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Tuesday, 11 March 2014
14:15 - 15:45 Real world power curves: A new era for wind resource assessments?
Resource Assessment  

Room: Tramuntana
Session description

In recent years substantial datasets of historic power performance tests have been assembled and these deliver a clear message: wind turbine performance in real world conditions can depart considerably from performance in idealised test conditions. A renewed sense of realism has been awakened in the resource assessment community which has led to the adoption of various methods for transposing an ideal warranted power curve into a real world power curve, i.e. a curve which represents a true central estimate of the power delivered in the site-specific wind speed, air density, turbulence, wind shear, inflow etc. These methods remain embryonic and there is much work left to do. Can the industry converge on accepted and standardised methods? Is it feasible to apply these new methods in everyday resource assessment calculations? Can these methods be shown truly to improve upon the simpler approaches they are superseding?

Learning objectives

  • Understand why the use of real world power curves is important
  • Apply the Inner-Outer range concept
  • Apply the turbulence renormalisation method
  • Apply the rotor equivalent wind speed (RESW) method
  • Apply the power matrix (proxy) method
  • Get an update of the progress of the Power Curve Working Group (PCWG)
Lead Session Chair:
Peter Stuart, RES, United Kingdom

Rozenn Wagner, DTU, Denmark
Frank Flottemesch Ecofys, The Netherlands
Erik Holtslag (1) F P Héctor Villanueva (1) Anthony Crockford (1)
(1) Ecofys, Utrecht, The Netherlands

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Frank Flottemesch works as a Senior Consultant in Ecofys’ Wind Energy unit since 2013
looking after new products, business development and WRA project management.
In 1993, Frank graduated at the Uni of Nuremberg, Germany with a diploma in geography &
climatology followed by GIS consultancy and land scape planning. In 2001, Frank joined
Enron/GE Wind for the setup of a Wind Analysis & Site Assessment team responsible for
Wind Resource Assessments globally involved in over 1000 site assessments and feasibility
studies. In January 2009, Frank joined PMSS Limited as a Resource Assessment Manager
helping establishing the German PMSS branch.


Considering real-world implications of equivalent wind speed for wras


The current IEC standard for wind turbine power curves (61400-12-1) considers only the hub-height wind speed and neglects shear. This will be addressed in the next edition of the standard, introducing the concept of ‘equivalent wind speed’, which will have a large impact on future wind resource assessment methods. Considering the significance of this change, Ecofys has begun proactively analysing the implications for on-going and future wind resource assessments.


Ecofys has used the equivalent rotor concept as developed by Rozenn Wagner at DTU and to be included in the next edition of IEC 61400-12-1. We have applied the methodology to a number of different scenarios to evaluate how it will impact project developers and wind turbine manufacturers when the standard comes into force. We consider the possibilities to use existing data (modified to the new concept) and the requirements for new data.

Main body of abstract

This analysis is divided into several scenarios.
1. Can wind resource assessments today already be improved based on this method? We analyse whether the methodology can be applied to current manufacturer power curves (currently based on hub-height wind speeds), in order to improve the accuracy of wind resource assessments at sites where shear is measured. We test different methods for modifying the standard WAsP wind atlas method to incorporate this concept.
2. Does this method require measurements across the full rotor? We evaluate whether measurements can instead be extrapolated from lower heights, and how this affects the uncertainty.
3. What requirements will be placed on future wind measurement campaigns? We consider whether this new standard will place additional requirements on project developers in the measurement phase.
4. How will projects benefit from lower uncertainty in the power curve? This new method is based on the principle that it lowers the uncertainty in the power curve, so we investigate how this will affect wind farms in terms of financing and operational performance.


A major change is coming in the definition of power curves, which will benefit project developers in terms of improved yield estimates and reduced uncertainties. However, it also presents several challenges – in terms of increased demands on measurement campaigns and in developing new methods to calculate the wind climate. We have demonstrated these impacts through case studies.

Learning objectives
This presentation will extend the industry dialogue regarding equivalent wind speed through concrete applications to projects in development. We will highlight benefits and challenges of this method, in advance of the finalisation of this new IEC standard.