Influence of ambient conditions on wind speed measurement: Impact on the annual energy production assessment

Abstract

The Annual Energy Production (AEP) estimations are crucial to analyze the potential of wind energy projects. To calculate the AEP of a wind farm, it is necessary to accurately measure the wind speed, because small errors in these measures lead to significant deviations in the wind turbine power curve. In-field wind speed is usually measured by means of cup anemometers, which are calibrated within wind tunnels. In-field ambient conditions differ from those at the laboratory, which increases the uncertainty of the wind speed measures performed at the location of the turbine. The present work is focused on analyzing the effect of the following ambient parameters on the cup anemometer behavior: temperature, humidity and atmospheric pressure. In order to reach this target, experimental tests in a wind tunnel were performed, which allows minimizing the effect of the rest of influence parameters: turbulence, average flow inclination angle and flow direction. With this work it is determined how flow air viscous forces affect the cup anemometers, changing its rotation frequency. This explanation concludes that a variation on air temperature, humidity and/or pressure modifies moist air kinematic viscosity, which leads to change the friction between air and cups and, consequently, the cup anemometers rotational frequency. In most cases, the kinematic viscosity is inversely proportional to the air density and therefore, higher in-field densities, compared to those at the laboratory where the anemometer was calibrated, lead to underestimate the wind speed, and vice versa. The fact that this effect has been quantified during the calibration process is quite important, since it allows removing the influence of the environmental parameters studied; so that by modifying the calibration methodology, the accuracy of cup anemometers would be optimized. In order to clarify how the moist air kinematic viscosity influences the calibration curve, the calibration measurements of a real cup anemometer are mapped into a new dimensionless abacus, with the Tip Speed Ratio (TSR) and the Reynold’s number as coordinates. The key idea is that the rotation frequency of the cup anemometer is related to both wind speed and moist air kinematic viscosity. This relation is mathematically described by the equation of a hyperbolic paraboloid surface up to a value of wind speed of 15.3 m/s.