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LORC tests and demonstrates technology for harvesting renewable energy offshore

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Tidal energy: Predictable, robust and easy to maintain

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mandag 14. november 2011

FACT BOX

SeaGen Strangford Narrows 1.2MW rated power installation

- Power to grid per year: potentially 5,000MWh/year
- Equivalent households: 1,500 (based on 3.33MWh per household per year (0.38kW continuous))

Tower
- Tower height above seabed: 40.7m
- Tower diameter: 3.025m

Drivetrain
- Rotor diameter: 16m (to be scaled up to 20m)
- Drivetrain weight: 27 tonnes
- Gearbox ratio: 69.9:1
- Tip speed: 12m/s
- Rated power: 1.2MW at or above 2.4 m/s - 2 x 600kW power units
- Nominal rotor speed: 14.3rpm at rated velocity
- Nominal generator speed: 1000rpm

Current on site
- Mean maximum spring current: 3.7m/s or 7.2 knots
- Max current: 4.8m/s or 9.3 knots
- Rated speed: 2.4 m/s or 4.7 knots

Learn more at www.seageneration.co.uk and at www.marineturbines.com


Tidal energy in short

Tidal streams are caused by the familiar rise and fall of the tides, caused by the moving gravitational fields of the moon and the sun which occurs twice a day. As water flows in and out of estuaries, through narrow straits and around headlands it carries energy. The amount of energy it is possible to extract depends on the speed of the flowing stream and the area intercepted by a tidal turbine; the swept area. Because water is about 800 times denser than air an equivalent amount of power can be extracted over smaller areas and at slower velocities than with wind turbines.

The density of sea water is 1027 kg/m^3 and the density of air is (@20degC) 1,2041 kg/m^3
Sea water has a factor 850 heavier than air.

Only the most energetic locations are suitable for cost-effective power generation. The basic requirement is to find sites with a mean maximum surface current at Spring tides exceeding around 5 knots or 2.5m/s. The faster the currents the more cost-effective can be any project.

MCT’s SeaGen technology ideally needs locations where the water depth is from 25 to 35m with sufficiently strong currents. Such places are not common but the UK sites are among the most energetic in Europe and the estimate is that there is the potential to install in the order of 5,000MW of turbines in UK waters; and possible ten times as many worldwide.

Tidal energy: The world’s first commercial tidal turbine, SeaGen, is now in its third year of operations, producing up to 5,000 MWh per year. This will open up oceans and attract investment for this novel method of clean electricity generation.

By Anne Korsgaard
Energy delivered to a timetable is inherently more valuable than randomly generated electricity. This is part of the reason why the SeaGen tidal energy system has confirmed that it is realistic to use renewable tidal energy from the seas. SeaGen is in principle like an underwater wind turbine but with the important advantage of having a predictable source of renewable energy to harvest: tidal currents. The design methods for the rotor blades were similar to those used for modern wind turbines whilst taking into account the much higher load cases due the higher density of water as compared to wind.

Strangford Narrows has one of the most energetic tidal flows in Europe, with a mean maximum spring current of 3.7m/s. At present, the 1.2MW SeaGen tidal energy extractor delivers the equivalent of the average needs of 1500 households with renewable and predictable electricity. 

"We are proud of the progress that we have made in tidal energy with the 1.2MW SeaGen installation and I am confident that tidal turbine technology will become competitive reasonably quickly. But at this stage, like with all new energy technologies, support is needed", says Technical Director of Marine Current Turbines, Peter Fraenkel. 

The ideal location
Surrounded by sea traffic and houses, only 400m from the shoreline and roughly 1km south of the ferry route between Portaferry and Strangford lies the SeaGen installation. Only the tower-like superstructure protrudes from the water. From the houses on shore it is barely perceivable and the interesting bit takes place underneath the surface of the water.

The tidal stream flows in and out through the narrows making the two rotors, each with a swept area of 201 square meters, of the Sea Gen power plant turn. The turbines have a patented feature by which the rotor blades can be pitched through 180o in order to allow them to operate efficiently in bi-directional flows – that is on both the ebb and the flood tides - which makes the plant produce for from 18 to 20 hours a day.

Thanks to the power conditioning enclosed in the structure, it produces fully grid compliant electricity. This means that multiple turbines can be easily interfaced to form an array. 

Monitored production
Because SeaGen is accredited by OFGEM (Office of the Gas and Electricity market regulator) as a UK electricity generator, the output to the grid is independently metered and audited through the local utility’s meter.

As a result, there are power curves and production figures for the power plant. SeaGen persistently reaches its rated power of 1.2MW at all times when the current exceeds 2.4 m/s. It has a mean rotor efficiency for both rotors on both tides averaging 48%. This efficiency level is on a par with the performance of the best modern wind turbines. Cut in is at 0.7m/s and there is no cut-out – it can run up to the maximum current which at Strangford is about 4.5m/s.

Det Norske Veritas (DNV), a world renowned classification society and authority on offshore engineering, has made a detailed review of the measurement program used by Marine Current Turbines for determining the performance of the SeaGen and has issued a certificate of approval to validate the measurement methods used.

"DNV’s endorsement provides a valuable confirmation of the accuracy of the team’s design assumptions for what remains the world’s first and only fully operational tidal current turbine", says Peter Fraenkel.

Operation and maintenance
MCT has designed SeaGen with a specific focus on easy and cost effective maintenance. For instance, underwater operations are avoided because it is equipped with a hydraulic lifting mechanism that lifts the rotors clear above the surface of the water to enable safe and low-cost repair and maintenance. 

"We have put a lot of emphasis on easy and low cost operations and maintenance because we believe that the O & M costs are what brings the cost of energy of many offshore renewable concepts up. Therefore it is an important parameter for the success of SeaGen", tells Peter Fraenkel.

Focus on maintenance aspects to bring down O&M costs. The arm can be hoisted up above sea level for repair and maintenance. The photo shows the SeaGen 1.2 MW installation at Strangford Narrows. Photo: Marine Current Turbines Limited.

Future focus on production and flexible design
Within the technical development area, MCT is working on a variation on SeaGen that uses exactly the same rotors, power trains and control systems but deploys them on a different support structure. The purpose is to increase the flexibility by which the SeaGen technology can be applied at for instance different depths and current strengths.

"The next phase for the company is to move on to scaling up the system from 1.2MW to 2.0MW and on production-engineering and large-scale manufacturing so as to seize the market opportunity", explains the new CEO, Andrew Tyler.

MCT is currently working on developing a larger 8MW array of four 2MW scaled up systems to be developed in Kyle Rea, Scotland as well as a 10MW array of tidal turbines in partnership with RWE npower renewables to be located off Anglesey in north Wales. The company is also working on a single SeaGen deployment in Canada's Bay of Fundy – which has the largest tidal range in the world. The Canadian project will introduce for the first time what MCT calls “SeaGen U”, where U stands for Underwater as it is a completely submerged version of SeaGen, which can be floated to the surface for maintenance.

With a production of between 2,500 and 5,000 MWh per year depending on site conditions, the 1.2MW version of SeaGen has proved itself to be a promising newcomer to the big market place of renewable energy concepts.

"I am excited at the prospect of building MCT into a significant technology provider to the power generation industry. We are currently focusing on the commercialization of our technology in order to reduce production costs and roll out more of our devices", explains the CEO Andrew Tyler.

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