Installed wind generation capacity is currently doubling about every three years. But even greater growth is now foreseeable as high capacity offshore turbines of 10 MW or greater start to become available.

Due to the development of such higher capacity wind energy systems, market forecasts for the nascent offshore industry have been revised dramatically upwards. The industry, which accounted for just 2 GW of the world's total 158 GW of wind power installed at the end of 2009, is expected to enter a period of rapid and prolonged growth beginning mid-decade. For example, industry research firm Emerging Energy Research currently projects that global offshore installed capacity will increase to approximately 20,000 MW by 2015 and rise sharply to 104 GW by 2025.

Until now, the greatest challenges to developing offshore wind have included the practical size and weight limitations of the wind turbine generators, gearboxes and blades, which must be transported over roadways and then erected hundreds of feet in the air.

The power density advantage of superconductor wires, however, is now being applied to the wind industry, offering an optimal solution to maximising the 'power per tower' for wind turbines, while also overcoming size and weight barriers — and reducing costs. With the ability to produce 10 MW of power or more per tower, American Superconductor Corporation's (AMSC) SeaTitan™ promises to be among the world's most powerful wind turbines.

The Next Target Application for Superconductors

SeaTitan's ability to maximise the power output per tower structure is facilitated by the elimination of copper from the generator's rotor and the use instead of high temperature superconductor (HTS) rotors. This enables the generator to be much smaller, lighter, more efficient and less expensive than conventional large-scale wind turbine generators. Efficiency is further enhanced — and manufacturing and maintenance costs kept down — by using a direct drive system, thus eliminating the complex turbine gearbox, which tends to be the most maintenance-intensive component.

AMSC's wholly owned Austria-based subsidiary AMSC Windtec™ has developed proprietary wind turbine designs that are utilised by more than a dozen manufacturers worldwide, including two of the world's top 10 wind turbine manufacturers. The company has also long been the leader in the superconductor arena — producing superconductor wire that can carry 150 times more power than conventional copper wire — and holds the world's deepest patent portfolio for superconductor rotating machines such as motors and generators.

Having invested over US$150 million in large superconductor rotating electric machines over the past two decades, a dramatic pay-off occurred in 2009 when the company completed full-power testing of an HTS motor designed for the US Navy.

Conducted at the US Navy's Integrated Power System Land-Based Test Site in Philadelphia, testing of the world's first 36.5 MW (49,000 horsepower) HTS ship propulsion motor doubled the Navy's power rating test record. Designed and built under a contract from the Office of Naval Research to demonstrate the efficacy of HTS motors as the primary propulsion technology for future Navy all-electric ships and submarines, Naval Sea Systems Command (NAVSEA) funded and led the testing of the motor.

This 36.5 MW superconductor ship propulsion motor by AMSC and Northrop Grumman is the basis of the SeaTitan

This same base platform is now being utilised for the HTS generators that will be at the heart of the SeaTitan wind turbines. These systems have been demonstrated to have as little as half the electrical losses of a conventional machine when at full power.

Superconductor generators additionally offer higher efficiencies than conventional machines over their full range of operation while delivering lower lifetime ownership costs, due in part to their enhanced reliability and improved Mean Time Before Failures (MTBF) figures. The HTS generator technology is also planned to be applied to onshore wind turbines to offer the same power density advantage.

Vast Potential Driving Interest in Offshore Wind

A key attribute of offshore wind energy is the fact that it is a large, and virtually untapped, clean energy resource. Offshore winds are typically stronger and more stable than onshore, resulting in significantly higher production per installed turbine. The North Sea, for example, has average wind speeds up to 12 m/s, but these winds are often located far from the coast.

These resources might be exploited in the future with the development of floating turbine foundations. Closer to the British coast, the wind speed falls to 8 m/s, about the average speed for a good onshore project. Offshore wind also offers a range of other advantages:

•No topographic effects and low surface roughness result in a steeper wind shear profile
•Low turbulence intensity, thus reducing loads on the turbine and usually increasing energy capture
•Minor variations in wind speed that result in a more predictable source of electricity that can compete more effectively with conventional power stations
•Capability of more than 3000 full load hours per year (h/y) versus the typical 2000—2300 full load h/y of onshore installations.

Because it is easier to transport very large turbine components by sea, offshore wind turbines can be larger than those on land. Larger sea-based machines are therefore ultimately more economical — although initial capital costs are more expensive, for example in foundations, these costs are recouped by higher energy yields. But the key to fully realising the potential of offshore resources is creating high-efficiency, lighter-weight, and lower-cost wind turbines that are reliable and robust.

Superconductors Offer a Game-changing Solution for Offshore

China, Denmark, Sweden, Belgium, the Netherlands, Germany and the UK have already built wind turbines in marine environments, both at sea or on harbour breakwaters. Further activity is also planned in most of these countries, as well as in Ireland, Italy and the US. Responding to this interest, several manufacturers are now offering machines specifically for the offshore marine market, but mostly in the range of 3—5 MW in capacity and typically using conventional technology with some design modifications for the marine environment. Such modifications typically include sealed nacelles and special crew access platforms for helicopters or ships for maintenance purposes.

Gearboxes for these conventional multi-MW turbines, however, are very heavy and there has been widespread speculation that some designs have unresolved reliability issues. Generators for multi-MW direct drive turbines are typically large and heavy as well. Allowances for tolerances and deformations in large generators reduces the effectiveness of permanent magnet generators. In addition, the potential unavailability of rare earth materials could prevent direct drive permanent magnet generators from being cost effective.

Deploying more advanced technology, such as a smaller superconductor generator with a large air gap, can effectively replace a permanent magnet machine for direct drive applications. In addition, with superconducting generators, tolerances, deformation and material availability are no longer an issue.

Multi-megawatt turbines, particularly those based on conventional turbine manufacturing technology, do not fall within the optimal 'yield curve' for turbines sited offshore. Since subsea structures and installation services can account for a majority of offshore wind farm costs, developers are seeking to maximise the 'power per tower' to get a more rapid return on investment. This requires significant reduction in the weight and bulk of the generator and ancillary components while at the same time maximising net electrical output.

Only fundamentally new approaches to turbine design, such as the SeaTitan, will achieve these goals.

Design Characteristics and Projected Benefits

The hub height of the SeaTitan HTS turbine is approximately 125 metres with a tower top diameter of 5 metres and a tower base diameter of 7 metres. The tubular steel tower can rest on conventional jacket foundations and deep-water foundations of various types. Technology, in conjunction with the HTS generator, is what sets this design apart from existing generators, however.

As turbine sizes approach 10 MW and beyond, other types of direct drive generators, such as permanent magnet and synchronous machines, get larger in diameter and weight, thus making them more expensive to integrate in comparison with HTS technology. Direct drive generators, in particular, obviate concerns over the effect of deflections in the main shaft since they do not influence gear-to-gear contact and are compensated for by free movement in a large air gap. Furthermore, vibrations set up by tooth engagement are eliminated, and lower rotational speeds reduce load cycles.

An artist's impression of the 10 MW SeaTitan machine from AMSC

AMSC Windtec, which provides licences and customised designs for onshore and offshore turbines, anticipates licensing the new SeaTitan technology to multiple manufacturers. Benefits include:

•High turbine power density: The HTS field winding produces magnetic fields higher than those of conventional machines resulting in much smaller size and weight.
•High partial load efficiency: HTS generators have higher efficiency at part load that results in a potential efficiency advantage of 10% or more at low speeds.
•Low noise: HTS generators have lower sound emissions than conventional machines.
•Harmonics: HTS generators have better power quality and in particular are free of harmonics.
•Maintenance: In addition to negating the need for a gearbox, direct drive HTS generators will not require the generator rotor overhaul, rewinding or re-insulation that is required with conventional generators.
•Simple mainframe: No decoupling between generator stator and mainframe housing is needed, because rotor deflections are effectively absorbed by the large air gap.
•Increased personal safety: The HTS generator rotor can be demagnetised during wind turbine maintenance or potential wind turbine repairs. This significantly increases the personal safety of service employees compared to wind turbines with permanent magnet generators.

The weight savings attributable to HTS technology allows the generator to be placed directly above the tower, enabling improved mainframe design and direct load transfer from hub to tower.

In most existing offshore wind turbines, a major failure mode is caused by the deflections of the rotor shaft, which occur under turbulent wind conditions. To reduce damage, the housing of the gearboxes or generators are decoupled from the mainframe in a complex way. This is not needed in an HTS generator, because the large air gap can absorb all likely deflections, and the generator housing can be directly integrated into the wind turbine mainframe.

This, combined with the small generator diameter, is the primary contributor to the strength and yet the light weight and comparatively small size of the SeaTitan design. Furthermore, the HTS design model requires only a single main bearing configuration as additional gearbox and generator bearings are not required.

In addition, conventional permanent magnet generators require grease for the bearings and tightening checks on fasteners, while conventional doubly-fed induction generators require cleaning and replacement of brushes on the slip ring.

AMSC has incorporated a number of design solutions that ensure redundancy in the SeaTitan's operation. In particular, the cryogenic cooling system must be as robust and reliable as possible so that customers are not involved in additional services or maintenance. The refrigeration system achieves high reliability by employing n+1 modular, single-stage GM coolers and long-life seals in its helium transfer coupling.

In AMSC's experience with cooling transfer systems in both HTS transmission and large rotating machines this component has presented excellent reliability in all cases. In addition, AMSC's design, which uses more than one cryogenically cooled surface, promotes efficiency and ease of maintenance. First, more than one cryogenically cooled surface in series allows each section to work less to lower the temperature of the cryogenic fluid. Also, if one cryogenically cooled surface malfunctions, the redundancy in the system is designed to be able to overcome the loss.

The refrigeration system additionally has no unusual environmental requirement or impact due to the required cryogenic cooling components for an HTS generator. In fact, most serviceable components are placed in the tower bottom for easier access and faster exchange. These accessible components include power converters, the compressors for cryogenic cooling, the control cabinet, and switchgear.

The Path Ahead for the Wind Industry

Maximising the potential of offshore wind power sites will require new technical approaches in turbine design to increase power density, reduce weight and lower maintenance costs. HTS technology, having been proven in large ship propulsion motors and in many other electric utility applications, is clearly one way to achieve these goals.

The SeaTitan will apply novel generator rotor technology and superconductor generator technology to reduce system size and weight and lifetime costs. Ultimately, significantly lower offshore wind development and maintenance costs will result from such developments. Indeed, the SeaTitan HTS generator represents a path forward to achieve wind generator power ratings in the 10 MW range and beyond.

Martin Fischer is vice president of American Superconductor, general manager of AMSC Windtec

Exploring the secrets of superconductor technology

The basic HTS wire substrate is nickel tungsten. Various buffer layers are then applied before the superconductor — yttrium barium copper oxide — and a very thin cap layer of silver are also applied.

Superconductor wire alongside conventional copper equivalents

The winding is then cooled to an operating temperature of 30—40°K or around -235°C. Compressors for the rotor refrigeration system represent a heat load which is about 0.4% of the machine rating or some 40 kW for the SeaTitan. Nonetheless, the overall losses from the generator (including the refrigeration units) are 40% less than those in conventional direct drive generators.

And there are significant advantages in terms of power density. For example, a 10 MW 10 rpm direct drive PMG would be 10—12 metres in diameter. An HTS generator with an equivalent capacity will be 4.5 metres in diameter.

At present the overall project is managed out of the company's AMSC Windtec division in Klagenfurt, Austria, and generator development is being conducted at facilities in Devens, Massachusetts, US.

The company is in active discussions with test centres both in North America and in Europe to perform the design testing and the intention is to commercialise these wind turbines by 2014 or 2015. This, of course, would require complete prototype testing prior to that.

Meanwhile, AMSC has recently announced an investment in advanced blade technology at Blade Dynamics, which provides a potential blade platform for the SeaTitan. AMSC Windtec is also lining up the other parts of the supply chain that would be needed for these wind turbines, like the towers, foundations and nacelle.