Vertical-axis wind turbine takes the plunge in deep water

Can the vertical-axis wind turbine find a niche for itself in deep offshore waters? Some wind engineers and developers think this design has advantages over the more commonly deployed three-blade, horizontal-axis wind turbine. (GW)

Deep-water vertical-axis wind turbine gets last dry run

Recharge
January 6, 2012

Construction will begin this summer in the south of France on the 2MW onshore prototype of a floating vertical-axis wind turbine (VAWT) that is on course to be the first full-scale machine of its kind operating offshore.

The VertiWind concept is the first fruit of a collaboration between French offshore oil and gas engineering giant Technip and compatriot start-up Nénuphar, which for the past three years has been road-testing a 1:10 scale version of its VAWT design, kitted out with a foundation with hydraulic jacks that simulate the marine environment.

The diagonally bladed concept, designed for deep-water use, is on the fast track. The companies plan to have a 25MW development consisting of 13 full-scale “multifloater” units on line by 2015.

The unit’s designers, aeronautical engineers Charles Smadja and Frédéric Silvert, came into offshore wind with the idea that although current utility-scale turbines were “optimal” for onshore and shallow waters, in deeper waters the machines would call for either “very large” floaters buoyed by massive ballasts or “very costly” tension-leg-type moorings. A vertical-axis rotor needs neither.

Technip had nailed its offshore colours to the mast off Norway in 2009 with the switch-on of Statoil’s spar-type Hywind turbine, which the contractor had designed, fabricated and installed in 220 metres of water off the southwest municipality of Karmøy.

“The starting point is certainly Hywind,” says Technip’s vice-president for renewable energy, Stéphane His. “It will always be seen as a key project in the history of our offshore wind strategy. Whether this sort of installation could be multiplied by a high number, however, unless certain optimisations were made [was a question].

“During this project there was some head-scratching on the subject of where we go from here. We were — and are — of the belief that floating offshore wind needs some sort of breakthrough and it needs something different to achieve this, something disruptive.”

Around this time, Technip got together with Nénuphar. It led quickly to the two companies taking the lead in an all-French consortium that is among the beneficiaries of the government’s Grand Emprunt €35bn ($45.3bn) industrial stimulus package.

Project partners for VertiWind include utility EDF; public-sector research, innovation and training centre IFP EN; and specialist deep­water contractor Seal Engineering, along with classification society Bureau Veritas.

The group’s 2MW offshore prototype is foreseen operating in water depths of at least 50 metres. Based on a three-­column semi-submersible concept, it will, like its onshore forerunner, feature a 50-metre-diameter Darrieus-type rotor consisting of three 70-metre-tall blades, each angled at 120 degrees, attached by struts to a pole at the centre of the floater.

The unit’s mooring system will be a chain-and-wire three-point spread, with standard drag anchors or piles, depending on a site’s soil conditions, securing it to the seabed.
Offshore, there will be an “air gap” — the distance between the rotor and the sea — of 25 metres.

Being a VAWT, the VertiWind has no yaw or pitch system — the 100-metre-high “omnidirectional” turbine can harness wind from any point on the compass; nor is there a gearbox, which is in line with a minimum-component philosophy.

The unit will be smaller than like-rated horizontal­axis models, with a power-production curve — according to Nénuphar’s output modelling — revved up by the faster, steadier winds found offshore, and an algorithm-based control system regulating rotor speed to improve the turbine’s efficiency.

His says: “The vertical-access turbine designs offer many advantages: high production output, operational stability, a low centre of gravity that means it can be built and installed in most places in the world, and a nacelle that is easily accessed, which is very important later, once the turbine is in operation offshore.”

The rotor, its shaft guided with two bearings enclosed in the mast, is connected via an elastic coupling to a direct-drive transmission, with a fail-safe emergency disc-and-caliper braking system stopping the turbine in any situation, including a network failure, as well as “parking” the rotor when the wind speed reaches cut-off.

The power take-off system will include a transformer to step-up electricity export over long distances through a dynamic subsea cable without a booster substation.

The floater’s generator, a 50-tonne permanent-magnet model built by France’s Alstom and Converteam, serves more than just the expected role of power producer, sitting 20 metres above the sea — 40 metres lower than on a conventional 100-metre-tall turbine — to give the floater a low centre of ­gravity.

VertiWind’s nine-metre draft is central to the commercial case behind its design.

Unlike spars, which need upwards of 100 metres of water during turbine ­mating and transport, VertiWind’s floating structure — fabricated with cylindrical steel columns set into hexagonal concrete heave plates — could be constructed and commissioned complete with turbine at the quayside. It would then be towed out to site with offshore service vessels for hook up to its mooring and electrical infrastructure, doing away with expensive heavy-lift crane work.

“[Deepwater ports] are not a situation we have everywhere in the world, so if you’re looking at many areas of the world where there is a demand for an easy-to-install solution, draft matters,” says His. “This opens up the possibilities for construction at a much larger number of ports.

“Working with Nénuphar has allowed us to think not only about the floating part of the system but also about the wind turbine itself, together, in an integrated way, along with construction and installation and operation.”

In operation offshore, the low-riding rotor lends stability to the floater in concert with the carbon­reinforced glass-fibre blades, minimising the gyroscopic effect on the structure by smoothing the torque dynamics and lessening the chances of a stall or blade-bending damage in high winds.

“Our rotor design evolved from one with straight, vertical blades as a response to the ­problem of torque variations during rotation, particularly in extreme wind velocities,” says Nénuphar’s Smadja. “In such a dynamic storm, the load occurs on the whole length of a straight blade at once; with our design, [the loads are distributed] along all the blades as they turn.”

The 35kW prototype that has been put through its paces since early 2009 at a site near the northern city of Boulogne-sur-Mer stands 12 metres tall, and is fitted with seven-metre blades on a six-metre-diameter rotor. Testing of the machine, built with engineering institute Arts et Métiers ParisTech, has concentrated on calibrating the power curve to the control system to hone efficiency and output, with refinements made to the aerofoil structure and the supporting struts.

The prototype’s base has a tilting mechanism that makes it possible for the turbine to operate in a “skewed flow”, as a floating VAWT might offshore, with the turbine axis being rotated up to 15 degrees. The “inclineable” foundation can also generate harmonic rotation to simulate motions and accelerations that mirror the offshore environment.

Cut-in speed on the prototype, which has been running at full load for the past 12 months, is four metres per second (m/s); cut-out is at “somewhat higher than 25m/s”.
“The measured output has matched very well with the calculations,” says Smadja. “We have also been able to prove, among other things, [that] this type of machine performs very well under skewed flow. On a floater, given the offshore wave and wind conditions, you often have skewed flow.

“Due to the architecture and the turbine design, the power production is not really impacted by the inclination of the turbine axis relative to the wind direction.”

Construction and operation of the full-scale onshore prototype will feed into fine-tuning and optimisation of the engineering for the building of the first offshore unit. An application has been made to the French authorities to install the flagship 2MW floater in 85 metres of water in the French ­Mediterranean, about 5km off the city of Fos-sur-Mer, where winds can whip through at 43m/s and waves of seven metres are not uncommon.

For the full-scale prototype — to be tested at a soon-to-be-announced site — Nénuphar has developed a composite blade technology that can be manufactured using an integrated, one-piece “monobloc” design, says ­Smadja. The resulting rotor blades are engineered to be light and of a stiffness tailored to the “dynamical and mechanical behaviour” of a VAWT, but with a manufacturing process that “remains as simple as for a straight blade”.

“I am amazed how far this project has come in the past three years,” Smadja remarks. “But this is just a start. What is coming is more challenging yet.”

The project timeline sees a second VertiWind turbine being floated out and connected with the flagship, followed, in 2015, by the 25MW development, dubbed VertiMed, which is being partly financed through the NER300 programme, a renewable-energy technology scheme managed jointly by the European Commission and European Investment Bank.

“Hywind has been an important project for [Technip] because it made us think about our involvement in offshore wind strategically,” notes His. “The acquisition earlier this year of [UK offshore installation contractor] Subocean, the creation of a distinct sector identity in TOW [Technip Offshore Wind] — and we are going to be very active in the French tender, supporting the Iberdrola-Areva bid.

“The VertiWind project shows how serious we continue to be about involving ourselves in this industry that is expanding so quickly and so broadly.

“We are approaching this with some humility; we know we have knowledge to build up, but we also have the confidence in the knowledge that we can bring to the [offshore wind] sector through our project-management experience in the offshore oil and gas industry.

“Projects are getting much bigger. You quickly get to projects costing €1bn and you can’t manage them in the way that onshore projects have been. Offshore wind is a different world.”