As a precursor to the European Wind Energy Association's EWEC conference – taking place in Marseille in March – it seems the perfect time to revisit some of the more ingenious devices that were on display in London, late last year, at the BWEA's 30-year anniversary event.
French company NHeolis, for example, featured a demonstration unit at the event, and claims a breakthrough in wind turbine design with an innovative blade form that amplifies the strength of the wind impinging on the blade, thus increasing rotor efficiency. Although in this case the rotor axis is still horizontal, the blades/sails are very different from normal blades.
A conventional wind turbine blade operates like an aircraft wing on its side, developing lift by virtue of its aerodynamic shape. Because the blade is attached to a horizontal shaft, this lift is converted into turning moment.
NHeolis blades do not generate lift in the same way but instead exploit the Venturi effect. This occurs when a “fluid” (in this case air) is driven down a tube that is wider at the entry end than at the exit, like a windsock at an airfield. The resulting constriction speeds up the flow and increases the pressure in the downstream end of the tube/windsock. Imagine a windsock being cut in half longitudinally, then take one of the concave halves and attach the wide end to a horizontal shaft such that the blade/”windsock” extends at an angle downwind and can rotate with the shaft (see diagram). Attach two more similar half “windsocks” to the shaft so that all three form the “blades” of a wind turbine.
Although each “windsock” blade lacks the forward facing half of a full tubular structure, the constricting effect on captured air is maintained by the pressure of the oncoming wind. Thus, if it is correctly aligned, thanks to the Venturi effect, the blade reacts to the wind by trying to move away from it, thus turning the shaft.
NHeolis produces its highly volumetric blades in carbon-epoxy composite, each half “windsock” sail (the French company calls it a shroud) being mounted to and stiffened by a composite tube or spar.
The rotor shaft drives a synchronous generator. The entire assembly is mounted on a vertical pivot so that it can rotate into wind, driven by a fixed rudder. The initial model develops 3kW (nominal) and can be mounted on a steel lattice tower or on a roof. It is said to behave well in turbulent flow so that nearby obstacles do not unduly impede operation. A cut-in wind speed of 2.5m/s (9km/s) and maximum speed, verified in wind tunnel testing, of 45m/s (164kph) are claimed, giving an unusually wide operating window so that an annual output in the region of 3400kWh might be expected.
Electromagnetic and aerodynamic brakes are incorporated. Noise is said to be minimal thanks to lower wind shear.
Each blade, the design of which was optimised with the help of ONERA, the French Aerospace Research Centre, and wind tunnel tested at CNES, the French Centre for Scientific Research, is two metres long and weighs some 21kg. Maximum rotor diameter (at the trailing edge) is 2.3m. NHelios says that the system is three or four times more compact than a standard turbine, but provides the same power. CNES is undertaking modelling studies to assess the viability of larger turbines.
Another application of intelligent science, at least according to Dallas, USA-based BroadStar Wind Systems, is the AeroCam wind turbine concept, which provided a compelling visual focus at BWEA 30.
This too has its rotor turning around a horizontal axis but is far from conventional. An AeroCam rotor is reminiscent of a water wheel, but more “articulate”. Each of the parallel “paddle” rotor blades alters its pitch continuously, according to where it is in its 360 degree rotational cycle. Those at the top of the trajectory and advancing into wind become near-horizontal so that optimum “wing” lift is generated over the aerodynamic profile, thereby exerting maximum turning moment on the rotor. Retreating blades become more nearly vertical, providing downwind “sails” so that they too contribute to the turning effect. The cyclical change of blade orientation is brought about mechanically by an offset cam system.
Like the NHeolis, BroadStar's patented system is said to be able to operate in wind extremes. Some models can extract energy from air flows as low as 4mph, to winds in excess of 80mph. Efficiency is said to be high because each blade sees air impinging at equal speed all the way along its length, though the design lacks the low airspeed, low-lift regime experienced near the hub of a conventional HAWT. Consequently, says BroadStar, AeroCam is approximately 30% smaller than a normal HAWT developing similar power.
Because the system's rotors are relatively small vertically, hub heights are low and special cranes are not needed for installation and maintenance. The horizontal orientation of the blades makes the system inherently stable, say the makers, enabling it to operate in turbulence with low noise, vibration and wear. Devices, particularly single-rotor types, are therefore readily integrated with low and high-rise urban roofs.
They can be deployed in architectural enclosures designed to concentrate airflows and increase wind speeds. According to Tom Stephens, BroadStar's vice president of research and design, “because AeroCam packs so much efficiency into its compact configuration, it's opening up opportunities to businesses and communities that could never have considered wind energy before. We believe this has the potential to redefine the market.”
“Because it can be deployed at low level, AeroCam can be used to capture surface wind energy without disrupting the airflows that larger turbines need to operate effectively. This makes them suitable, says the company, for installing between large utility turbines at new and existing wind farms, thereby offering a means to enhance energy production. Barriers to wind energy development might be overcome by installing low-profile wind farms based on AeroCam, providing a higher power yield per square mile than conventional turbines. Small-scale generation can be contemplated by municipalities, making property assets from low-rise to commercial buildings and shopping malls to city parks their own generators.
This point is emphasised by a recent agreement signed with US retailer JCPenney. AeroCams will be installed at a 1.6m ft2 distribution centre in Reno, Nevada. According to Stephen Else, president of BroadStar, his company can deliver 250kW and 500kW machines for US$250,000 and US$500,000 respectively, making it the “first turbine to break the US$1/watt cost barrier”. Else adds that AeroCam is well suited to off-grid distributed electricity generation, though with the additional possibility, where appropriate, of connecting to the grid via smart metres.
A third turbine concept aired at the BWEA show was Nordic Windpower's N1000 1MW two-blader. This company, originally Swedish but now a US enterprise, has revisited the two-blade WT configuration that fell out of favour two decades or so ago, arguing that this is the time to resurrect its advantages. Disadvantages, most notably fatigue and noise originally experienced with smaller “farm-scale” two-bladers, are scarcely noticed at utility scale, says the company. Visual disturbance that some members of the public felt with fast revolving small-scale rotors is less evident in slower revolving large rotors.
One important advantage is that, unlike a three-blade rotor that has to be lifted to the hub by crane, a two-blader can be mounted to the nacelle on the ground, reducing deployment time and cost. Another is that, with only two blades to look after rather than three, maintenance costs are reduced. A third plus is that, thanks to the lack of a third blade, head weight is reduced. This means that, for the same power output, the drive train and gearbox can be less highly rated and therefore lighter.
Basic wind turbine science says that, to capture a certain amount of wind, a rotor needs a certain aerodynamic area. Loss of area occasioned by adopting a two-blade configuration rather than three is compensated for by increasing the blade's chord (edge-to-edge dimension or width). Blade thickness is then increased to maintain a blade thickness-to-chord ratio of about 15%–20%. A thicker blade is inherently a stronger one. Because of this form strength, less structural material can be used, making the blades both lighter and cheaper to produce than conventional equivalents. This, along with the decrease in the number of blades needed, reduces blade cost overall.
Although a two-bladed turbine is less efficient than a three-blader, Nordic Windpower says that the difference in annual energy production of around 2%–3% can readily be compensated for by a 1% increase in turbine diameter. According to ceo Steve Taber, two-bladers originally fell out of favour less because of the disadvantages noted above, rather because the two-blade configuration had not successfully been modelled in software and was therefore more difficult to engineer. Now, he says, the company operates such a programme, developed and refined in Sweden over almost three decades.
Taber points out that a necessary prerequisite of a successful two-blade design is a teetering hub – a term adopted from the helicopter world meaning that the blades are attached to the turbine shaft via hinges. Although these have only a small range of movement, +/− 2 degrees, this motion has a decisive influence on the loads acting on the turbine system. Associated damping ensures that dynamic loads are passed into the drive train more evenly, mitigating the vibration and fatigue that would otherwise ensue. These are further reduced by enclosing the gearbox, drive shaft and generator within a unified tubular housing which serves to retain the system and allows forces to dissipate away from the gearbox.
Two twin-blade turbines, of 2MW and 3MW, operated for six years to 1988, provided invaluable base data. A second, subsequently commissioned 3MW prototype is still operating and has become the world record holder, claims Nordic, for power production and accumulated operating hours. Steve Taber indicates that it has seen 11 years in service with no major component failures and almost zero maintenance. The only attention required, he asserts, was an occasional service of the elastomeric hinges. Currently four N-1000 turbines are operating in Sweden.
Caution shown by investors daunted by unfamiliar technology led to the unfortunate bankruptcy of the original Swedish company, which was subsequently acquired by US interests. Recently, investor Goldman Sachs took a gamble on the two-blade concept, injecting funds to enable the 1MW model to progress to market.
The company itself is determined to move forward with its design. Alex Potier, global sales manager for Nordic Windpower, says, “with an innovative and cost-effective technology, our order book is growing strong. Over the next several years, the company is poised to earn the trust and respect of wind farm developers around the world as we continue to deliver new and reliable wind turbines designs.”
The world's largest turbine?
A fourth wind turbine concept, aired in an entertaining “Dragon's Den” type session at BWEA 30 (Dragon's Den being an English TV programme in which a panel of millionaire investor “dragons” pass judgment on entrepreneurial ideas), breaks the mould on scale alone, though its configuration is conventional.
The Britannia 7.5MW machine, currently under development by Clipper Wind Power Inc, will have a 150m diameter rotor and a tower more than 100m high, substantially higher than Big Ben. As if this were not enough, “presenter” David Steele spoke of a potential 10MW machine currently under consideration. Advocating the merits of scale, Steele said, “we're talking long distances offshore for machines like this. The expense of working in deep water many miles from shore means it makes sense to maximise the available power for each installation. Thanks to advanced design, our 7.5MW machine will have a similar weight to a typical 5-6MW machine of today.”
The technology of the world's largest turbine is being adapted from that of Clipper's Liberty 2.5MW turbine, which has earned plaudits from the US Department of Energy. A novel drive train architecture is said to be able to accommodate the low rotor speed, high torque and high point loads that characterise large-scale conventionally-configured machines. The Clipper solution is to mount the main rotor shaft firmly within twin tapered-roller main bearings, so that shaft axial movement and misalignment are prevented, and split the input torque to four permanent magnet generators. Together with two-stage helical gearing, this approach greatly reduces the stresses found in standard three-stage planetary gearboxes used in today's multi megawatt machines.
Additionally, it provides redundancy in the load path and in case of generator outage. Other forward looking features likely to influence “Project Britannia” include the placing of high-speed gear sets in “cartridges” that can be replaced without gearbox removal, the provision of multiple inspection ports, an advanced gearbox health monitoring system, and an on-board jib hoist to facilitate on-site maintenance without the need for a crane. Permanent magnet generators are relatively simple, should require little maintenance and generate direct current, which is then converted by high-power electronics to alternating current at the required frequency.
The first 7.5MW Britannia is already spoken for, having been ordered by the UK's Crown Estate so that it can better appreciate the challenges faced in developing wind turbines for deep water deployment. Development is now proceeding at Clipper Wind Power's British base at Blythe, Northumberland, with support from the UK's North-East Regional Development Agency. The Blyth-based New and Renewable Energy Centre (NaREC) is to evaluate the drive train, rotor and generators.
An erected prototype is expected to go on line in 2012. It will be interesting to see how Clipper's concept fares against the direct drive (gearless) technology being investigated by Siemens Energy as it installs two machines, of 2.3 and 3.6MW power, off Denmark for a two-year test period. Siemens' cto, Henrik Stiesdal, has suggested that direct drive could compete with geared drive in large turbine sizes.
A vertical axis
Vertax Wind Ltd believes a better answer for offshore could be the vertical axis wind turbine, engineered in sizes up to 10MW.
Peter Hunter of Vertax told BWEA 30 delegates that a large VAWT would be a “steady plodder” compared with the more efficient “Formula 1” HAWT, but is just what the offshore sector needs because, with its relatively few moving parts and lower stresses, it could serve for 40 years with minimal maintenance and high reliability.
The system could, says Hunter, keep working over a higher band of wind speed than conventional HAWTs, resulting in a higher capacity factor. A VAWT could drive a permanent magnet generator directly rather than through a gearbox. Much of the support structure could be concrete, saving cost and avoiding metal supply chain constraints. A stall regulation system is being trialled.
Installing foundations in the seabed for enormous offshore machines could give engineers an equally enormous headache. One way around this is to avoid seabed structures altogether by installing turbines on floating platforms, in the manner of many offshore oil and gas installations.
Blue H, a firm registered in the UK but based in the Netherlands, has built a 25m wide prototype platform and located it in 113m of water off Brindisi, Italy, some 21km from shore. The platform carries a modest demonstration turbine, but the company hopes to build a full-scale 90MW wind farm at 120m depth in the region.
As Neil Bastick of Blue H told the BWEA Dragons' Den audience, “we think this is offshore wind's secret hope for grid parity. There's no assembly out at sea, no seabed preparation. Standard cranes can lower the tower and wind turbine aboard the platform in harbour or close to shore. You can do the work in the winter and tow the platform to its intended location in summer when the weather is kinder.”
The platform is held in position by chains attached to counterweights on the sea bed. Bastick believes the system is best suited to depths over about 35m.
And just this week, it has been announced that Project Deepwater Turbine, a consortium led by Blue H and including BAE Systems, EDF Energy, CEFAS, SLP Energy and Romax, has been selected by the UK's Energy Technology Institute (ETI) as one of the first projects to receive funds as part of its £1.1 billion initiative. This specific project will aim to develop an integrated solution for a 5MW floating turbine deployed offshore, in waters between 30 and 300 metres deep.