Wind Power: Thinking Big

By Robert Evatt, Online Editor, Power Engineering

Engineers are known for taking on numerous technical issues to improve the efficiency and output of electrical generators.

But for large-scale wind turbines, much of the improvement comes from just one factor – size.

The continued growth of wind turbine generation has largely mirrored the growth of turbine blades, the MW rating and the height of the hub. The first units are now dwarfed by today’s larger wind turbines.

Recent research from the journal Nature Energy suggested both land-based and offshore turbines have the potential to grow even further, which would continue to lower overall wind power costs as the average per-turbine generation grows.

The study, which surveyed 163 wind power experts, demonstrates just how quickly wind turbines have grown. The very first wind turbines in the 1980s featured an average rotor diameter of 18 meters. That grew to an average rotor diameter of 73 meters in 2005, and 102 meters in 2015, or large enough to sweep an area 50 percent larger than a football field.

The world’s largest turbine blade, manufactured by LM Wind Power, stretches a full 88.4 meters long, allowing for a total diameter of nearly 177 meters. The blade may have been the largest object ever transported on roads in Denmark. Its 218-kilometer trip to a testing facility in Aalborg took just six hours, but it required nine months of planning and coordinating to pull off. Photo courtesy: LM Wind Power

Yet that’s not the projected limit. By the 2030s, that average rotor diameter is expected to reach 135 meters. Siemens has a rotor diameter of 142 meters being offered today for onshore.

The length of offshore wind turbines could grow even more dramatically. These types of wind generators featured a rotor diameter of 90 meters in 2005 and 119 meters in 2015. By the 2030s, that rotor diameter could reach 190 meters.

Ed Hall, general manager of engineering for onshore wind at GE, said the larger nature of offshore turbines comes down to a lack of neighbors that might be inconvenienced and the economics of scale.

“Since the infrastructure for offshore turbines is very expensive, you’ll want to put as large a turbine as you can to recoup the costs,” he said.

Hub heights have grown as well. On land, the current average of 82 meters is expected to give way to an average of 115 meters in 2030. Offshore hub heights have grown slower to a current average of 90 meters, but that’s expected to give way to an average of 125 meters in the 2030s.

The result is improved turbine performance and lower generation costs, even though the average turbine capacity in the U.S. has remained roughly the same since 2011.

Last year, Siemens developed and erected a 115-meter concrete hub for MidAmerican Energy in Adams County, Iowa. Taller hub heights don’t just allow for longer blades. The taller heights allow turbines to access stronger and more consistent winds at higher elevations, which can boost power production by 10 percent or more, according to Siemens. Photo courtesy: Siemens.

Michael McManus, head of business development and strategy of onshore Americas at Siemens Windpower, said his company has greatly increased the size of land-based turbines even as the general capacity remained the same.

“If you look at the pretty recent past, we had a 2 MW class turbine that’s evolved from an 83-meter rotor to a 120-meter rotor, which is a significant jump in rotor diameter.”

Turbines with larger capacities can handle even larger blades, McManus said. Siemens’ 3-MW class direct-drive turbine now sports both a 130-meter rotor and 142-meter rotor.

Vikaas Rao-Aourpally, vice president of sales and business at Goldwind Americas, said the rate of growth has been staggering.

“Even over the last five years, the sizes that are coming out are remarkable,” he said. “When I started at Goldwind six years ago, the average size was maybe 100 meters at most. Now, we have a variant of a 3-MW turbine that has a 140 meter diameter.”

As of right now, the world’s largest turbine blade, manufactured by LM Wind Power, stretches a full 88.4 meters long, allowing for a total diameter of nearly 177 meters. That blade, designed for an Adwen AD8-180 wind turbine, is expected to provide 25 years of use.

Goldwind’s GW3S turbine – a 140-meter-rotor prototype in Hebei province of China, with a rated capacity of 3.4MW. Photo courtesy: Goldwind

LM noted such a large blade design needed to balance swept area, energy production and weight as well as the load transferred to the wind turbine. The blade is now undergoing fatigue testing to simulate high wind conditions.

Though at the rate wind companies are refining their manufacturing processes, the current blade size record might not stand for long. Siemens is also examining methods for lengthening turbine blades, which requires a careful examination of the entire manufacturing process and logistics, and finding ways to create new structures that can withstand increased loads.

“We are bound by these limits, though we continue to research and test new materials that could enable larger higher-performance structures,” McManus said.

The growing size of the rotors has become the main method for increasing the overall energy capture of the wind turbine. McManus said bigger rotor diameters greatly impact the amount of energy generated.

As a result, Siemens plans to continue expanding rotor diameters for all of its turbine classes as much as possible.

In the U.S., specific power – the ratio of generator size to rotor area – reached 250 W/m² by 2015.

Of course, energy companies can’t automatically put the biggest rotors everywhere. McManus said a number of factors can influence what types of turbines clients choose to install in their wind facilities. The primary factor is the cost of electricity. The influencers include climatic, logistical and regulatory contributors.

Rao-Aourpally said the U.S. has some federal regulations on hub heights that much of the rest of the world doesn’t have, though that isn’t stopping some wind developers.

“The restrictions aren’t a hard-and-fast rule, but just make for a longer development and permitting process,” he said. “Customers are starting to explore that.”

Additionally, turbines have to be specifically designed to handle the weight of bigger blades, even if the capacity rating remains the same, Rao- Aourpally said. Today, Goldwind and other companies are designing turbines with growth in mind.

“New turbines are, from the drawing board, being built to support larger blade sizes,” he said.

GE’s Digital Wind Farm hardware and software package allows engineers to capture fast-flowing data from the turbine, the facility as a whole and the grid to further optimize wind energy production. Photo courtesy: GE

Hall said turbine manufacturers are now working more closely with blade manufacturers to ensure blades and turbines can improve in sync and eliminate any potential bottlenecks.

But one factor in particular has emerged as a key concern – logistics. Wind turbine blades are generally manufactured in one piece to ensure the most durable structure possible, which makes travel from the manufacturing site to the wind facility a challenge.

Siemens often has to move 53-meter to 70-meter blades long distances and find specialized trucks and trailers to safely handle them. The company increasingly relies on the expertise of shipping companies to ensure blades can grow.

“We partner with transportation companies to determine the most cost-effective way to ship the turbine components,” McManus said. “That has a big impact on our work. We’re now turning to shipping by rail.”

At this point, Siemens hasn’t yet created a wind turbine blade too big to ship.

The gargantuan 88.4-meter LM Wind Power blade may have been the largest object ever transported on roads in Denmark. Its 218-kilometer trip to a testing facility in Aalborg took just six hours, but it required nine months of planning and coordinating to pull off.

LM carefully planned the route to ensure the blade wouldn’t run into an impassible curve or low bridge, but the company still had to dismantle guardrails and sign posts at certain points to make room.

Even with new logistical challenges that come from enlarging the equipment, the economy of scale and new construction techniques are driving down the up-front costs of wind development.

For example, construction of hubs tall enough and strong enough to support heavy equipment operating in high winds can become increasingly cost-prohibitive, McManus said.

“With steel towers, as you go taller, the cost increases exponentially due to economic shipping constraints combined with the increased cost of the foundation to support the structure,” he said.

One solution Siemens offers to neutralize this problem calls for switching from flanged, interlocking steel tubes to a tower constructed of match casted concrete segments. The patented match-casting process allows for construction by segments, without the use of grout in the joints during construction which provides a rapid construction cycle equal to steel towers at a lower cost of electricity at high hub heights.

Siemens put that technique to use last year for a 115-meter hub at a MidAmerican wind facility in Adams County, Iowa.

Hub heights don’t just allow for longer blades. McManus said taller heights allow turbines to access stronger and more consistent winds at higher elevations, which increases the annual energy production of the turbine and thus lowers the cost of electricity for customers.

The Nature Energy research indicates the offshore wind market is less mature, with relatively slower blade and hub growth. Though transportation logistics allow for turbines with much larger capacities – usually in the 6 MW to 8 MW range – the cost of the foundation and installation are much higher than on land. As a result, turbines with much larger capacities are generally necessary to cover up-front costs.

Though the continued growth in wind equipment size creates issues with physical scaling laws or transportation, the Nature Energy survey indicates wind experts are confident turbine developers will be able to overcome them.

Though size is the most useful method of improving wind power production, companies aren’t neglecting other methods. Rotor design enhancements, improved component reliability and reduced financing costs are expected to help as well, according to the Nature Energy Study.

Rao-Aourpally said Goldwind, as well as other companies, are switching away from traditional gearboxes within rotors to permanent magnet direct-drive technology, which turns a magnet-based generator. Not only does the new technology improve efficiency, it also results in lower maintenance and less downtime.

“The lack of a gearbox means one less item for annual maintenance,” he said.

McManus said digital controls can improve performance as well. As wind itself can behave unpredictably, finding ways to automatically adjust and harness how currents are moving at any given moment can help keep production steady.

“There’s a high degree of focus put into the controls of the machine and the park as a whole to optimize the performance of the machine at any given wind condition,” McManus said.

Hall said GE’s Digital Wind Farm hardware and software package allows engineers to capture fast-flowing data from the turbine, the facility as a whole and the grid to further optimize wind energy production.

“Every wind current is of interest to engineers, and analyzing them could enable us to extract 1-2 percent more power from the machine,” he said. “We can even forecast energy production from the farm and use that to determine how to satisfy power use for the grid.”

Thanks to improved efficiencies and lower up-front costs, the survey suggests the total cost of land-based wind power should fall by 24 percent by 2030 and 35 percent by 2050. Offshore wind prices should fall even faster, with a 30 percent decline by 2030 and 41 percent by 2050.

With that kind of motivation, wind power platforms should continue to evolve quickly, and growth will remain a popular solution, Rao- Aourpally said.

“The trend of going bigger and larger is going to continue,” he said.

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