Abstract

Whilst the total capacity of offshore wind farms in the North Sea continues to increase, there are considerable uncertainties in the prediction of actual power produced by them and how to improve their efficiency. The difficulty stems from the fact that the aerodynamics of large wind farms involves complex flow interactions across a wide range of scales, not only between neighbouring turbines within a wind farm but also between wind farms and the atmospheric boundary layer.

In this talk, I will present a combined theoretical and computational study of large wind farms conducted through a collaboration between Oxford and the UK Met Office over the last few years. I will start with a simple theoretical consideration of momentum balance across a large wind farm to derive a key relationship between a handful of non-dimensional parameters. Most of these parameters seem to depend largely on either ‘internal’ or ‘external’ characteristics of wind farm flow and not both, explaining why this so-called ‘two-scale momentum’ approach may help reduce the complexity of otherwise formidable multiscale flow problem. Some computational results are then presented to demonstrate the importance of an external parameter called ‘wind extractability’ factor, and an analytical model derived from the two-scale momentum approach is shown to predict a practical upper limit to the efficiency of large wind farms for a given wind extractability condition. The talk will end with some suggestions for future work, including data-driven modelling of wind extractability and its application to wind farm optimisation.