New tidal-turbine ideas
Some large commercial-scale tidal-current turbine projects are now being installed, such as Open Hydro’s device in an 8MW project in Brittany, and MCT’s 1.2 MW Seagen has been feeding power to the Northern Ireland grid for several years. Next a 10MW SeaGen array is planned off N Wales. Many other designs are under test, at around 1MW scale, including the Atlantis AK 1000 and Hammerfest Strom’s device.
However, tidal current turbine technology is still at the stage when there are many new ideas emerging at various scales. Some are specifically designed for shallow water, like the Pulse Tidal hydrovane system: www.pulsetidal.com/
Some others are designed for low-speed water flows. Although the resource is large, low-speed projects are usually seen as much less economically viable - the power available is proportional to the cube of the water speed. But if cheap robust designs can be developed, there may be a niche for them even so. For example, the Hales Tidal Turbine uses a simple flat plate vertical axis rotor.
The Hales team says that ‘Many tidal turbine designs being tested at the present time have very slim propeller or foil blades which require water flow speeds in the 3 to 4 m/s range to be effective, these tidal speeds only happen in a very few locations around the world, limiting their deployment’. But they say tidal turbines like theirs ‘have much larger blade areas and can operate successfully in water flows between 1 and 2 m/s’, with these lower-speed water flows being found ‘in a great many tidal areas of the world’. They add that the turbines rotate slowly, in a drag type mode, so the blades ‘can be made stronger to withstand the high stress loads created by the water flow, unlike propellers and foils which also try to produce a lift effect to improve their performance’. A prototype is being developed with help from Kingston University, with trials in the Thames: www.hales-turbine.co.uk/
Simplicity of design does of course also mean that the device should be cheaper. Hydro-Gen, based in France, have produced a low-cost low-maintenance floating marine or river-current energy-converter system, mounted under a platform supported by a catamaran, with a slide-able turbine and generator unit that can be raised out of the water for maintenance on a tower, and which can also be folded down for transport by truck. It can run at between 0.5-3.5m/s. There are 10-100kW bespoke ‘tailor made’ options available. A version with an annular collar/duct is being looked at. They have also built a floating-paddle wheel system for shallow/turbulent water. www.hydro-gen.fr
More exotically, in the USA, Green Hydropower Inc., are promoting the idea of using vessels anchored in a swift moving body of tidal water as a base for tidal power generation via drag devices trailed behind. Lines from the stern of the vessel would have parachute-like drag anchors, which would create extreme amounts of line pull, this being converted onboard into rotational power that will then produce electricity. This idea avoids having to fix tidal devices to the seabed, e.g. by very large, costly and invasive, gravity anchor systems or monopiles driven into the seabed. By contrast, a vessel based tidal power unit is they say ‘able to move into a body of tidal waters, anchor the vessel, operate the drag device systems, pick up anchor and move out in a fashion that allows the seabed and surrounding underwater landscape to remain very much unaltered and available for sealife and marine mammals to thrive, seasonal fisheries to operate and seasonal migration patterns to go undisturbed’. Though the developers accept that standard rotors have a much higher energy conversion efficiency than drag devices, they say ‘what a drag device lacks in conversion efficiency can be made up for in quantity and size’.
They have patented some configurations with multiple parachute-like drag anchors mounted on a continuous belt system running out behind the ship, and around a drum on board - the parachutes collapse in the flow on the return journey. A bit of a long-shot perhaps, with fish and debris perhaps being trapped in the drag parachutes. But some river tests have been done: www.youtube.com/watch?v=HCCE0JhvkGo
If that seems too far fetched, then how about the ‘Tidal Kite’ idea, developed by Minesto. It’s an aerofoil wing, with a tidal rotor and generator mounted on it, which is tethered to the seabed and free to move under the tidal flow. However it doesn’t just stay in one place, but moves rapidly in a figure of 8 pattern under the influence of a rudder and the tether and lift forces created by the tidal flow. That means the rotor turns faster than if it was simply in the tidal flow - in fact, it’s claimed, up to 10 times faster. Given that, unlike other tidal devices, it doesn’t need expensive foundations or towers, it ought to be cheaper, and less invasive, and there should be many locations where it could extract power from relatively low tidal flows - thus, in effect expanding, the potential tidal resource. The strains on the tethering cable are going to be high, but Minesto plans to test a prototype off Northern Ireland www.minesto.com
Some of these more radical ideas may be non-starters. Most existing tidal currents devices have been based on horizontal axis propeller-type designs, much like wind turbines, mounted on fixed towers or on the sea-bed, sometimes with ducts to enhance the flow. However vertical axis H or V shaped devices, as now being developed for offshore wind farms, have the advantage that they can accept flows from any direction. That may be less important with tides than with wind, since tidal flows are very consistently along one path - just changing direction along it every 6 hours or so. Nevertheless, a large Kobald vertical axis turbine was tested in the Straits of Messina in 2004 and in 2009 a version was also being developed for Indonesia: www.pontediarchimede.it
In the UK, in addition to the Hales device mentioned above, Neptune’s 1 MW Proteus has vertical axis rotors in a duct system, and is now on test in the Humber: www.neptunerenewableenergy.com
Vertical axis turbines don’t have to be mounted vertically - cross flow turbines are also an option, with a horizontal shaft, like Ocean Renewable Power Company’s version of the Gorlov spiral rotor: http://www.orpc.co/orpcpowersystem_tidgenpowersystem.aspx
Finally what about the idea of using wave action as well as tidal flows? C-Energy, in the Netherlands, have developed a vertical axis tidal turbine the blades of which rotate in the tidal flow, while the horizontal struts linking these blades to the central axis are shaped so as to lift the whole assembly up and down when acted on by wave motion: www.c-energy.nl/
It’s early days yet, and the above review only covers a few of the many ideas now under test, but it could be that some of the new designs may win out.
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