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Rotor blades for wind-energy generators have now reached such a size that it is essential they be constructed from lightweight materials. However, even the slightest error in production can have a seriously detrimental effect on their function. Foremost in this respect have been concerns over control of the exact volume of resin used to impregnate the glass-reinforced plastic (GRP) matting that forms the blades, and over the ability to guarantee the mixing ratio, regardless of the actual flow rate of the mixed resin. These fears have now been laid to rest by the introduction of fully automated, purpose-designed metering and mixing systems.
(Published on May 2008 – JEC Magazine #40)
Fabrication of GRP rotor blades that can be more than 60 metres in length is a complex process. Achieving the specified lifetime requires a combination of the latest material technology, together with the most recent developments in automated metering and mixing systems(Figure 1).
A common method of construction for rotor blades is to produce them in two halves, subsequently bonding them together and finally smoothing the outer surface of the joints with a gelcoat. Alternatively, the rotor blades can be cast in a single piece which, although technically more challenging, has the advantage of requiring neither the bonding nor the gelcoating processes.
Equal infusion of the fibre mat
Two alternative production techniques are available when using the two-half method. Either fibre mats impregnated with partially cured thermosetting resins – known as prepregs – can be used, or a two-component reactive epoxy resin can be infused into the fibre mats under vacuum, directly on-site.
These processes may sound simple enough, but in reality, the sheer scale of the rotor blades demands a high degree of technical expertise. For example, the infusion process for a typical 50-metrelong blade will take upwards of six hours to dispense and use around 1,000 kg of resin, whilst 600 kg of resin is used on average for bonding the two halves together.
The surface finish of the rotor blade is a direct consequence of the polyurethane mould, which can be reused many times. The rotor blade itself is hollow in section and consists of different layers, with a total wall thickness of just a few centimetres. The internal space is a complex composition of laminated parts, including balsa wood for reinforcement. A foil is used for support during the pressurised infusion process.
In a comparison of dispensing systems currently available to undertake this process, capacity is very similar. The difference is in the flexibility and speed of the electronics that control the metering and mixing accuracy, and therefore are responsible for the reliability of the bonding.
Naturally, every machine manufacturer asserts that the mixing ratio of their system is accurate during the entire production cycle, but this can now be precisely verified by means of a glass transition temperature test. This test defines at what temperature the material has the highest deformation and clearly identifies the point between elastic and brittle states.
Consequently, the availability for on-site testing of potential dispensing machines is an important component of the end user’s choice in today’s competitive marketplace, where performance is no longer evaluated from data sheets, but from actual tests with the systems at their plant before the investment is made. This has been the experience of the Hilger u. Kern / Dopag Group with systems developed specifically for use in the manufacture of rotor blades, and as a result, both systems initially loaned to one of the largest manufacturers of rotor blades remain permanently integrated into their production process. The results have been extraordinary, according to the manufacturer.
Precise viscosity adjustment
The Hilger u. Kern / Dopag Group is able to offer a complete range of systems for all three processes used with the two-half technique.
All three systems comprise a resininfusion system (figure 2) to apply the epoxy resin to the fibre mat, a gluing system (figure 3) to bond the two halves together, and a gelcoat system (figure 4) to process the thixotropic filling compound. The epoxy resins utilised in these processes are standard products, widely available from several suppliers. So, for example, the mixed viscosity of the infusion resin needs to be only a few hundred mPa s to guarantee optimal vacuum-supported infusion into the fibre mats, whereas the gluing resin must be of a high viscosity in order to bond the two halves together.
The Hilger u. Kern / Dopag Group infusion system features magnetically coupled, leakage-free axial piston pumps that typically feed the resin at 20 litres per minute, although even greater flow rates of up to 60 litres per minute are possible as a function of pump selection, if required. Such an output would be almost impossible using gear pumps.
The system comprises a switchgear cabinet with cooling, three pressure-feed containers and three axial piston pumps, one for the A component and two for the B components (figure 5). The use of two hardeners allows the user to cater to changes in environmental influences that might alter the reaction and curing time of the mixed resin, such as temperature and humidity.
Even if many industrial applications specify the use of dynamic mixers, static mixers are perfectly adequate for this infusion system, since the mixed material is also subsequently fed to an intermediate vessel and additionally mixed again in the output hoses.
Similarly, static mixing is preferred for use with the bonding resin application, since the shearing action of a dynamic mixer would precipitate a reduction in the viscosity of the resin. Output from the metering pumps is around 12 litres per minute, which is still in the range where gear-type pumps are perfectly appropriate.
Material supply is normally by eccentric pump, although when very low viscosity resins are used, a gear pump is used instead. All pumps are protected against abrasive wear through a specialised sealing technique.
Rapid control circuits
Today, an output rate of 20 litres per minute is not unusual for a resin infusion system, and is indeed concurrent with market expectations. However, what makes the Hilger u. Kern / Dopag Group infusion system particularly interesting is its use of the MR20 metering computer, also developed by Hilger u. Kern. The real innovation of this computer is the new software that has been especially developed for the metering and mixing systems used in this type of application. The computer has served the market for some time, designed as it was to specifically control mixing and proportioning systems, for which it offers the user a multiplicity of functions and programs that can be easily chosen, thanks to simple operator guidance and a touch-screen panel.
The software can be easily adapted to many different requirements, and offers comprehensive functions from monitoring through the control unit to production data capture. All the interfaces necessary to allow for integration into fully automated production processes or a master computer exist as a standard function. Due to its rapid control circuit, the MR20 offers precisely the right control of the production process required for accurate repeatability of the mixing ratio. Only if this can be guaranteed can the prescribed mechanical limit values of the rotor blades be achieved.
When the blade is in use, normal air flow, which approaches the blade from the front and induces it to turn, does not pose a problem regarding the prescribed mechanical limits. It is only when the blades are subjected to gusting during stormy weather that the upper limits can be reached and problems may occur.
This is why it is vitally important that the production process conform to the exact specifications, not only in compliance with the correct mixing ratios, but also in the flexibility of adjustment. For instance, during the infusion process, the material is often not deposited at a constant flow rate. When this occurs, rapid readjustment by the MR20 computer is paramount, in order to conform to actual flow rates. Additionally, processes and guidelines often change, and users expect a system that can be easily and quickly readjusted to suit the new parameters.