WIND ENERGY IN COLD CLIMATES
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Icing of the blades causes production losses from wind turbines. This is the case even with slight icing as the aerodynamic properties of the blade are sensitive to minor changes in the blade profile. Heavy icing can result in a total stop of the turbine. The duration of ice on the blades can be considerably longer than the time of icing conditions. Downtimes of several weeks with a single icing incident have been reported in Southern Germany.
The structural loads of a turbine may be significantly increased due to icing of the blades. Icing usually sheds from the blades unevenly and this results in increased loading on the turbine .
Ice thrown off the blade may also pose a safety risk even in areas where icing is infrequent, specifically when the turbines are situated close to a public road, or by skiing resorts, for example.
Ice shedding off the tower or the nacelle can also pose a similar though a more limited risk especially for the service personnel and the public. There are also cases when icing of the yaw gear has resulted in the damage of yawing motor.
Icing also affects the wind sensors, both in resource estimation and controlling the turbine. A wind turbine with an iced control anemometer may not start even in strong winds, which results in production losses. Increased loads are caused if a pitch control system is based on information of an iced anemometer. A wind vane stuck by icing means operation in misaligned yaw or a production stop due to the misalignment.
Low temperatures effect materials, in the case of wind turbines primarily the plastics, steel and lubricants, as low temperatures cause brittle fracture of materials. Insufficient lubrication of bearings and the gearbox is the result of oils getting too stiff. Malfunctioning of hydraulics and electronics have been reported. When changing the hydraulic oil to a stronger arctic version, the tubes, valves and equipment associated may also have to be changed or modified. However, even when cold rated lubricants are used, adverse impacts on unit performance have been identified. One clear case is low speed startup of turbines in cold environments, specifically for turbines that freewheel up to synchronous speed. In such cases, freewheel start-up may be retarded due to the high viscosity of the gearbox oil.
When going to very low temperatures, the need for cold weather or weather resistant materials extends for both the steel and plastics used in the system fabrication but also wires and other turbine parts not considered in most system impact assessments. Wires who's insulation becomes brittle and fractures, leading to shorting, has caused many problems in turbines that have been designed for cold climates. Every piece of equipment, even the most trivial, must be assessed for flexibility and usability at extreme temperatures.
Also service and monitoring under difficult conditions has to be taken into account. This may result in increased O&M costs or extended downtime of the turbine.
Another factor that has been identified is the increased system loading due to the high density of cold air masses. It is not uncommon to have (stall controlled) turbines produce over 20% rated capacity due to the air density. Several cases of generator overheating have been reported in Canada and Finland caused by overproduction due to high air density . This leads to production losses and probably has lead to generator failures . Impacts on the gearbox and breaking systems will likewise need to be considered as the higher loading conditions will impact unit life. However, due to the complexity of these systems, specific tests and the impact of cold temperatures on these subsystems have not generally been carried out.
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Mail: Tomas Wallenius