In one of its brochures of the early 1990s, US firm Kenetech Windpower reprinted an enthusiastic quote from The Wall Street Journal about its advanced, lightweight wind-turbine design. "Energy experts say this new 'five-cent machine' could do for wind energy what Henry Ford did for the car - (it) puts wind on par with coal power, the cheapest traditional alternative.

Moreover, it makes wind power cheaper than coal if air pollution and other environmental costs are taken into account." Unfortunately the KVS-33 model did not live up to expectations and soon after its introduction developed serious and costly technology problems that it failed to overcome. The company filed for bankruptcy in 1996.

Undaunted by this public failure, the pioneering spirit in the US continues. California-based Makani Power is one example of a US start-up at the forefront of next-generation, high-altitude power-generation, with an innovative airborne wind turbine comprising a tethered, rigid wing with multiple turbines.

More traditional advances in wind turbine technology are being produced by US giant General Electric (GE). It entered the market ten years ago by acquiring Enron Wind's wind-power activities and a 1.5MW variable-speed turbine model, which GE developed into what has become a genuine wind-industry success story, with more than 16,500 units installed worldwide. The company's 2010 trend-setting 1.6MW model with 100-metre rotor diameter offers a specific power rate of 0.20kW per square metre.

While the generator power increased by only 6.7%, the rotor-swept area increased on the original 1.5MW turbine's 65-metre rotor diamater by a factor of 2.37, greatly increasing energy capture. These "radical" configurations offer a huge boost to achievable full-load hours and annual yield at low and medium wind-speed sites, driving down the cost of energy. Initially met with scepticism, the product apparently sold out within months.

In the same year GE announced a 2.75MW model featuring unusually shaped "aero-elastically tailored" curved blades and a 112-metre rotor diameter, but these plans were discontinued. According to unconfirmed US sources, GE is working on a new 2.75MW turbine with a 118-metre rotor diameter, but no further details are yet known.

Last year GE Global Research announced plans to develop a next-generation, high-temperature superconductor, or HTS generator, for 10-15MW wind turbine applications. Based on superconducting-magnet technology previously used in healthcare MRI systems, GE claims the superconductor will enable significant efficiency improvements and gearbox elimination. The key design objectives are to reduce the generator size and weight while lowering speed and increasing torque density, and at the same time becoming less dependent on the rare-earth materials used, for example, in copper-based permanent magnet generators. The combination of increased power rating and superior energy-conversion efficiency is claimed to offer better economies of scale and reduced energy costs.

Established in the market

With a heritage dating back more than 35 years, Northern Power Systems (NPS) is one of the world's oldest wind-turbine manufacturers, as well as a highly specialised power-electronics expert. For years the company relied on a 100kW Northwind 100 stall-regulated (fixed blade angle), variable-speed turbine with a 21-metre rotor diameter. An Arctic version for cold-weather applications and a wind-diesel hybrid system for remote regions supplement the standard Northwind 100 model for grid connection.

NPS's product range was expanded in 2010 with a 2.3MW direct-drive model as part of a family of multiple products in development. In early development stages is NPS's next major leap forward - an 8MW direct-drive offshore turbine with a 175-metre rotor diameter.

Both smaller and bigger models feature a standardised generator comprising a liquid-cooled stator built in segments, and an air-cooled inner rotor with permanent magnets. The major benefit for offshore application is that individual generator stator segments, as well as converter modules, can be exchanged with the aid of an onboard crane.

Licensing business model

Wind-turbine technology developer and veteran superconductor specialist AMSC - through its Austrian subsidiary, Windtec Solutions - offers a range of conventional geared models with power ratings ranging from 1.65MW to 5.5/6MW. These models are offered as licensed or co-development products to third-party suppliers located mainly in Asia. Until recently, China's Sinovel was AMSC's main technology client.

AMSC is working on an innovative 10MW direct-drive offshore turbine design - known as SeaTitan - fitted with an high-temperature superconductor generator and a 190-metre rotor diameter.

The licensing/co-development formula offers wind-industry entrants access to state-of-the-art technology and an existing supply chain, a fast time to market and a limited risk profile. It is not known whether any US wind companies are among AMSC's current technology clients.

Testing innovation

A 150-ton rig designed for testing offshore turbine drive trains up to 7.5MW is to come online by the end of the year at Clemson University's Restoration Institute in South Carolina. According to an Associated Press article, a larger testing unit suitable for 15MW will follow. The identity of the clients for even the smaller of the test rigs remains uncertain.

While there are no offshore turbines operational yet in US waters, the Virginia Marine Resources Commission has authorised Spanish manufacturer Gamesa to perform the scientific surveys required prior to installing its offshore prototype. Gamesa opened a research centre in Chesapeake, Virginia, last February, where it designs offshore wind turbines together with US shipbuilder Northrop Grumman. The partners expect to install the first 5MW G128-5.0 medium-speed prototype in the fourth quarter of 2012.

CUTTING EDGE FLEXIBLE ROTOR BLADE DESIGNS AIM TO SLICE COSTS AND BOOST STRENGTH

When developing its now-discontinued 10MW Britannia offshore turbine, Clipper Windpower had initial plans to apply a patented retractable or telescopic blade concept, where blade length could vary depending on the prevailing wind speed. The aim was to increase productivity at low wind speeds with the blade at its maximum length, without the load increases attached to a large rotor during high winds. However, these plans were dropped in favour of a conventional single-piece blade design.

Just as innovative but far more realistic is an idea by California-based Modular Wind Energy (MWE), founded in 2008. It has developed a segmented rotor-blade design of 45 metres or more that enables production and easy transportation in individual 15-metre sections.

Outside the bolted circular root joint, the blade structure is similar to conventional blades: shear web reinforcements connect the primary spar caps, which are covered by a moulded aerodynamic skin.

The clever feature is the way the sections are assembled. At the joint, the blade sections are put together using a patented two-metre-long truss system that comprises three equally spaced sandwich-type, airfoil-shape ribs. Aluminium truss bars forming structurally stiff triangles with the spar caps interconnect these individual ribs, and a main function of the truss system itself is to relieve skin loading between blade sections.

According to MWE's chief commercial officer Warren Ault, this performance is achieved without the significant weight penalty associated with "traditional" bolted segmented blades. MWE instead adhesively bonds individual primary load-carrying spar cap "planks" into laminate beams, which in turn bond to the airfoil skins.

MWE claims that the benefits of its blade include greater stiffness and strength, and up to 20% lighter design compared with conventional blades of similar length and structural strength. This in turn enables greater lengths and, ultimately, lower energy costs.

The first 45-metre MWE 45 prototype set is scheduled to be installed in Europe by the third quarter of 2012, supplemented with five additional series sets in the US by end of the year.