Back to the programme printer.gif Print




Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'How does the wind blow behind wind turbines and in wind farms?' taking place on Tuesday, 11 March 2014 at 16:30-18:00. The meet-the-authors will take place in the poster area.

Ioannis Antoniou Siemens Wind Power A/S, Denmark
Co-authors:
Ioannis Antoniou (1) F P Jochen Cleve (1) Apostolos Piperaas (1) Julija Tastu (1)
(1) Siemens Wind Power A/S, Ballerup, Denmark

Printer friendly version: printer.gif Print

Abstract

Advocating for the replacement of the hub height wind speed with the equivalent wind speed

Introduction

Traditionally power curve measurements have taken place using a cup anemometer atop a met mast at hub height.
The assumption behind this has been that the measurement is representative of the wind profile over the whole rotor.
The large rotor evolution has created the need for a more representative measurement of the wind over the whole rotor.
Lidar remote sensing devices measure the wind speed at more heights and offer thus the possibility.
As a consequence, the concept of the energy equivalent wind speed has been proposed to replace the measurement of the wind speed at hub height.

Approach

We present a number of power curve campaign results from generally flat terrain sites (on-shore and near-shore
locations) characterized by different roughness and climatic conditions. In these campaigns the wind speed has
been measured using both cup anemometers mounted on a met mast reaching hub height and lidar devices
covering the measurement of the wind speed over the whole rotor. Both the met masts and the lidars have been
situated at IEC 61400-12-1 (1) allowed distances from the turbines. The lidars allow the measurement of the wind
profile characteristics over the whole rotor allowing the calculation of the equivalent wind speed over the rotor, as
suggested in the draft of the new revision of the 12-1 document:

where A Is the rotor area, R the rotor radius, and φ the wind direction difference between the wind at a height and the wind at hub height.

In the special case of a flat profile with zero wind veer, the measurement of the equivalent wind speed coincides with the measurement of the wind speed at hub height. Besides the equivalent wind speed, a lidar can provide information relating the wind profile characteristics over the rotor (turbulence, the wind shear exponent and the wind veer distribution) to the differences between the equivalent wind speed and the wind at hub height on the one hand and the turbine performance on the other.
The analysis of the results shows that clearly the wind speed measurement using an anemometer at hub height is not adequate for describing the performance of turbines with large rotors and thereby supports the introduction of the equivalent wind speed instead.


Main body of abstract

In all described measurement campaigns, the lidar measures the wind speed at ten heights evenly distributed
over the turbine rotor with one of the measurements being made at hub height. In order to make compatible the
comparison of the results of the cup and the equivalent wind speed, all lidar measurements have been
normalized to the cup anemometer wind speed at hub height. In this way we assume that the lidar measures
the wind speed with the same accuracy at all heights. In this way the lidar measurement at hub height becomes
equal to the cup anemometer wind speed.

In Figure 1, the difference between the cup and the equivalent wind speed is presented for an near-shore
location.
The difference is seen to assume substantial values and it can be expected that these differences will influence
both the power curve and the AEP of the measured turbine.

Figure 1 Difference between cup (hub height) and equivalent wind speed

The analysis of most of the collected wind profiles confirms that strong wind shear and veer variations can exist
both above and below hub height. When analyzing the influence of atmospheric stability on the wind turbine
performance, turbulence intensity has been often used as a proxy. In Figure 2 we present the wind profiles for
two different turbulence levels at hub height corresponding to a stable (left) and a near neutral/unstable
situation (right) from the same measurement campaign. Likewise in Table 1, the differences between the hub
height and the equivalent wind speed as a function of the turbulence intensity are presented. Figure 2 results
indicate large wind profile differences between the low and the high turbulence situations. More intensive
mixing due to high TI results in more uniform profiles with height (right), whereas large wind speed variations
with height are seen for low TIs. It is again obvious that the hub height wind speed is not an adequate
description of the wind speed of the wind profile over the whole rotor, nor the description of the Power curve or
the turbine AEP.

Figure 2 Wind profile modifications as a result of the atmospheric stability

Table 1 Difference between the cup and the equivalent wind speed as a function of the TI at hub height.

It has been argued extensively, e.g. [2,3], that atmospheric stability affects the wind turbine power production.
The results of the present paper question this conclusion in the sense that the differences in power production
are documented to actually be attributed to a large degree on the lacking description of the energy of the wind
profile. The conclusions of the present study are also applicable and can readily be transferred to other terrain
types.

In Figure 3 the wind profiles and the wind veer measured using a lidar is presented. The measurement
campaign was conducted in a fairly flat inland site and shows how the specific site is dominated by a high wind
veer. The figure serves as an example of the impact which measurements both above and below hub height may
have when the energy potential of a site needs to be calculated. It is realistic to expect that the high veer needs
to be accounted for in order to achieve a better estimation of the site’s energy potential.

The present paper in its final form will also provide analysis on the envelope of the applicability of the equivalent
wind speed in the case where high wind shear and veer are experienced

Conclusion

We have analyzed a number of power curve measurement campaigns where the wind has been measured with
the help of a cup anemometer at hub height and a lidar remote sensing device. To make the results of the
two measurement methods compatible we have normalized the lidar to the cup anemometer data.

The analysis shows that the wind speed at hub height cannot describe the impinging wind profile which, on
occasions and even in fairly flat terrain, can be characterized by both high wind shear and veer. Thus the
assumption behind the measurement of the wind speed at hub height can only under very specific atmospheric
wind conditions be considered as representative of the wind speed over the whole rotor.

The analysis of the power curve campaigns (to be presented in the final paper) shows clearly that there is
a difference in the power curve and the AEP depending whether both quantities are presented as a function
of the equivalent wind speed or the wind speed at hub height.

Both campaign data as well as aeroelastic calculations (to be presented in the final paper) using high wind
veer and shear data, show an impact on the power production of the turbine and likewise the analysis
suggests that the knowledge of the wind speed and the rest of the wind profile characteristics will contribute
in a more accurate determination of the energy potential of a site.

Following the analysis of the campaign results, the findings suggest that the principle of the equivalent
wind speed is a more pragmatic approach for both power curve and siting measurements.



Learning objectives
1. Gaining a better understanding of the available wind resource.
2. Benefits and limitations of the equivalent wind speed method.
3. Gaining a better understanding of the turbine’s interaction with the wind.



References
1. “Power performance measurements on electricity producing wind turbines”, IEC 61400-12-1 (2005).
2. “Atmospheric Stability Affects Wind Turbine Power Collection”, Lundquist, J. et al, AWEA Windpower 2012.
3. “Ground-based Remote Sensing to examine the Impact of Atmospheric Stability on the Wind Profile at Wind Power Project Sites”, Moore, Kathleen E. et al, AWEA Wind Resource & Project Energy Assessment Seminar, AWEA Pittsburgh 2012.