The challenges of maintaining the leading edge02 July 2018

Daniel Petherbridge, technical service engineer at Belzona Polymerics, looks at the challenges of repairing and protecting wind turbine blades’ leading edges

Wind energy is no longer just a novel way to generate electricity for a local area. The rapidly increasing size, efficiency and number of wind turbines is fast making wind a lucrative method for substantial power generation. In January 2018, a new European record was set for power generated by wind in a single day. In fact, wind power is now the largest source of renewable energy in Europe, overtaking coal on its way to becoming the second largest source overall. Therefore, the importance of a high quality and easy-to-apply leading edge protection (LEP) coating has never been more important to keep up with the demand from this blossoming industry.

Precipitation and power generation
Although it may seem innocuous, the impact of debris and weather on a blade (in particular rain) can cause significant damage. Using some rudimentary mathematics, the impact pressure of rain droplets can be estimated using modified water hammer equations. Based on a 2mm diameter droplet and an 80m/s tip speed, the pressure imparted by the rain drop is estimated at 120MPa. This value is already higher than the yield stress quoted for some blade materials.

This type of damage manifests itself as pitting on the blade’s surface, especially on the leading edge, where the most impact will occur. This deterioration causes a reduction in aerodynamic efficiency and subsequently a loss in operating efficiency. Some studies show that leading edge erosion can result in a 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 reasonably be expected to perform continuously for 15 years, this is a significant problem for turbine operators.

O&M overheads
A variety of studies have investigated the costs and strategies of operations and maintenance (O&M) for wind power, and some have found that these O&M ventures 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 costs mean 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. An investigation into the impact velocity versus the rain flow rate found that impact velocity will cause more damage than increased rain flow. 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 the rain droplets.

The challenge
The difficulty with blade maintenance is not only finding suitable materials and methods for protecting new blades but, more commonly, repairing damage to those already in the field. The associated challenges can be twofold:
● Firstly, materials science: developing materials and techniques which will protect blades throughout their useful lifetime.
● Secondly, application: applying the protective measures in the field, often in difficult conditions, narrows the available maintenance windows.

There is already an extensive amount of research in the materials science field for ultimate performance. So, here we will address the second major challenge, applying protective materials in situ.

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 LEP will be sensitive to moisture, temperature and humidity. These elements are very difficult to control, especially for the duration of the repair. Therefore, a material which is less sensitive to these conditions is ideal for conducting repairs in-situ. As an example, let us look at the UK climate.

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 its materials specifically to overcome these issues and make application as easy as possible. These LEP materials cure without the requirement for external heat or UV, in some cases down to temperatures as low as 5°C (41°F) and the majority from 10°C (50°F).

The table, left, shows Belzona materials compared with some established LEP systems and their minimum recommended application temperatures. Evidently, most materials require the application environment to reach at least 15°C (59°F). However, lower temperature thresholds can increase the days of the year where maintenance can take place. As can be seen from the table, on average the temperatures in the UK are pushing these materials to their limit.

Breaking this down further, the benefits of using materials which can be applied at lower temperatures becomes more apparent. Let’s take the upcoming Hornsea offshore wind project in the UK (set to be the world’s largest offshore wind farm) as an example. The table shows the average maximum monthly temperature at the nearest weather station in Bridlington. Using this data as an estimate of expected temperatures when conducting blade maintenance, Belzona materials can increase the maintenance window of four to five months of the year to approximately nine – an increase of 100%. Theoretically, in the case of Belzona 1111 (Super Metal), maintenance could be conducted all year round.

This not only makes it easier to schedule O&M activities, but 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 which 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 the system 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. These simple ratios make it easy for any applicator to ensure the correct mix is achieved. In addition to this, these materials also come in a variety of unit sizes, this means it is possible that part mixing may not even be required. Additionally, only a brush or roller is necessary for application – no need for injection cartridges or specialist equipment.

Ease of application is important, however it needs to be backed up by material performance. ASTM G73 is the most commonly used test method to assess performance to determine a material's ability to withstand rain erosion. This test involves samples of the coating being rotated at high velocity through a series of water droplets impacting the sample. 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 or no damage to the coating at tip speeds of up to 143m/s. In comparison, coating B showed signs of significant degradation through the length of the sample.

A stateside solution
At a wind farm near Des Moines in Iowa, US, some of the turbines’ 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. Consequently, Belzona 1341 was chosen because of its positive results in internal Belzona testing (ASTM G73 & G76).

Although Belzona 1341 was originally designed for fluid handling equipment, this material has since been used on different machinery and structures all around the world. As with many Belzona systems, its potential for many types of industries is limitless.

The application took place at the end of October 2017. To access the blades, a suspended platform system was used. Initially, the pitted substrate was resurfaced with a filler product to restore the correct dimensions of the blade. Then, upon curing, this surface was sanded and cleaned before a thick coat of Belzona 1341 was applied.

All in all, the project manager was highly impressed with how easy and user-friendly the Belzona application was. Another benefit of the solution was the fact that it came in 500g module packaging. This made measuring and mixing, while on a suspended platform, practical and easy. It was also the perfect amount for necessary coverage of the blades. As for the performance of the coating, to date it is performing perfectly, with official test data being collected over the next few months.

Dig deeper than the datasheets
In summary, wind power is an ever expanding and growing industry. One of the biggest drivers of cost is the operations and maintenance of the turbines once they are in the field generating electricity. Leading edge erosion caused by impact from rain droplets is one of the key issues facing the sector. There are a variety of polymeric coatings available for leading edge protection, however the issue of temperature is often overlooked. All materials have a recommended minimum application temperature, which often does not account for the low temperatures experienced in many wind farm installations. Materials, such as Belzona Polymerics, are designed for ease of application and effective curing at lower temperatures. This not only gives a larger window of application but also makes it easier for the technician to apply in the field ensuring the quoted performance is achieved.

Adam Offord

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