Dispensing systems for bonding rotor blades

Over the decades to come, we will see a continuous increase in the proportion of wind power in the global energy mix. The goal is for wind farms to supply electricity at a price lower than that of fossil energy. As a result, the manufacturers of rotor blades and of dispensing and mixing systems for adhesives will have a number of challenges to overcome in the future.

The key considerations when manufacturing rotor blades for wind turbines are to shorten the production time while reducing production costs. At the same time, wind farm operators require a warran ty on the blades lasting for more than 20 years. In order to fulfil all of these require ments, the different disciplines involved need to pool their expertise and work to gether efficiently. The dispensing, mixing and application of adhesives in particular play an important role (Figure 1).

Increasing amounts of adhesives

In the case of rotor blades that are more than 100 meters in length, up to 1200 kilo grams of adhesive are needed to bond the two halves of the blades together. Both the length of the blades and the amount of adhesive required are likely to increase in the future. The open time of the adhesive is generally around 45 minutes at 25 °C. Regardless of the blade size, the bonding process must be completed within this time. The adhesive is measured using highresolution volume counter units with a closedloop regulation system for each of the adhesive components. Sensors can also be integrated to monitor the application temperature within the required tolerances.

Where smaller sections of the rotor blades have to be bonded toward the end of the adhesive application, a fast-curing adhesive speeds up the process. In the case of larger components, such as the main mold (Figure 2), a slow-curing adhesive allows for a careful, thorough bonding process.

The option of choosing different adhesives not only improves the overall efficiency of the bonding process, but also offers a range of opportunities for adapting the procedure. As a result, users can fulfill the varying requirements of the different components of the rotor blades.

Accelerating the processes

Because of the properties of the materials, accelerating the bonding process represents a major challenge. The curing of materials is a continuous process and reliable adhesion depends on the open time of the mixed adhesive, which is a crucial aspect of its pot life. As already mentioned, the application cycle must be completed within 45 minutes, regardless of the blade size. Because of the increasing length of rotor blades, the bonding process needs to be speeded up.

The most recently developed high-performance dispensing pumps with highly accurate process settings allow adhesive to be dispensed at a speed of over 30 kilograms per minute. Furthermore, machines with variable flow rates and additional material buffers are available which do not need to be refilled during the process. With the help of this new technology, adhesives can be dispensed, mixed and applied within the specified time window.

Application with different bead profiles

Different bead profiles are needed when the adhesive is applied to the various sections of the rotor blade: the main mold, leading edge and trailing edge. To ensure that the application process meets the varying requirements of the bonding gap, the nozzle often has to be changed – up to nine times for each rotor blade. This is highly time-consuming and involves a great deal of work. It makes up a significant part of the bonding process and may prevent rotor blades from being increased in size any further. In addition, the bonding points sometimes require manual reworking (shape, air bubbles etc.), which results in a considerable increase in the amount of work in the overall process.

The Fraunhofer Institute for Wind Energy Systems IWES has developed a variable nozzle for the application of adhesives that can apply different profiles in one step. The nozzle can automatically adjust the profile of the adhesive bead applied to different sections of the rotor blade, which results in a significant reduction in the time needed. The use of sensors allows this process to be partially or fully automated and the quantity of material used from the start of the bonding process can be accurately measured (Figures 4 and 5).

This technical development makes it possible to apply only the specified amount of adhesive to each section with the correct bead profile, which guarantees the highest possible quality and keeps waste to a minimum. Rotor blades bonded in this way are also lighter in weight. Dopag has incorporated the variable adhesive application nozzle into its Gluemix metering and mixing system (Figure 3).

Consistent mixing ratio

Manufacturers provide a warranty on their rotor blades for a period of more than 20 years. One important factor in meeting this commitment is the permanent bond between the two halves of the blade. This can only be guaranteed if the specified mixing ratio for the material components remains consistent during the application of the adhesive. The main adhesive used in the wind power industry is a two-component epoxy resin. The tolerances for the mixing ratio are very tight. The ratio must remain within the range of ±2 parts hardener to 100 parts resin. The consistency of the mixing ratio is guaranteed by high-precision volume counters which are used to control and operate the metering pumps for each component independently as part of a closed loop regulation system.

Dopag uses spindle flow meters to ensure that the pressure drop in the volume counter is kept to a minimum when compared with other available solutions. This functionality allows for end-to-end monitoring and control of the mixing ratios.

In addition, the systems have an automatic pre-calibration mechanism that relies on weighing systems. This results in a significant increase in the reliability of the process, because the ratios can be maintained accurately and consistently. Dopag has integrated these technologies into its Gluemix system.

Online monitoring and predictive maintenance

The continuous convergence of the systems and processes allows the different elements of the production chain to be connected with one another. This ongoing process (Industry 4.0) includes the integration of data analyses and intelligent technologies with the aim of creating a more efficient and reliable production environment. The increasing connectivity also enables seamless communication between the systems in the mixing process for adhesives for rotor blades. This increases the efficiency of the process and makes it possible to respond to changes in real time.

The Dopag Life Cycle Logger (LCL) allows for online monitoring and predictive maintenance of the system which represents a significant advance in the bonding process. In the past, it was rarely possible to carry out reliable analyses of historical operating data and draw conclusions about maintenance intervals and the service life of components. The new module gives machine operators and service staff a detailed insight into all the process parameters and usage profiles of the metering components. These include material quantities and pressures, temperature and acceleration data, test bench results concerning efficiency and other operating data. As a result, users can respond immediately to anomalies and ensure a high level of process reliability. This also brings benefits for predictive maintenance. By comparing the data, operators can identify at an early stage any potentially critical areas of wear and gradually emerging faults.

In the context of overall equipment effectiveness (OEE) (Figure 6), all the key figures concerning availability, performance and quality can be accurately documented. OEE is a globally recognized standard for evaluating the productivity of industrial plants and plays a central role in improving operational efficiency. The data provided by the Life Cycle Logger forms the basis for the strategic planning and monitoring of the real-time productivity of dispensing machines and it can be seamlessly integrated into modern lean management systems.