A key consideration that goes behind blade design and manufacturing is related to coming up with a reliable efficient power generation.

This means low maintenance of machinery, low variation in weight and mechanical performance of blades to reduce variable results and of course high power output in lower wind speeds to optimise the power curve.

Yet, blade failure, which is one of the main reasons behind wind turbine accidents, can arise due to numerous reasons or from possible sources, and results in either whole blades or pieces of blade being thrown from the turbine.

There are far too many blade failures, or blades in need of repair, replacement or a remedy, says David Cripps, Strategic Account Manager - Wind Energy, Gurit USA Inc.

“The work going on now in this respect is due to either manufacturing defects or design defects. In many cases, the defect has been caused by a valid design, but one that just proved impractical to manufacture to the tolerance level or accuracy required,” said Cripps.

Commenting on how much one can relate to the pressure to rapidly grow output from blade factories, Cripps said a lot has to do with the difficulty that new manufacturing teams can have with building to the accuracy, or defect level required, whilst it being driven to lower costs and increased production throughput.

“Also with the rapid rate of introduction of new blade designs, it is always possible that design errors creep in,” acknowledged Cripps.

According to experts from the industry, structural flaws have been encountered, particularly with the blades. Cracks sometimes appear soon after manufacturing. Mechanical failure, due to alignment and assembly errors, is common. Electrical sensors frequently fail because of the power surges. Non-hydraulic brakes tend to be reliable, but hydraulic braking systems often cause problems.

Manufacturing flaws can cause problems during normal operation. For example, it has been pointed out that blades can develop cracks at the edges, near the hub or at the tips. Fibre glass rotor blades have been considered to be the most susceptible components of a wind turbine. Typical manufacturing flaws in case of the blades may be summarised as delaminations, adhesive flaws and resin-poor areas.

 
New designs and flaws

Designing new turbine blades is a complex process in which the quality of the end result depends very much on the reliability of the design details, the materials and the production processes.

The industry has also witnessed the emergence of new wind turbine blade designs, based on new concepts.

For instance, a couple of years back, researchers at Sandia National Laboratories, in partnership with Knight & Carver (K&C), developed a blade design, promising to reduce the cost-of-energy (COE) of wind turbines at low-wind-speed sites. This design allows the blade to twist more than traditional designs, thus relieving some of the effects of gusty turbulent wind on blade life.

A company like LM Glasfiber says its design for the blade root (where the blade is fitted to the wind turbine) is thoroughly tested and adapted to ensure it can support the blades that are becoming longer and heavier. The root solutions for LM blades are robust, and are based on the principle of mechanical locking. Steel bushings are cast into the root and form an integral part of the blade.

Still, manufacturers have been admitting that fiberglass-coated turbine blades can develop cracks because of a design flaw.

“Design flaws are just one part of the cause of defective blades today. Depending on the nature of the flaw (and whether it originates in design or manufacturing), it is often entirely possible to retrofit blades with extra laminate, reinforcing weak areas. That is one of the great flexibilities of working with composite materials,” says Cripps.

However, he said it is important to recognise that the repair material (typically fiberglass and resin that is applied by hand) does not come with a set of inherent structural properties. The structural properties of the repair laminate are effectively created by the people applying the laminate materials and so the resultant properties are only as good as the people doing the repairs.

Furthermore, the repair laminates are often being applied in the field, away from many of the controls inherent in a factory.

“For example, in a field repair resins are usually mixed by hand, instead of by the machines used in the original blade factory. Mixing resin and hardener off-ratio can have disastrous results on the resultant laminate properties. Curing the laminate is also often difficult to achieve or control in a field environment. These factors will all influence the effectiveness of the resultant repair or upgrade,” explained Cripps.

Emerging manufacturing technologies

According to Gurit, in today’s production, there are two clear technologies - resin infusion and prepregs.  Gurit is heavily involved with materials and design for both types of manufacturing, and it uniquely sees both sides of the technologies.

Both manufacturing technologies are perfectly valid ways to build blades and companies are using both very successfully, according to the company.

“In terms of new blade plant start-ups, many of these companies are trying to use infusion. This is mainly because it is initially perceived to be a less expensive process, with lower initial tooling costs. However, infusion is not always as easy to accomplish successfully as it first may appear,” said Cripps.

According to him, there is a severe lack of experienced people to help the blade industry with this process. Those who do have good infusion experience are usually stretched very thin by the high demands of needing to train a large number of new people in what is a sensitive and relatively operator-dependent process, with risk of complete scrap if it goes wrong. With blade plants often sited in very remote locations, with no nearby composite skill base, the training demands become extremely high.

“As a result there have been plenty of expensive manufacturing mistakes and scrapped blades which, if they are not caught in the factory, end up as even more expensive in-field repair or upgrade jobs,” added Cripps. 

With prepreg blade manufacturing, there is still the need for operator care and training but at least some of the blade manufacturing process steps are 'contracted out' to the material suppliers.