Thanks to taller towers, larger rotors and improved controller software, wind turbines’ capacity factors (CFs) have risen dramatically over the past few years. Indeed, technological advancements have made wind turbines more reliable and, in turn, more bankable.
Increases in CFs – the ratio of the actual energy produced in a given period, to the hypothetical maximum possible – contribute to more attractive project economics, according to Blake Nixon, president of Minnesota-based Geronimo Wind. More electricity produced, he adds, leads to increased revenue from power purchase agreements and helps to lower the cost of energy.
Additionally, technological upgrades – most notably, for turbines that operate in low wind speeds – have made several North American sites once deemed off limits to wind energy viable for development, according to Ryan Wiser, senior scientist at Lawrence Berkeley National Laboratory (LBNL) and co-author of the 2011 Wind Technologies Report.
According to the report, CFs have gradually increased, from about 25% in 1999 to nearly 40% for projects installed in 2011 – thanks, in part, to larger rotors, taller hub heights and software upgrades in controller packages.
Notably, the report finds that CFs stagnated between 2009 and 2010, primarily due to a build-out of wind projects in weaker-resource areas. However, looking at only the CF number obfuscates the very real advances made in rotors, tower and controller software, Wiser says.
Larger rotors. According to Wiser, a key reason for increased CFs involves larger rotors. In fact, LBNL research shows that the average rotor diameter has increased by 86% since 1999. The average rotor diameter ballooned to 89 meters in 2011, up from 84.3 meters in 2010 and 81.6 meters in 2009.
Manufacturers are using this development to their benefit. For example, over the past decade, the original GE 1.5 MW turbine with a 70.5-meter rotor has gone through a number of iterations to the 1.6-100, the most current iteration.
Compared to the 1.5 machine, the 1.6-100 has 100% more swept area, 50% more annual energy production (AEP), and a 38% higher capacity factor in the low-wind-speed environment for which it was designed, according to Keith Longtin, wind product line leader at GE Energy, who adds that there are sites with GE’s 1.6-100s running at over 55% capacity factor.
“With such high performance,” he explains, “the 1.6-100 has enabled otherwise uneconomic projects in low-wind-speed sites to succeed.”
Meanwhile, Dan Broderick, U.S. resident engineering manager at Gamesa, explains that the Langhorne, Pa.-based manufacturer will soon roll out a 114-meter rotor, which represents a considerable upgrade over the 80-meter rotor that Gamesa has used since entering the North American market in 2005. According to Broderick, the larger rotor helps produce more electricity from the same wind resource.
“If you were to take down a G80 in Class III wind site, for example, and replace that turbine with a G114, you would achieve a 59 percent power increase,” Broderick says. “Therefore, if that G80 had a 30 percent capacity factor, it would increase 59 percent with the increased rotor, to a CF of 48 percent.”
Tall towers. There has recently been an increased emphasis on capturing more energy at higher hub heights, and demand for towers exceeding 100 meters is continually growing, says Dan Shreve, managing director at MAKE Consulting.
At Gamesa, for example, tower heights have increased in lockstep with industry trends, Broderick says. With the launch of its new 4.5 MW turbine platform, Gamesa is planning to roll out a series of towers that are significantly taller than 78 meters, which is roughly the height that is considered the industry standard.
“As we move forward, the turbines reach higher and capture more energy at a given site and, thereby, increase the capacity factor,” Broderick says, adding that Gamesa has plans to introduce 90-, 100-, 120- and, eventually, 140-meter towers.
In the quest for taller towers, some manufacturers, such as Acciona, are deploying hybrid towers made of concrete and steel.
Improved controller software. Advanced control capabilities are being developed to tie these technological advancements together, most notably in the areas of load reduction via independent pitch control and turbine lifetime monitoring, Shreve notes, adding that new control strategies aim to maximize the efficiency of new and existing wind farms.
Technological advancements have made wind turbines more reliable and, in turn, more bankable.
Gamesa is also working on blade pitch technology. “In the G114, we are implementing independent blade pitch control, and we have implemented similar technology in our 4.5 MW-G128 turbine,” Broderick says. “Independent blade pitch allows each blade to pitch independently from the other two blades during turbine operation, as opposed to all three blades pitching in unison. This technology improves turbine life, reduces loads, helps to improve energy capture and, thereby, increases the capacity factor.”
GE’s Longtin says that blade pitch technology deserves credit for boosting CF. He credits the company’s research-and-development team for developing patented controls that optimize wind turbine performance, energy capture and loads.
“Utilizing sensors, the control system measures and calculates the effects of the wind throughout the rotor rotation, and responds by pitching each blade individually and adjusting generator torque and speed to minimize loads while increasing annual energy production.”
Other improvements in turbine technology are also playing a significant role in boosting CFs, Shreve notes. For instance, new direct-drive concepts offer significantly improved torque-to-weight ratios and do not rely on high-cost rare-earth materials, such as neodymium, which is used in the production of permanent-magnet direct-drive generators.
Despite the advances, increased turbine size could also lead to limitations. In fact, spacial impediments could eventually slow down technological development.
Broderick notes that the sheer size of turbine components will eventually become an issue. In Gamesa’s case, he says, future turbines atop 140-meter towers will encroach on Federal Aviation Administration (FAA) restricted airspace. For example, FAA rules prohibit the tip of a turbine blade from reaching 500 feet above ground. Although he failed to give specifics, Broderick says Gamesa is working with the FAA on the issue.
In addition, logistical limitations are coming into play for turbine makers. However, many suppliers are developing ways to address these issues. For example, Gamesa has already made its two-piece blade technology commercially available on its 4.5 MW platform and plans a wider rollout in the future. w
Industry At Large: Turbine Technology
Turbine Advances Fuel Rise In Capacity Factors
By Mark Del Franco
From taller towers to more intuitive controller software, several technological breakthroughs have boosted wind turbine capacity factors.
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