Wind power has become an indispensable part of an environmentally friendly power supply. Therefore, it is important for this growth market that wind turbines become more efficient, more reliable, and more durable. Weak points in blade production, for example, could result in unplanned additional operation and maintenance costs over the entire service life of the turbine.

The radar module is extremely light and compact thanks to its monolithically integrated construction, in which different components and functions are integrated into a single chip. (©Fraunhofer IAF)

The rotors, which usually are equipped with three blades, are the central component of all wind turbines. They convert wind into rotational energy, and then into electricity. Much like the wings on an aircraft, the blades are subjected to enormous external loads, and therefore must be designed to be extremely robust. Modern wind turbine blades are mainly constructed from glass fiber and carbon fiber reinforced plastics (GFRP/CFRP) so that they can elastically absorb the wind energy from strong gusts without breaking. For a single blade, up to 100 sheets of glass fiber webbing are layered on top of each other, shaped, and then glued together with epoxy resin. Quality control is essential at this stage in production; however, difficulty lies in layering the glass fiber sheets flat before they are glued, without creating undulations and folds, and avoiding the formation of lumps of resin or sections of laminate that don't set when applying the epoxy. These defects, as well as delaminations or fractures, can be identified on a large scale using infrared thermography.

To increase the efficiency and reliability of wind turbines, a material scanner was developed for checking the quality of rotor blades. Using radar-based technology, defects in the material composition of the wind turbine blades can be detected in even greater detail.

At the core of the material scanner is a high-frequency radar that operates in the W-band between 85 and 100 GHz, with only very few watts of transmitting power. Specialized software is then used to process the transmitter and receiver signals, and visualize the measurement results. This generates a cross-sectional view of the blade in which defects can be identified in the millimeter range, making the scanner significantly more accurate than conventional methods.

The radar module is based on indium gallium arsenide (InGaAs) semiconductor technology. It is extremely light and compact thanks to its monolithically integrated construction in which different components and functions are integrated into a single chip. Measuring 42 × 28 × 79 mm, it is approximately the size of a pack of cigarettes and weighs 160 grams. It has low power consumption of approximately 5 watts, and is fitted with an integrated microcontroller that emits measurement signals via an Internet interface. Future improvements will see the module's frequency range extended to 260 GHz into the H-band, quadrupling the bandwidth of the module from 15 GHz to more than 60 GHz.

In addition to its use in the production of rotor blades, in the future, the material scanner may also find a role in maintenance, where it could be used to classify defects such as those caused by the impact of birds. Currently, the routine testing of rotor blades is mainly performed by hand — an expert knocks on the blade with a hammer and can tell from the tone whether there are any defects in that section. An automated solution, supplemented by the new radar technology, could vastly reduce the downtime of wind turbines and thus save costs. This is particularly true for the manual maintenance of offshore wind turbines, which must be reached by boat, sometimes on harsh seas.

Alternative testing technologies, such as ultrasound solutions, are extremely difficult to integrate into maintenance procedures. Water or gel has to be utilized as a coupling agent, as every air pocket between the sensor and measured part muffles the ultrasound signal to a considerable extent. While this entails certain side effects, it is nonetheless possible when checking for defects during rotor blade production. Applying water or gel to wind turbine blades that are 100 meters in the air is extremely complicated. Because it allows for non-contact remote sensing, radar is the optimal solution in this case.

The radar scanner can contribute to the development of innovative material inspections in other industries as well; for example, the aircraft industry. In newer aircraft such as the Boeing 787 Dreamliner or the Airbus A350, the wings in particular are mainly built out of lightweight composite materials. Accurate and rapid defect testing during both production and maintenance can save costs and prevent damage caused by material fatigue.

For more information, contact Michael Teiwes at +49 761 5159-450, or visit here.