Technology

In TrueWind we aim to increase accuracy of mast top-mounted cup anemometer measurements by 1% as an average for flat and complex sites, when compared with existing solutions. The project will introduce the following two key products in the wind energy market:

  1. Optimized cup anemometer
  2. Lidic system for calibration of anemometers in wind tunnels - 

 
Cup anemometry 

is an old wind measurement technology with high accuracy and confidence in the wind industry. Nevertheless, cup anemometry accuracy can be improved significantly today. A cup anemometer is influenced by turbulence, flow inclination angle, temperature and air density. These influential effects are investigated and cup anemometers are given a classification index according to the IEC61400-12-1 standard. The best cup anemometers today have classification indices of 0.9A for flat terrain and 3.0B for complex terrain. This means that additional 0.9% or 3.0% have to be added to the calibration uncertainty of 0.025m/s to account for the free field operational effects. 
WindSensor,has already a highly competitive cup anemometer on the market with a classification of 1.3A for flat terrain and 3.7B for complex terrain. However, this cup anemometer can be significantly improved. The European ACCUWIND project, has shown how characteristics of a cup anemometer can be improved. Design of a new cup rotor will significantly improve tilt response to a classification index of expectedly 0.3A for flat terrain and 2.4B for complex terrain. 
 
Lidics
The potential to improve the cup anemometer technology is available now with Lidics, very short range lidars (2-10m range). They measure wind speed along a laser beam in a small volume of about 10cm length, comparable to the measurement volume of a cup anemometer (ground based and nacelle based Lidars have measurement volumes of 40-200 m). The Lidic is, because of the similar measurement volume, very useful for calibration purposes for cups, both in wind tunnels and in the field.
The advantage of Lidic is the significantly higher accuracy. Lidics are not associated with flow distortion as a pitot tube, and it can be traceable calibrated by a calibration wheel to a higher accuracy. Accuracy can be made comparable to LDA (Laser Doppler Anemometry). LDA, however, needs seeding of particles in the air and it is not useful in day to day calibrations or in field measurements.

This project will compare field measurements with the traceable wind tunnel calibrated cup anemometers. In this comparison the cup anemometer model being used in the classification process will be evaluated and improved in order to guarantee more exact classification indices. Furthermore, the model will be used to develop an algorithm that can correct cup anemometer output data to achieve even more accurate field measurements.
 
Inter comparisons in anemometer calibrations
MEASNET has arranged regular inter comparisons in anemometer calibrations for many years. The most recent state-of-the art deviations are at the order of ±1%. However, the wind energy community requires this deviation to be further reduced down to at least ±0.5%. An issue is the interference effects between tunnel boundaries and the rotating cups which are not fully understood, and this turns out to be a crucial issue for smaller wind tunnels. Newly larger twin wind tunnels, constructed by Svend Ole Hansen ApS present a chance to carry out investigations on the influence of the tunnel size, tunnel boundary effects and of turbulence. The influence of the turbulence spectrum on calibrations has never been investigated before. Test methods with a turbulence generator to generate full turbulence spectra, especially including the low frequency turbulence, normally missing in the spectrum, will be implemented into the wind tunnel and investigations on the influence of turbulence will be documented. 

The lidic technology is complementary to the similarly accurate LDA-technology

- as it can be combined with LDA to measure the full 3D variability of the wind tunnel flow. Both measurement technologies can be traceable calibrated with flywheels. In this project the Lidic and LDA technologies will be integrated into a new product for wind tunnel measurements. By combining a 1D Lidic and a 2D LDA, all three velocity components of the wind vector can be measured from a single head. This is a huge practical benefit compared to the traditional way of aligning two or three heads which may be far apart. It also simplifies movement of the measurement position because the need for a rigid structure to connect optical heads, or a complex traverse system to track the measurement volumes of several optical heads is eliminated. Absolute precision measurements of the three-dimensional wind vector in the open air can be determined with very high accuracy (< 0.1%).

 
The achievements of this project are to maintain commercial and technological leadership supporting the wind energy activities:

  • Significantly improved accuracy in wind tunnel calibration of cup anemometers using the Lidic system, increasing market share of wind tunnel calibrations
  • An improved cup anemometer with significantly improved classification characteristics and lower calibration uncertainty with significantly increased market share
  • Establishment of an open air traceable calibration facility with three Lidics, and verification and improvement of the cup anemometer model to further increase accuracy of cup anemometer measurements
  • Implementation of Lidic technology in combination with LDA technology, providing a new product portfolio to maintain global market leadership in wind tunnel measurement equipment.