Mitigation of offshore wind farm cluster wake effects by low-specific-rating, low-induction turbines

Abstract. One of the major challenges for the large-scale expansion of offshore wind energy across several countries is the strong interaction between wind farm clusters within designated areas of high power density. The resulting cluster wake effects significantly reduce the annual energy production, especially during periods with low to medium wind speeds.

We analyse a potential mitigation strategy by comparing prospective expansion scenarios for the entire available area in the German Bight, employing a low-specific-rating, low-induction turbine concept, against a conventional 15MW reference design. While maintaining the rated power and wind farm layout, the innovative turbine, denoted the hybrid-lambda concept, has a 35 % larger rotor diameter with a corresponding higher hub height than the reference machine and a significantly reduced thrust coefficient above 6.8 ms^-1.

We perform mesoscale simulations with the induction-modified Fitch wind farm parametrisation for one entire year, representative of multi-decadal wind speed and direction statistics. We investigate boundary-layer interaction, inner- and inter-wind-farm wake effects, as well as power performance over the entire year and during critical grid situations, such as doldrums.

Maintaining a prescribed power density, the low-specific-rating, low-induction turbine provides an increase of 19.4 % in annual energy generation, the corresponding capacity factor, and the number of full-load hours compared to the conventional concept. The surplus is mainly caused by the larger swept rotor area, rather than by the increased hub height and the reduced inner-farm wake effects due to the low-induction rotor design, while we observed stronger wind speed deficits outside of the clusters.

Especially during periods of very low power production, our results show lower cluster-wake-induced wind speed reductions and higher generated power with lower fluctuations from hybrid-lambda turbines. In consequence, doldrums, i.e. periods when the total generated power of the German Bight is below a certain fraction of the installed power, are less frequent and have shorter duration.

In another simulation variant, the number and total installed power of the hybrid-lambda turbines were reduced by 22.0 % while still achieving the same energy output as generated by the original number of conventional turbines. Additional simulation setups show a lower decrease in performance in the German sector equipped with hybrid-lambda turbines compared to the reference when adding additional wind farm clusters in the prevailing wind direction within the Dutch exclusive economic zone. We conclude that the deployment of such innovative low-specific-rating, low-induction turbines can provide higher energy production at a prescribed power density or require a smaller number of installed turbines to achieve a given annual energy production especially in dense area with significant cluster wake effects. Overall, such turbines feature higher feed-in at low wind speeds and more stable power output, requiring less reserve power and possibly contributing to improved grid stability.