Design, Analysis, and Testing of Leading-Edge Protection Tapes for Wind Turbine Blades

Restricted (Penn State Only)
Author:
Major, Desirae
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 26, 2019
Committee Members:
  • Jose Palacios, Thesis Advisor
  • Sven Schmitz, Thesis Advisor
  • Amy Ruth Pritchett, Committee Member
Keywords:
  • Renewable Energy
  • Wind Turbines
  • Annual Energy Production
  • Leading-edge Protection Tape
  • Wind Turbine Leading-edge Erosion
Abstract:
One of the sources of wind turbine blade damage is erosion of the surface at the leading edge. Depending on the location, wind farms are exposed to various environmental hazards. The impact of particles such as sand or rain at the blade leading edge during operation erodes the surface over time. High rotational speeds and a high impact count make the leading edge at the outboard 40% of the blade the most susceptible to severe damage. Besides posing structural concerns, leading-edge erosion degrades the aerodynamic performance of the blades by notably decreasing lift and increasing profile drag. Aerodynamic degradation of eroded blades results in notable annual energy production (AEP) losses for utility-scale wind turbines. To avoid these losses and protect the blades, leading-edge protection (LEP) tapes have so far proven to be a reliable and affordable solution. Tapes impact AEP as well, though losses are notably smaller than those for eroded blades. The mechanisms that degrade rotor performance when LEP tape is applied is not, however, a well-studied phenomenon for utility-scale wind turbines. Research was conducted in conjunction with 3M, an industry leader in LEP tapes, to identify the performance degrading mechanism and develop new tape designs that minimize the impact of LEP tapes on wind turbine AEP. Cross-sectional parameters of the LEP tape such as maximum thickness at the center of the tape, width of the maximum thickness, minimum height of the backward-facing step at the tape edge, and taper angle from the maximum thickness to the minimum height are varied. Numerical CFD models are developed to estimate the effect of both standard and new tape designs on lift, drag, and cl/cd for a NACA 64-618 airfoil, a common wind turbine tip section airfoil. With transition modeling included, CFD predicts that the performance of LEP tapes compared to a clean airfoil is independent of height and width of maximum thickness, but is controlled by the height of the backward-facing step. Standard LEP tapes, with a backward-facing step height of 0.350 mm or 0.500 mm, increase drag 40% to 115% and decrease cl/cd by 25% to 55% relative to a clean airfoil. For tapered LEP tapes, with a 0.075 mm backward-facing step height by comparison, drag increases 1% to 15% and cl/cd decreases only 5% to 10% compared to a clean airfoil. CFD models predict that below a certain backward-facing step height the boundary layer does not trip, minimizing the aerodynamic degradation compared to a clean 2-D airfoil. Two tapered LEP tape designs are manufactured by 3M for experimental verification on a full-scale chord model at Re = 1 million, 2 million, and 3 million, and at angle of attack = 0 degrees. Wake probe measurements of profile drag show a 50% and 80% increase in profile drag for a 0.350 mm and 0.500 mm backward-facing step, respectively. Comparatively, a prototype tapered LEP tape with a 0.075 mm backward-facing step increased the profile drag of the full-scale chord model by 30%, though oil visualization of the flow over the model revealed that - when applied cleanly - tapered LEP tapes do not transition the boundary layer at the tape step. A critical transition criterion for the backward-facing step of a LEP tape is determined from experimental data using the method of Knox and Braslow. Using experimental data for a 0.350 mm backward-facing step, the critical roughness height Reynolds number required for premature boundary-layer transition at the backward-facing step height is estimated to be Re_{k,crit} = 200. The computed local roughness height Reynolds number at the height of the backward-facing step for a tapered LEP tape falls well below the critical transition criterion for the range of free-stream Reynolds numbers observed along the span of a representative 1.5 MW utility-scale wind turbine rotor blade. The wind turbine design and analysis code XTurb-PSU is used to predict the power output of a representative utility-scale 1.5 MW wind turbine with the various LEP tape designs applied to the rotor to estimate how the impact on wind turbine AEP changes by tapering the cross-section of LEP tapes. Under eroded conditions, notable lift decreases and profile drag increases result in a 5% AEP decrease compared to a clean rotor. Applying a standard LEP tape improves AEP output, though AEP still decreases by 2% to 3%, for a 0.350 mm and 0.500 mm backward-facing step height respectively. By tapering LEP tapes and reducing the height of the backward-facing step to 0.075 mm, AEP loss due to tape application is eliminated for a representative 1.5 MW pitch-controlled wind turbine rotor. Examining the trend of percent change in AEP versus average percent change in profile drag, AEP decreases linearly with increasing profile drag in the range examined in this work. Even for damaged tapered LEP tapes, the experimentally observed 30% increase in profile drag is predicted to result in only a 1% decrease in AEP compared to a clean rotor, still less than half the AEP loss associated with standard LEP tapes on the market today.