D1.3 Benchmark report on wake models at the wind farm scale

This report is a short summary of the activities done in the task T1.1 and an introduction to some of the articles and reports produced for the task 1.1.

Task 1.1 focuses on benchmarking the existing wind farm scale wake models of the EERA-DTOC partners. The ultimate goal of this task is to provide guidelines for the industry users on which model to use in a specific context and how to quantify the uncertainty of the results produced by the wind farm flow models. In order to draw this conclusion two benchmark campaigns on two different Danish offshore wind farms (Horns Rev and Lillgrund) were carried out in close collaboration with another international collaborative project the IEA-Task 31: WakeBench. The IEA-Task 31 has many similar goals as the Task 1.1 of EERA-DTOC, with a wider scope (also including wind farm in complex terrain, and in the vicinity of forest) and a wider range of participants (over 60 participants from various countries all over the world) that runs from 2012 to 2014. The purpose of the IEA-Task 31 is to reach an international consensus and establish guidelines on operation of wind farm flow models. Most participants in EERA-DTOC T1.1 are also participating in the IEA-Task 31, including CENER, which is leading the IEA-Task 31 and DTU, who processed the two common offshore wind farm benchmarking campaigns between the two projects. This gave EERA-DTOC a key position within IEA-Task 31 to influence the offshore benchmarks definitions and which metrics to use in the benchmark. It also provided an opportunity to EERA-DTOC participants to use datasets from offshore wind farms that are owned by companies that are not partners of EERA-DTOC (DONG Energy: Horns Rev and Vattenfall: Lillgrund).

The results of the two benchmarking campaigns indicate that many of the models significantly overpredict the maximum wake losses in comparison with the measurements. This is particularly the case for the CFD models that simulate the wake shape exactly, and less so for more empirically-based models. However, recent findings obtained within this project indicate that the discrepancy between the measurements and model results could be caused by a high uncertainty in the measurement of the wind direction. This high uncertainty causes the analysis of the measured data to produce artificially low power losses in the wake center because of direction variability. This finding challenges the traditional methods of comparing wind farm SCADA measurements with wind farm flow models. It has created a considerable amount of discussion within the wind farm flow modeler community, and this has been identified as one of the major tasks to be addressed within the IEA-Task 31: WakeBench project.

Because of this situation, we consider that it is necessary to reach some sort of an international consensus on how to properly validate wind farm flow models with measurements with high degree of uncertainty. We therefore recommend to the industry users to follow the guidelines planned to be established within the IEA-Task 31: WakeBench project.