Round Robin Numerical Flow Simulation in Wind Energy Final Report Francesco Durante, Volker Riedel Ulrike Bunse, Peter Busche, Harald Mellinghoff, Dr. Kai Mönnich, Till Schorer DEWI GmbH - Deutsches Windenergie-Institut Wilhelmshaven, Germany 2008-03-08 This project was sponsored by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, FKZ 0329965. Round Robin Numerical Flow Simulation in Wind Energy DEWI GmbH - Partial Reproduction not Permitted 2 / 173 Round Robin Numerical Flow Simulation in Wind Energy Disclaimer The following results refer to one single site with three met masts installed, they refer to the model in the version and configuration used in the test and they refer to a specific model operator/team. Under no circumstances can these results be considered as a general statement about the quality of the service provided by each company, nor can these results be considered as a general quality assessment of the computer model used. In particular, if a third party flow model is used by the participant, the results cannot be considered as a statement of quality of the third party model. The reasons are as follows: 1. Flow models in general have many parameters to set and simulation design decisions have to be made, depending on the site under consideration and depending on the experience of the model operator. It is not clear if the model operator during the Round Robin Test will be the same than during normal business model operation. 2. Wind farm sites are very different, especially regarding orographic complexity. The results that refer to this site are not transferable to any other site and are valid only for this specific site. 3. Measurement uncertainties specified below refer to the standard uncertainty, assuming a normal distributed error. Although less likely, the actual deviation between measurement value and real value can be higher than the standard error. DEWI GmbH - Partial Reproduction not Permitted 3 / 173 Round Robin Numerical Flow Simulation in Wind Energy DEWI GmbH - Partial Reproduction not Permitted 4 / 173 Round Robin Numerical Flow Simulation in Wind Energy Table of Content 1 Preface...........................................................................................................................................9 2 Sponsors.......................................................................................................................................10 3 Setup of the Round Robin test.......................................................................................................11 3.1 The aim...................................................................................................................................11 3.2 The choice of the test site.......................................................................................................11 3.3 The dummy Run......................................................................................................................12 4 Description of the Participating Models.........................................................................................13 5 Description of the Main Test.........................................................................................................20 5.1 Input Data-set.........................................................................................................................20 5.1.1 Elevation..........................................................................................................................21 5.1.2 Land Cover.......................................................................................................................23 5.1.3 Meteorological data.........................................................................................................25 5.1.4 Coordinates.....................................................................................................................28 5.2 Output Data-set......................................................................................................................28 5.2.1 Mean Horizontal Wind Speed for terrain-following surfaces............................................28 5.2.2 Mean Wind Speed for the target locations.......................................................................29 5.2.3 Wind Statistics for target locations in 80 m above Ground Level......................................29 6 Assessment of the Results.............................................................................................................30 6.1 Weibull A- and k-Parameter....................................................................................................30 6.2 Mean Horizontal Wind Speed for a terrain-following surface at different heights...................33 6.3 General Remarks on the Evaluations Regarding Wind Speed...................................................43 6.3.1 Technical details on the assessment of the statistics........................................................43 6.4 Important Remarks regarding the Results of the Participant ANM...........................................46 6.5 Incomplete Data Submitted by Participant UNJ.......................................................................49 6.6 Incomplete Data Submitted by Participant REP.......................................................................49 6.7 Evaluations Regarding Mean Wind Speed...............................................................................50 6.7.1 Quantitative Evaluations with Respect to the Sector-wise Speed-Up Factors....................51 6.7.2 Deviations from all Participants Mean Values...................................................................54 DEWI GmbH - Partial Reproduction not Permitted 5 / 173 Round Robin Numerical Flow Simulation in Wind Energy 6.7.3 Mean Absolute Values of Sector-wise Errors....................................................................55 6.7.4 Overall Direction Independent Wind Speed Prediction Error............................................56 6.8 Evaluations Regarding Mean Squared Wind Speed..................................................................67 6.9 Evaluations Regarding Mean Cubed Wind Speed....................................................................77 6.10 Evaluations Regarding Wind Direction....................................................................................86 6.10.1 Qualitative Evaluation Regarding Veer Angles..................................................................86 6.10.2 Qualitative Evaluation Regarding Veer Angles: Target Mast 1...........................................86 6.10.3 Qualitative Evaluation Regarding Veer Angles: Target Mast 2...........................................87 6.10.4 Evaluations Regarding the Overall Wind Direction Distributions.......................................88 6.10.5 Assessment of Average Sector-Wise Wind Direction Errors..............................................90 6.10.6 Assessment of the Directional Dispersion of the Single Sector Wind Direction Results.....90 6.11 Single Turbine AEP................................................................................................................104 6.12 Energy Yield Results: Single Turbine AEP...............................................................................105 6.13 Wind Farm Energy Yield Results............................................................................................113 7 Conclusions.................................................................................................................................121 8 Appendix....................................................................................................................................125 8.1 Overview of the properties of the on-site measurements.....................................................125 8.1.1 Wind Direction Effects...................................................................................................129 8.1.2 Speed-up Factors...........................................................................................................132 8.1.3 Turbulence Intensity......................................................................................................135 8.1.4 Sector-wise Wind Speed Correlation Properties.............................................................142 8.1.5 Filling of a Gap in Target Mast 1 Data.............................................................................148 8.1.6 Filling of a Gap in Target Mast 2 Data.............................................................................152 8.2 Issues related to the site selection process............................................................................156 8.2.1 General Considerations..................................................................................................156 8.2.2 Wind Speed Measurement............................................................................................157 8.2.3 Spatial Arrangement......................................................................................................158 8.2.4 Wind Direction Measurements......................................................................................158 8.2.5 Exemplary illustrations of some problematic effects found during site search................161 8.2.6 Maps.............................................................................................................................166 DEWI GmbH - Partial Reproduction not Permitted 6 / 173 Round Robin Numerical Flow Simulation in Wind Energy 8.2.7 Forest............................................................................................................................166 8.2.8 Coordinates...................................................................................................................167 8.2.9 Data from Numerical Site Calibrations...........................................................................168 8.3 Output data Format..............................................................................................................170 8.3.1 Mean Horizontal Wind Speed for terrain following surfaces - Data Format.....................170 8.3.2 Mean Wind Speed for target locations - Data Format.....................................................170 8.3.3 Wind Statistics for target locations - Data Format..........................................................171 9 References..................................................................................................................................173 DEWI GmbH - Partial Reproduction not Permitted 7 / 173 Round Robin Numerical Flow Simulation in Wind Energy DEWI GmbH - Partial Reproduction not Permitted 8 / 173 Round Robin Numerical Flow Simulation in Wind Energy 1 Preface Wind energy has successfully developed world wide during the last years. Along with this good development, there have been continuous improvements of the tools used for the simulation of the wind flow conditions. Future installations are foreseen to concentrate over off-shore areas and to develop more and more over internal complex terrain sites. With the exploitation of wind energy over internal areas, the complexity of the surface and flow conditions makes the requirements for a proper wind resource assessment and for a correct definition of the financing risks even higher. Despite a general improvement of several aspects of resource assessment methods (such as the analysis of the anemometers effects, the long term correlation techniques etc. ), the methods for extrapolation of wind conditions from a measurement mast to a wind turbine site has remained more or less unchanged since the early nineties: The European Wind Atlas is a „de facto“ standard method for most of the energy yield calculations. Limitations and weaknesses of this methodology are well known and under complex terrain conditions (or other non-standard situations) large systematic errors are to be expected. An alternative solution to improve the reliability of resource assessment over complex orography is represented by the use of fluid dynamical numerical models. Presently, different typologies of numerical models and methodologies are adopted for wind resource evaluation. Most of them have been initially developed for purposes marginally related to wind energy applications such as automotive industry or weather forecasting. While in other sectors of research and engineering the numerical solution of the fluid dynamic equations has become a standard approach for the derivation of the flow properties, in wind energy the adoption of numerical flow models as an accepted standard method still lacks the necessary experience and still requires a systematic classification and a scientific evaluation of errors and of the related uncertainties. Such an evaluation would support decisions for a wide range of wind energy actors like banks, financing companies, developers, etc. It is believed that the presence of a scientific classification and an independent assessment of the myriad of different wind simulation methods will eventually improve the acceptability of the concept of numerical simulation on the whole. This project aims, as much as possible, to fill the aforementioned gaps. DEWI GmbH - Partial Reproduction not Permitted 9 / 173 Round Robin Numerical Flow Simulation in Wind Energy 2 Sponsors This research was sponsored by the following parties: - German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety - Bremer Landesbank - Commerzbank - Deutsche Immobilien Leasing GmbH - GE Energy - HSH Nordbank AG - Nord/LB - Ostwind Verw.ges. - WPD Beteiligungs GmbH - DEWI GmbH DEWI GmbH - Partial Reproduction not Permitted 10 / 173