See also:

• Tidal Force and Acceleration


• Barrage Tidal Power

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 to windmills that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact 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 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 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.

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

• Pentland Firth in Scotland
• Dee estuary in Wales
• Pembrokeshire in Wales ^[4]
• River Severn between Wales and England ^[5]
• 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^[6] 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^[7] in Canada.
• East River^[8] in New York City
• Vancouver Island in Canada
• Strait of Magellan south of mainland Chile
• Golden Gate in the San Francisco Bay^[9]
• Piscataqua River in New Hampshire^[10]

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^[11] and 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 fasted flows ever recorded (11 m/s, 21 knots) – two small turbines will provided 3.5 MW. Another larger 5 meter diameter turbine, capable of 800kW in 4m/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

SeaGen , the world's first commercial tidal stream generator 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. Another British device, the Hydro Venturi, is to be tested in San Francisco Bay.^[12]

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.^[13][14]

SeaGen, a commercial prototype has been installed by Marine Current Turbines Ltd in Strangford Lough in Northern Ireland in April 2008. The turbine is expected to generate 1.2MW and is being connected to the grid. It is currently the only commercial scale device to have been installed anywhere in the world. ^[15]

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.^[16]

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. ^[17]

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 Pembrokshire 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 metres long and 15 metres 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^[18] 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 and Alderney Renewable Energy Ltd for the supply of tidal turbines in the Channel Islands. Open Hydro

Shrouded tidal energy 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 ^[19] of 59.3%) in one case nearly 4 times higher power output ^[20] 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.

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 with a smaller turbine 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^[21]in Australia make use of the design and Lunar Energy (http://www.lunarenergy.co.uk/duct.htm) use a double ended shroud. The Tidal Energy Pty Ltd tidal turbine is multi directional able to face up-stream in any direction and the Lunar Energy turbine 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 is a unique case with a pivoting base. Lunar Energy use a wide angle diffuser to capture incoming flow that may not be inline 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 scrutinised. Claims vary from a 15%-25% [4] to a 384% [5] 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 may need expensive active devices 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. [6] One design yaws the shrouded turbine using a turntable [7].

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 water ways. ^[22]
• Hidden in a shroud, a turbine is less likely to be damaged by floating debris.
• Bio-fouling is also reduced as 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 as organisms are unable to attached at increased velocities. ^[23]
• Described as 'eco-benign', the slow r.p.m. of tidal stream turbines does not interfere with marine life or the environment and has little or no visual amenity impact.

Disadvantages

• Most shrouded turbines are directional, although one exception is the version^[24] 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 like a wind sock.
• 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 and or including structural mounts in the shroud geometry performs two functions, that of supporting the turbine and providing 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.

Energy calculations

Various turbine designs have varying efficiencies and therefore varying power output. If the efficiency of the turbine "Cp" is known the equation below can be used to determine the power output.

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

• P = Cp x 0.5 x ρ x A x V³ ^[25]

where:

Cp is the turbine coefficient of performance

P = the power generated (in watts)

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 efficiencies as much as 3 to 4 times the power of the same turbine in open flow. ^[26]

Price calculations

Prices paid for electricity varies around the globe. The kilowatt price can be 10-15 British Pence in the UK, or 30-40 US cents or more in remote areas.

The following equation can be used to calculate the revenue from a tidal stream turbine. By substituting variables such as the efficiency, 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 only. 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 turbine - up to 60% for a shrouded turbine)
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. This may vary by manufacturer.
Gearbox and Generator Losses* = x .95 (multiply by .95) assuming 5% for gearbox and generator losses
$ = the price per kilowatt hour that would be paid (prices vary with location)
Year = 350 days (allowing 15 days per year for maintenance if necessary)

Shrouded turbines can produce 3 to 4 times as much revenue as a free stream turbine.

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

Annual Revenue = Cp x 0.5 x ρ x A x V³ x Hr x LL x GGL x $ x Y

Annual Revenue = 0.20 x 0.5 x 1025 x 27 x 12 x 0.95 x 0.95 x 0.10/1000 x 350

Revenue Revenue = $10,490.22 (or $31,470.62 for a shrouded turbine)

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. A run of river turbine would operate for as long as the river flows, which is obviously 24 hours per day. For example a commercial sized turbine with a 100m² sweep area would therefore return $1,049,022.00 per annum (or $3,147,062.00 for a shrouded turbine with 60% efficiency)

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 revenue increases by 8 times (as power is a function of the velocity cubed). The same commercial turbine given in the example above, if installed in a 6 m/s velocity flow, would return $8,392,000 (or $25,176,000 for a shrouded turbine) for every square meter of sweep area of the turbine. It's not hard to see the commercial attraction of tidal stream turbines.

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 classified as a renewable energy source.

Notes