The value of Leading Edge Protection has never been greater now wind power is the largest renewable energy source in Europe

Although it may seem innocuous, the impact of debris and weather (in particular rain) on a wind turbine blade can cause significant damage. Based on a 2mm diameter droplet and an 80m/s tip speed, the pressure imparted by a raindrop is estimated at 120MPa – higher than the yield stress quoted for some blade materials.

This causes pitting on the blade’s surface, especially on the leading edge, where most impacts occur. This reduces aerodynamic efficiency and subsequently losses in operating efficiency. Some studies show that leading edge erosion can result in drag increase of up to 500%, culminating in a decrease in annual energy output of up to 20%. The effects of this damage can be apparent in as little as two years. As wind turbines can operate continuously for 15 years, this is a significant problem for turbine operators.

O&M Overheads

It is said that the costs and strategies of operations and maintenance (O&M) for Wind Power can account for as much as 30% of the overall per-MWh-cost for wind turbines. Some studies have looked at failures on a component-by-component basis. Depending on the type of turbine, the blades can account for up to 22% of failures. The resulting high cost means many companies are moving towards a preventative and predictive approach, especially in offshore markets.

As well as being more costly to maintain, offshore wind turbines are also more susceptible to damage. As offshore wind is not limited by acoustic emission, the tip speeds and blade lengths tend to be much larger, thereby increasing the impact velocity of rain droplets.

The Challenge

The difficulty with blade maintenance is not only finding suitable materials and methods for protecting new blades but also repairing damage to those already in the field. The associated challenges are, firstly, developing materials and techniques which will protect blades throughout their lifetime. Secondly, applying these protective measures in the field, often in difficult conditions, narrows the available maintenance windows.

Climatic Conditions

Climatic conditions are often the driving factor when considering an in-situ repair. The weather will not only affect a technician’s ability to access the blades but also the applicability of the protective materials.

Nearly all materials used in Leading Edge Protection (LEP) will be sensitive to moisture, temperature, and humidity. Therefore, a material which is less sensitive to these conditions is ideal.

Temperature is a key factor with any application of a coating or tape system. The temperature will affect the viscosity, working life (pot life) and eventual cure time. All manufacturers will give recommendations on minimum application temperatures, if used below these temperatures the product will be extremely difficult to handle and apply.

A Simple Solution

Belzona Polymerics manufactures and designs materials specifically to overcome these issues. These easy to apply LEP materials cure without the requirement for external heat or UV, in some cases down to temperatures as low as 5°C and the majority from 10°C whereas most other LEP materials cure at 10-15°C. Lower temperature thresholds can increase the number of days per year where maintenance can take place.

The benefits of using materials which can be applied at lower temperatures become more apparent in the waters around the UK. Belzona says its materials can increase the maintenance window here from four to five months of the year to approximately nine. Theoretically, in the case of Belzona 1111 (Super Metal), which has a minimum application temperature of 5°C, maintenance could be conducted all year round. This not only makes it easier to schedule O&M activities, it could also reduce costs as less equipment will be required to maintain climate controls during certain parts of the year.

It is not only the climatic conditions and parameters of a material that make it easier to apply. There is also the physical aspect to consider. The easier it is for a technician to apply, the more likely it will be effective. Belzona LEP materials all have simple mixing ratios, Belzona 1111, Belzona 1331 and Belzona 1341 have mixing ratios of 3:1, 2:1 and 1:1 respectively. In addition, they come in a variety of unit sizes, meaning that part mixing may not be required. Plus, only a brush or roller is necessary for application – no need for injection cartridges or specialist equipment.

Ease of application is important, but it must be backed up by material performance. ASTM G73 is the most commonly used test method to assess performance and determine a material’s ability to withstand rain erosion. This test involves samples of the coating being rotated at high velocity through water droplets that impact them. Tip speeds in the region of 80m/s are common in many current wind turbine designs. Belzona 1341 has undergone testing to this standard and showed little to no damage at tip speeds of up to 143m/s.

A Stateside Solution

At a wind farm near Des Moines, Iowa, some turbine blades were experiencing leading edge damage. The project manager in charge of the farm wanted a long-term LEP solution which could repair and preserve the blades. Downtime was a critical factor and the unpredictable October weather meant that an easy-to-apply system was essential. Belzona 1341 was chosen because of its positive ASTM G73 & G76 test results.

Initially, the pitted substrate was resurfaced with a filler to restore the correct dimensions of the blade. This surface was simply sanded and cleaned before a thick coat of Belzona 1341 was applied.

Another benefit was that in came in 500g module packaging that made measuring and mixing while on a suspended platform, practical and easy. It was also enough for necessary coverage of the blades.

Dig Deeper than the Datasheets

Wind power is an ever expanding and growing industry. One of the biggest drivers of cost is the operation and maintenance of the turbines once they are in the field generating electricity.

Harnessing wind power involves exposure to the elements, which can wreak havoc on the turbines and blades. Issues such as corrosion, erosion and environmental damage can lead to deterioration and costly implications accompanied with unwanted downtime.

For both onshore and offshore environments, choosing the correct solution is essential to repair and protect assets from these problems, whilst simultaneously making wind farm maintenance procedures more efficient and less time-consuming.

Author
Tom Austin-Morgan

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