Energy Dynamics of an Infinitely Large Offshore Wind Farm

Understanding of the energy dynamics of large offshore wind farms is particularly important given the scale of planned developments in Europe. At the end of 2008, almost 1.5GW of offshore wind farms had been installed, virtually all in Europe. According to the European Wind Energy Association the reference scenario expects 35 GW of offshore wind capacity in 2020 and 120 GW in 2030.

This is not unrealistic given targets for individual member countries e.g. the UK expects to install 33 GW by 2020 and Germany plans 25 GW by 2030. Given the economics of offshore wind energy the majority of development, particularly in the near-term, can be expected in the higher resource, moderate water depths of the North Sea rather than the Mediterranean.

There should therefore be significant interest in understanding the energy dynamics of the infinitely large wind farm – how wakes behave and whether the extraction of energy by wind turbines over a large area has a significant and lasting impact on the atmospheric boundary layer. Here we focus on developing understanding of the infinite wind farm through a combination of theoretical considerations, data analysis and modeling.

Initial evaluation of power losses due to wakes in the large Danish offshore wind farms at Horns Rev and Nysted indicated that losses were larger than expected. Temporary solutions have been found to account for this in wind farm models including use of an ‘added roughness’ block around the offshore wind farm.

In the long-term however physical understanding of wake interactions must be built and new models currently being developed must be evaluated at wind farms with different wind turbines, wind farm layouts and wind climates. Empirical information available indicates that the deficit in the row of rotors rather quickly asymptotes to a level which is of about the same magnitude as for the infinitely large wind farm.

We will examine whether this can be proved theoretically or is indicated by data currently available. We will also evaluate whether energy extraction at the likely scale of development in European Seas can be expected to modulate the downstream wind climate by consideration of i) the amount of energy extracted in comparison to the kinetic and total energy available within the boundary layer and ii) consideration of the turbulence added by large wind farm developments.