 13, 2003, in the Kvalsund, south of Hammerfest, Norway.

In late April 2008, Ocean Renewable Power Company, LLC (ORPC) successfully completed the testing of its proprietary turbine-generator unit (TGU) prototype at ORPC’s Cobscook Bay and Western Passage tidal sites near Eastport, Maine. The TGU is the core of the OCGen™ technology and utilizes advanced design cross-flow (ADCF) turbines to drive a permanent magnet generator located between the turbines and mounted on the same shaft. ORPC has developed TGU designs that can be used for generating power from river, tidal, and deep water ocean currents.

A commercial prototype, called SeaGen, has been installed by Marine Current Turbines Ltd. in Strangford Lough in Northern Ireland in April 2008. The turbine is expected to generate 1.2 MW and was reported to have fed 150 kW into the grid for the first time on July 17, 2008. It is currently the only commercial scale device to have been installed anywhere in the world.

RWE's NPower announced that it is in partnership with Marine Current Turbines to build a tidal farm of SeaGen turbines off the coast of Anglesey in Wales, though strictly speaking this is not a prototype, but a commercial farm.

British Columbia Tidal Energy Corp. plans to deploy at least three 1.2 MW turbines in the Campbell River or in the surrounding coastline of British Columbia by 2009. 

In November 2007, British company Lunar Energy announced that, in conjunction with E.ON, they would be building the world's first tidal energy farm off the coast of Pembrokeshire in Wales. It will be the world's first deep-sea tidal energy farm and will provide electricity for 5,000 homes. Eight underwater turbines, each 25 m long and 15 m high, are to be installed on the sea bottom off St. David's peninsula. Construction is due to start in the summer of 2008, and the proposed tidal energy turbines, described as "a wind farm under the sea," should be operational by 2010.

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

OpenHydro, an Irish based company, exploiting the Open-Centre Turbine turbine developed in the US, has a prototype being tested at the European Marine Energy Centre (EMEC), in Orkney, Scotland. Nova Scotia Power has selected their turbine for a tidal energy demonstration project in the Bay of Fundy, Nova Scotia, Canada.

Shrouded Tidal Energy Turbines

An emerging tidal stream technology is the shrouded tidal turbine enclosed in a Venturi-shaped shroud or duct. Shrouded tidal energy turbines produce a subatmosphere of low pressure behind the turbine, which allows the turbine to operate at higher efficiency (than the Betz limit of 59.3%). In one case a turbine of this type produced a nearly 4 times higher power output than the same minus the shroud.

The Race Rocks Tidal Current Generator before installation.
This working example of a shrouded turbine in the photo was deployed by Clean Current Power at Race Rocks in southern British Columbia in 2006. It operates bi-directionally and has proven to be efficient in contributing to the integrated power system of Race Rocks. The turbine was removed in May 2007 so that the bearing system could be redesigned.

Considerable commercial interest has been shown in shrouded tidal stream turbines due to the increased power output. They can operate in shallower, slower-moving water, and smaller turbines can be used at sites where large turbines are restricted. Arrayed across a seaway or in fast-flowing rivers, shrouded turbines are cabled to shore for connection to a grid or a 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 used for 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. Tidal Energy Pty. Ltd. in Australia make use of the design, and Lunar Energy uses a double-ended shroud. The Tidal Energy Pty. Ltd. tidal turbine is multi-directional (able to face upstream in any direction), and the Lunar Energy turbine is bi-directional. All tidal stream turbines constantly need to face at the correct angle to the water stream in order to operate. The Tidal Energy Pty. Ltd. tidal turbine is unique because it has a pivoting base. Lunar Energy uses a wide-angle diffuser to capture incoming flow that may not be in line with the long axis of the turbine. A shroud can also be built into a tidal fence or barrage, increasing the performance of turbines.

Types of Shroud

Not all shrouded turbines are the same—the performance of a shrouded turbine varies with the design of the shroud. Not all shrouded turbines have undergone independent scrutiny of claimed performances, as companies closely guard their respective technologies, so quoted performance figures need to be closely scrutinized. Claims vary from a 15%–25% to a 384% improvement over the same turbine without the shroud. Shrouded turbines do not operate at maximum efficiency when the shroud does not intercept the current flow at the correct angle, which can occur as currents eddy and swirl, resulting in reduced operational efficiency. At lower turbine efficiencies the extra cost of the shroud must be justified, while at higher efficiencies the extra cost of the shroud has less impact on commercial returns. Similarly the added cost of the supporting structure for the shroud has to be balanced against the performance gained. Yawing (pivoting) the shroud and turbine at the correct angle, so it always faces upstream like a wind sock, can increase turbine performance but expensive active devices may be required to turn the shroud into the flow. Passive designs can be incorporated, such as floating the shrouded turbine under a pontoon on a swing mooring, or flying the turbine like a kite under water. One design yaws the shrouded turbine using a turntable.

Advantages

• A shroud of suitable geometry can increase the flow velocity across the turbine by 3 to 4 times the open or free stream velocity, allowing the turbine to produce 3 to 4 times the power than the same turbine without the shroud.
• More power generated means greater returns on investment.
• The number of suitable sites is increased as sites formerly too 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 waterways. 
• Hidden in a shroud, a turbine is less likely to be damaged by floating debris.
• Bio-fouling is also reduced, since the turbine is shaded from natural light in shallow water.
• The increased velocities through the turbine effectively water-blast the shroud throat and turbine clean, since organisms are unable to attach at increased velocities. 
• Described as "eco-benign," tidal stream turbines have a slow rpm that does not interfere with marine life or the environment and that has little or no visual amenity impact.

Disadvantages

• Most shrouded turbines are directional, although one exception is the version off Southern Vancouver Island in British Columbia. One-direction 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 yawed 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.
• Shrouded turbines need to be below the mean low water level.
• Shrouded turbine loads are 3 to 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 or including structural mounts in the shroud geometry performs two functions: It supports the turbine and also provides a net benefit of 3 to 4 times the power output.
• Shrouded turbines may be hazardous to marine life, as fish or marine mammals can get sucked into the turbine blades through the venturi.

Source of the Energy

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 is classified as a renewable energy source.

Barrage Tidal Power

Rance tidal power plant

 

An artistic impression of a tidal barrage, including embankments, a ship lock, and caissons housing a sluice and two turbines

With only three operating plants globally—a large 240 MW plant on the Rance River in France and two small plants, one on the Bay of Fundy and the other across a tiny inlet in Kislaya Guba, Russia—the barrage method of extracting tidal energy involves building a barrage across a bay or river. Turbines installed in the barrage wall generate power as water flows in and out of the estuary basin, 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 the basin or lagoon changes relative to the water level inside, the turbines are able to produce power. The installation on the Rance River has been in operation since 1966 and has 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 caissons (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 also by 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, the turbines are disconnected, and the basin is filled again. The cycle repeats itself. Ebb generation (also known as outflow generation) gets 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, 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, since 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), the level 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 two-basin type. In this configuration, one basin 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 favorable 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 a smaller volume of water being exchanged between the basin and the sea. This allows 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, and this changes also affects the ecosystem. "Tidal lagoons" do not suffer from this problem.

Sediment Movements

Estuaries often have a 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. The open-center turbine reduces this problem by allowing fish to pass through the open center of the turbine. Recently a run of the river type turbine has been developed in France. This basically is a very large, slow-rotating Kaplan type turbine mounted on an angle. Testing for fish mortality has indicated much lower mortality figures (less than 5%). This concept seems very suitable for adaption to marine current/tidal turbines.

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 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 Modeling of Tidal Schemes

In mathematical modeling of a scheme design, the basin is broken into segments, each maintaining its own set of variables. Time is advanced in steps. Every step, neighboring 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 modeling 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. It has been projected that the proposed Severn Barrage could save 18 million tons of coal per year of operation, decreasing the output of greenhouse gases into the atmosphere.

If fossil fuel resources decline during the 21st century, as predicted by Hubbert peak theory, tidal power is one of the alternative sources 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. 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 18 MW installed capacity.
• 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 0.5 MW installed capacity. In 2006 it was upgraded with 1.2MW experimental advanced orthogonal turbine.
• Another 12MW project at Kislaya Guba in Russia with orthogonal turbines is under construction.
• 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.
• 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 fuel power station.
• 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.