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25th Anniversary Special - Technology - Much more complex than at first sight

Effective wind turbine design was not the result of a quick fix revolution but a learning-by-doing evolution. Modern wind turbine design started with the oil crisis of the 1970s, when "revolutionary" initiatives were fast and big, but ultimately expensive. Thereafter, step-by-step progress followed as lessons were learned.

Early attempts to find shortcuts to giant turbines provided useful insight, says David Milborrow, but steady evolution won the day.

Effective wind turbine design was not the result of a quick fix revolution but a learning-by-doing evolution. Modern wind turbine design started with the oil crisis of the 1970s, when "revolutionary" initiatives were fast and big, but ultimately expensive. Thereafter, step-by-step progress followed as lessons were learned.

Several government-funded research and development programs began in the 1970s, particularly in the United States, Denmark, Sweden, the Netherlands and the United Kingdom, which focused on developing large wind energy conversion systems with rotor diameters around 50-60 metres and one megawatt power output.

The big-scale strategy was to reduce the number of machines needed to generate large quantities of electricity and minimise the environmental impact. And this "revolutionary" approach believed that contemporary advances in science and technology would enable the jumbo jet of wind turbines to be built, skipping the painstaking steps that had started with aircraft of much smaller dimensions. But it proved too ambitious and few of the large machines spawned from these programs were a commercial success.

The knowledge gained, however, in aerodynamics, control and materials science was vital to the development of a proper understanding of wind turbine design.

 

The first market

Support for wind power technology development in Denmark started around 1977, but it was in the United States in 1978 that the first real market for turbines was created with tax credits in California and attractive purchase rates for electricity. That led to the construction of thousands of small machines. In the mid- 1970s, 30 kW and 50 kW were popular sizes, growing in ten years to 80 kW. By 1988, California had 17,000 machines, averaging 110 kW, with a combined capacity of 1450 MW.

While thousands of fast-spinning early turbines were going up in California, another school of thought still believed in a revolutionary approach and big scale wind turbines. There were around 18, averaging 1.7 MW per machine - smaller than today's most popular commercial models but, in terms of size, way ahead of the smaller machines on the slower moving evolutionary track. Some of the biggest had ratings as high as 4 MW (large even by today's standards) with rotor diameters from 38 metres to 100 metres. But size did not say it all - the most powerful machine, the Hamilton Standard's WTS4, with a rotor diameter of 78 metres, was a good bit smaller than the large Growian, with a 100 metre span.

The large machines nearly all had a horizontal axis and two blades right up to the 1990s, although there were exceptions to this rule (table below). Some had rotors downwind, facing away from the wind, but these proved to be noisy, as the passage of the rotor through the low velocity wake in the lee of the tower caused unacceptable low frequency sounds.

By the early 1990s several countries were following California's example and stimulating markets for wind turbine sales. Denmark's first program, the forerunner for others in the Netherlands, Germany, Spain, Britain, India and elsewhere, encouraged a more evolutionary approach. But revolutionary machines were still being built with the aim of forcing the natural evolutionary pace - electric utilities, after all, dealt in megawatts, not kilowatts.

Some of the resulting machines were modest enlargements of existing commercial designs and more likely to be commercially successful. Others aimed to significantly push up the power rating or, perhaps, validate novel concepts, such as the one-blade machines built by German manufacturer MBB with Riva Calzoni of Italy.

 

Evolutionary road

Meanwhile, the natural evolution was ongoing, producing smaller machines in many guises in the early 1980s, with a particularly wide spread of designs attracted to the lucrative California market. Small and unreliable, some of these smaller new turbines had poor energy productivity by modern standards.

But competition among manufacturers resulted in gradual improvements and by the mid-1980s, the remaining turbine suppliers - who by this time had reduced in numbers - offered more reliable and cost-effective machines. Around 200 of the machines operating in 1984 are still operating today. The most common models in California included the ubiquitous US Windpower W56-100 turbine and two Vestas machines (table above).

The extremely rapid development of the American market, almost exclusively in California, took capacity to around 1800 MW by 1992, but after withdrawal of the more favourable tax incentives and enhanced energy payments, growth slowed to a crawl. Although further additions to the wind turbine fleets were made, these were more than offset by the decommissioning of many of the early machines, so American wind energy capacity declined during the mid- 1990s.

The Danish market continued to expand steadily and, as more financial incentives were introduced, other European markets opened up. Wind power growth in Germany became particularly strong following the introduction of the Electricity Feed Law in 1991 that required the utilities to pay wind turbine owners a guaranteed price for their generation equal to 90% of the retail price of electricity. Attractive loan conditions and tax incentives also encouraged a wide variety of individuals and organisations to invest in wind energy.

 

Growth continues steadily

Machine sizes have steadily increased from 1992 to 2008, a trend that has been particularly clear in Germany (graph below). By the turn of the century, the average machine size installed in Denmark was about 860 kW, and in Germany it was 1110 kW. By last year that had risen to 1920 kW and it still increasing. Commercial models of 2 MW to 3.6 MW are now available and popular among customers.

At the top of the range, a small handful of wind turbine suppliers are now offering machines rated at around 5 MW, with several already sold commercially for offshore use. Two German manufacturers are testing prototype 6 MW machines, which have evolved from their smaller models. So, the path of natural evolution is now delivering machines that are more powerful than the 'revolutionary' machines of the early days, but it has taken about 20 years for the crossover point to be reached.

 

Design Possibilities

The design of engineering equipment tends to converge towards standardised concepts. Cars no longer have under frames - monocoque construction is almost universal because it is cheap and robust. Wind turbine design technology, however, has not yet converged and it remains to be seen if it ever will.

Many of the early wind turbines had two blades; most were pitch controlled and delivered power through induction or synchronous generators. Significantly, that list of early models, while containing design diversions long abandoned, also included machine concepts that have emerged as commercial options much more recently. The US MOD-2 designs were among early machines with partial span pitch control, while the direct drive Canadian Eole, which coupled the rotor directly to the generator, bypassed the need for a gearbox. They are both concepts firmly back on the market today.

 

Diversity in design

So while concepts such as Eole's vertical axis have now almost disappeared, the only firm conclusion is that diversity has increased. The last few years have seen the emergence, or re-emergence, of several novel concepts, including three types of wind turbines without gearboxes and flexible downwind rotors. Not all have made it to commercial success, however.

Some of these features may have been incorporated as a result of experience gained from the early revolutionary machines, but other features, such as the early preference for two-blade machines, did not catch on. The principal features to emerge over time are machines with three blades that face into the wind, with power control and drivetrains still evolving (table below).

The further from the shore, the higher the wind speeds - and the higher the costs. While the world's total wind capacity is expected to reach around 150 GW this year, up from 121 GW at the end of last year, offshore wind capacity accounts only for about 1% of the total.

While no truly offshore machines featured among the first big-scale revolutionary designs, early strategic thinking did consider the exploitation of offshore wind, partly because of the huge resource and partly because of their less visible impact on the landscape.

Early research and development studies looked mainly at the feasibility and cost of installing the big revolutionary machines offshore, as it was unlikely to make sense to install small machines there. Yet the first truly offshore wind farm, built in 1991 at Vindeby in southern Denmark, ended up using "marinised" commercially-available 400 kW machines. In time, offshore wind farms have used commercially available machines and have consequently seen an upward trend in machine sizes.

 

The future

A trend in wind turbine design that seems set to continue is a leaning towards bigger turbines with larger rotors. The use of progressively larger machines, which are cheaper to make per unit of installed generating capacity than smaller turbines and deliver more energy, have contributed, in part, to the dramatic reduction in wind energy prices that has occurred over the years. The improvement in energy yield is partly because the rotor is located higher from the ground and intercepts higher velocity winds, and partly because they are slightly more efficient.

The growth in turbine sizes cannot, of course, continue indefinitely but, in the short to medium term, it is likely to progress upwards. Only ten years ago it was perceived that 70 metres might be an upper limit, yet that figure has long been surpassed. There is now a feeling that the trend may level out with turbine rotors around 7-10 MW output, but only time will tell.

 

THE PATH OF WIND TURBINE TECHNOLOGY

- Features of typical components of today

Rotor diameter - A gradual increase along the "evolutionary" path, now up to 112 metres

Number of blades - Most now have three, though a few have two.

Blade material - Many early machines had steel spars. Glass-reinforced plastic is now common, along with wood- epoxy.

Rotor orientation - Usually upwind of tower. Some were downwind machines.

Rotational speed - Many early "revolutionary" machines were variable speed. Most early "evolutionary" designs were fixed speed, but variable speed gradually became established and is now common.

Power control - Most common methods are stall control, where blades are fixed but stall in high winds, and pitch control, where all or part of the blade is rotated to limit power. "Revolutionary" machines mostly used pitch control, while "evolutionary" machines used both. Now, stall control is much less common.

Power train - Gearboxes to step up the speed of the rotor to that of the generator were common in the early days for all types, but direct drive (no gearbox) is now becoming increasingly common.

Generator - Induction machines were common in the early days. Variable speed machines use AC/DC/AC systems. Double- fed induction generators are growing in popularity but the latest trend is towards permanent magnet generators.

Yaw control - Sensors monitor wind direction, and the rotor is moved under power to line up with the wind. Free (passive) yaw was used in early machines, but is now rare.

Towers - Lattice towers were used in early machines but cylindrical steel construction is widespread. Some "revolutionary" machines used concrete towers and these are making a comeback in reaction to high steel costs.

- David Milborrow is an independent renewable energy consultant.

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