In a growing trend, low and medium wind-speed sites across the globe are being targeted for development. In the US, as well as in southern Europe, China and India, it is particularly prevalent. Low wind speeds are common in many parts of the world. In the US, for example, only around 10% of the country has high winds, while some 50% has low wind.
For wind development, a low wind speed, or Class III, is generally defined as 6.5-7 metres per second (m/s) on average and a medium wind speed, or Class II, is 7-8.5m/s (see table, below).
Many of the windiest or Class I sites with crucial transmission access have already been developed, notably in the US and coastal Europe. Power-purchase agreements for sites near high-load centres — such as cities — in more moderate wind resource areas can be easier to clinch and more lucrative per kilowatt hour due to the laws of supply and demand.
Conversely, electricity prices are still flagging because of new reserves of natural gas from shale rock and faltering demand due to the recession — so the pressure is on to maximise the amount of energy that can be gained from a site.
Wind technology has advanced to the point that turbines can now offer a 20-30% production gain compared with a year or two ago, making lower-wind sites in Class II and III areas more economically feasible. Turbines such as GE’s 1.6-100 and Vestas’ V100 1.8MW, the leading turbines of their type, have been designed specifically to achieve this. A typical low-wind machine might have more efficient and longer rotors — perhaps containing carbon fibre — a modestly sized generator and a higher hub height. It could also have a lower noise profile, making it better suited for more populated areas.
Pick and mix
In today’s buyer’s market, wind-turbine manufacturers must also offer exactly what the customers need. Indeed, an array of "suites" featuring different configurations of turbine heights and rotor diameters are on offer in order to make the most of low-to-medium wind sites.
Policy has had an impact. In the US, where the low-to-medium trend is especially pronounced, renewable-energy regulations in lower-wind states such as Illinois, Indiana, Ohio and Missouri are helping increase the rush. Federal policies have also been influential. The generous investment tax credit, created under the 2009 economic-stimulus programme, allows for the reimbursement of about 30% of the project construction costs.
For Spanish developer Gamesa, low and medium wind sites are a cornerstone of its business. According to Juan Diego Diaz, the company’s global marketing director, more than 90% of its global portfolio of unfilled orders are for such sites. Last year, a hefty 85-87% of total company sales were for low and medium sites, while in India 100% of the company’s sales were for low wind-speed sites. "We saw very clearly that the future is in low- and medium-wind classes," says Diaz, speaking of the company’s recent offerings of suitable turbines such as the G97 and G10X.
In the US, too, the writing is on the wall. "Development has really shifted to areas that used to be ignored, such as Illinois," says Andy Cukurs, wind-power company Suzlon’s CEO in North America. Suzlon’s S9X range, launched in April, includes the S95 and S97, designed to work in medium and low wind conditions respectively. In June, the company won its first North American order for S9X turbines —the S95 with a 90-metre hub height for a 31.5MW project in Nova Scotia, Canada, being developed by Sprott Power Corp, a Canadian renewables developer.
Other recent project announcements that confirm the moderate-wind trend include GE supplying 94 of its 1.6-100 wind turbines for the Osage project in Oklahoma, US, which will provide power to Associated Electric Cooperative. A prototype of the 1.6-100 is operating in Tehachapi, California. The turbine offers a capacity factor — output over a given period of time — of more than 45% at 7m/s. This is similar to that of the Nordex N117/2400, also designed for lower wind speed sites, and compares favourably with the 30% typically offered by many previous-generation turbines.
In France, the Germinon project using German wind-turbine builder Nordex’s medium-wind precursor to the N117, the N100, was inaugurated this spring. The site has a mean wind speed of 7.4m/s.
And in July, Spanish renewable-energy firm EDP Renovaveis (EDPR) commissioned the 99MW Timber Road project in north-western Ohio, US, with the Vestas V100 1.8 with 95-metre towers, the state’s first utility-scale wind project.
"This would not have been viable previously," says Gabriel Alonso, chief executive of EDPR North America. "Four to five years ago, we were looking only for the windiest spots." Alonso says the energy price in a power purchase agreement can be as much as 50-100% higher in Ohio compared with, for example, Oklahoma, where EDPR is developing a "very windy" site.
According to Matt DaPrato, a wind analyst at IHS Emerging Energy Research, the technological advances that have enabled the trend have been part of a steady stream of improvements rather than a breakthrough in technology, with blades increasing incrementally in size. Jesse Broehl, an analyst at Make Consulting, notes that a 5-metre increase in rotor diameter for a 1.5MW machine with a 77-metre rotor diameter, the annual energy produced (AEP) at a medium wind-speed site will increase 5-7%, and by 8-9% at a low wind-speed site.
But for all the benefits of Class III sites, a number of challenges remain. The main one is the obvious lack of wind resource, notes Dan Bernadett, chief engineer at renewable-energy consultancy AWS Truepower. Indeed something like 20-30% more wind resource can be harvested at a Class I wind site, says Gamesa’s Diaz. Hence the constant stretching of blades — a limit has not yet been reached technologically, says Bernadett — and also of tower height.
Extreme wind gusts can challenge turbines designed for lower wind-speed areas, especially in an area with hurricane potential such as the south-eastern US. "Big rotors also capture more wind when you don’t want them to," says Bernadett, whose company produces an extreme wind map.
Other challenges include the transportation to the site of larger towers — with wider and/or heavier bases — and longer, heavier rotors. If a blade is more than 40 or 45 metres long, a truck can only carry one rather than two as is usually the case, says Broehl. That will increase a developer’s transport costs.
Because of the size of low-wind towers, possible solutions are being considered such as concrete bases cast in place, a steel-concrete hybrid, a three-legged "tripod" base and a six-sided bolt-together tower. Another option is the "Hi-Jack" technology developed by GE subsidiary Wind Tower Systems allowing the nacelle and hub to be installed on the towers without the need for large cranes. Construction of a low-wind project may also require, for example, larger cranes.
The larger blades can also be a problem. They can be so cumbersome when transported by road that Nordex is considering moving blades in two parts.
Another challenge is the height limitation for aviation. In the US, the general restriction established by the Federal Aviation Administration is 500 feet (152.4 metres) for tip height — lower than in Germany. A variance on that limit can be obtained, although that is less likely nearer population centres and airports. Indeed Nordex’s N117, with a hub height of 91 metres, is advertised as being especially suitable for the US market as well as "below the critical threshold of 150 metres". Suzlon subsidiary Repower uses a turbine with a 143-metre hub height on a revamped version of its 3.2M11. This pushes the height limit even in Germany.
Local opposition can be stronger for low wind-speed sites if they are closer to built-up areas, notes Alonso of EDPR. And wind-resource prediction at proposed sites for taller turbines can be more uncertain, says Clint Johnson, a senior vice-president at renewable-energy consultancy GL Garrad Hassan North America. That is because more extrapolation of data is needed to measure wind generation from a higher hub height or larger rotor diameter. Remote sensing measurements, such as sonic detection and ranging or light detection and ranging, have the potential to mitigate some of the uncertainty as the industry adapts to incorporate remote-sensing data into assessment methodologies, he says.
In contrast, servicing of the turbines can actually be easier in lower-wind sites, says Cukurs. "It’s easier to maintain than on a mountain when the wind is screaming all the time." This in turn makes it easier to have higher reliability, he adds.
In terms of gaining backing for low-wind projects, the financial community has adapted well. It is possible to get project backing, says Jim Tynion, a project finance expert at US legal firm Foley & Lardner. But during due diligence, he says, the data must be "drilled into deeper", to ensure the project is viable.
In fact some "micro-siting" has even been used over the past two to three years, whereby different configurations of turbines are used for different parts of the site. Turbines must also be priced so that projects make financial sense even if gross revenue is lower. Manufacturers are offering low- to medium-speed turbines at attractive prices but not cut rates, Tunion says.
Will the pendulum swing back to Class I wind sites if power prices increase? Mike Revak, vice-president of sales in North America for Siemens Wind Power, thinks so. "The trend is only increasing, although it won’t be overwhelming, " he says, noting that Class I sites are still being developed. That is especially so in north Africa and Central and South America and also in parts of North America, Europe, China and India.
MEETING A NEED — NEWEST MODELS PUT LONG BLADES ON LOWER-RATED MACHINES, WRITES EIZE DE VRIES
As the wind industry seeks to utilise sites with low to medium wind-speed (IEC WC II and III) sites, an industry trend has been to fit large rotors on turbines with modest power ratings.
Vestas started this process in 2009 by introducing a 1.8MW turbine with a 100-metre rotor diameter, which was followed in 2010 by a GE 1.6MW model featuring a similar rotor size.
Earlier this year Nordex launched a 2.4MW turbine with a 117-metre rotor diameter. Similarly, Siemens introduced a 2.3MW direct-drive machine with a 113-metre rotor diameter. Finland’s WinWind presented a new 3MW geared medium-speed turbine with a 120-metre rotor. Germany’s Fuhrländer has a 3MW model, also with a 120-metre rotor diameter, while Gamesa has added a G136-4.5MW sister model version of the 2009 G128-4.5MW with a 136-metre diameter blade. This is currently the wind market’s largest rotor diameter.
Enercon’s new 3MW E-101 turbine does not fit into the trend, but efficient rotor blades combined with up to 135-metre hub height can also suit Class III performance demands.
Wind expert and Enercon Belgium representative Bernhard Fink in principle supports the large rotor trend but warns of potential added risks with regard to safety.
"During heavy storms in 2010 and this year over Germany, Belgium and the Netherlands we recorded very high wind speeds on our turbines at 100-metre hub height," he says. "These storm conditions typically occur several times a year and the measured wind speeds exceed maximum IEC WC III values. This for me raises a question whether turbines designed for IEC WC III can be safely exposed to occasional WC II conditions."
Lower power but for longer
A lower specific power ratio — the relation between power rating and rotor-swept area — on these newer models means that they can produce more full-load compared to older models of similar megawatts. This in turn produces a higher capacity factor — the measure by which the industry rates a turbine’s output.
According to industry insiders there is another, less well-known motive behind the trend for large rotor blades. It is also designed to enhance competitiveness by offering improved lifecycle performance. This strategy, popular with established European and US suppliers, is said to be partly fuelled by fears of increased pressure from lower-cost Asian suppliers.
Technically, lowering turbine- specific power ratings offers several additional benefits such as improved utilisation of the electricity distribution system. It also contributes to higher grid network base-load levels, providing substantial savings on required (fossil-based) reserve power capacity.
In practical terms, rotor diameters of 2MW turbines have increased from about 70-80 metres a decade ago to around 90-100 metres today. Power values for wind turbines at the turn of the century were typically 0.40-0.50 kilowatt per square metre, which is significantly more than the 0.19 kW and 0.31kW given for a number of the newly introduced or announced Class III turbines.
The latest models
AMSC Windtec 3MW
US-based AMSC Windtec issues technology licenses and turbine co-development packages to third-party clients. The first wind farm comprising 3MW fast-speed geared turbines based on AMSC technology, and a turbine co-development with Sinovel, was built in 2009. Further development of the 3MW technology also aims to fit much larger rotors. Last year AMSC Windtec took a 25% share in advanced rotor-blade developer and manufacturer Blade Dynamics.
Power rating: 3MW
Rotor diameter: 140 metres
Specific power rating: 0.19kWh/m2
Drive system: Fast speed geared
Status: In development
Spain’s Gamesa early 2009 installed the prototype of a new 4.5MW onshore turbine. In March this year a second G128-4.5MW prototype was completed, while series production was planned to start by the end of the year. The G136-4.5MW is a Class III sister model version of the G128-4.5MW.
Power rating: 4.5MW
Rotor diameter: 136 metres
Specific power rating: 0.31 kWh/m2
Drive system: Two-stage medium speed geared with permanent magnet generator
Special feature: Segmented composite rotor blades
Series start: Unknown
The 1.6-100 builds on experience with more than 16,500 GE 1.5MW turbines and can be viewed as evolutionary product development. The initial 1.5MW Tacke TW 1.5 prototype was fitted with a 65-metre diameter rotor, and this over time evolved in several increments from 70.5 metres to 77 metres then to 82.5 metres with the power rating unchanged.
Power rating: 1.6MW
Rotor diameter: 100 metres
Specific power rating: 0.20kWh/m2
Drive system: Fast speed geared with doubly-fed induction generator
Series start: 2012
At Class III sites, N117/2400 turbines are claimed to be capable of achieving some 3,500 full-load hours annually, corresponding to a 40% capacity factor. The model will be fitted with new-generation slender blades, developed in-house and aimed at minimising loads despite a substantial rotor-diameter increase.
Power rating: 2.4MW
Rotor diameter: 117 metres
Specific power rating: 0.22kWh/m2
Drive system: Fast speed geared with doubly-fed induction generator
Prototype: Q4 2011
Series production start: In the US, July 2012
In late 2009 Siemens installed a 3MW prototype based on a new lightweight design concept. The 2.3MW SWT-2.3-113 is a first sister product of the SWT-3.0-101, characterised by a reduced power rating and increased rotor diameter for low- and medium-speed wind sites. The generator is kept unchanged. The turbine is fitted with a new-generation slender Quantum Blade that does not contain carbon fibres.
Power rating: 2.3MW
Rotor diameter: 113 metres
Specific power rating: 0.23kWh/m2
Drive system: Liquid-cooled, Siemens-design direct drive with permanent-magnet generator
Series start: 2012
CHINA CHANGES COURSE — TRANSMISSION WOES MAKE LOW-WIND AREA MORE ATTRACTIVE, WRITES WU QI
China is shifting away from its strategy of developing projects in sparsely populated high-wind-speed areas with poor transmission links. According to the country’s latest Five Year Plan 2011-2015, China is to develop more projects in Class III low-wind locations closer to the densly populated central and southern areas.
Despite an installed capacity of more than 6GW, the concept of building projects on low-wind sites close to transmission links is fairly new for China. The first low-speed wind farm, a Longyuan-developed 198MW project near Chuzhou city in Anhui province in the east of the country, was only built at the end of 2010.
Since July 2009, China has categorised its mainland areas into four types of wind-power resource with four benchmark feed-in tariffs. These are CNY 0.51 ($0.08) CNY 0.54, CNY 0.58 and CNY0.61 per kilowatt hour. The first three categories all have rich wind resource, have been under development since 2003 and tend to be located in north-east and north-western areas such as Inner Mongolia, Xinjiang and Hebei.
In the fourth category the wind speed averages 6-8 metres per second while the annual wind utilisation ranges from under 2,000 hours a year. This compares favourably with high-speed areas. According to a study by power producer Guodian Corporation, turbines at its Inner Mongolian wind farms were only on for 2,000 hours a year due to grid issues.
Low wind-speed areas account for more than 60% of Chinese mainland areas and include south-east China’s Fujian province and Yunnan province in the south-west. Around 20GW is likely to be built here in the next five years, around a fifth of the total capacity planned over that period.
Additionally, in early August this year, the National Energy Bureau approved 28.8GW wind-power projects, the first phase of the 12th Five Year Plan and to be completed before the end of 2012. Out of this, 10.6GW is in low wind-speed areas.
Xie Changjun, general manager of Longyuan, says it costs only 5% more to construct wind farms in low wind-speed areas than in northern China. Xie says Longyuan plans to invest more developing wind farms in low wind speed areas in the populous Anhui, Liaoning and Shandong provinces in the next two to three years. In August 2010, Guodian subsidiary United Power signed an agreement with Tianchang city in Anhui province to develop wind farms with a total capacity of 150MW, which are expected to bring about 12% return on investment, according to officials at the company.
United Power has signed an exclusive agreement with Siyang county in Jiangsu province to set up 150MW wind farms in low wind-speed areas. Early this year, Guodian undertook a feasibility study to develop 49.5MW wind farms in low wind-speed areas in Qingjing county, Fujian province.
Turbine manufacturers looking to develop low-wind products include Goldwind, which is set to go into production with a version of its 1.5MW turbine that has an 87-metre blade. In April, Sinovel signed an agreement with Linfen in Shaxi province to build a factory for low-wind 1.5MW turbines.
In addition, blade manufacturers Zhongcai Technology, Zhongfu Lianzhong and Shidai Xincai have all developed blades for low-wind turbines. The majority of these blades are in the 82-metre diameter range, although Zhongfu Lianzhong is said to be working on a 50-metre blade. Last year, Siemens opened its first blade factory in China, to produce 49-metre blades for its 2.3MW and 3.6MW machines.