This executive summary comes from a cold climate report issued by a Finnish wind authority VTT.
The industry now has about 10 years of experience in cold climates. Modern turbines are often adapted to low temperatures and the recorded accumulated down time has been relatively low.
Wind turbines in cold climates may experience significant periods in icing events or low temperatures outside the operational limits of standard wind turbines. Apart from lower energy production, which directly influences a wind farm’s economy, there are legal issues, such as ice throw, increased noise, fatigue loading and O&M aspects that must be considered. Wind turbines operating in cold climates are located in Asia, Europe, Oceania, and North America.
Low temperatures and icing set additional requirements for wind resource measurements. Special measurement equipment should be designed for use in low temperature and icing climate. Anemometers and wind vanes in particular should be selected with such conditions in mind. A small amount of ice may significantly reduce the measured wind speed and large ice accretions may stop the anemometer. Just a small amount of rime ice on the cups, for example, and shaft of an anemometer may lead to an underestimation of the wind speed by about 30% at a speed of 10 m/s.
Temperature recordings allow estimating extreme temperatures and durations for low temperatures. Icing measurements are, however, rare and not included in standard meteorological measurements. It is possible to estimate in-cloud icing from visibility observations, which include cloud base height measurements. These measurements are usually made only at airports. Therefore the coverage and accuracy of this method are only adequate. If icing is expected to deteriorate power performance, it is advisable to add icing measurements to resource estimations. Suitable ice detectors can provide direct icing measurements. An increased risk for icing can be estimated with a dew point detector. Icing may also be evaluated using two anemometers side-by-side, with one properly heated and the other unheated.
Another method may include three anemometers. The third unit is heated only if there is a significant difference between the outputs of the two. Meteorologists have developed models for estimating different types of atmospheric icing and their effects. Models developed by the aviation industry calculate weight and shape of ice accumulations on the leading edge of a wing. Those computer codes have been modified for wind turbines. Due to complexity of icing and aerodynamics, the development of more accurate models has been slow. Maps to describe annual icing time have been developed but standardised methods to calculate the local icing time based on meteorological measurements are still lacking.
Technical solutions are available for wind turbines operating at low temperature and in icing climates. Low-temperature materials and oils should be used when temperatures outside the standard limits are probable. Many wind-turbine manufacturers have low-temperature versions of their standard lines. Cold-weather turbines are also often equipped with gearbox and pitch accumulator heaters. For sites where icing conditions prevail, ice detectors, hydrophobic blade coatings, and anti-icing or deicing systems are slowly becoming available.
The severity of icing varies depending on local conditions. In particular, the site altitude compared to the average height of the terrain has a great effect on the severity of icing. It can retard energy production at elevated sites in Scandinavia, Alpine regions of Europe as well as at elevated sites in North America in Canada and Alaska. During wind-power production in Norway, for example, icing has not had a negative affect, even though turbines are locate up to 200 meter above sea level and higher in Finland. Icing and snow can considerably extend the duration of maintenance and repair in winter. Snow may even prevent access to a site. Systems that keep blades free of ice are probably the only solution to profitable wind-power production in areas with severe icing. Such sites can be found from Scandinavia to North America.
Read the full report at: http://www.vtt.fi/inf/pdf/workingpapers/2010/W152.pdf
VTT Technical Research Centre of Finland
Filed Under: Policy
