CUTTING EDGE ON THE LEADING EDGE
The growth of wind energy is exceptional – both in terms of installations and physical size. In Europe alone, it’s predicted to overtake gas installations and become the largest form of power generation by 2020, supported firmly by a mammoth addition of 4.9GW in capacity during just the first half of 2019. Meanwhile, the size of wind turbines has reached incredible heights, with LM Wind Power unveiling the world’s longest blade at a staggering 107 meters in length (McPhee, D. 2019). 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 (particularly rain) on a blade can cause considerable 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 raindrop 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% (Sareen et al. 2014), 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.
A variety of studies have investigated the costs and strategies of operations and maintenance (O&M) for wind power. Some have found that these O&M ventures can account for as much as 30% of the overall per-MWh-cost for wind turbines. Other 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 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 the rain droplets.
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 that 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. Here, we will address the second major challenge; applying protective materials in situ.
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 ability to apply the protective materials.
Nearly all materials used in LEP will be sensitive to damp surfaces, temperature and humidity. These elements are very difficult to control, especially for the duration of the repair. Therefore, a material that is less sensitive to these conditions is ideal for conducting repairs in situ.
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.
Belzona Polymerics manufactures and designs its materials specifically to overcome these issues and make application as easy as possible. Especially in the case of its newest system – Belzona 5721 - designed to deliver effective in-situ leading edge protection.
INSIGHT FROM THE LAB
Richard Collett, Belzona Polymerics’ Chief R&D Chemist, has been prominent in this latest development. He said, “From the outset, our aim was to develop a solution that provided excellent resistance to rain erosion and abrasion, whilst being easy to apply in situ.
“Traditionally, the application window available for wind turbine blade maintenance is narrow, requiring favorable weather conditions and adequate temperatures to carry out repairs. We wanted to be able to extend that application window further, providing more opportunities to carry out blade repair and protection.
“Belzona 5721 helps to maximize available maintenance windows in a number of ways. Firstly, it can be applied by brush as a single coat system. This reduces downtime waiting for overcoating numerous layers and also makes it suitable for application from a platform or even rope access. What’s more, it cures without the requirement for external heat or UV, even at temperatures as low as 5⁰C (41⁰F). This means it is touch dry in approximately 30-45 minutes – a major benefit that minimizes downtime.”
PUTTING THE THEORY INTO PRACTICE
The benefits of using materials that can be applied at lower temperatures become apparent if we look at a UK case study. For example, the Hornsea offshore wind farm, which once fully constructed, will have a planned capacity of up to 6 gigawatts (GW). Here, the average maximum monthly temperature at the nearest weather station in Bridlington varies throughout the year, ranging from less than 10°C to 20°C. 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 5721, 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 that make it easier to apply. There is also the physical aspect to consider. The easier it is for a technician to apply, the greater the likelihood that the system will perform as intended. Belzona LEP materials all have simple mixing ratios. Belzona 1221 and Belzona 5721 have volume mixing ratios of 1:1 and 1.75:1 respectively. These simple ratios make it easy for any applicator to ensure the correct mix is achieved. In addition to this, the materials also come in a variety of unit sizes, which means it is possible that part mixing may not even be required. Additionally, only a brush is necessary for the application – no need for injection cartridges or specialist equipment.
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 the impact of 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 those that Belzona Polymerics provides are designed for ease of application and effective curing at lower temperatures. This not only gives a longer window of application but also makes it easier for the technician to apply in the field, minimizing application downtime considerably.
For more information about Belzona’s range of repair and protection options for wind turbine maintenance, visit: https://www.belzona.com/en/focus/wind_power.aspx
McPhee, D. (2019). World's largest wind turbine blade arrives for UK testing - News for the Oil and Gas Sector. [online] Energy Voice. Available at: https://www.energyvoice.com. [Accessed 7 Oct. 2019].
Sareen, Agrim & Sapre, Chinmay & Selig, Michael. (2014). Effects of leading-edge erosion on wind turbine blade performance. Wind Energy. 17. 10.1002/we.1649.