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Harnessing wind energy in icy climes

WORLDWIDE: Wind projects located in areas with low temperatures and high chances of ice and snow would not seem to be the ideal choice. Expensive to construct and maintain, hard to access, with higher safety risks to employees and returns that are complex to predict, such projects are a highly specialist area.

Arctic straits… Cold climates have huge potential, but realising it is challenging (pic:VTT)
Arctic straits… Cold climates have huge potential, but realising it is challenging (pic:VTT)

But cold-climate projects comprise a staggering 25% of the world wind market, according to consultancy BTM in its latest World Market Update. In the first calculation of the size of the cold-climate market ever undertaken, BTM expects an installed capacity of 69GW by the end of the year, adding an additional 50GW by 2017.

cold climate capacity chart

Cold-climate sites do have some advantages. They have high wind resource, especially during winter. Human population is typically sparse, giving the potential to build large wind farms. But the technical challenges are huge and, although a lot of research and development is being undertaken, most experts believe there is still some way to go before these challenges can be overcome and the potential market fully realised.

Special equipment is required for wind resource measurements as small amounts of ice, even on support structures, may significantly disturb measurements.

Larger accumulations of ice may stop anenometers and wind vanes working altogether. Dedicated ice detectors can be used for measuring whether there is ice, its intensity, type and weight. Meteorological masts typically need to be very slender, however in heavy icy conditions a mast with a mass of around 1,000kg can collect 5,000kg of ice. The International Energy Agency (IEA) wind special task force on cold climate projects recommends lattice towers for mounting measuring instruments, although these increase costs significantly. If the cups of an anemometer gain even a small amount of ice, wind speeds can be underestimated by about 30%. Heated anenometers are available and can be used to detect ice in combination with an unheated anemometer — when there is ice they will show different readings.

Other significant challenges include access to sites - for both construction and operation and maintenance — and safety for both employees and the public. A particular problem is ice detaching from blades as it melts, known as "ice throw". A study of the Enercon E-40 600KW machine at Gutsch in the Swiss Alps found that although around 50% of the pieces were found within 20 metres or less of the turbine, some were found up to 92 metres away. The maximum weight of a piece of ice thrown from blades was 1.8kg.

Maintenance challenges

Ongoing maintenance of cold-climate wind projects is vital. Up until now, relatively little maintenance has been undertaken on blades, but as the sector matures, significant problems with blades are being discovered. "Blades will probably be the biggest source of issues in the years to come," says Ville Karkkolainen, managing director of specialist maintenance company Blade Defence. "We see serious issues all round, especially when you're talking about these megawatt-class, 40-metre plus blades when they are five to seven years of age."

All brands of blade show problems, the only difference is the level of problems, Karkkoleinen reports. Cold climates accelerate the degradation of the blade structure. "Flying ice particles have very similar properties to sand, so you have high erosion on both the leading edge and trailing edge," he explains. Then the blade surface cracks, exposing the laminate. Once water enters the structure, it expands as it freezes and cracks develop and can grow rapidly. "It pretty much underlines the need for constant maintenance," he says. "No-one has experience in running 40 metre plus blades for significant amounts of time. Lab analysis and simulation don't see the full picture unfortunately."

A blade covered with 5mm of ice can reduce the power curve by as much as 80%, Karkkoleinen says.

He gives the example of a 1.5MW turbine running on full power producing 280kW. "There was no torque because the lift that the blades should create was significantly disrupted."

Blade lifetimes are being notably reduced. "If you ask by how much, no-one knows. A manufacturer would probably say that it was insignificant and I would say something else. No-one has the right answer, except that it's significant," Karkkoleinen says.

There are a variety of different approaches to removing ice from blades. IEA Wind recommends heating the outer surfaces of the blade where there is heavy ice. Antifreeze coatings are another option, but although performance has been promising in laboratory tests, none has proved to be functional or robust enough in field conditions, according to the task force report.

Despite the issues associated with cold climates, some developers have gone one further and built wind farms in mountainous areas, which pose even greater challenges. Developer ImWind was one of the first with its Tauernwindpark in Austria, which came online in 2002. However, managing director Johannes Trauttmansdorff is frank about the potential for alpine projects.

"We have worked on the development of quite a few other projects in the alpine region and there is a definite reason why we haven't built them. Our experience made us step back from 90% of other, similar projects," Trauttmansdorff says.

The Tauernwindpark was only economically feasible because it benefitted from subsidies both from the European Union and the Austrian government, Trauttmansdorff explains. "With today's investment costs, the project would not be feasible."

Mountain issues

Despite five years of extensive wind-resource studies involving world experts on the subject, ultrasound, Sodar (sonic detection and ranging) and Lidar (light detection and ranging) wind measurement technology at 11 different altitudes on three different areas of the wind farm, the project produces the amount of energy from 13 turbines that was expected for 11. "There was a lot less (energy produced) than this extremely cautious and realistic number. Conditions in mountainous terrain are almost unpredictable," he says.

Projects other developers have attempted in the mountains are "crazy", Trauttmansdorff says. People look at sites that are local to them for political reasons, not the ones that would be best suited for a wind farm. "They are site-driven, not brain-driven," he says.

Manufacturers are busy with research and development (R&D) to solve the problems associated with cold climates. There is also a large amount of work under way at institutions around the world that have set up working groups to collaborate on the issue. A hub for such activity is Finland, where all wind projects come under the cold climate definition. The IEA Wind special task force is co-ordinated by the Finnish VTT Technical Research Centre. Swedish R&D body Elforsk is working on the issue, as is the Technocentre Eolien in Quebec, Canada.

Tomas Wallenius, research scientist at VTT, says that in the past four years there has been a leap forward in the development of antiand de-icing technology, and many manufacturers now offer these options. Experience in the field is providing a lot of information on how technology is performing in situ. "The industry is learning a lot, and we are winning those challenges from the technical point of view," says Wallenius.

Manufacturer Repower, which has around 400MW of turbines installed in cold-climate conditions, has been focussing on how to operate turbines with ice on the blade in a safe way. Helmut Herold, CEO and managing director of Repower's Canadian subsidiary, reports that there has been an improvement in validating performance since last winter when a large number of turbines were installed. It is also researching how to remove ice safely, including heating the blades from the inside.

But Herold warns that there is a long way to go. "There are still challenges, and I'm not sure we can cover all the potential. The cold areas are usually far in the north, so the grid challenges come with that. Transport issues might limit a few areas.

"We are still in the beginning of testing these solutions on a broader basis. There are challenges with all the technologies and there might be an adjustment coming up based on real experience in big projects," Herold says.

Official guidance

There are very few guidelines, recommendations, certification or standards that cover wind energy in cold climates. International certification body Germanischer Lloyd has the only official technical note that addresses component, type and project certification for turbine survival below -20C and operation below -15C.

The International Electrotechnical Commission (IEC) is working on the next edition of its IEC 61400-1 standard, which sets out minimum requirements for turbines internationally. Edition four, due for publication in 2015/16, will address low temperatures and ice conditions for the first time. Although it is still a work in progress, the standard will cover structural design, site assessment issues and the effects of cold climate on the control, protection, mechanical and electrical systems of the turbine.

The standard will introduce a class for cold-climate turbines, similar to those used for wind speeds. Currently, the IEC committee is proposing three classes, which will be determined by duration of ice on the turbine rotor and the minimum ambient temperature, according to Ville Lehtomaki, research scientist at VTT Technical Research Centre and co-ordinator of the IEC cold climate sub committee.

Despite all the work being undertaken on cold climate wind, it could be that there is a need for even greater collaboration if projects in cold climates are to realise their potential. If the industry can overcome competitiveness and validate the experiences of all the operators, it may come up with a solution more quickly. "The industry as a whole needs to find a solution to unlock the potential of the north," says Herold.

COLD CLIMATE WHAT DOES IT MEAN?

  • - Wind projects are considered to be in cold climates when they are located in areas where there is a significant amount of time or frequency of either temperatures lower than the operational limits of standard wind turbines, or of ice and snow, or both.
  • - Specially adapted turbines are needed where minimum temperatures below -20 Celsius have been recorded on an average of more than nine days a year, according to certification provider GL in Technical note 067, revision 4.
  • - There is no typical site - some may have low temperatures but no atmospheric icing, while another might have a mild annual average temperature but periods of heavy icing.

cold climate turbines

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