Rain Erosion Load and Its Effect on Leading-Edge Lifetime and Potential of Erosion-Safe Mode at Wind Turbines in the North Sea and Baltic Sea

Abstract

Leading-edge erosion at wind turbine blades cause a loss in profit for wind farm owners, in particular offshore. The characterization of the rain erosion environmental load at wind turbine blades is based on the long-term rain rate and wind speed observations at 10-minute resolutions at coastal stations around the North Sea, Baltic Sea, and inland. It is assumed that an IEA Wind 15 MW turbine is installed at each station. The leading-edge lifetime is found to increase from the South to the North along the German and Danish North Sea coastline from 1.4 to 2.8 years. In the Danish and German Baltic Sea, the lifetime in the West is shorter (~2 years) than further East (~3 to 4 years). It is recommended to use a time series of 10 years or longer because shorter time series most likely will cause an overestimation of the lifetime. The loss in profit due to leading-edge erosion can potentially be reduced by ~70% using the erosion-safe mode, i.e., reduce the tip speed during heavy rain events, to reduce blade erosion, aerodynamic loss, repair costs, and downtime during repair. The aerodynamic loss for the 18 stations is on average 0.46% of the annual energy production.

A Comprehensive Analysis of Wind Turbine Blade Damage

“…The terminal velocity of a falling raindrop is also heavily dependent on the climatic conditions. The maximum free falling terminal velocity levels out at around 9 m/s for diameters in excess of about 3.5 mm [50,61]. Assuming a rain droplet with a terminal velocity of 8 m/s, fully entrained in a horizontal 20 m/s wind (i.e., assuming that the droplet is also travelling at this speed horizontally), strikes a rotating blade with a 90 m/s tangential tip speed, it is calculated that the impact velocity between the rain and blade does not drop below 80 m/s [45].…”

“…The first impact of this exposure is a gradual increase in the blade's surface roughness, which negatively affects the blade's aerodynamic performance by increasing its friction drag [49], and aerodynamic loss on the scale of 0.45-0.50% [50]. Depending on the level of leading edge erosion, drag can increase from 6 to 500% [51,52], leading in turn to an approx.…”

“…In extreme cases of leading edge erosion, the structural integrity of the blade can be affected (Figure 11). Leading edge erosion can become an issue after only 2 years of turbine operation [50], much sooner than expected, with the tip being most susceptible to wear, but with erosion also exhibited on the more inboard portions of the blade [55]. As with all forms of environmental exposure, leading edge erosion is heavily site-dependent.…”

“…In general, the diameter of raindrops depends on the climatic conditions under which they are formed and the conditions of their transportation in the air. Typical raindrop diameters are commonly cited from 0.5 mm to 5 mm [50,61], while for mild to moderate rain rates, raindrop diameters range from 0.5 mm to 3 mm. Assuming water density at approximately 1000 kg/m 3 , the mass of a raindrop of spherical shape with a diameter of 3 mm is calculated at m = 0.014 gr.…”

“…Using Equation (3), assuming a density of 900 kg/m 3 for the hailstone (this value varies widely) and of 1225 kg/m 3 for the air, and a perfectly spherical hailstone shape and thus a drag coefficient of 0.5, the theoretical velocity for a range of hailstone diameters from 5 mm to 90 mm is calculated to be between 10 m/s and 40 m/s, respectively. Given the above hailstone velocities, it is conclusively proved that the maximum calculated impact velocity of a 15 mm and 30 mm diameter hailstone, impacting a blade tip with a tip speed of 90 m/s, in a 20 m/s wind field varies from 70 m/s to 120 m/s [50,61]. Given the above assumptions for the hailstones, the exerted forces can be calculated using the methodology for Equation (1).…”