Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Real world power curves: A new era for wind resource assessments?' taking place on Tuesday, 11 March 2014 at 14:15-15:45. The meet-the-authors will take place in the poster area.

Simon Feeney Renewable Energy Systems Limited, United Kingdom
Co-authors:
Simon Feeney (1) ^{F P} Alan Derrick (1) Alastair Oram (1) Iain Campbell (1) Gail Hutton (1) Chris Slinger (2) Michael Harris (2) John Medley (2) Edward Burin des Roziers (2) Greg Powles (1)
(1) Renewable Energy Systems Limited, , United Kingdom (2) ZephIR LiDAR, , United Kingdom

Project Cyclops - the way forward in power curve measurements?

Introduction

It is now widely accepted within the industry that turbine power output is a function of a complex set of site conditions (shear, turbulence, veer, inflow angle, terrain complexity...) which are not captured in the warranted curve. This project investigates an alternative technique to measure turbine performance in a real world setting. This is achieved through the use of nacelle LiDAR measurements at multiple distances in front of the rotor. The technique has the potential to provide power curve measurements that are repeatable under a wide range of conditions and are less onerous and expensive than current industry standards.

Approach

RES have deployed a ZephIR DM LiDAR on the nacelle of a wind turbine in Southern England. This type of scanning LiDAR can measure wind speeds across the full rotor disk, allowing hub height and rotor-equivalent power curves to be determined. Five months of measurements were captured and analysed to assess how the wind field conditions change as the incident flow approaches the rotor. This interaction was studied over a wide variety of atmospheric conditions, directly measured by the LiDAR, and used to characterise and understand the effects of varying wind fields on turbine power production.

Main body of abstract

The industry standard IEC61400-12-1 stipulates power curve wind measurements at a distance of between two and four rotor diameters (2-4D) away from the test turbine, such that the measured wind speed is relatively unaffected by turbine rotor blockage and is representative of the free stream. This approach assumes a simplified propagation of the wind field as it approaches the rotor, and can be inaccurate in complex terrain. The use of a scanning nacelle LiDAR, measuring at multiple distances upwind of the rotor, allows the wind field to be directly measured as it approaches the rotor while maintaining a link with the free stream wind speed outside the blade-induction zone.
Measuring the wind field close to the rotor increases the correlation between the measured wind speed and the power output. This is a novel approach to measuring power curves which is less sensitive to “non-standard” flow conditions.
However the blade induction effect substantially changes the shape of the power curve. Approaches for correcting this power curve to the free stream wind speed, in order to integrate such power curves into resource assessments, are investigated.
The approach is tested under a variety of site conditions, and an optimum distance and measurement configuration is suggested for this site. The vision is that this will form the start of a best practice guideline for the industry to build on and test whether this approach is suitable for all sites and wind turbine models.


Conclusion

This research details a new measurement approach for power curves, which is less susceptible to site complexity and “non-standard” inflow conditions. It compares the approach with hub height mast based power curves, and power curves generated through ground based vertically profiling LiDARs. It investigates how each measurement technique can determine which parameters ultimately have the most effect on power output of a wind turbine. It recommends techniques for correcting the “blocked” wind speed to a free stream wind speed that can be used in resource prediction.


Learning objectives
Guidelines for the use of circular scan continuous wave nacelle LiDARS in power performance assessments.
Whether measurements closer than two rotor diameters can be used as a measurement technique that is less susceptible to terrain complexity and “non-standard” inflow conditions.
How different measurement strategies can offer insights into turbine performance, in isolation as well as in conjunction
How the wind field approaching the turbine changes as it interacts with the rotor through the “blocking effect”