The average service life of wind turbine blades is about 20 years. Traditionally, their integrity control is performed by industrial climbers, and such surveys directly affect the electricity generation profitability. The monitoring has to be performed regularly, at least once every four years; however, it is highly complex and expensive when it comes to offshore wind energy. Particular weather conditions often lead to changes in survey schedules, which may give rise to structural damage entailing drastic service life reduction of the entire turbine.
The best performance is achieved with the maximum smoothness of the blade surface. When the turbine is in operation, the blades rotate at a speed of up to 300-350 km/h (comparable to Formula 1 racing cars). Any damage, even the tiniest, leads to turbulence build-up, efficiency reduction and, of course, early wear-out of the turbine. In the extreme operating conditions of marine power plants, the blades are exposed to more serious impact of ultraviolet radiation, wind, rain, and salt-loaded air (as compared to onshore locations).
In the conventional method, industrial climbers, who monitor wind turbines, look for damages by tapping surfaces. There are more challenges with surveying rotor blades of marine power plants than those of onshore plants. Thus, in order to reduce the turbine downtime, minimize breakage risks and improve performance, researchers of the Fraunhofer research centre along with some industrial companies developed a joint project called Thermoflight. The project provides for testing new methods of continuous monitoring of turbine blades and other structures without actual presence of men. The essence of this new approach lies in the use of special drones outfitted with thermal imaging cameras and installation of acoustic monitoring systems on rotor elements.
The acoustic control method provides for installation of sensors (acoustic and piezoelectric) on blade inner surfaces and inherently weak spots of the structure, providing for detection of miscellaneous structural damage. The method is based on the fact that structural damage causes changes in tensile forces inside of the structure resulting in release of energy in the form of heat, and in formation of surface acoustic waves. Acoustic radiation sensors analyse wave time delay and transmit the data to a digital gauge installed on the turbine hub, which collects and analyses the data. The data analysis allows determining, in which part of the structure damage is developing, and how grave the damage is. Acoustic monitoring is then used as an early warning system. In addition, thermography made it possible to identify damage of composite material of the inner and outer structure, such as flaking, foreign inclusions and other defects reducing the service life of turbine elements.
The thermographic monitoring method has been used for some time for onshore wind power plants; it has proven efficient, and now researchers are adapting it to marine conditions. The most reliable thermal imaging camera types and drone models will be selected after the pilot phase.
This, in addition to reducing the monitoring costs, the new turbine integrity control techniques facilitate major improvements in energy generation, since they help reduce turbine downtime. While it would take an industrial climber an entire day to survey a marine turbine, with a drone this time is reduced to one hour, while an acoustic system would take even less time. The operating company would have all required data on land. Specialist surveys are still unavoidable, of course, however, with preliminary data on hand surveyors will perform their surveys faster, and plan for them more efficiently.