The aim of IEA Wind Annex 32 is to address the very fast development of wind lidar technologies and their applicability for more accurate measurement of wind characteristics relevant for a more reliable deployment of wind power systems. The purpose is to bring together the present actors in the research community and industry to create synergies in the many R&D activities already on-going in this very promising and new measurement technology.

 

Operating Agent organization

ForWind - Center for Wind Energy Research
Research Group Wind Energy Systems
Carl von Ossietzky University of Oldenburg

Representative: Prof. Martin Kühn

Introduction

In spring 2011 ForWind – Oldenburg proposed together with the Danish Technical University (DTU) Wind Energy and the Stuttgart Wind Energy (SWE) at the University of Stuttgart to the Executive Committee of IEA Wind (ExCo) to start a new project intended to disseminate the state of the art knowledge concerning the application of lidars for wind energy. The same year in autumn, the ExCo officially approved the proposal and start the 32nd IEA Wind Annex “Wind lidar systems for wind energy deployment".

Project scope

The main objective of the new Annex is the publication of, experimentally tested, recommended practices for wind lidar measurements. This should build up based on the joint experience of the participants. The recommendations will be benchmarked with measured data collected at various meteorological and lidar operational conditions. The selected data are mutually shared by the participants.

The expected outcome is the formulation of guidelines and expert reports dealing with the evaluation of performance of different systems applied in different selected areas. The planned IEA Recommended Practices (RP) are supposed to complement recent documents available in regards to other applications of wind lidar systems including the IEC 61400-12-1 Annex L on power curve measurement.

The RPs are planned to be published in two parts during the three years course of the Annex. The first part focuses on the standard procedures that ensure the best precision of lidar measurements in flat terrain, while the second part dedicates to issues of measurement with lidars in complex terrain. The first part has been initiated in the IEA Wind Annex 11 and the participants of Annex 32 supported the finalization of that document during the first year of work. The final document IEA Wind RP 15 “Ground-based vertically-profiling remote sensing for wind resource assessment” has reached its last stage for approval by the Executive Committee of the IEA Wind. The publication is planned to take place by mid 2013. The workpackages in Annex 32 towards evaluation of present measurement procedures and formulation of the second part of the Recommended Practices on lidars have been defined during 2012. The main topics of interest will be the use of lidar for the assessment of wind condition in complex terrain, as well as the assessment of the power curve and loads of wind turbines. Work is planned on both; ground-based as well as nacelle-based measuring techniques.

Work plan

The work plan is subdivided in four subtasks as follows (see also Figure 1):

  • Subtask 1: Lidar calibration and classification;

  • Subtask 2: Lidar procedures for site assessment;

  • Subtask 3: Lidar procedures for turbine assessment;

  • Subtask 4: Data management (data exchange of wind, power and load measurements).

The activities are coordinated by ForWind – University of Oldenburg, which acts as Operating Agent. The work is supported by the DTU Wind Energy in Denmark, the National Renewable Energy Laboratory in the USA and the Endowed Chair of Wind Energy at the University of Stuttgart, who coordinate subtasks 1 to 3 respectively.Subtask 4 is coordinated by DTU Wind Energy.

Subtask I will concern itself with coordinating efforts concerning the accuracy of wind lidars and assessing and developing calibration methods and uncertainty budgets based on the concepts presented in IEC61400-12-1 Version 2 CD, Annex L. The work of the sub-task will be made available in expert reports and will hopefully contribute significantly to future revisions of IEC61400-12-1 Annex L.

As lidars become more routinely used in more novel applications such as floating offshore or nacelle-mounted, Subtask I will act as a forum to try and form consensus concerning calibration techniques for these new devices. Expert reports will be prepared as appropriate and it is hoped that these will form a useful starting point for future standardization efforts.

Subtask II dedicates to the coordination of work concerning the proper application of lidar measurements in site assessment activities. The objective is the formulation of best practices measurement of conventional quantities such us mean wind speed, turbulence intensity and wind direction. Additionally, there is an interest in exploring the assessment of non-conventional variables such us wind shear. The work will cover the operation of lidar systems in flat terrain, as well as in complex flow, such as in complex terrain and offshore measurements.

Several applications of wind data in the wind energy industry rely not only on the mean wind, but also on the turbulence intensity. Typically, this value is directly derived from the statistics (e.g. the standard deviations) of the fluctuations about the mean value from a cup anemometer measurement. Since lidars sample a volume at several positions in order to evaluate the wind vector, the wind, the turbulence observed by a wind lidar and a conventional anemometer will be different, even for the same deterministic wind. These are issues that could refrain some users from adoption of lidar systems since it affects directly the comparability against conventional systems. This activity will give insight in the present knowledge of this and other issues related to wind lidar measurements.

Subtask III focuses on the integration of wind lidar in the standard procedures for the assessment of wind turbines. In particular it is intended to extend to loads estimation the already aknowledged application of lidars for testing the power performances of wind turbines. The activity of this task is divided in nacelle- and ground-based topics as well as in power performance and load estimation issues. Additionally, possible methods for wind field reconstruction from nacelle-based lidar data are going to be reviewed and disseminated.

Coordinators

Subtask I & IV
Mike Courtney
DTU Wind Energy, Denmark

Subtask III
Andrew Clifton
National Renewable Energy Laboratory (NREL), Colorado (USA)

Subtask III
Andreas Rettenmeier
WindForS, Germany