Using wind speed from a blade-mounted flow sensor for power and load assessment on modern wind turbines

Abstract

Abstract.In this paper an alternative method to evaluate power performance and loads on wind turbines using a blade-mounted flow sensor is investigated. The hypothesis is that the wind speed measured at the blades has a high correlation with the power and loads such that a power or load assessment can be performed from a few hours or days of measurements.In the present study a blade-mounted five-hole pitot tube is used as the flow sensor as an alternative to the conventional approach, where the reference wind speed is either measured at a nearby met mast or on the nacelle using lidar technology or cup anemometers. From the flow sensor measurements, an accurate estimate of the wind speed at the rotor plane can be obtained. This wind speed is disturbed by the presence of the wind turbine, and it is therefore different from the free-flow wind speed. However, the recorded wind speed has a high correlation with the actual power production as well as the flap-wise loads as it is measured close to the blade where the aerodynamic forces are acting.Conventional power curves are based on at least 180 h of 10 min mean values, but using the blade-mounted flow sensor both the observation average time and the overall assessment time can potentially be shortened. The basis for this hypothesis is that the sensor is able to provide more observations with higher accuracy, as the sensor follows the rotation of the rotor and because of the high correlation between the flow at the blades and the power production. This is the research question addressed in this paper.The method is first tested using aeroelastic simulations where the dependence of the radial position and effect of multiple blade-mounted flow sensors are also investigated. Next the method is evaluated on the basis of fullscale measurements on a pitch-regulated, variable-speed 3.6 MW wind turbine.It is concluded that the wind speed derived from the blade-mounted flow sensor is highly correlated with the power and flap-wise bending moment and that the method has advantages over the traditional approach where the met-mast wind speed is used as reference, e.g. the capability of measuring the shear, veer and turbulence. The aeroelastic simulations show that the assessment time can be reduced, but this reduction cannot be confirmed from the current measurement database due to sensor problems and practical circumstances. Measuring the wind speed at the rotor plane comes with a price as the wind speed is affected by the induction which may be sensitive to the changes you want to evaluate, e.g. different vortex generator configurations. Furthermore it is concluded that a robust instrument and measurement system is required to obtain accurate and reliable wind speed recordings from pitot-tube measurements.Published by Copernicus Publications on behalf of the European Academy of Wind Energy e.V.

Ability of the e-TellTale sensor to detect flow features over wind turbine blades: flow stall/reattachment dynamics

“…Some field measurement campaigns were punctually performed for research purposes using pressure probes around the blades on dedicated blade manufacturing with however only weak potential of these sensors to be used in a day-to-day operation of wind turbines [26]. Some solutions were explored such as tufts or stall flags glued on the blade correlated with positions of the flow separation [25,20] [7]. However, these methods need a mounted camera on the turbine with its associated drawbacks (fragility of the camera, vision at night ...).…”

Free flow wind speed from a blade-mounted flow sensor

“…Shen et al (2006Shen et al ( , 2009, Guntur and Sørensen (2014) and Rahimi et al (2017) present several methods to calculate the flow near the airfoil that also take 3D effects into account, but the methods require information that cannot be obtained directly from a BMFS. Pedersen et al (2017) describes how to obtain the effective 3D inflow from the relative wind speed and two perpen- From the relative velocity, V rel , the wind speed at the rotor plane, V r , is found by subtracting the velocity of the sensor, V s :…”

“…These relative curves can be used to investigate e.g. aerodynamic modifications or detect performance 20 issues (Pedersen et al, 2017).…”

Free-flow wind speed from a blade-mounted flow sensor

“…Shen et al (2006Shen et al ( , 2009, Guntur and Sørensen (2014), and Rahimi et al (2018) present several methods to calculate the flow near the airfoil that also take 3-D effects into account, but the methods require information that cannot be obtained directly from a BMFS. Pedersen et al (2017) describes how to obtain the effective 3-D inflow from the relative wind speed and two perpendicular angles measured by a blade-mounted five-hole pitot, including compensation for bound circulation. The compensation method uses a look-up table generated by 2-D computational fluid dynamic (CFD) simulations, thus neglecting 3-D effects and tip and root vortices.…”

“…Furthermore, the load distribution along the blade should be taken into account, such that the sensor measures the inflow where the largest loads occur. In Pedersen et al (2017), different radial positions are investigated and 50-67 was found to be optimal.…”