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United States

Research paves the way for US wind expansion

The robust whir of powerful wind turbines has reached ears in Washington DC, where federal officials are directing their labs to think bigger, taller and mightier. The US Department of Energy's (DOE's) Energy Efficiency and Renewable Energy office has declared that a target of 20% wind energy by 2030 is feasible, providing that there are enough incentives to promote research, development and manufacturing. That goal assumes a doubling of annual wind turbine manufacture, then a doubling again and again - a rapid growth that is expected to generate hundreds of thousands of jobs and encourage the development of ever-bigger turbines.

As a result, in 2009, the National Renewable Energy Laboratory (NREL) received more than $100 million from the American Recovery and Reinvestment Act (ARRA). Overseen by the DOE, the NREL is the only national laboratory in the US devoted exclusively to renewable energy, and the injection of funds reflects the Obama administration's emphasis on clean energy and on generating jobs.

Of this stimulus money, some $10 million has gone towards an NREL project to build a new 5MW dynamometer powerful enough to test tens of millions of inch-pounds of torque on large drive trains.

Meanwhile, in a test that represented a real coming together of giants this year, NREL's current 2.5MW dynamometer blasted 12.6 million inch-pounds of torque at Samsung's 2.5MW drive train. It was the largest full-scale dynamometer test of a wind turbine drive train that has ever been carried out in the US.

The NREL dynamometer is outfitted with a powerful 3,550-horsepower electric motor that can produce speeds up to 146 revolutions per minute, simulating everything from soft breezes to gales. It can simulate worst-case wind conditions 24 hours a day. With a few months of testing, manufacturers can learn whether their gearboxes, bearings and cog wheels can stand up to two decades' worth of real-world conditions. The new 5MW dynamometer will be twice as powerful and able to test larger turbines.

Enhanced testing facilities
The DOE has paid out a further $45 million ARRA grant in support of the plans of South Carolina's Clemson University to set up a $98 million wind turbine drive-testing facility. The new facility will be able to test 5MW to 15MW drive trains for land-based and offshore wind turbines.

NREL will act as technical adviser on the project, along with Cener, Spain's national renewable energy centre. Housed in an old naval storage depot, the South Carolina facility is due to be up and running by the end of 2012.
Meanwhile, in Boston, Massachusetts, last December, work began on a new facility to test rotor blades the length of a football field. The DOE channelled $25 million of ARRA funds to the new Wind Technology Testing Center (WTTC), to be built on the edge of Boston Harbour.

The new WTTC facility will test blades up to 90 metres long for structural integrity and durability. According to NREL's Derek Berry, engineering supervisor at the site, each doubling of blade length means that it can capture four times more energy.

When completed at the end of this year, the facility will be able to conduct static tests, pulling blades in different directions to find their breaking points. Fatigue tests will simulate wear and tear, in a way that is comparable to bending a paper clip repeatedly back and forth.

The National Wind Technology Center (NWTC) in Colorado, previously the country's biggest test facility, can handle blades up to 50 metres long. But the largest turbine blades now in production measure 62 metres, so a new facility was needed. One reason Boston was chosen is that it has a deep-water port and can welcome the giant blades at the harbour from manufacturers all over the world.

Reliable longer blades should accelerate the move towards large offshore turbines, expected to generate about 60GW of power by 2030 - about one-sixth of the total US wind power production. However, developers are reluctant to install turbines in deep water until they know they can rely on the design codes, or simulation models, used to assess offshore turbine designs.

The prospect of wind turbines set in deep ocean water came closer in 2009, when NREL researchers from the laboratory's offshore code comparison collaboration team refined codes and models for the far-offshore behemoths.

Gearbox lifespan under scrutiny  
Meanwhile, the march towards larger turbines is getting a boost from a body that examines gearbox longevity. In 2007, the NWTC formed the Gearbox Reliability Collaborative (GRC), bringing together scientists, engineers, manufacturers, utilities and suppliers to examine why gearboxes built to last 20 years fail prematurely.

Gearboxes are expensive - and expensive to repair hundreds of feet above the ground. Throw in unplanned turbine downtime when they are not working properly and you get a significant addition to the cost of energy. Since gearbox failure is widespread across the industry, the suspicion is that the trouble lies in the design process itself and the tools used. To that end, the GRC has designed a generic gearbox, representative of the megawatt-scale class but not associated with any particular manufacturer.

The team is using comprehensive analysis, field testing and dynamometer testing to understand the loading conditions that are driving system failure, and to assess load distribution, deformation and internal behaviour as factors in gearbox fatigue. They also want to establish whether gear contact stress could lead to micro-pitting, in which fatigue creates microscopic weak spots in gears. The tests will also measure how well improved filter systems are keeping oil clean by removing wear particles generated under normal and extreme loading conditions.

The first results are expected shortly and will be available in the public domain through the NWTC. All turbine manufacturers should benefit from free access to the GRC's work, but none should gain a competitive edge since the gearbox being tested is generic.

The NWTC last summer installed a 1.5MW GE turbine with 38.5-metre blades and a 2.3MW Siemens turbine with 50-metre blades. Researchers are studying them for aerodynamics, fatigue, power characteristics and noise.

They are also scrutinizing the microclimates in which the huge turbines operate, the foundations that support them and the thermal performance of the power cables that pass underground to deliver electricity to the grid. These big turbines can weigh in excess of 500 tonnes and are expected to be the workhorses of the next decade.

Huge wind potential
The US wind energy industry shattered records for installation in 2009, even though the manufacture of new turbines slipped as the global financial economy receded. But with federal incentives and the ARRA dollars steered towards wind projects, the DOE expects the manufacturing dip to be a short one.

In February, the DOE issued a new estimate of US potential for wind-generated energy, based on a survey by NREL and consultant AWS Truewind. It involved 48 states, all deemed suitable for wind development. The previous assessment, in 1993, predicted average speed at 50 metres above the ground. But with today's turbines typically 80 metres above the ground, the survey found a lot more wind suitable for capture.

The 48 contiguous states have the potential to generate up to 37,000TWh of wind energy annually, according to the survey. That is nine times the gigawatts from all sources used by US citizens in 2009. Just so long as the wind keeps blowing.   

William Scanlon is a science writer for the National Renewable Energy Laboratory in Golden, Colorado

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