Twin corkscrew turbines under water for Wave/Tidal/Hydro Power
The 600kW Flumill 2 tidal turbine is lowered into the water at the Emec test site Photograph: Flumill
In Depth: Turning a twin-corkscrew turbine into ‘beautiful’ reality
The first of Flumill’s twin-corkscrew tidal-power turbines
is now turning at the European Marine Energy Centre (Emec) testing site off Scotland’s Orkney Islands, having gone from drawing board to deployment of a full-scale prototype in less than 15 months.
With a nameplate capacity of 600kW, the helical Flumill 2 (F2) will be the smallest of the company’s commercial tidal systems, rated at as low as 120kW in the 1.5 metre-per-second (m/s) current that passes through Emec’s Shapinsay Sound “nursery”, where it was installed in September.
Flumill’s turbine has a unique look. Using a design adapted from excess-flow valves found in gas-distribution systems, the seabed-mounted device draws power out of the current as it moves up the spiral of the glass-reinforced plastic (GRP) screw, turning a gearless permanentmagnet generator (PMG).
The concept, which Flumill is planning to scale up to models of 12 metres in diameter, has the advantage of having turbines that, because they rotate in opposite directions, make the device hydrodynamically stable. This allows for easy transport to site and early kick-in in streams slower than 1m/s.
Development of the flagship F2 has been a joint Norwegian-UK effort. The 17.8-metre-long GRP helix tower and top-fin elements were fabricated by Norway’s Sørkomp in the town of Arendal, with the PMG and telemetry system from compatriots SmartMotor and Scanmatic, respectively. The 121-tonne carbon steel and magnetite concrete gravity base came from Orkney firm Currie Brothers.
The turbine has been performing “beautifully” at Emec, according to Flumill president Tony Trayner. “In less than 15 months we have taken the idea that was Flumill and produced a robust, environmentally friendly and economically viable system that is operating to expectations at Emec — in fact, it has been producing about 4% better than expected. The Flumill could significantly improve the cost basis of tidal energy worldwide.”
After component reassembly and commissioning at Hatston Pier, near Kirkwall, the Orkney capital, and tow-out to Emec, installation of the F2 took two days.
Flumill worked exclusively with Orkney contractors for the prototype installation, with Leask Marine handling deployment, Heddle Construction the onshore crane support and transport, and underwater surveys carried out by Roving Eye Enterprises.
“We wanted to work with local industry, as well as prove the point that this turbine can be installed without relying on heavy-lift vessels; the fleet that is already here in Orkney was all we needed,” says Trayner. The savings can be seen in the fact that Flumill’s most expensive vessel had a day rate of £3,000 ($4,775), whereas a heavy-lift barge or the like would start at £150,000 a day.
Flumill has been feeding all tidal and wave readings collected during testing at Shapinsay directly to the Emec databank, so that the information can be stored for use by future turbine developers.
“We struggled to compile the detailed data we needed to help us develop our technologies,” says Trayner. “We have had to study data from some 220 tidal sites around the world to piece together what we know on the subject and demonstrate how our device should perform.”
Computational fluid dynamic (CFD) analysis of the design as early as 2002 picked it out as “very prospective”. Because of the device’s unusual shape, however, Flumill had to have a tidal-testing tank custom-built to double-check the results with a 1:10 version of the turbine hooked up to a generator, before a second round of CFD was run.
A programme of tests of the one-tenth-scale device and then tow-testing of a full-size commercial version, certified by classification body DNV, took place in southern Norway’s Tromøysund fjord before the turbine was transported to Emec.
Flumill has also built an F8 — 44 metres long, eight metres in diameter and able to operate in tidal flow of 8m/s — as a “base model”, to assure itself that building the full range of machines up to the F12 would not pose any major problems.
The Flumill 8, though “midrange”, is a monster. Installed in the Pentland Skerries, just south of the Orkneys, where tides rip through at better than 6m/s, the company calculates such a machine would flow 14GWh a year.
“I don’t care much for nameplate capacities,” says Trayner. “I would like my nameplate capacity to be as low as possible and my gigawatt-hour output to be as high as possible.”
One of the strengths of the Flumill turbine is that it has the potential of big production numbers, while causing only slight disruption in a tidal current. Since the helical design creates little wake, a farm of Flumills could be arrayed with less lost space between them and without sacrificing energy capture.
“We have an impact in the tidal stream that is very small,” Trayner states. “From our second programme of CFD we could prove that because we don’t rotate faster than the flow of water, we don’t cavitate the water, it just spirals off. This means we can put more units in a set stream of water.”
Arrays are on the way. Flumill’s plan is to move swiftly into serial production of the machine once it is proven at Emec. By 2014, the company aims to have, as a proof of concept, a 10MW pilot tidal farm consisting of “five to eight” F4s, F6s and F8s operating in a 3.3m/s stream off the Norwegian city of Tromsø,
in water depths of 65 metres. Consents have been applied for from the Norwegian authorities, and funding for the development, which would be Europe’s first fully fledged tidal farm, has “been agreed”.
Across the Atlantic, meanwhile, Flumill is in negotiation with Maine Public Utilities Commission to install two F6 systems in the Lubec Narrows and Cobscook Bay. Installation of a multi-unit tidal farm in ten metres of water in the Gulf of Maine could begin by 2015.
“We have proven the system works, now we are working hard to prove the cost base and make the system bankable,” says Trayner.
Twin corkscrew turbines under water for Wave/Tidal/Hydro Power
http://www.archimedeshydroscrew.com/twin-corkscrew-turbines-under-water-for-wavetidalhydro-power/
http://www.archimedeshydroscrew.com/archimedes-hydro-screw-industry-gets-a-much-needed-boost/
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