Shrouded tidal turbine

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 [ [http://www.ndsu.nodak.edu/ndsu/klemen/Perfect_Turbine.htm Betz Limit] ] of 59.3%) in one case nearly 4 times higher power output [ [http://www.cyberiad.net/library/pdf/bk_tidal_paper25apr06.pdf Brian Kirke's published article Developments in Ducted Water Turbines] ] than the same minus the shroud.

Principles

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.

cite web
publisher= Clean Current Power Systems Incorporated
url= http://www.cleancurrent.com/technology/rrproject.htm
title = The Race Rocks Tidal Energy Project
accessdate=2008-07-09] ]

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 [ [http://tidalenergy.net.au/ Tidal Energy] ] 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% [http://www.lunarenergy.co.uk/duct.htm] to a 384% [http://peswiki.com/index.php/Directory:Davidson_Hill_Venturi_by_Tidal_Energy_Pty_Ltd] 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. [http://uekus.com/] One design yaws the shrouded turbine using a turntable [http://tidalenergy.net.au] .

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. [http://www.verdantpower.com/what-initiative Verdant Power ] ]
* 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. [ [http://www.cyberiad.net/library/pdf/bk_tidal_paper25apr06.pdf Brian Kirke's PhD Thesis] ]
* 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 [ [http://www.racerocks.com/racerock/energy/tidalenergy/tidalenergy.htm deployed at Race Rocks] ] 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.

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.Fact|date=October 2007

The following equation can be used to calculate the revenue from a tidal stream turbine.Fact|date=October 2007 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.Fact|date=October 2007

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.

References