Ice detection on rotor blades
The winter months are the windiest in the whole year. All the more annoying when the wind turbines have to be shut down due to ice accretion. Operators are interested in running their wind turbines as long as possible, to produce as much energy as possible, while the regulatory requirements often demand for an early shutdown when there is risk (of ice shedding). Precise ice detection on rotor blades like „Rotor Ice Control“ can minimize this problem.
Shutdowns can be avoided completely if it can be ensured that there are no ice accumulations. In passive anti-icing measures the surface is changed, usually through the application of an additional layer. To ensure lightning protection, only metal-free materials can be used, which however does not have the desired material fatigue properties. Therefore, ice could still adhere to the blade as many of the coatings erode quickly and the Anti-Ice Effect is lost.
Active anti-icing measures are often implemented by a preventive heating of the rotor blades.
There are approaches only to heat the leading edge of the rotor blades, e.g. used by Nordex, to reduce energy consumption significantly and increase economic viability. To detect the right start point and a successful end of the heating cycle, the combination with an ice detection system is necessary.
Rotor blade heaters can also be used for de-icing iced blades. If the blades are heated from the inside, as is the case for example at Enercon, a simple heater fan can be used. After initial problems with choosing the right heater fan, this technology is now well understood. Only the high energy consumption prevents efficient operation of these systems.
Alternatively, if a little icing is allowed on the rotor blade, it makes sense to activate the rotor blade heaters only when a shutdown is imminent. Minor ice accumulations detach from the blades by the centrifugal forces when the rotor rotates. However, if there are massive icing conditions, it is often more usefulto delay the de-icing in order not to waste energy unnecessarily. For an efficient control of the rotor blade heating, it is essential to be able to detect the current ice mass on the rotor blade and the increase in the ice mass precisely.
Ice detection systems
For ice detection, there are three different approaches:
- Nacelle-based ice detection systems
- Ice detection systems based on performance curves
- Ice detection on rotor blades
In order to ensure the safety of the turbine and its environment, a maximum acceptable mass of ice per rotor blade is specified by the authorities. If this amount of ice is reached, the system must be switched off. Often a specification of thickness is used instead of weight that is considered as harmless.
Nacelle-based ice detection systems
Nacelle-based ice detection systems determine at the nacelle whether the conditions for a blade icing are present. There are several options available:
Comparison of two anemometers, ultrasound measurement or capture meteorological data such as humidity. All approaches have in common that flow conditions around the nacelle are used and they differ from the conditions around the rotor blade as rotor blades extend to higher heights and rotate. Possibly, the waste heat of the nacelle could also affect the measurement results. To safely predict ice accumulation, these systems need to be sufficiently sensitive. Therefore, these systems are relatively cost-effective, however, they are known to stop too early and to restart too late. For wind turbines with few not severe icing cases per year, these systems are still the first choice.
Ice detection systems based on performance curves
Ice detection systems based on the performance curve of the wind turbine tend to be even more economical because they generally work without any additional hardware. It is possible to detect ice accumulation by comparing the expected performance with the actual performance of the wind turbine by using sophisticated algorithms. The major disadvantage is that the comparison is only possible while the wind turbine is rotating and the power difference can be determined. In most cases, authorities are requiring a second ice detection system, which closes this gap.
Ice detection on rotor blades
Stricter requirements from the authorities and a rising cost pressure has led to increasing technological developments in recent years. The trend is clearly towards sensor on and in the rotor blade. The most common method is the vibration measurement on the rotor blade. An iced and thus heavier rotor blade vibrates slower than an ice-free rotor blade. The vibrations are detected with different sensors:
- Electrical accelerometers
- Fiber optic accelerometers
fos4X offers the solution Rotor Ice Control, a system for ice detection on rotor blades based on fiber optic vibration sensors. When selecting sensors for installation in the rotor blade, it is necessary to pay particular attention to lightning protection. Here are passive, fiber optic sensors in advantage to electrical sensors. Further important criteria in the selection of suitable ice detection systems are:
- Calibration: How fast the system can be calibrated to achieve the specified accuracy? What information is necessary for the wind turbine operator, the manufacturer and of the system itself? How often is calibrated and must be recalibrated? Is the calibration made manually or automatically?
- Installation effort: Is it possible to retrofit the system or are cost-intensive lifting platforms or specially trained personnel necessary? How much downtime is necessary for the installation?
- Data connection: Which data connection is available? Is the connection needed for the active operation or just for maintenance?
- Certification: Is the system certified?
- Automatic restart: Which conditions are needed to allow automatic restart?
- Operating conditions: In which operating conditions the system works? When not?
- Time: How fast can the system reliably detect ice after it is started?
- Interfaces: Which software interfaces are provided by the system?
- Maintenance: Which components need to be serviced in which time intervals? Are there any components included with typically low life times? Are the sensors easily replaceable in case of a defect?
- Maintenance Agreement: Can the system be included in the maintenance contract?
- Isolated solution or intensive functional extensions available: Is the system an isolated solution for ice challenges or are there additional functions available, e.g. condition monitoring or possibilities for analysis of the operation of turbines in wind park configurations?
Ice detection on rotor blades is important for increased functionality in order to minimize downtime of wind turbines during the winter. The balancing act between the required safety and the minimizing possible loss of income is difficult, but especially in regions with high risk of icing, new approaches can provide this balancing act.
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