Safety concerns over flying ice shards have sparked improvements in ice detection and removal devices on turbine blades.
By Sam Phipps
 
Ice build-up on wind turbines is a serious problem that affects not just efficiency of power generation but also safety. The renewables industry has therefore devoted considerable resources to improving both detection and removal via various methods.

Falling shards of ice pose an obvious risk to people, buildings and vehicles if the turbines are close to habitations or roads. In the UK several instances of this kind were reported last winter, the ice sometimes landing on homes and in gardens in chunks of up to 2ft. Residents were wary of venturing out until the turbines were switched off.

Several European wind turbines are also sited close to ski slopes, making “ice throw” a recognised issue. Shards from an Enercon E-40 turbine, which was installed on Gütsch mountain in the Swiss Alps five years ago have been found almost 100 metres from the blades, weighing up to 1.8kg.

Prevention better than cure

Ice accretion can also modify the original aerodynamic profile of the blade, with potentially drastic effects on the structural loading of the entire rotor. Detection is therefore vital. However, the question of whether it is then necessary to remove the ice ¬ and to invest in the technology which enables ice removal¬ depends largely on geography.

In a moderate climate such as the UK, where cold snaps of a few days are more common than prolonged freezing, standard practice is to switch off the turbines until meteorological conditions improve.

This month, delegates have been meeting in Andermatt, Switzerland to share expertise on the issue. The 13th International Workshop on Atmospheric Icing of Structures has brought together many of the leading players.

Barry Cooper of Finnish company Labkotec says its LID-3210C ice detector and remover has been installed widely in Scandinavian and Russian wind farms over the last few years. “We don¹t generally get the long-term severe weather to make it worthwhile in the UK but E.ON still want to trial it in Scotland. It’s true that the ice that can come off these turbines is a hell of a size.”

The LID-3210C detects ice before it reaches 0.5mm thickness, and activates de-icing foils on the blades. As soon as the first ice has been detected and melted, the system switches to anti-icing, which eliminates the need for further de-icing, thus saving energy. It also detects when anti-icing is no longer necessary either, cutting out any unnecessary heating.
If the ice is exceptionally thick, the device will stop the turbine, which will only be reactivated once it has melted naturally.

Ice detection

Insensys, by contrast, a technology company based in Hampshire, has developed a device purely designed to detect ice, not to remove it. Again, the logic is that a temporary shutdown is preferable.

“Our Rotor Monitoring System has just been certified for safe detection of ice by Germanischer Lloyd,” says Julie Turner of Insensys, referring to Europe’s leading certification organisation for wind energy.

RMS is a load-measuring tool that uses fibre optic sensors to detect the increase and decrease of mass, or ice, on a wind turbine blade. Using information sent by fibre optics patches, customised algorhythms send data to the turbine control system.

The system allows remote monitoring of turbines so that operators can get early detection not just of ice but also blade damage, rotor imbalance and lightning strike. The equipment can be retrofitted to existing wind turbines.

Insensys says the early restarting that is enabled by RMS after shutdown can increase generating revenue by up to 10,000 Euro a year per average turbine.

Likewise, German company Infralytic also produces real-time information on icing by means of fibre optics. It too says it can avoid unnecessary standstills by replacing the estimates that used to be the norm when sensors were not integrated into the blade.

Meanwhile, Igus, also based in Germany, uses BLADEcontrol apparatus to measure natural oscillation in the rotor blades. Piezo-electric acceleration sensors are attached to the blades. When the blades’ frequency spectrums reach the same levels as those stored under various disturbance conditions, including ice, the alert is sounded and the system can be shut down.

From airplanes to turbines

The basis of most ice detection technology is transferred from the aviation industry. For instance, Goodrich ice detectors, which are distributed worldwide by Campbell Scientific Canada Corp, have a long history of use on aircraft. “It makes sense for CSC to market this same technology for ground based applications,” a spokesman says.

The Goodrich detector vibrates ultrasonically at a nominal frequency of 40kHz. As ice accretes on the probe, the frequency falls and when this reduction is equivalent to 0.5mm of thickness, the ice signals are activated for 60 seconds and the detector activates a de-icing cycle. If another icing encounter is detected within those 60 seconds, the timer is reset to zero and the ice signals remain activated for another 60 seconds.
As the blade de-icing market gains pace, several companies are known to be developing or improving blade-heating equipment, including Vestas, Bonus, NEG Micon, Enercon and Lagerwey.