While many wind turbine components still include such an age-old element as iron, there is nothing of the kind to be found in rotor blades – they are all about chemistry.  This opens up opportunities if repairs are required because engineers can rely on repair kits with a good track record. 

Rotor blades are designed to last about 20 to 25 years. During this period, downtimes which are less hard on the blades alternate with periods of very heavy usage.  And then there is the huge impact of lightning or heavy hail. Wear and tear during operation and the normal aging process take their toll when it comes to how long the rotor blades last.

Stefan Brassel is Manager of Rotor Service and Sales at Deutsche Windtechnik GmbH. In his experience human error is another reason why damage is caused: “­Transport and assembly also cause damage.  And repairs that haven’t been done properly”. And the latter has the knock-on ­effect of one repair after the other. Brassel believes that damage caused by human error is up to 80 % – or in other words more than three quarters.

Human errors lead to damage

Even without looking at relevant surveys or statistics, the strong human factor that results in damage is obvious. “Other causes of damage are manufacturing defects that only become apparent during turbine operation,” points out service expert Brassel. Design flaws are among the top three causes of damage. Brassel goes on to say that “a lightning protection system that is badly designed results in frequent and severe lightning damage. Using too much adhesive at the trailing edge of the shell bonding leads to stress or shrinkage fractures that prompt a large number of cross cracks at the trailing edge”.

All these detrimental influences require a proactive ­maintenance strategy – which doesn’t cover all ­incidences of damage by a long chalk. Therefore, a number of repair companies with a good track record have sprung up that can repair damage of up to several square meters without having to dismantle the blades. And they are very quick too. Customers expect repair teams to have the ­necessary expertise and to keep turbine downtime as short as ­possible. Because every repair doubles the costs as ­operations are interrupted.

How do repair teams reach the sites? The choice is ­between rope access or elevated working platforms. Marko Wicha, Project Manager at Windigo GmbH, believes there’s no clear-cut case for either. The Berlin-based company relies on both. He explains: “There are pros and cons to both technologies. We recommend rope access for small repairs and elevated working platforms for large ones”. He goes on to say that rope access is not practical for large-scale or even structural repairs of laminate. He is not worried by the usual objection raised against using elevated platforms due to the wind. “When it comes down to it, engineers using rope access will be more exposed to the wind, for example when repairing blade tips”. And he points out that wind speed of 15m/sec would bring both methods to a halt.

Prevention is better than cure

Even before the rotor blade’s material is affected, internal and external impact damages the blade’s coating. Solid matter in the air, like sand or coarse particles of dust, are hard on the inflow edges in particular.  However, there is one thing Brassel is surprised about: “Contrary to all ­expectations, erosion at desert sites is not as bad as you might think considering the amount of sand there”. Brassel believes that the minor damage done to the ­inflow edges is due to the dust particles’ low weight.

However, the coating’s worst enemy is rain. Because of their relatively large weight and the high kinetic ­energy, raindrops have a damaging impact on the coating. In the end, erosion takes place in the fibre-reinforced ­composites. Erosion is especially strong when the ­kinetic energy generated from the drop height coincides with the rotating blades’ maximum energy – or in other words at the blade tips. The erosion is detrimental to aerodynamic properties and significantly diminishes energy yields.

While damage can be rectified through a general overhaul of the rotor blade, dismantling, repairing and ­assembling blades costs a lot of time and therefore money. The ­alternative is a polyurethane-based (PUR) coating on the suspended blade. This is usually sprayed on. PUR coatings are considered especially durable and have high UV ­resistance. The elasticity of the coating is one of the rotor blades key features. It absorbs the blade’s deformation well during a cycle without leading to crack formations. The usual thickness of a layer amounts to no more than just 300 micrometres.

Standardisation around the corner

Although the PUR coating needs to be renewed at regular intervals, it lasts two or three times longer than ­conventional coatings, according to Christian Claus who heads the Renewable Energy Business Division at 3M. In a coating magazine Claus says : “Prevention is more cost effective than repair at a later date” and gives an ­example. He states that the costs for a coating amount to ­approximately € 1,000 and goes on to say that by ­regularly ­carrying out maintenance and applying a PU-coating alone the increase in revenue could be € 100,000.

But it is not just damage caused that means the blade ­coating has to be superb.  Because the top coating ­dazzled it wasn’t popular in the past. Matt coatings ­provided the answer.

The blade coating has to suit such a variety of different requirements and applications that standardisation is a must. A standards committee has been working on a new version and summary of the relevant standard for several years. Some of the multi-section Coating Materials DIN EU ISO 4628 has already been published, but the ­majority can still be consulted by the public. Final publication – and therefore the point when it will come into effect – is expected this autumn.

Pioneering concepts create new opportunities

In order to minimize the risk of damage, defining a ­quality standard and enhancing the quality of ­manufacturing, ­assembly and transport is vital. Particularly since Brassel believes the job will not get any easier “because the blades are getting more complicated”. But he is also aware of the manufacturers‘ efforts to make the blades less repair-prone.  “An interesting aspect is that Siemens is making rotor blades without any bonded areas,” he says. “These blades are heavy, but work fine”.

Siemens is breaking new ground. The essential elements of a conventional rotor blade are two half shells made of glass-fibre reinforced plastics (GFRP). To achieve the finished result, engineers place the glass-fabrics together with the pre-fabricated reinforcing belts in a half-shell mould and evacuate the textile composite. Because of the low pressure, the resin is infused into the fabrics within two to three hours. The half-shell is then hardened at approximately 70 C° for several hours. Only the main belt inside the blade is made of CFRP. 

The high-tech material still has one major disadvantage,  CFRP is costly. Moreover, soaking the fibres during the infusion process is difficult. Currently, the ­manufacturers only make some of the rotor blade from CFRP. So the ­repair teams can carry on as they were for some time yet.

Jörn Iken